of 
 

 
 
 
 
FRAGMENTS OF SCIENCE 
 
 FOR 
 
 UNSCIENTIFIC PEOPLE: 
 
 A SERIES OF DETACHED ESSAYS, LECTURES, 
 AND REVIEWS. 
 
 BY 
 JOHN TYNDALL, LL. D., F. R. S., 
 
 ATITIIOE OF "HEAT AS A "** OF, MfflifTTr." "T-""~^"^" ON SOUKD," ETC., ETC. 
 
 ORK: 
 D. APPLETON AND COMPANY, 
 
 549 & 551 BROADWAY. 
 
 1871. 
 
PEEFACE. 
 
 MY MOTIVE in writing these papers was mainly that 
 which prompted the publication of my Royal Institu- 
 tion lectures ; a desire, namely, to extend sympathy for 
 science beyond the limits of the scientific public. 
 
 The fulfilment of this desire has caused a tempo- 
 rary and sometimes reluctant deflection of thought 
 from the line of original research. But considering 
 the result aimed at, and in part I trust achieved, I do 
 not regret the price paid for it. 
 
 I have carefully looked over all the articles here 
 printed, added a little, omitted a little in fact, tried 
 as far as my time permitted to render the work pre- 
 sentable. Most of the essays are of a purely scien- 
 tific character, and from those which are not, I have 
 endeavored, without veiling my convictions, to exclude 
 every word that could cause needless irritation. 
 
 From America the impulse came which induced me 
 to gather these " Fragments " together, and to my 
 friends in the United States I dedicate them. 
 
 JOHN TYNDALL. 
 
 ATHEN.EUM CLUB : March, 1871. 
 
CONTENTS. 
 
 I. THE CONSTITUTION OF NATURE, . . 1 
 
 II. THOUGHTS ON PRAYER AND NATURAL LAW, . . 33 
 
 III. MIRACLES AND SPECIAL PROVIDENCES, . . , .43 
 
 ^ IV. MATTER AND FORCE, . . . . . 71 
 
 V. AN ADDRESS TO STUDENTS, . . . . .95 
 
 VI. SCOPE AND LIMIT OF SCIENTIFIC MATERIALISM, . 107 
 
 x VII. SCIENTIFIC USE OF THE IMAGINATION, . . .125 
 
 VIII. ON RADIATION, ..... 1C7 
 
 IX. ON RADIANT HEAT IN RELATION TO THE COLOR AND CHEMI- 
 CAL CONSTITUTION OF BODIES, . . .211 
 1L ON CHEMICAL RATS AND THE STRUCTURE AND LIGHT OF 
 
 THE SKY, . . . . . .235 
 
 XI. DUST AND DISEASE, ..... 275 
 
 ADDITION TO "DUST AND DISEASE," . . . 323 
 
 XII. LIFE AND LETTERS OF FARADAY, . . . 329 
 
 XIII. AN ELEMENTARY LECTURE ON MAGNETISM, . . 353 
 XIV. SHORTER ARTICLES : 
 
 SLATES, 377 
 
 DEATH BY LIGHTNING, .... 397 
 
 SCIENCE AND SPIRITS, .... 402 
 
 VITALITY, . . . . . . 410 
 
 ADDITIONAL REMARKS ON MIRACLES, . 418 
 
I. 
 
 THE CONSTITUTION OF NATURE. 
 
 AN ESSAY. 
 [Fortnightly Review, vol. iii. p. 129.] 
 
" The gentle Mother of all 
 Showed me the lore of colors and of sounds ; 
 The innumerable tenements of beauty ; 
 The miracle of generative force ; 
 Far-reaching concords of Astronomy 
 Felt in the plants and in the punctual birds ; 
 Mainly, the linked purpose of the whole ; 
 And, chiefest prize, found I true liberty 
 The home of homes plain-dealing Nature gave." 
 
 RALPH WALDO EMERSON. 
 
I. 
 
 THE CONSTITUTION OF NATURE. 
 
 WE cannot think of space as finite, for wherever in 
 imagination we erect a boundary we are compelled to think 
 of space as existing beyond that boundary. Thus by the 
 incessant dissolution of limits we arrive at a more or less 
 adequate idea of the infinity of space. But though com- 
 pelled to think of space as unbounded, there is no mental 
 necessity to compel us to think of it either as filled or as 
 empty ; whether it is filled or empty must be decided by 
 experiment and observation. That it is not entirely void, 
 the starry heavens declare ; but the question still remains, 
 Are the stars themselves hung in vacuof Are the vast 
 regions which surround them, and across which their light 
 is propagated, absolutely empty ? A century ago the 
 answer to this question would be, " No, for particles of 
 light are incessantly shot through space." The reply of 
 modern science is also negative, but on a somewhat differ- 
 ent ground. It has the best possible reasons for rejecting 
 the idea of luminiferous particles ; but, in support of the 
 conclusion that the celestial spaces are occupied by matter, 
 it is able to offer proofs almost as cogent as those which 
 can be adduced for the existence of an atmosphere round 
 the earth. Men's minds, indeed, rose to a conception of 
 the celestial and universal atmosphere through the study 
 of the terrestrial and local one. Frqm the phenomena of 
 sound as displayed in the air, they ascended to the phe~ 
 
10 FRAGMENTS OF SCIENCE. 
 
 nomena of light as displayed in the ether / which is the 
 name given to the interstellar medium. 
 
 x The notion of this medium must not be considered as 
 \ a vague or fanciful conception on the part of scientific men. 
 Of its reality most of them are as convinced as they are of 
 the existence of the sun and moon. The luminiferous ether 
 has definite mechanical properties. It is almost infinitely 
 more attenuated than any known gas, but its properties are 
 those of a solid rather than of a gas. It resembles jelly 
 rather than air. A body thus constituted may have its 
 boundaries ; but, although the ether may not be coexten- 
 sive with space, we at all events know that it extends as 
 far as the most distant visible stars. In fact, it is the ve- 
 hicle of their light, and without it they could not be seen. 
 This all-pervading substance takes up their molecular tre- 
 mors, and conveys them with inconceivable rapidity to our or- 
 gans of vision. It is the transported shiver of bodies count- 
 less millions of miles distant, which translates itself in human 
 consciousness into the splendor of the firmament at night. 
 
 If the ether have a boundary, masses of ponderable 
 matter might be conceived to exist beyond it, but they 
 could emit no light. Beyond the ether dark suns might 
 burn ; there, under proper conditions, combustion might be 
 carried on ; fuel might consume unseen, and metals be 
 heated to fusion in invisible fires. A body, moreover, once 
 heated there, would continue forever heated ; a sun or 
 planet, once molten, would continue forever molten. For, 
 the loss of heat being simply the abstraction of molecular 
 motion by the ether, where this medium is absent no cool- 
 ing could occur. A sentient being, on approaching a heated 
 body in this region, would be conscious of no augmentation 
 of temperature. The gradations of warmth dependent on 
 the laws of radiation would not exist, and actual contact 
 would first reveal the heat of an extra ethereal sun. 
 
 Imagine a paddle-wheel placed in water and caused to 
 
THE CONSTITUTION OF NATURE. 11 
 
 rotate. From it, as a centre, waves would issue in all 
 directions, and a wader, as he approached the place of dis- 
 turbance, would be met by stronger and stronger waves. 
 This gradual augmentation of the impressions made upon 
 the wader's body is exactly analogous to the augmentation 
 of light when we approach a luminous source. In the one 
 case, however, the coarse common nerves of the body suf- 
 fice ; for the other we must have the finer optic nerve. But 
 suppose the water withdrawn; the action at a distance 
 would then cease, and, as far as the sense of touch is con- 
 cerned, the wader would be first rendered conscious of the 
 motion of the wheel by the actual blow of the paddles. 
 The transference of motion from the paddles to the water 
 is mechanically similar to the transference of molecular 
 motion from the heated body to the ether ; and the propa- 
 gation of waves through the liquid is mechanically similar 
 to the propagation of light and radiant heat. 
 
 As far as our knowledge of space extends, we are to 
 conceive it as the holder of the luminiferous ether, through 
 which are interspersed, at enormous distances apart, the 
 ponderous nuclei of the stars. Associated with the star 
 that most concerns us we have a group of dark planetary 
 masses revolving at various distances round it, each again 
 rotating on its own axis ; and, finally, associated with some 
 of these planets we have dark bodies of minor note the 
 moons. Whether the other fixed stars have similar plane- 
 tary companions or not is to us a matter of pure conjecture, 
 which may or may not enter into our conception of the 
 universe. But, probably, every thoughtful person believes, 
 with regard to those distant suns, that there is in space 
 something besides our system on which they shine. 
 
 Having thus obtained a general view of the present 
 condition of space, and of the bodies contained in it, we 
 may inquire whether things were so created at the begin- 
 ning. Was space furnished at once, "by the fiat of Omnipo- 
 
12 FRAGMENTS OF SCIENCE. 
 
 tcnce, with these burning orbs ? To this question the man 
 of science, if he confine himself within his own limits, will 
 give no answer, though it must be remarked that in the 
 formation of an opinion he has better materials to guide 
 him than anybody else. He can clearly show, however, 
 that the present state of things may be derivative. He 
 can even assign reasons which render probable its deriva- 
 tive origin that it was not originally what it now is. At 
 all events, he can prove that out of common non-luminous 
 matter this whole pomp of stars might have been evolved. 
 
 The law of gravitation enunciated by Newton is, that 
 every particle of matter in the universe attracts every other 
 particle with a force which diminishes as the square of the 
 distance increases. Thus the sun and the earth mutually 
 pull each other ; thus the earth and the moon are kept in 
 company ; the force which holds every respective pair of 
 masses together being the integrated force of their com- 
 ponent parts. Under the operation of this force, a stone 
 falls to the ground and is warmed by the shock ; under its 
 operation meteors plunge into our atmosphere and rise to 
 incandescence. Showers of such doubtless fall incessantly 
 upon the sun. Acted on by this force, were it stopped in 
 its orbit to-morrow 7 , the earth would rush toward, and finally 
 combine with, the sun. Heat would also be developed by 
 this collision, and Mayer, Helmholtz, and Thomson, have 
 calculated its amount. It would equal that produced by 
 the combustion of more than five thousand worlds of solid 
 coal, all this heat being generated at the instant of collision. 
 In the attraction of gravity, therefore, acting upon non- 
 luminous matter, we have a source of heat more powerful 
 than could be derived from any terrestrial combustion. And 
 were the matter of the universe cast in cold detached frag- 
 ments into space, and there abandoned to the mutual gravi- 
 tation of its own parts, the collision of the fragments would 
 in the end produce the fires of the stars. 
 
THE CONSTITUTION OF NATURE. 13 
 
 The action of gravity upon matter originally cold may, 
 in fact, be the origin of all light and heat, and the proxi- 
 mate source of such other powers as are generated by 
 light and heat. But we have now to inquire what is the 
 light and what is the heat thus produced ? This question 
 has already been answered in a general way. Both light 
 and heat are modes of motion. Two planets clash and 
 come to rest; their motion, considered as masses, is de- 
 stroyed, but it is really continued as a motion of their 
 ultimate particles. It is this motion, taken up by the 
 ether, and propagated through it with a velocity of one 
 hundred and eighty-five thousand miles a second, that comes 
 to us as the light and heat of suns and stars. The atoms 
 of a hot body swing with inconceivable rapidity, but this 
 power of vibration necessarily implies the operation of 
 forces between the atoms themselves. It reveals to us 
 that, while they are held together by one force, they are 
 kept asunder by another, their position at any moment de- 
 pending on the equilibrium of attraction and repulsion. The 
 atoms are virtually connected by elastic springs, which op- 
 pose at the same time their approach and their retreat, but 
 which tolerate the vibration called heat. When two bod- 
 ies drawn together by the force of gravity strike each other, 
 the intensity of the ultimate vibration, or, in other words, 
 the amount of heat generated, is proportional to the vis 
 viva destroyed by the collision. The molecular motion 
 once set up is instantly shared with the ether, and diffused 
 by it throughout space. 
 
 We on the earth's surface live night and day in the 
 midst of ethereal commotion. The medium is never still. 
 The cloud-canopy above us may be thick enough to shut 
 out the light of the stars, but this canopy is itself a warm 
 body, which radiates its motion through the ether. The 
 earth also is warm, and sends its heat-pulses incessantly 
 forth. It is the waste of its molecular motion in space 
 
14 FRAGMENTS OF SCIENCE. 
 
 that chills the earth upon a clear night ; it is the return 
 of its motion from the clouds which prevents the earth's 
 temperature on a cloudy night from falling so low. To the 
 conception of space being filled, we must therefore add 
 the conception of its being in a state of incessant tremor. 
 The sources of vibration are the ponderable masses of the 
 universe. Let us take a sample of these and examine it in 
 detail. When we look to our planet we find it to be an 
 aggregate of solids, liquids, and gases. When we look at 
 any one of these, we generally find it composed of still 
 more elementary parts. We learn, for example, that the 
 water of our rivers is formed by the union, in definite pro- 
 portions, of two gases, oxygen and hydrogen. We know 
 how to bring these constituents together, and to cause them 
 to form water : we also know how to analyze the water, 
 and recover from it its two constituents. So, likewise, as 
 regards the solid proportions of the earth. Our chalk-hills, 
 for example, are formed by a combination of carbon, oxy- 
 gen, and calcium. These are elements the union of which, 
 in definite proportions, has resulted in the formation of 
 chalk. The flints within the chalk we know to be a com- 
 pound of oxygen and silicium, called silica ; and our or- 
 dinary clay is, for the most part, formed by the union of 
 silicium, oxygen, and the well-known light metal, alumin- 
 ium. By far the greater portion of the earth's crust is 
 compounded of the elementary substances mentioned in 
 these few lines. 
 
 The principle of gravitation has been already described 
 as an attraction which every particle of matter, however 
 small, has for every other particle. With gravity there is 
 no selection ; no particular atoms choose, by preference, 
 other particular atoms as objects of attraction ; the attrac- 
 tion of gravitation is proportional to the quantity of the 
 attracting matter, regardless of its quality. But in the 
 molecular world which we have now entered matters are 
 
THE CONSTITUTION OF NATURE. 15 
 
 otherwise arranged. Here we have atoms between which 
 a strong attraction is exercised, and also atoms between 
 which a weak attraction is exercised. One atom can jostle 
 another out of its place in virtue of a superior force of at- 
 traction. But though the amount of force exerted varies 
 thus from atom to atom, it is still an attraction of the same 
 mechanical quality, if I may use the term, as that of grav- 
 ity itself. Its intensity might be measured in the same 
 way, namely, by the amount of motion which it can impart 
 in a certain time. Thus the attraction of gravity at the 
 earth's surface is expressed by the number thirty-two, be- 
 cause, when acting freely on a body for a second of time, 
 it imparts to the body a velocity of thirty-two feet a second. 
 In like manner the mutual attraction of oxygen and hydro- 
 gen might be measured by the velocity imparted to the 
 atoms in their rushing together. Of course such a unit of 
 time as a second is not here to be thought of, the whole 
 interval required by the atoms to cross the minute spaces 
 which separate them not amounting probably to more than 
 an inconceivably small fraction of a second. 
 
 It has been stated that when a body falls to the earth 
 it is warmed by the shock. Here we have what we may 
 call a mechanical combination of the earth and the body. 
 Suffer the falling body and the earth to dwindle in imagi- 
 nation to the size of atoms, and for the attraction of grav- 
 ity substitute that of chemical affinity, which is the name 
 given to the molecular attraction, we have then what is 
 called a chemical combination. The effect of the union in 
 this case also is the development of heat, and from the 
 amount of heat generated we can infer the intensity of the 
 atomic pull. Measured by ordinary mechanical standards, 
 this is enormous. Mix eight pounds of oxygen with one 
 of hydrogen, and pass a spark through the mixture ; the 
 gases instantly combine, their atoms rushing over the little 
 distances between them. Take a weight of forty-seven 
 
16 FRAGMENTS OF SCIENCE. 
 
 thousand pounds to an elevation of one thousand feet above 
 the earth's surface, and let it fall ; the energy with which 
 it would strike the earth would not exceed that of the eight 
 pounds of oxygen atoms as they dash against one pound 
 of hydrogen atoms to form water. 
 
 It is sometimes stated that the force of gravity is dis- 
 tinguished from all other forces by the fact of its resisting 
 conversion into any other. Chemical affinity, it is said, 
 can be converted into heat and light, and these again into 
 magnetism and electricity. But gravity refuses to be so 
 converted ; it is a force which maintains itself under all 
 circumstances, and is not capable of disappearing to give 
 place to another. If by this is meant that a particle of 
 matter can never be deprived of its weight, the assertion 
 is correct ; but the law which affirms the convertibility of 
 natural forces was never meant, in the minds of those who 
 understood it, to affirm that such a conversion as that here 
 implied occurs in any case whatever. As regards converti- 
 bility into heat, gravity and chemical affinity stand on pre- 
 cisely the same footing. The attraction in the one case is 
 as indestructible as in the other. Nobody affirms that 
 when a stone rests upon the surface of the earth the mutual 
 attraction of the earth and stone is abolished ; nobody 
 means to affirm that the mutual attraction oi oxygen for 
 hydrogen ceases after the atoms have combined to form 
 water. What is meant in the case of chemical affinity is, 
 that the pull of that affinity, acting through a certain space, 
 imparts a motion of translation of the one atom toward the 
 other. This motion of translation is not heat, nor is the 
 force that produces it heat. But when the atoms strike and 
 recoil, the motion of translation is converted into a motion 
 of vibration, and this latter motion is heat. But the vibra- 
 tion, so far from causing the extinction of the original at- 
 traction, is in part carried on by that attraction. The atoms 
 recoil in virtue of the elastic force which opposes actual 
 
THE CONSTITUTION OF NATURE. 17 
 
 contact, and in the recoil they are driven too far back. The 
 original attraction then triumphs over the force of recoil, 
 and urges the atoms once more together. Thus, like a pen- 
 dulum, they oscillate, until their motion is imparted to the 
 surrounding ether ; or, in other words, until their heat be- 
 comes radiant heat. 
 
 In this sense, and in this sense only, is chemical affinity 
 converted into heat. There is, first of all, the attraction 
 between the atoms ; there is, secondly, space between them. 
 Across this space the attraction urges them. They collide, 
 they recoil, they oscillate. There is a change in the form 
 of the motion, but there is no real loss. It is so with the 
 attraction of gravity. To produce motion here, space must 
 also intervene between the attracting bodies : when they 
 strike motion is apparently destroyed, but in reality there 
 is no destruction. Their atoms are suddenly urged together 
 by the shock ; by their own perfect elasticity these atoms 
 recoil ; and thus is set up the molecular oscillation which 
 announces itself to the nerves as heat. 
 
 It was formerly universally supposed that by the colli- 
 sion of unelastic bodies force was destroyed. Men saw, for 
 example, when two spheres of clay, or painter's putty, or 
 lead, were urged together, that the motion possessed by 
 the masses prior to impact was more or less annihilated. 
 They believed in an absolute destruction of the force of 
 impact. Until recent times, indeed, no difficulty was ex- 
 perienced in believing this, whereas, at present, the ideas 
 of force and its destruction refuse to be united in most 
 philosophic minds. In the collision of elastic bodies, on the 
 contrary, it was observed that the motion with which they 
 clashed together was in great part restored by the resiliency 
 of the masses, the more perfect the elasticity the more com- 
 plete being the restitution. This led to the idea of perfectly 
 elastic bodies bodies competent to restore by their recoil 
 the whole of the motion which they possessed before impact. 
 
18 FRAGMENTS OF SCIENCE. 
 
 Hence the idea of the conservation of force, as opposed 
 to the destruction of force, which was supposed to occur 
 when inelastic bodies met in collision. 
 
 We now know that the principle of conservation holds 
 equally good with elastic and unelastic bodies. Perfectly 
 elastic bodies develop no heat on collision. They retain 
 their motion afterward, though its direction may be changed ; 
 and it is only when sensible motion is, in whole or in part, 
 destroyed that heat is generated. This always occurs in 
 unelastic collision, the heat developed being the exact 
 equivalent of the motion extinguished. This heat virtually 
 declares that the property of elasticity, denied to the masses, 
 exists among their atoms, and by their recoil and oscillation 
 the principle of conservation is vindicated. 
 
 But ambiguity in the use of the term " force " has been 
 for some time more and more creeping upon us. We called 
 the attraction of gravity a force without any reference to 
 motion. A body resting on a shelf is as much pulled by 
 gravity as when after having been pushed off the shelf it 
 falls toward the earth. We applied the term force also to 
 that molecular attraction which we called chemical affinity. 
 When, however, we spoke of the conservation of force in 
 the case of elastic collision, we meant neither a pull nor a 
 push, which, as just indicated, might be exerted upon inert 
 matter, but we meant the moving force, if I may use the 
 term, of the colliding masses. 
 
 What I have called moving force has a definite me- 
 chanical measure in the amount of work that it can perform. 
 The simplest form of work is the raising of a weight. A 
 man walking up-hill or up-stairs with a pound weight in 
 his hand, to an elevation say of sixteen feet, performs a cer- 
 tain amount of work over and above the lifting of his own 
 body. If he ascend to a height of thirty-two feet, he does 
 twice the work ; if to a height of forty-eight feet, he does 
 throe times the work ; if to sixty-four feet, he does four 
 
THE CONSTITUTION OF NATURE. 19 
 
 times the work, and so on. If, moreover, he carries up two 
 pounds instead of one, other things being equal, he does 
 twice the work ; if three, four, or five pounds, he does three, 
 four, or five times the work. In fact, it is plain that the 
 work performed depends on two factors, the weight raised 
 and the height to which it is raised. It is expressed by the 
 product of these two factors. 
 
 But a body may be caused to reach a certain elevation 
 in opposition to the force of gravity, without being actually 
 carried up to the elevation. If a hodman, for example, 
 wished to land a brick at an elevation of sixteen feet above 
 the place where he stands, he would probably pitch it up to 
 the bricklayer. He would thus impart, by a sudden effort, 
 a velocity to the brick sufficient to raise it to the required 
 height; the work accomplished by that effort being pre- 
 cisely the same as if he had slowly carried up the brick. 
 The initial velocity which must be imparted in the case here 
 assumed, is well known. To reach a height of sixteen feet, 
 the brick must quit the man's hand with a velocity of 
 thirty-two feet a second. It is needless to say that a body 
 starting with any velocity, would, if wholly unopposed or 
 unaided, continue to move forever with the same velocity. 
 But when, in the case before us, the body is thrown upward, 
 it moves in opposition to gravity, which incessantly retards 
 its motion, and finally brings it to rest at an elevation of 
 sixteen feet. If not here caught by the bricklayer, it would 
 return to the hodman with an accelerated motion, and 
 reach his hand with the precise velocity it possessed on 
 quitting it. 
 
 Supposing the man competent to impart to the brick, at 
 starting, a speed of sixty-four feet a second, or twice its 
 former speed, would the amount of work performed in this 
 effort be only twice what it was in the first instance ? No ; 
 it would be four times that quantity. A body starting with 
 twice the velocity of another, will rise to four times the 
 
20 FRAGMENTS OF SCIENCE. 
 
 height; in like manner, a threefold velocity will give a 
 ninefold elevation, a fourfold velocity will give a sixteen- 
 fold elevation, and so on. The height attained, then, or the 
 work done, is not proportional to the velocity, but to the 
 square of the velocity. As before, the work is also pro- 
 portional to the weight elevated. Hence the work which 
 any moving masses whatever are competent to perform,, by 
 the motion which they at any moment possess, is jointly 
 proportional to the weight and the square of the velocity. 
 Here, then, we have a second measure of work, in which we 
 simply translate the idea of height into its equivalent idea 
 of motion. 
 
 In mechanics, the product of the mass of a moving body 
 into the square of its velocity, expresses what is called the 
 vis viva, or living force. It is also sometimes called the 
 "mechanical effect." If, for example, we point a cannon 
 upward, and start a ball with twice the velocity imparted 
 by a second cannon, the ball will rise to four times the 
 height. The speedier ball, if directed against a target, will 
 also do four times the execution. Hence the importance 
 of imparting a high velocity to projectiles in war. Having 
 thus cleared our way to a perfectly clear conception of the 
 vis viva of moving masses, we are prepared for the an- 
 nouncement that the heat generated by the collision of a 
 falling body against the earth is proportional to the vis 
 viva annihilated. In point of fact it is not an annihilation 
 at all, but a transference of vis viva from the mass, to its 
 ultimate particles. This, as we now learn, is proportional 
 to the square of the velocity. In the case, therefore, of two 
 cannon-balls of equal weight, if one strike a target with 
 twice the velocity of the other, it will generate four times 
 the heat ; if with three times the velocity, it will generate 
 nine times the heat, and so on. 
 
 Mr. Joule has shown that in falling from a height of 772 
 feet, a body will generate an amount of heat sufficient to 
 
THE CONSTITUTION OF NATURE. 21 
 
 raise its own weight of water one degree Fahrenheit in 
 temperature. We have here the mechanical equivalent of 
 heat. Now, a body falling from a height of 772 feet, has, 
 upon striking the earth, a velocity of 223 feet a second ; 
 and if this velocity were imparted to a body, by any other 
 means, the quantity of heat generated by the stoppage of 
 its motion would be that stated above. Six times that ve- 
 locity, or 1,338 feet, would not be an inordinate one for a 
 cannon-ball as it quits the gun ; but if animated by six 
 times the velocity, thirty-six times the heat will be gener- 
 ated by the stoppage of its motion. Hence a cannon-ball 
 moving with a velocity of 1,338 feet a second, would, by 
 collision, generate an amount of heat competent to raise its 
 own weight of water 36 degrees Fahrenheit in tempera- 
 ture. If composed of iron, and if all the heat generated 
 were concentrated in the ball itself, its temperature would 
 be raised about 360 degrees Fahrenheit ; because one de- 
 gree in the case of water is equivalent to about ten de- 
 grees in the case of iron. In artillery practice the heat 
 generated is usually concentrated upon the front of the 
 bolt, and on the portion of the target first struck. By this 
 concentration the heat developed may become sufficiently 
 intense to raise the dust of the metal to incandescence, a 
 flash of light often accompanying collision with the target. 
 
 Let us now fix our attention for a moment on the gun- 
 powder which urges the cannon-ball. This is composed of 
 combustible matter, which if burnt in the open air would 
 yield a certain amount of heat. It will not yield this 
 amount if it performs the work of urging a ball. The heat 
 then generated by the gunpowder will fall short of that 
 produced in the open air, by an amount equivalent to the 
 vis viva of the ball ; and this exact amount is restored by 
 the ball on its collision with the target. In this perfect 
 way are heat and mechanical motion connected. 
 
 Broadly enunciated, the principle of the conservation 
 
22 FRAGMENTS OF SCIENCE. 
 
 of force asserts that the quantity of force in the universe is 
 as unalterable as the quantity of matter ; that it is alike 
 impossible to create force and to annihilate it. But in 
 what sense are we to understand this assertion ? It would 
 be manifestly inapplicable to the force of gravity as New- 
 ton defined it ; for this is a force varying inversely as the 
 square of the distance, and to affirm the constancy of a 
 varying force would be self-contradictory. Yet, when the 
 question is properly understood, gravity forms no exception 
 to the law of conservation. Following the method pur- 
 sued by Helmholtz, I will here attempt an elementary ex- 
 position of this law, which, though destined in its applica- 
 tions to produce momentous changes in human thought, is 
 not difficult of comprehension. 
 
 For the sake of simplicity we will consider a particle of 
 matter, which we may call F, to be perfectly fixed, and a 
 second movable particle, D, placed at a distance from F. 
 We will assume that these two particles attract each other 
 according to the Newtonian law. At a certain distance the 
 attraction is of a certain definite amount, which might be 
 determined by means of a spring-balance. At half this dis- 
 tance the attraction w r ould be augmented four times ; at a 
 third of the distance it would be augmented nine times ; at 
 one-fourth of the distance sixteen times, and so on. In 
 every case the attraction might be measured by determin- 
 ing, with the spring-balance, the amount of tension which 
 is just sufficient to prevent D from moving toward F. 
 Thus far we have nothing whatever to do with motion ; we 
 deal with statics, not with dynamics. We simply take into 
 account the distance of D from Jf 9 and the pull exerted by 
 gravity at that distance. 
 
 It is customary in mechanics to represent the magni- 
 tude of a force by a line of a certain length, a force of 
 double magnitude being represented by a line of double 
 length, and so on. Placing then the particle D at a dis- 
 
THE CONSTITUTION OF NATURE. 23 
 
 tance from F, we can in imagination draw a straight line 
 from D to F, and at D erect a perpendicular to this line, 
 which shall represent the amount of the attraction exerted 
 on D in this position. If D be at a very great distance 
 from F the attraction will be very small, and the perpendic- 
 ular consequently very short. Let us now suppose that at 
 every point in the line joining F and D a perpendicular is 
 erected proportional in length to the attraction exerted at 
 that point ; we should thus obtain an infinite number of 
 perpendiculars of gradually increasing length as D ap- 
 proaches F. Uniting the ends of all these perpendiculars, 
 we should obtain a curve, and between this curve and the 
 straight line joining F and D we should have an area con- 
 taining all the perpendiculars placed side by side. Each 
 one of this infinite series of perpendiculars representing an 
 attraction, or tension as it is sometimes called, the area just 
 referred to represents the total effort capable of being ex- 
 erted by the tensions upon the particle D, during its pas- 
 sage from its first position up to F. 
 
 Up to the present point we have been dealing with ten- 
 sions, and not with motion. Thus far vis viva has been 
 entirely foreign to our contemplation of D and F. Let us 
 now suppose D placed at a practically infinite distance from 
 F ; here the pull of gravity would be nothing, and the per- 
 pendicular representing it would dwindle to a point. In 
 this position the sum of the tensions capable of being ex- 
 erted on D would be a maximum. Let D now begin to 
 move in obedience to the attraction exerted upon it. Mo- 
 tion being once set up, the idea of vis viva arises. In 
 moving toward F the particle D consumes, as it were, the 
 tensions. Let us fix our attention on D at any point of the 
 path over which it is moving. Between that point and F 
 there is a quantity of unused tensions ; beyond that point 
 the tensions have been all consumed, but we have in their 
 place an equivalent quantity of vis viva. After D has 
 
24 FRAGMENTS OF SCIENCE. 
 
 passed any point, the tension previously in store at that 
 point disappears, but not without having added, during the 
 infinitely small duration of its action, a due amount of 
 motion to that previously possessed by D. The nearer D 
 approaches to F, the smaller is the sum of the tensions re- 
 maining, but the greater is the living force ; the farther D 
 is from F, the greater is the sum of the unconsumed ten- 
 sions, and the less is the living force. Now the principle 
 of conservation affirms not the constancy of the value of the 
 tensions of gravity, nor yet the constancy of the vis viva, 
 taken separately, but the absolute constancy of the value 
 of the sum of both. At the beginning the vis viva was 
 zero and the tension area was a maximum ; close to F the 
 vis viva is a maximum, while the tension area is zero. At 
 every other point the work-producing power of the particle 
 D consists in part of vis viva and in part of tensions. 
 
 If gravity, instead of being attraction, were repulsion, 
 when the particles are in contact, the sum of the tensions 
 between two material particles D and F would be a maxi- 
 mum, and the vis viva zero. If D, in obedience to the 
 repulsion, moved away from F, vis viva would be gener- 
 ated ; and the farther D retreated from F the greater 
 would be its vis viva, and the less the amount of tension 
 still available for producing motion. Taking repulsion into 
 account as well as attraction, the principle of the conserva- 
 tion of force affirms that the mechanical value of the ten- 
 sions and vires vivce of the material universe is a constant 
 quantity. The universe, in short, possesses two kinds of 
 property which are mutually convertible, at an unvarying 
 rate. The diminution of either carries with it the enhance- 
 ment of the other, the total value of the property remain- 
 ing unchanged. 
 
 The considerations that we have here applied to gravity 
 apply equally to chemical affinity. In a mixture of oxygen 
 and hydrogen the atoms exist apart, but by the application 
 
THE CONSTITUTION OF NATURE. 25 
 
 of proper means they may be caused to rush together 
 across the space that separates them. While this space 
 exists, and as long as the atoms have not begun to move 
 toward each other, we have tensions and nothing else. 
 During their motion toward each other the tensions, as in 
 the case of gravity, are converted into vis viva. After 
 they clash we have still vis viva, but in another form. It 
 was translation, it is vibration. It was molecular transfer, 
 it is heat. The same considerations apply to a mixture of 
 hydrogen and chlorine. When these gases are mingled in 
 the dark they remain separate, but if a sunbeam fall upon 
 the mixture the atoms rush together with detonation. 
 Here also we have tension converted into molecular trans- 
 lation, and molecular translation into heat and sound. 
 
 It is possible to reverse these processes, to unlock the 
 embrace of the atoms and replace them in their first posi- 
 tions. But to accomplish this as much heat would be re- 
 quired as was generated by their union. Such reversals 
 occur daily and hourly in Nature. By the solar waves, the 
 oxygen of water is divorced from its hydrogen in the leaves 
 of plants. As molecular vis viva the waves disappear, but 
 in so doing they reendow the atoms of oxygen and hydro- 
 gen with tension. The atoms are thus enabled to recom- 
 bine, and when they do so they restore the precise amount 
 of heat consumed in their separation. The same remarks 
 apply to the compound of carbon and oxygen, called car- 
 bonic acid, which is exhaled from our lungs, produced by 
 our fires, and found sparingly diffused everywhere through- 
 out the air. In the leaves of plants the sunbeams also 
 wrench these atoms asunder, and sacrifice themselves in the 
 act ; but when the plants are burnt the amount of heat 
 consumed in their production is restored. 
 
 This, then, is the rhythmic play of Nature as regards 
 her forces. Throughout all her regions she oscillates from 
 tension to vis viva, from vis viva to tension. We have 
 2 
 
26 FRAGMENTS OF SCIENCE. 
 
 the same play in the planetary system. The earth's orbit 
 is an ellipse, one of the foci of which is occupied by the 
 sun. Imagine the earth at the most distant part of the 
 orbit. Her motion, and consequently her vis viva, is then 
 a minimum. The planet rounds the curve, and begins to 
 approach the sun. In front it has a store of tensions, 
 which is gradually consumed, an equivalent amount of vis 
 viva being generated. When nearest to the sun the mo- 
 tion, and consequently the vis viva, is a maximum. But 
 here the available tensions have been used up. The earth 
 rounds this portion of the curve and retreats from the sun. 
 Tensions are now stored up, but vis viva is lost, to be again 
 restored at the expense of the complementary force on the 
 opposite side of the curve. Thus beats the heart of the 
 universe, but without increase or diminution of its total 
 stock of force. 
 
 I have thus far tried to steer clear amid confusion by 
 fixing the mind of the reader upon things rather than upon 
 names. But good names are essential ; and here, as yet, 
 we are not provided with such. We have had the force of 
 gravity and living force two utterly distinct things. We 
 have had pulls and tensions ; and we might have had the 
 force of heat, the force of light, the force of magnetism, or 
 the force of electricity all of which terms have been em- 
 ployed more or less loosely by writers on . physics. This 
 confusion is happily avoided by the introduction of the 
 term " energy," embracing under it both tension and vis 
 viva. Energy is possessed by bodies already in motion ; 
 it is then actual, and we agree to call it actual or dynamic 
 energy. It is our old vis viva. On the other hand, energy 
 is possible to bodies not in motion, but which, in virtue of 
 attraction or repulsion, possess a power of motion which 
 would realize itself if all hinderances were removed. 
 Looking, for example, at gravity, a body on the earth's 
 surface in a position from which it cannot fall to a lower 
 
THE CONSTITUTION OF NATURE. 27 
 
 one possesses no energy. It has neither motion nor power 
 of motion. But the same body suspended at a height 
 above the earth has a power of motion though it may not 
 have exercised it. Energy is possible to such a body, and 
 we agree to call this potential energy. It embraces our old 
 tensions. We, moreover, speak of the conservation of en- 
 ergy instead of the conservation of force ; and say that the 
 sum of the potential and dynamic energies of the material 
 universe is a constant quantity. 
 
 A body cast upward consumes the actual energy of 
 projection, and lays up potential energy. When it reaches 
 its utmost height all its actual energy is consumed, its 
 potential energy being then a maximum. When it re- 
 turns, there is a reconversion of the potential into the 
 actual. A pendulum at the limit of its swing possesses 
 potential energy ; at the lowest point of its arc its energy 
 is all actual. A patch of snow resting on a mountain-slope 
 has potential energy ; loosened, and shooting down as an 
 avalanche, it possesses dynamic energy. The pine-trees 
 growing on the Alps have potential energy ; but rushing 
 down the Holzrinne of the wood-cutters they possess actual 
 energy. The same is true of the mountains themselves. 
 As long as the rocks w T hich compose them can fall to a 
 lower level, they possess potential energy, which is con- 
 verted into actual when the frost ruptures their cohesion 
 and hands them over to the action of gravity. The hammer 
 of the great bell of Westminster, when raised before strik- 
 ing, possesses potential energy ; when it falls, the energy 
 becomes dynamic ; and after the stroke, we have the 
 rhythmic play of potential and dynamic in the vibrations 
 of the bell. The same holds good for the molecular oscilla- 
 tions of a heated body. An atom is pressed against its 
 neighbor, and recoils. But the ultimate amplitude of the 
 recoil is soon attained, the motion of the atom in that 
 direction is checked, and for an instant its energy is all 
 
28 FRAGMENTS OF SCIENCE. 
 
 potential. It is then drawn toward 'its neighbor with 
 accelerated speed, thus, by attraction, converting its poten- 
 tial into dynamic energy. Its motion in this direction is 
 also finally checked, and, for an instant, again its energy is 
 all potential. It again retreats, converting, by repulsion, 
 its potential into dynamic energy, till the latter attains a 
 maximum, after which it is again changed into potential 
 energy. Thus, what is true of the earth, as she swings to 
 and fro in her yearly journey round the sun, is also true of 
 her minutest atom. We have wheels within wheels, and 
 rhythm within rhythm. 
 
 When a body is heated, a change of molecular arrange- 
 ment always occurs, and to produce this change heat is 
 consumed. Hence, a portion only of the heat communi- 
 cated to the body remains as dynamic energy. Looking 
 back on some of the statements made at the beginning of 
 this article, now that our knowledge is more extensive, we 
 see the necessity of qualifying them. When, for example, 
 two bodies clash, heat is generated ; but the heat, or molec- 
 ular dynamic energy, developed at the moment of collision, 
 is not the equivalent of the sensible dynamic energy de- 
 stroyed. The true equivalent is this heat, plus the potential 
 energy conferred upon the molecules by the placing of 
 greater distances between them. This molecular potential 
 energy is afterward, on the cooling of the body, converted 
 into heat. 
 
 Wherever two atoms capable of uniting together by 
 their mutual attractions exist separately, they form a store 
 of potential energy. Thus our woods, forests, and coal- 
 fields on the one hand, and our atmospheric oxygen on the 
 other, constitute a vast store of energy of this kind vast, 
 but far from infinite. We have, besides our coal-fields, 
 bodies in the metallic condition more or less sparsely dis- 
 tributed in the earth's crust. These bodies can be oxidized, 
 and hence are, so far as they go, stores of potential energy. 
 
THE CONSTITUTION OF NATURE. 29 
 
 But the attractions of the great mass of the earth's crust 
 are already satisfied, and from them no further energy can 
 possibly be obtained. Ages ago the elementary constitu- 
 ents of our rocks clashed together and produced the motion 
 of heat, which was taken up by the ether and carried away 
 through stellar space. It is lost forever as far as we are 
 concerned. In those ages the hot conflict of carbon, 
 oxygen, and calcium, produced the chalk and limestone 
 hills which are now cold ; and from this carbon, oxygen, 
 and calcium, no further energy can be derived. And so it 
 is with almost all the other constituents of the earth's 
 crust. They took their present form in obedience to mo- 
 lecular force ; they turned their potential energy into dy- 
 namic, and gave it to the universe ages before man 
 appeared upon this planet. For him a residue of potential 
 energy remains, vast truly in relation to the life and wants 
 of an individual, but exceedingly minute in comparison 
 with the earth's primitive store. 
 
 To sum up. The whole stock of energy or worJeing- 
 power in the world consists of attractions, repulsions, and 
 motions. If the attractions and repulsions are so circum- 
 stanced as to be able to produce motion, they are sources 
 of working-power, but not otherwise. As stated a moment 
 ago, the attraction exerted between the earth and a body 
 at a distance from the earth's surface is a source of working- 
 power; because the body can be moved by the attraction, 
 and in falling to the earth can perform work. When it 
 rests upon the earth's surface it is not a source of power or 
 energy, because it can fall no farther. But though it has 
 ceased to be a source of energy, the attraction of gravity still 
 acts as & force, which holds the earth and weight together. 
 
 The same remarks apply to attracting atoms and mole- 
 cules. As long as distance separates them, they can move 
 across it in obedience to the attraction, and the motion 
 thus produced may, by proper appliances, be caused to 
 
30 FRAGMENTS OF SCIENCE. 
 
 perform mechanical work. When, for example, two atoms 
 of hydrogen unite with one of oxygen, to form water, the 
 atoms are first drawn toward each other they move, they 
 clash, and then, by virtue of their resiliency, they recoil and 
 quiver. To this quivering motion we give the name of 
 heat. Now this atomic vibration is merely the redistribu- 
 tion of the motion produced by the chemical affinity ; and 
 this is the only sense in which chemical affinity can be said 
 to be converted into heat. We must not imagine the 
 chemical attraction destroyed, or converted into any thing 
 else. For the atoms when mutually clasped to form a 
 molecule of water, are held together by the very attraction 
 which first drew them toward each other. That which has 
 really been expended is the pull exerted through the space by 
 which the distance between the atoms has been diminished. 
 
 If this be understood it will be at once seen that gravity 
 may in this sense be said to be convertible into heat ; that 
 it is in reality no more an outstanding and inconvertible 
 agent, as it is sometimes stated to be, than chemical affin- 
 ity. By the exertion of a certain pull through a certain 
 space a body is caused to clash with a certain definite 
 velocity against the earth. Heat is thereby developed, 
 and this is the only sense in which gravity can be said to 
 be converted into heat. In no case is the force which pro- 
 duces the motion annihilated or changed into any thing 
 else. The mutual attraction of the earth and weight exists 
 when they are in contact as when they were separate ; 
 but the ability of that attraction to employ itself in the 
 production of motion does not exist. 
 
 The transformation, in this case, is easily followed by 
 the mind's eye. First, the weight as a whole is set in 
 motion by the attraction of gravity. This motion of the 
 mass is arrested by collision with the earth, being broken up 
 into molecular tremors, to which we give the name of heat. 
 
 And when we reverse the process, and employ those 
 
THE CONSTITUTION OF NATURE. 31 
 
 tremors of heat to raise a weight, as is done through the 
 intermediation of an elastic fluid in the steam-engine, a 
 certain definite portion of the molecular motion is de- 
 stroyed in raising the weight. In this sense, and this sense 
 only, can the heat be said to be converted into gravity, or, 
 more correctly, into potential energy of gravity. It is not 
 that the destruction of the heat has created any new 
 attraction, but simply that the old attraction has now a 
 power conferred upon it, of exerting a certain definite pull 
 in the interval between the starting-point of the falling 
 weight and its collision with the earth. 
 
 When, therefore, writers on the conservation of energy 
 speak of tensions being " consumed " and " generated," 
 they do not mean thereby that old attractions have been an- 
 nihilated, and new ones brought into existence, but that, 
 in the one case, the power of the attraction to produce 
 motion has been diminished by the shortening of the dis- 
 tance between the attracting bodies, and that in the other 
 case the power of producing motion has been augmented 
 by the increase of the distance. These remarks apply to 
 all bodies, whether they be sensible masses or molecules. 
 
 Of the inner quality that enables matter to attract 
 matter we know nothing; and the law of conservation 
 makes no statement regarding that quality. It takes the 
 facts of attraction as they stand, and affirms only the con- 
 stancy of working-power. That power may exist in the 
 form of MOTION ; or it may exist in the form of FORCE, with 
 distance to act through. The former is dynamic energy, 
 the latter is potential energy, the constancy of the sum of 
 both being affirmed by the law of conservation. The con- 
 vertibility of natural forces consists solely in transforma- 
 tions of dynamic into potential, and of potential into dy- 
 namic energy, which are incessantly going on. In no other 
 sense has the convertibility of force, at present, any scien- 
 tific meaning. 
 
II. 
 
 THOUGHTS ON PRAYER AND NATURAL 
 LAW. 
 
 AN EXTRACT. 
 [Mountaineering in 1861, p. 33.] 
 
'Aber im stillen Gemach entwirft bedcutendc Zirkel 
 
 Sinnend der Weise. 
 
 Folgt durcli die Liifte dem Klang, folgt durch den Aether dem Stralil, 
 
 Sucht das vertraute Gesetz in des Zufalls grausenden Wundern, 
 
 Sucht den ruhenden Pol in der Erscheinungen Flucht." 
 
 SCHILLER. 
 
II. 
 
 PRAYER AND NATURAL LAW. 
 
 THE aspects of Nature are more varied and impressive 
 in Alpine regions than elsewhere. The mountains in their 
 setting of deep-blue sky ; the glow of firmament and peaks 
 at sunrise and sunset ; the formation and distribution of 
 clouds ; the descent of rain, hail, and snow ; the stealthy 
 slide of glaciers and the rush of avalanches and rivers ; 
 the fury of storms ; thunder and lightning, with their 
 occasional accompaniment of blazing woods all these 
 things tend to excite the feelings and to bewilder the mind. 
 In this entanglement of phenomena it seems hopeless to 
 seek for law or orderly connection. And before the 
 thought of law dawned upon the human mind men natu- 
 rally referred these inexplicable effects to personal agency. 
 The savage saw in the fall of a cataract the leap of a spirit, 
 and the echoed thunder-peal was to him the hammer-clang 
 of an exasperated god. Propitiation of these terrible 
 powers was the consequence, and sacrifice was offered to 
 the demons of earth and air. 
 
 But observation tends to chasten the emotions and to 
 check those structural efforts of the intellect which have 
 emotion for their base. One by one natural phenomena 
 have been associated with their proximate causes; and 
 the idea of direct personal volition mixing itself in the! 
 economy of Nature is retreating more and more. Many of 
 us fear this tendency ; our faith and feelings are dear to us, 
 
36 FRAGMENTS OF SCIENCE. 
 
 and we look with suspicion and dislike on any philosophy, 
 the apparent tendency of which is to dry up the soul. 
 Probably every change from ancient savagery to our present 
 enlightenment excited, in a greater or less degree, a fear 
 of this kind. But the fact is, that we have not yet deter- 
 mined whether the form under which they now appear in 
 the world is necessary to the life and warmth of religious 
 feeling. We may err in linking the imperishable with the 
 transitory, and confound the living plant with the decaying 
 r -pole to which it clings. My object, however, at present is 
 j not to argue, but to mark a tendency. We have ceased to 
 propitiate the powers of Nature ceased even to pray for 
 things in manifest contradiction to natural laws. In Prot- 
 estant countries, at least, I think it is conceded that the 
 age of miracles is past. 
 
 The general question of miracles is at present in able 
 and accomplished hands ; and were it not so, my polemical 
 acquirements are so limited, that I should not presume to 
 enter upon a discussion of this subject on its entire merits. 
 But there is one little outlying point,' which attaches itself 
 x to this question, on which a student of science, without 
 quitting the ground which strictly belongs to him, may 
 offer a remark. 
 
 At the auberge near the foot of the Rhone glacier, I 
 met, in the summer of 1858, an athletic young priest, who, 
 after a solid breakfast, including a bottle of wine, informed 
 me that he had come up to " bless the mountains." This 
 was the annual custom of the place. Year by year the 
 Highest was entreated, by official intercessors, to make 
 such meteorological arrangements as should insure food 
 and shelter for the flocks and herds of the Valaisians. A 
 diversion of the Rhone, or a deepening of the river's bed, 
 would have been of incalculable benefit to the inhabitants 
 of the valley at the time I now mention. But the priest 
 would have shrunk from the idea of asking the Omnipo- 
 
PRAYER AND NATURAL LAW. 37 
 
 tent to open a new channel for the river, or to cause a 
 portion of it to flow over the Grimsel Pass, and down the 
 vale of Oberhasli to Brientz. This he would have deemed 
 a miracle, and he did not come to ask the Creator to per- 
 form miracles, but to do something which he manifestly 
 thought lay quite within the bounds of the natural and 
 non-miraculous. A Protestant gentleman, who was present 
 at the time, smiled at this recital. He had no faith in the 
 priest's blessing, still he deemed his prayer different in 
 kind from a request to open a new river-cut, or to cause 
 the water to flow up-hill. 
 
 In a similar manner we Protestants smile at the honest 
 Tyrolese priest, who, when he feared the bursting of a 
 glacier-dam, offered the sacrifice of the mass upon the ice 
 as a means of averting the calamity. That poor man did 
 not expect to convert the ice into adamant, or to strengthen 
 its texture so as to enable it to withstand the pressure of 
 the water ; nor did he expect that his sacrifice would cause 
 the stream to roll back upon its source and relieve him, by 
 a miracle, of its presence. But beyond the boundaries of 
 his knowledge lay a region where rain w T as generated, he 
 knew not how. He was not so presumptuous as to expect 
 a miracle, but he firmly believed that in yonder cloud-land 
 matters could be so arranged, without trespass on the 
 miraculous, that the stream which threatened him and his 
 flock should be caused to shrink within its proper bounds. 
 
 Both these priests fashioned that which they did not 
 understand to their respective wants and wishes. In their 
 case imagination wrought, unconditioned by a knowledge 
 of laws. A similar state of mind was long prevalent 
 among mechanicians ; many of whom, and some of them 
 extremely skilful ones, were occupied a century ago with 
 the question of a perpetual motion. They aimed at con- 
 structing a machine which should execute work without 
 the expenditure of power ; and many of them went mad 
 
38 FRAGMENTS OF SCIENCE. 
 
 in the pursuit of this object. The faith in such a consum- 
 mation, involving as it did immense personal interest to 
 the inventor, was extremely exciting, and every attempt to 
 destroy this faith was met by bitter resentment on the 
 part of those who held it. Gradually, however, as men 
 became more and more acquainted with the true functions 
 of machinery, the dream dissolved. The hope of getting 
 work out of mere mechanical combinations disappeared; 
 but still there remained for the speculator a cloud-land 
 denser than that which filled the imagination of the Tyrol- 
 ese priest, and out of which he still hoped to evolve per- 
 petual motion. There was the mystic store of chemic 
 force, which nobody understood; there were heat and 
 light, electricity and magnetism, all competent to produce 
 mechanical motions. 1 Here, then, is the mine in which 
 we must seek our gem. A modified and more refined form 
 of the ancient faith revived ; and, for aught I know, a rem- 
 nant of sanguine designers may at the present moment be 
 engaged on the problem which like-minded men in former 
 years left unsolved. 
 
 And why should a perpetual motion, even under modern 
 conditions, be impossible ? The answer to this question is 
 the statement of that great generalization of modern sci- 
 ence, which is known under the name of the Conservation 
 of Energy. This principle asserts that no power can make 
 its appearance in Nature without an equivalent expenditure 
 of some other power ; that natural agents are so related to 
 each other as to be mutually convertible, but that no new 
 agency is created. Light runs into heat ; heat into elec- 
 tricity ; electricity into magnetism ; magnetism into me- 
 chanical force ; and mechanical force again into light and 
 heat. The Proteus changes, but he is ever the same ; and 
 his changes in Nature, supposing no miracle to supervene, 
 are the expression, not of spontaneity, but of physical neces- 
 1 See Helmholtz " Wechselwirkung der Naturkrafte." 
 
PRAYER AND NATURAL LAW. 39 
 
 sity. A perpetual motion, then, is deemed impossible, be- 
 cause it demands the creation of force, whereas the principle 
 of Conservation is, no creation but infinite conversion. 
 
 It is an old remark that the law which moulds a tear 
 also rounds a planet. In the application of law in Nature 
 the terms great and small are unknown. Thus the principle 
 referred to teaches us that the Italian wind gliding over 
 the crest of the Matterhorn is as firmly ruled as the earth 
 in its orbital revolution round the sun ; and that the fall of 
 its vapor into clouds is exactly as much a matter of neces- 
 sity as the return of the seasons. The dispersion, there- 
 fore, of the slightest mist by the special volition of the 
 Eternal, would be as much a miracle as the rolling of the 
 Rhone over the Grimsel precipices and down Haslithal to 
 Brientz. 
 
 It seems to me quite beyond the present power of 
 science, to demonstrate that the Tyrolese priest, or his 
 colleague of the Rhone valley, asked for an " impossibility " 
 in praying for good weather ; but science can demonstrate 
 the incompleteness of the knowledge of Nature w r hich 
 limited their prayers to this narrow ground ; and she may 
 lessen the number of instances in which we " ask amiss," 
 by showing that we sometimes pray for the performance 
 of a miracle when we do not intend it. She does assert, 
 for example, that, without a disturbance of natural law, 
 quite as serious as the stoppage of an eclipse, or the rolling 
 of the St. Lawrence up the Falls of Niagara, no act of 
 humiliation, individual or national, could call one shower 
 from heaven, or deflect toward us a single beam of the sun. 
 
 Those, therefore, who believe that the miraculous is still 
 active in Nature, may, with perfect consistency, join in our 
 periodic prayers for fair weather and for rain : while those 
 who hold that the age of miracles is past, will refuse to 
 join in such petitions. And if these latter wish to fall back 
 upon such a justification, they may fairly urge that the 
 
40 FRAGMENTS OF SCIENCE. 
 
 latest conclusions of science are in perfect accordance with 
 the doctrine of the Master Himself, which manifestly was 
 that the distribution of natural phenomena is not affected 
 by moral or religious causes. " He maketh His sun to rise 
 on the evil and on the good, and sendeth rain on the just 
 and on the unjust." Granting " the power of Free-will in 
 man," so strongly claimed by Professor Mansel in his ad- 
 mirable defence of the belief in miracles, and assuming the 
 efficacy of free prayer to produce changes in external 
 Nature, it necessarily follows that natural laws are more or 
 less at the mercy of man's volition, and no conclusion 
 founded on the assumed permanence of those laws would 
 be worthy of confidence. 
 
 It is a wholesome sign for England that she numbers 
 among her clergy men wise enough to understand all this, 
 and courageous enough to act up to their knowledge. 
 Such men do service to the public character by encourag- 
 ing a manly and intelligent conflict with the causes of 
 disease and scarcity, instead of a delusive reliance on 
 supernatural aid. But they have also a value beyond this 
 local and temporary one. They prepare the public mind 
 for changes which, though inevitable, could hardly, without 
 such preparation, be wrought without violence. Iron is 
 strong ; still, water in crystallizing will shiver an iron 
 envelope, and the more unyielding the metal is, the worse 
 for its safety. There are men among us who would encom- 
 pass philosophic speculation by a rigid envelope, hoping- 
 thereby to restrain it, but in reality giving it explosive 
 force. If we want an illustration of this we have only to 
 look at modern Rome. In England, thanks to men of the 
 stamp to which I have alluded, scope is gradually given to 
 thought for changes of aggregation, and the envelope 
 slowly alters its form in accordance with the necessities o/ 
 the time. 
 
THE proximate origin of the foregoing slight article, and probably the 
 remoter origin of the next following one, was this : Some years ago, a 
 day of prayer and humiliation, on account of a bad harvest, was ap- 
 pointed by the proper religious authorities ; but certain clergymen of the 
 Church of England, doubting the wisdom of the demonstration, declined 
 to join in the services of the day. For this act of nonconformity they 
 were severely censured by some of their brethren. Rightly or wrongly, 
 my sympathies were on the side of these men ; and, to lend them a help- 
 ing hand in their struggle against odds, I inserted the foregoing chapter 
 in the little book mentioned on the title-page. Some time subsequently 
 I received from a gentleman of great weight and distinction in the scien- 
 tific world, and, I believe, of perfect orthodoxy in the religious one, a 
 note directing my attention to an exceedingly thoughtful article on 
 Prayer and Cholera in the Pall Mall Gazette. My eminent correspondent 
 deemed the article a fair answer to the remarks made by me in 1861. 
 I also was struck by the temper and ability of the article, but I could 
 not deem its arguments satisfactory, and, in a short note to the editor of 
 the Pall Mall Gazette, I ventured to state so much. This letter elicited 
 some very able replies, and a second leading article was also devoted to 
 the subject. In answer to all, I risked the publication of a second letter, 
 and soon afterward, by an extremely courteous note from the editor, the 
 discussion was closed. 
 
 Though thus stopped locally, the discussion flowed in other directions. 
 Sermons were preached, essays were published, articles were written, 
 while a copious correspondence occupied the pages of some of the re- 
 ligious newspapers. It gave me sincere pleasure to notice that the dis- 
 cussion, save in a few cases where natural coarseness had the upper 
 hand, was conducted with a minimum of vituperation. The severity 
 shown was hardly more than sufficient to demonstrate earnestness, while 
 gentlemanly feeling was too predominant to permit that earnestness to 
 contract itself to bigotry or to clothe itself in abuse. It was probably 
 the memory of this discussion which caused another excellent friend of 
 mine to recommend to my perusal the exceedingly able work which in 
 the next article I have endeavored to review. 
 
IIL 
 MIRACLES AND SPECIAL PROVIDENCES. 
 
 A REVIEW. 
 [Fortnightly Review, New Series, vol. i., p. 645.] 
 
" Mr. Mozley's book belongs to that class of writing of which Butler 
 may be taken as the type. It is strong, genuine argument about difficult 
 matters, fairly tracing what is difficult, fairly trying to grapple, not with 
 what appears the gist and strong point of a question, but with what really 
 at bottom is the knot of it. It is a book the reasoning of which may not 
 satisfy every one. . . . But we think it is a book for people who wish to 
 see a great subject handled on a scale which befits it, and with a percep- 
 tion of its real elements. It is a book which will have attractions for 
 those who like to see a powerful mind applying itself, without shrinking 
 or holding back, without trick, or reserve, or show of any kind, as a 
 wrestler closes body to body with his antagonist, to the strength of an 
 adverse and powerful argument." The Times, Tuesday, June 5, 1866. 
 
 " We should add, that the faults of the work are wholly on the surface 
 and in the arrangement ; that the matter is as solid and as logical as that 
 of any book within recent memory, and that it abounds in striking pas- 
 sages, of which we have scarcely been able even to give a sample. No 
 future arguer against miracles can afford to pass it over." Saturday Re- 
 view, September 15, 1866. 
 
III. 
 
 MIRACLES AND SPECIAL PROVIDENCES. 
 
 IT is my privilege to enjoy the friendship of a select 
 number of religious men, with whom I converse frankly 
 upon theological subjects, expressing without disguise the 
 notions and opinions I entertain regarding their tenets, and 
 hearing in return these notions and opinions subjected to 
 criticism. I have thus far found them liberal and loving 
 men, patient in hearing, tolerant in reply, who know how 
 to reconcile the duties of courtesy with the earnestness of 
 debate. From one of these, nearly a year ago, I received 
 a note, recommending strongly to my attention the volume 
 of " Bampton Lectures " for 1865, in which the question of 
 miracles is treated by Mr. Mozley. Previous to receiving 
 this note, I had in part made the acquaintance of the work, 
 through the able and elaborate review of it which had ap- 
 peared in the Times. The combined effect of the letter 
 and the review was to make the book the companion of 
 my summer tour in the Alps. There, during the wet and 
 snowy days which were only too prevalent last year, and 
 during the days of rest interpolated between days of toil, 
 I made myself more thoroughly conversant with Mr. Moz- 
 ley's volume. I found it clear and strong an intellectual 
 tonic, as bracing and pleasant to my mind as the keen air 
 of the mountains was to my body. From time to time I 
 jotted down my thoughts regarding it, intending afterward, 
 if time permitted, to work them up into a coherent whole. 
 
46 FRAGMENTS OF SCIENCE. 
 
 Other duties, however, interfered with the carrying out of 
 this intention, and what I wrote last summer I now pub- 
 lish, not hoping within any reasonable time to be able to 
 render my defence of scientific method more complete. 
 
 Mr. Mozley refers at the outset of his task to the move- 
 ment against miracles which of late years has taken place, 
 and which determined his choice of a subject. He acquits 
 modern science of having had any great share in the pro- 
 duction of this movement. The objection against miracles, 
 he says, does not arise from any minute knowledge of the 
 laws of Nature, but simply because they are opposed to 
 that plain and obvious order of Nature which everybody 
 sees. The present movement is, he thinks, to be ascribed to 
 the greater earnestness and penetration of the present age. 
 Formerly miracles were accepted without question, because 
 without reflection ; but the exercise of what Mr. Mozley 
 calls the historic imagination is a characteristic of our own 
 time. Men are now accustomed to place before themselves 
 vivid images of historic facts, and when a miracle rises to 
 view, they halt before the astounding occurrence, and real- 
 izing it with the same clearness as if it were now passing 
 before their eyes, they ask themselves, " Can this have 
 taken place ? " In some instances the effort to answer this 
 question has led to a disbelief in miracles, in others to a 
 strengthening of belief. The end and aim of Mr. Mozley's 
 lectures is to show that the strengthening of belief is the 
 logical result which ought to follow from the examination 
 of the facts. 
 
 Attempts have been made by religious men to bring 
 the Scripture miracles within the scope of the order of 
 Nature, but all such attempts are rejected by Mr. Mozley 
 as utterly futile and wide of the mark. Regarding mira- 
 cles as a necessary accompaniment of a revelation, their 
 evidential value in his eyes depends entirely upon their 
 deviation from the order of Nature. Thus deviating, they 
 
MIRACLES AND SPECIAL PROVIDENCES. 47 
 
 suggest and illustrate to him a power higher than Nature, 
 a " personal will ; " and they commend the person in whom 
 this power is vested as a messenger from on high. With- 
 out these credentials such a messenger would have no right 
 to demand belief, even though his assertion regarding his 
 divine mission were backed by a holy life. Nor is it by 
 miracles alone that the order of Nature is, or may be, dis- 
 turbed. The material universe is also the arena of " spe- 
 cial providences." Under these two heads Mr. Mozley dis- 
 turbs the total preternatural. One form of the preternatural 
 may shade into the other, as one color passes into another 
 in the rainbow ; but while the line which divides the spe- 
 cially providential from the miraculous cannot be sharply 
 drawn, their distinction broadly expressed is this, that 
 while a special providence can only excite surmise more 
 or less probable, it is " the nature of a miracle to give 
 proof, as distinguished from mere surmise of divine de- 
 sign." ^ 
 
 Mr. Mozley adduces various illustrations of what he re- 
 gards to be special providences as distinguished from mira- 
 cles. " The death of Arius," he says, " was not miraculous, 
 because the coincidence of the death of a heresiarch taking- 
 place when it was peculiarly advantageous to the orthodox 
 faith .... was not such as to compel the inference of ex- 
 traordinary Divine agency ; but it was a special providence, 
 because it carried a reasonable appearance of it. The mir- 
 acle of the Thundering Legion was a special providence, 
 but not a miracle, for the same reason, because the coinci- 
 dence of an instantaneous fall of rain in answer to prayer 
 carried some appearance, but not proof, of preternatural 
 agency." The eminent lecturer's remarks on this head 
 brought to my recollection certain narratives published in 
 Methodist magazines, which I used to read with avidity 
 when a boy. The title of these chapters, if I remember 
 right, was " The Providence of God asserted," and in them 
 
48 FRAGMENTS OF SCIENCE. 
 
 the most extraordinary and exciting escapes from peril were 
 recounted and ascribed to prayer, while equally wonderful 
 instances of calamity were adduced as illustrations of Di- 
 vine retribution. In such magazines, or elsewhere, I found 
 recorded the case of the celebrated Samuel Hick, which, as 
 it illustrates a whole class of special providences, approach- 
 ing in conclusiveness to miracles, is worthy of mention here. 
 It is related of this holy man and I, for one, have no doubt 
 of his holiness that flour was lacking to make the sacra- 
 mental bread.- Grain was present, and a windmill was 
 present, but there was no wind to grind the corn. With 
 faith, undoubting Samuel Hick prayed to the Lord of the 
 winds : the sails turned, the corn was ground, after which 
 the wind ceased. According to the canon of the Bampton 
 Lecturer, this, though carrying a strong appearance of an 
 immediate exertion of Divine energy, lacks by a hair's- 
 breadth the quality of a miracle. For the wind might have 
 arisen, and might have ceased, in the ordinary course of 
 Nature. Hence the occurrence did not " compel the infer- 
 ence of extraordinary Divine agency." In like manner Mr. 
 Mozley considers that "the appearance of the cross to 
 Con stan tine was a miracle, or a special providence, ac- 
 cording to which account of it we adopt. As only a mete- 
 oric appearance in the shape of a cross it gave some token 
 of preternatural agency, but not full evidence." 
 
 In the Catholic canton of Switzerland where I now 
 write, and still more among the pious Tyrolese, the moun- 
 tains are dotted with shrines, containing offerings of all 
 kinds, in acknowledgment of special mercies legs, feet, 
 arms, and hands of gold, silver, brass, and wood, according 
 as worldly possessions enabled the grateful heart to express 
 its indebtedness. Most of these offerings are made to the 
 Virgin Mary. They are recognitions of "special provi- 
 dences," wrought through the instrumentality of the Mother 
 of God. Mr. Mozley's belief, that of the Methodist chron- 
 
MIRACLES AND SPECIAL PROVIDENCES. 49 
 
 icier, and that of the Tyrolese peasant, are substantially the^ 
 same. Each of them assumes that Nature, instead of flow- 
 ing ever onward in the uninterrupted rhythm of cause and 
 effect, is mediately ruled by the free human will. As re- 
 gards direct action upon natural phenomena, man's will is 
 confessedly powerless, but it is the trigger which, by its 
 own free action, liberates the Divine power. In this sense, 
 and to this extent, man, of course, commands Nature. 
 
 Did the existence of this belief depend solely upon the 
 material benefits derived from it, it could not, in my opinion, 
 last a decade. As a purely objective fact we should soon 
 see that the distribution of natural phenomena is unaffected 
 by the merits or the demerits of man ; that the law of gravi- 
 tation crushes the simple worshippers of Ottery St. Mary, 
 while singing their hymns, just as surely as if they were 
 engaged in a midnight brawl. The hold of this belief upon 
 the human mind is not due to outward verification, but to 
 the inner warmth, force, and elevation with which it is com- 
 monly associated. It is plain, however, that these feelings 
 may exist under the most various forms. They are not 
 limited to Church of England Protestantism they are not 
 even limited to Christianity. Though less refined, they are 
 certainly not less strong, in the heart of the Methodist and 
 the Tyrolese than in the heart of Mr. Mozley. Indeed, those 
 feelings belong to the primal powers of man's nature. A 
 " skeptic " may have them. They find vent in the battle- 
 cry of the Moslem. They take hue and form in the hunting- 
 grounds of the red Indian ; and raise all of them, as they 
 raise the Christian, upon a wave of victory, above the ter- 
 rors of the grave. 
 
 The character, then, of a miracle, as distinguished from 
 a special providence, is that the former furnishes proof, 
 while in the case of the latter we have only surmise. Dis- 
 solve the element of doubt, and the alleged fact passes from 
 the one class of the preternatural into the other. In other 
 3 
 
50 FRAGMENTS OF SCIENCE. 
 
 words, if a special providence could be proved to be a spe- 
 cial providence, it would cease to be a special providence 
 and become a miracle. There is not the least cloudiness 
 about Mr. Mozley's meaning here. A special providence is 
 a doubtful miracle. Why, then, not use the correct phra- 
 seology ? The term employed conveys no negative sug- 
 gestion, whereas the negation of certainty is the peculiar 
 characteristic of the thing intended to be expressed. There 
 is an apparent unwillingness on the part of Mr. Mozley to 
 call a special providence what his own definition makes it 
 to be. Instead of speaking of it as a doubtful miracle, he 
 calls it " an invisible miracle." He speaks of the point of 
 contact of supernatural power with the chain of causation 
 being so high up as to be wiiolly, or in part, out of sight, 
 whereas the essence of a special providence is the uncer- 
 tainty whether there is any contact at all, either high or 
 low. By the use of an incorrect term, however, a grave 
 danger is avoided. For the idea of doubt, if kept system- 
 atically before the mind, would soon be fatal to the special 
 providence as a means of edification. The term employed, 
 on the contrary, invites and encourages the trust which is 
 necessary to supplement the evidence. 
 
 This inner trust, though at first rejected by Mr. Mozley 
 in favor of external proof, is subsequently called upon to 
 do momentous duty with regard to miracles. Whenever 
 the evidence of the miraculous seems incommensurate with 
 the fact which it has to establish, or rather when the fact 
 is so amazing that hardly any evidence is sufficient to estab- 
 lish it, Mr. Mozley invokes "the affections." They must 
 urge the reason to accept the conclusion from which unaided 
 it recoils. The affections and emotions are eminently the 
 court of appeal in matters of real religion, which is an affair 
 of the heart, but they are not, I submit, the court in which 
 to weigh allegations regarding the credibility of physical 
 facts. These must be judged by the dry light of the intel- 
 
MIRACLES AND SPECIAL PROVIDENCES. 51 
 
 lect alone, appeals to the affections being reserved for cases 
 where moral elevation, and not historic conviction, is the 
 aim. It is, moreover, because the result, in the case under 
 consideration, is deemed desirable that the affections are 
 called upon to back it. If undesirable, they would, with 
 equal right, be called upon to act the other way. Even to 
 the disciplined scientific mind this would be a dangerous 
 doctrine. A favorite theory the desire to establish or 
 avoid a certain result can so warp the mind as to destroy 
 its power of estimating facts. I have known men to work 
 for years under a fascination of this kind, unable to extri- 
 cate themselves from its fatal influence. They had certain 
 data, but not, as it happened, enough. By a process exactly 
 analogous to that invoked by Mr. Mozley they supplemented 
 the data, and went wrong. From that hour their intellects 
 were so blinded to the perception of adverse phenomena 
 that they never reached truth. If, then, to the disciplined 
 scientific mind, this incongruous mixture of proof and trust 
 be fraught with danger, what must it be to the indiscrimi- 
 nate audience which Mr. Mozley addresses ? In calling 
 upon this agency he acts the part of Frankenstein. It is 
 the monster thus evoked that we see stalking abroad, in 
 the so-called spiritualistic phenomena of the present day. 
 Again, I say, where the aim is to elevate the mind, to 
 quicken the moral sense, to kindle the fire of religion in the 
 soul, let the affections by all means be invoked ; but they 
 must not be permitted to color our reports, or to influence 
 our acceptance of reports of occurrences in external Nature. 
 Testimony as to natural facts is usually worthless when 
 wrapped in this atmosphere of the affections, the most 
 earnest subjective truth being thus rendered perfectly com- 
 patible with the most astounding objective error. 
 
 There are questions in judging of which the affections 
 or sympathies are often our best guides, the estimation of 
 moral goodness being one of these. But at this precise 
 
52 FRAGMENTS OF SCIENCE. 
 
 point, where they are really of use, Mr. Mozley excludes 
 the affections, and demands a miracle as a certificate of 
 character. He will not accept any other evidence of the 
 perfect goodness of Christ. " No outward life or conduct," 
 he says, " however irreproachable, could prove His perfect 
 sinlessness, because goodness depends upon the inward 
 motive, and the perfection of the inward motive is not 
 proved by the outward act." But surely the miracle is an 
 outward act, and to pass from it to the inner motive im- 
 poses a greater strain upon logic than that involved in our 
 ordinary methods of estimating men. There is, at least, 
 moral congruity between the outward goodness and the 
 inner life, but there is no such congruity between the mira- 
 cle and the life within. The test of moral goodness laid 
 down by Mr. Mozley is not the test of John, who says, " He 
 that doeth righteousness is righteous ; " nor is it the test 
 of Jesus " By their fruits ye shall know them ; do men 
 gather grapes of thorns, or figs of thistles ? " But it is the 
 test of another : " If thou be the Son of God, command that 
 these stones be made bread." For my own part, I prefer 
 the attitude of Fichte to that of Mr. Mozley. " The Jesus 
 of John," says this noble and mighty thinker, " knows no 
 other God than the True God, in whom we all are, and live, 
 and may be blessed, and out of whom there is only Death 
 and Nothingness. And he appeals, and rightly appeals, in 
 support of this truth, not to reasoning, but to the inward 
 practical sense of truth in man, not even knowing any other 
 proof than this inward testimony, 4 If any man will do the 
 will of Him who sent me, he shall know of the doctrine 
 whether it be of God." ' 
 
 Accepting Mr. Mozley's test, with which alone I am now 
 dealing, it is evident that, in the demonstration of moral 
 goodness, the quantity of the miraculous comes into play. 
 Had Christ, for example, limited Himself to the conversion 
 of water into wine, He would have fallen short of the per- 
 
MIRACLES AND SPECIAL PROVIDENCES. 53 
 
 formance of Jannes and Jambres, for it is a smaller thing to 
 convert one liquid into another than to convert a dead rod 
 into a living serpent. But Jannes and Jambres, we are in- 
 formed, were not good. Hence, if Mr. Mozley's test be a 
 true one, a point must exist, on the one side, of which 
 miraculous power demonstrates goodness, while on the other 
 side it does not. How is this " point of contrary flexure " 
 to be determined? It must lie somewhere between the 
 magicians and Moses, for within this space the power passed 
 from the diabolical to the Divine. But how to mark the 
 point of passage how, out of a purely quantitative differ- 
 ence in the visible manifestation of power we are to infer a 
 total inversion of quality it is extremely difficult to see. 
 Moses, we are informed, produced a large reptile, Jannes 
 and Jambres produced a small one. I do not possess the 
 intellectual faculty which would enable me to infer from 
 those data either the goodness of the one or the badness of 
 the other ; and in the highest recorded manifestations of the 
 miraculous I am equally at a loss. Let us not play fast and 
 loose with the miraculous ; either it is a demonstration of 
 goodness in all cases or in none. If Mr. Mozley accepts 
 Christ's goodness as transcendent, because He did such 
 works as no other man did, he ought, logically speaking, to 
 accept the works of those who, in His name, had cast out 
 devils, as demonstrating a proportionate goodness on their 
 part. But it is people of this class who are consigned to 
 everlasting fire prepared for the devil and his angels. Such 
 zeal as that of Mr. Mozley for miracles tends, I fear, to eat 
 his religion up. The logical threatens to stifle the spiritual. 
 The truly religious soul needs no miraculous proof of the 
 goodness of Christ. The words addressed to Matthew at 
 the receipt of custom required no miracle to produce obedi- 
 ence. It was by no stroke of the supernatural that Jesus 
 caused those sent to seize Him to go backward and fall to 
 the ground. It was the sublime and holy effluence from 
 
54 FRAGMENTS OF SCIENCE. 
 
 within, which needed no prodigy to commend it to the rev- 
 erence even of his foes. 
 
 As regards the function of miracles in the founding of a 
 religion, Mr. Mozley institutes a comparison between the 
 religion of Christ and that of Mahomet, and he derides the 
 latter as " irrational " because it does not profess to adduce 
 miracles in proof of its supernatural origin. But the re- 
 ligion of Mahomet, notwithstanding this drawback, has 
 thriven in the world, and at one time it held sway over 
 larger populations than Christianity itself. The spread and 
 influence of Christianity are, however, brought forward by 
 Mr. Mozley as " a permanent, enormous, and incalculable 
 practical result" of Christian miracles; and he actually 
 makes use of this result to strengthen his plea for the mirac- 
 ulous. His logical warrant for this proceeding is not clear. 
 It is the method of science, when a phenomenon presents 
 itself, to the production of which several elements may con- 
 tribute, to exclude them one by one, so as to arrive at 
 length at the truly effective cause. Heat, for example, is 
 associated with a phenomenon ; we exclude heat, but the 
 phenomenon remains : hence, heat is not its cause. Mag- 
 netism is associated with a phenomenon ; we exclude mag- 
 netism, but the phenomenon remains : hence, magnetism is 
 not its cause. Thus, also, when we seek the cause of the 
 diffusion of a religion whether it be due to miracles or to 
 the spiritual force of its founders we exclude the miracles, 
 and, finding the result unchanged, we infer that miracles 
 are not the effective cause. This important experiment 
 Mahometanism has made for us. It has lived and spread 
 without miracles ; and to assert, in the face of this, that 
 Christianity has spread because of miracles, is not more op- 
 posed to the spirit of science than to the common-sense of 
 mankind. 
 
 The incongruity of inferring moral goodness from mirac- 
 ulous power has been dwelt upon above ; in another par- 
 
MIRACLES AND SPECIAL PROVIDENCES. 55 
 
 ticular also the strain put upon miracles by Mr. Mozley is, 
 I think, more than they can bear. In consistency with his 
 principles, it is difficult to see how he is to draw from the 
 miracles of Christ any certain conclusion as to His Divine 
 nature. He dwells very forcibly on what he calls " the argu- 
 ment from experience," in the demolition of which he takes 
 evident delight. He destroys the argument, and repeats it 
 for the mere pleasure of again and again knocking the 
 breath out of it. Experience, he urges, can only deal with 
 the past ; and the moment we attempt to project experience 
 a hair's breadth beyond the point it has at any moment 
 reached, we are condemned by reason. It appears to me 
 that, when he infers from Christ's miracles a divine and 
 altogether superhuman energy, Mr. Mozley places himself 
 precisely under this condemnation. For what is his logical 
 ground for concluding that the miracles of the New Testa- 
 ment illustrate Divine power ? May they not be the result 
 of expanded human power ? A miracle he defines as some- 
 thing impossible to man. But how does he know that the 
 miracles of the New Testament are impossible to man ? 
 Seek as he may, he has absolutely no reason to adduce save 
 this that man has never hitherto accomplished such things. 
 But does the fact that man has never raised the dead prove 
 that he can never raise the dead ? " Assuredly not," must 
 be Mr. Mozley's reply ; " for this would be pushing experi- 
 ence beyond the limit it has now reached which I pro- 
 nounce unlawful." Then a period may come when man 
 will be able to raise the dead. If this be conceded and I 
 do not see how Mr. Mozley can avoid the concession it 
 destroys the necessity of inferring Christ's divinity from his 
 miracles. He, it may be contended, antedated the humanity 
 of the future ; as a mighty tidal-wave leaves high upon the 
 beach a mark which by-and-by becomes the general level 
 of the ocean. Turn the matter as you will, no other warrant 
 will be found for the all-important conclusion that Christ's 
 
56 FRAGMENTS OF SCIENCE. 
 
 miracles demonstrate Divine power, than an argument 
 which has been stigmatized by Mr. Mozley as " a rope of 
 sand " the argument from experience. 
 
 The learned Bampton Lecturer would be in this posi- 
 tion even if he had seen with his own eyes every miracle 
 recorded in the New Testament. But he has not seen these 
 miracles; and his intellectual plight is, therefore, worse. 
 He accepts these miracles on testimony. Why does he be- 
 lieve that testimony ? How does he know that it is not 
 delusion ; how is he sure that it is not even falsehood ? 
 He will answer that the writing bears the marks of sobriety 
 and truth ; and that, in many cases, the bearers of this mes- 
 sage to mankind sealed it with their blood. Granted with 
 all my heart ; but whence the value of all this ? Is it not 
 solely derived from the fact that men, as we know them, do 
 not sacrifice their lives in the attestation of that which they 
 know to be untrue ? Does not the entire value of the tes- 
 timony of the apostles depend ultimately upon our expe- 
 rience of human nature ? It appears, therefore, that those 
 who alleged to have seen the miracles based their inferences 
 from what they saw on the argument from experience ; and 
 that Mr. Mozley bases his belief in their testimony on the 
 same argument. The weakness of his conclusion is aug- 
 mented by this double insertion of a principle of belief to 
 which he flatly denies rationality. His reasoning, in fact, 
 cuts two ways if it destroys our trust in the order of Na- 
 ture, it far more effectually abolishes the basis on which 
 Mr. Mozley seeks to found the Christian religion. 
 
 Over this argument from experience, which, at bottom, 
 is his argument, Mr. Mozley rides rough-shod. There is a 
 dash of scorn in the energy with which he tramples on it. 
 Probably some previous writer had made too much of it, 
 and thus invited his powerful assault. Finding the diffi- 
 culty of belief in miracles to arise from their being in con- 
 tradiction to the order of Nature, he set himself to examine 
 
MIRACLES AND SPECIAL PROVIDENCES. 57 
 
 the grounds of our belief in that order. With a vigor of 
 logic rarely equalled, and with a confidence in its conclu- 
 sions never surpassed, he disposes of this belief in a manner 
 calculated to startle those who, without due examination, 
 had come to the conclusion that the order of Nature was 
 secure. 
 
 What we mean, he says, by our belief in the order of 
 Nature, is the belief that the future will be like the past. 
 There is not, according to Mr. Mozley, the slightest rational 
 basis for this belief. 
 
 " That any cause in Nature is more permanent than its existing and 
 known effects, extending further, and about to produce other and more 
 instances besides what it has produced already, we have no evidence. 
 Let us imagine," he continues, " the occurrence of a particular physical 
 phenomenon for the first time. Upon that single occurrence we should 
 have but the very faintest expectation of another. If it did occur again, 
 once or twice, so far from counting on another occurrence, a cessation 
 would occur as the most natural event to us. But let it continue one 
 hundred times, and we should find no hesitation in inviting persons from 
 a distance to see it ; and if it occurred every day for years, its occur- 
 rence would be a certainty to us, its cessation a marvel. . . . What 
 ground of reason can we assign for an expectation that any part of the 
 course of Nature will be the next moment what it has been up to this 
 moment, i. e., for our belief in the uniformity of Nature ? None. No 
 demonstrative reason can be given, for the contrary to the recurrence of 
 a fact of Nature is no contradiction. No probable reason can be given, 
 for all probable reasoning respecting the course of Nature is founded upon 
 this presumption of likeness, and, therefore, cannot be the foundation 
 of it. No reason can be given for this belief. It is without a reason. 
 It rests upon no rational grounds, and can be traced to no rational prin- 
 ciple." 
 
 " Every thing," Mr. Mozley, however, adds, " depends 
 upon this belief, every provision we make for the future, 
 every safeguard and caution we employ against it, all cal- 
 culation, all adjustment of means to ends supposes this be- 
 lief; and yet this belief has no more producible reason for 
 it than a speculation of fancy. ... It is necessary, all-im- 
 
58 FKAGMENTS OF SCIENCE. 
 
 portant for the purposes of life, but solely practical, and 
 possesses no intellectual character. . . . The proper func- 
 tion," continues Mr. Mozley, " of the inductive principle, 
 the argument from experience, the belief in the order of 
 Nature by whatever phrase we designate the same instinct 
 is to operate as a practical basis for the affairs of life and 
 the carrying on of human society." To sum up, the belief 
 in the order of Nature is general, but it is " an unintelligent 
 impulse, of which we can give no rational account." It is 
 inserted in our constitution solely to induce us to till our 
 fields, to raise our winter fuel, and thus to meet the future 
 on the perfectly gratuitous supposition that that future will 
 be like the past. 
 
 " Thus, step by step," says Mr. Mozley, with the empha- 
 sis of a man who feels his position to be a strong one, " has 
 philosophy loosened the connection of the order of Nature 
 with the ground of reason, befriending in exact proportion 
 as it has done this the principle of miracles." For " this 
 belief, not having itself a foundation in reason, the ground 
 is gone upon which it could be maintained that miracles, 
 as opposed to the order of Nature, are opposed to reason." 
 When we regard this belief in connection with science, " in 
 which connection it receives a more imposing name, and is 
 called the inductive principle," the result is the same. 
 " The inductive principle is only this unreasoning impulse 
 applied to a scientifically ascertained fact. . . . Science has 
 led up to the fact, but there it stops, and for converting 
 this fact into a law a totally unscientific principle comes 
 into play, the same as that which generalizes the common- 
 est observation of Nature." 
 
 The eloquent pleader of the cause of miracles passes 
 over without a word the results of scientific investigation 
 as proving any thing rational regarding the principles or 
 methods by which such results have been achieved. Here, 
 as before, he declines the test, " By their fruits shall ye 
 
MIRACLES AND SPECIAL PROVIDENCES. 59 
 
 know them." Perhaps the best way of proceeding will be 
 to give one or two examples of the mode in which men of 
 science apply the unintelligent impulse with which Mr. 
 Mozley credits them, and which shall show by illustration 
 the surreptitious character of the method by which they 
 climb from the region of facts to that of laws. 
 
 It was known before the sixteenth century that, the end 
 of an open tube being dipped into water, on drawing an 
 air-tight piston up the tube the water follows the piston, 
 and this fact had been turned to account in the construction 
 of the common pump. The effect was explained at the 
 time by the maxim, " Nature abhors a vacuum." It was 
 not known that there was any limit to the height to which 
 the water would ascend, until, on one occasion, the garden- 
 ers of Florence, while attempting to raise the water a very 
 great elevation, found that the column ceased at a height 
 of thirty-two feet. Beyond this all the skill of the pump- 
 maker could not get it to rise. The fact was brought to 
 the notice of Galileo, and he, soured by a world which had 
 not treated his science over-kindly, is said to have twitted 
 the philosophy of the time by remarking that Nature evi- 
 dently abhorred a vacuum only to a height of thirty-two 
 feet. But Galileo did not solve the problem. It was taken 
 up by his pupil Torricelli, who pondered it, and while he 
 did so various thoughts regarding it arose in his mind. It 
 occurred to him that the water might be forced up in the 
 tube by a pressure applied to'the surface of the water out- 
 side. But where, under the actual circumstances, was such 
 a pressure to be found ? After much reflection, it flashed 
 upon Torricelli that the atmosphere might possibly exert 
 the pressure ; that the impalpable air might possess weight, 
 and that a column of water thirty-two feet high might be 
 of the exact weight necessary to hold the pressure of the 
 atmosphere in equilibrium. 
 
 There is much in this process of pondering and its 
 
60 FRAGMENTS OF SCIENCE. 
 
 results which it is impossible to analyze. It is by a kind of 
 inspiration that we rise from the wise and sedulous con- 
 templation of facts to the principles on which they depend. 
 The mind is, as it were, a photographic plate, which is 
 gradually cleansed by the effort to think rightly, and which 
 when so cleansed, and not before, receives impressions from 
 the light of truth. This passage from facts to principles is 
 called induction, which in its highest form is inspiration ; 
 but, to make it sure, the inward sight must be shown 
 to be in accordance with outward fact. To prove or dis- 
 prove the induction, we must resort to deduction and ex- 
 periment. 
 
 Torricelli reasoned thus : If a column of water thirty- 
 two feet high holds the pressure of the atmosphere in 
 equilibrium, a shorter column of a heavier liquid ought to 
 do the same. Now, mercury is thirteen times heavier than 
 water ; hence, if my induction be correct, the atmosphere 
 ought to be able to sustain only thirty inches of mercury. 
 Here, then, is a deduction which can be immediately sub- 
 mitted to experiment. Torricelli took a glass tube a yard 
 or so in length, closed at one end and open at the other, 
 and filling it with mercury, he stopped the open end with 
 his thumb, and inverted it in a basin filled with the liquid 
 metal. One can imagine the feeling with which Torricelli 
 removed his thumb,, and the delight he experienced when 
 he found that his thought had forestalled a fact never before 
 revealed to human eyes. The column sank, but ceased to 
 sink at a height of thirty inches, leaving the Torricellian 
 vacuum overhead. From that hour the theory of the pump 
 was established. 
 
 The celebrated % Pascal followed Torricelli with a still 
 further deduction. He reasoned thus : If the mercurial 
 column be supported by the atmosphere, the higher we 
 ascend in the air the lower the column ought to sink, for 
 the less will be the weight of the air overhead. He ascend- 
 
MIRACLES AND SPECIAL PROVIDENCES. 61 
 
 ed the Puy de Dome, carrying with him a barometric 
 column, and found that as he ascended the mountain the 
 column sank, and that as he descended the column rose. 
 
 Between the time here referred to and the present, 
 millions of experiments have been made upon this subject. 
 Every village pump is an apparatus for such experiments. 
 In thousands of instances, moreover, pumps have refused 
 to work ; but on examination it has infallibly been found 
 that the well was dry, that the pump required priming, or 
 that some other defect in the apparatus accounted for the 
 anomalous action. In every case of the kind the skill of 
 the pump-maker has been found to be the true remedy. In 
 no case has the pressure of the atmosphere ceased ; con- 
 stancy, as regards the lifting of pump-water, has been 
 hitherto the demonstrated rule of Nature. So also as regards 
 Pascal's experiment. His experience has been the universal 
 experience ever since. Men have climbed mountains, and 
 gone up in balloons ; but no deviation from Pascal's result 
 has ever been observed. Barometers, like pumps, have 
 refused to act; but instead of indicating any suspension of 
 the operations of Nature, or any interference on the part of 
 its Author with atmospheric pressure, examination has in 
 every instance fixed the anomaly upon the instruments 
 themselves. It is this welding, then, of rigid logic to veri- 
 fying fact that Mr. Mozley refers to an " unreasoning im- 
 pulse." 
 
 Let us now briefly consider the case of Newton. Before 
 his time men had occupied themselves with the problem of 
 the solar system. Kepler had deduced, from a vast mass 
 of observations, the general expressions of planetary motion 
 known as " Kepler's laws." It had b,een observed that a 
 magnet attracts iron ; and by one of those flashes of inspi- 
 ration which reveal to the human mind the vast in the 
 minute, the general in the particular, it occurred to Kepler, 
 that the force by which bodies fall to the earth might also 
 
62 FRAGMENTS OF SCIENCE. 
 
 be an attraction. Newton pondered all these things. He 
 had a great power of pondering. He could look into the 
 darkest subject until it became entirely luminous. How 
 this light arises we cannot explain ; but, as a matter of 
 fact, it does arise. Let me remark here, that this power of 
 pondering facts is one with which the ancients could be 
 but imperfectly acquainted. They found the uncontrolled 
 exercise of the imagination too pleasant to expend much 
 time in gathering and brooding over facts. Hence it is that 
 when those whose education has been derived from the 
 ancients speak of " the reason of man," they are apt to 
 omit from their conception of reason one of its greatest 
 powers. "Well, Newton slowly marshalled his thoughts, or 
 rather they came to him while he "intended his mind," 
 rising one after another like a series of intellectual births 
 out of chaos. He made this idea of attraction his own. 
 But to apply the idea to the solar system, it was necessary 
 to know the magnitude of the attraction and the law of its 
 variation with the distance. His conceptions first of all 
 passed from the action of the earth as a whole, to that of 
 its constituents particles, the integration of which composes 
 the whole. And persistent thought brought more and 
 more clearly out the final divination, that every particle of 
 matter attracts every other particle by a force which varies 
 inversely as the square of the distance between the par- 
 ticles. This is Newton's celebrated law of inverse squares. 
 Here we have the flower and outcome of his induction ; and 
 how to verify it, or to disprove it, was the next question. 
 The first step of Newton in this direction was to prove, 
 mathematically, that if this law of attraction be the true 
 one ; if the earth be constituted of particles which obey 
 this law ; then the action of a sphere equal to the earth in 
 size on a body outside of it, is the same as that which 
 would be exerted if the whole mass of the sphere were 
 contracted to a point at its centre. Practically speaking, 
 
MIRACLES AND SPECIAL PROVIDENCES. 63 
 
 then, the centre of the earth is the point from which 
 distances must be measured to bodies attracted by the 
 earth. This was the first-fruit of his deduction. 
 
 From experiments executed before his time, Newton 
 knew the amount of the earth's attraction at the earth's sur- 
 face, or at a distance of 4,000 miles from its centre. His 
 object now was to measure the attraction at a greater dis- 
 tance, and thus to determine the law of its diminution. 
 But how was he to find a body at a sufficient distance ? 
 He had no balloon, and even if had, he knew that any height 
 which he could attain would be too small to enable him to 
 solve his problem. What did he do ? He fixed his thoughts 
 upon the moon a body at a distance of 240,000 miles, or 
 sixty times the earth's radius from the earth's centre. He 
 virtually weighed the moon, and found that weight to be 
 3-gVo^h of what it would be at the earth's surface. This is 
 exactly what his theory required. I will not dwell here upon 
 the pause of Newton after his first calculations, or speak of 
 his self-denial in withholding them, because they did not 
 quite agree with the observations then at his command. 
 Newton's action in this matter is the normal action of the 
 scientific mind. If it were otherwise if scientific men were 
 not accustomed to demand verification if they were satis- 
 fied with the imperfect while the perfect is attainable, their 
 science, instead of being, as it is, a fortress of adamant, 
 would be a house of clay, ill-fitted to bear the buffetings of 
 the theologic storms to which it has been from time to time, 
 and is at present exposed. 
 
 Thus, we see, that Newton, like Torricelli, first pondered 
 his facts, illuminated them with persistent thought, and 
 finally divined the character of the force of gravitation. But 
 having thus travelled inward to the principle, he had to re- 
 verse his steps, carry the principle outward, and justify it 
 by demonstrating its fitness to external Nature. This he 
 did by determining the attraction of the earth and moon. 
 
64 FRAGMENTS OF SCIENCE. 
 
 And here, in passing, I would notice a point which is 
 well worthy of attention. Kepler had deduced his laws 
 from observation. As far back as those observations ex- 
 tended, the planetary motions had obeyed these laws ; and, 
 neither Kepler nor Newton entertained a doubt as to their 
 continuing to obey them. Year after year, as the ages 
 rolled, they believed that those laws would continue to 
 illustrate themselves in the heavens. But this was not suf- 
 ficient. The scientific mind can find no repose in the mere 
 registration of sequence in Nature. The further question 
 intrudes itself with resistless might : whence comes the se- 
 quence ? What is it that binds the consequent with its an- 
 tecedent in Nature ? The truly scientific intellect never can 
 attain rest until it reaches the forces by which the observed 
 succession is produced. It was thus with Torricelli ; it was 
 thus with Newton ; it is thus preeminently with the real 
 scientific man of to-day. In common with the most igno- 
 mnt, he shares the belief that spring will succeed winter, 
 that summer will succeed spring, that autumn will succeed 
 summer, and that winter will succeed autumn. But he 
 knows still further and this knowledge is essential to his 
 intellectual repose that this succession, besides being per- 
 manent, is, under the circumstances, necessary that the 
 gravitating force exerted between the sun, and a revolving 
 sphere with an axis inclined to the plane of its orbit, must 
 produce the observed succession of the seasons. Not until 
 this relation between forces and phenomena has been es- 
 tablished is the law of reason rendered concentric with the 
 law of Nature, and not until this is effected does the mind 
 of the scientific philosopher rest in peace. 
 
 The expectation of likeness, then, in the procession of 
 phenomena is not that on which the scientific mind founds 
 its belief in the order of Nature. If the force be permanent 
 the phenomena are necessary, whether they resemble or do 
 not resemble any thing that has gone before. Hence, in 
 
MIRACLES AND SPECIAL PROVIDENCES. 65 
 
 judging of the order of Nature, our inquiries eventually 
 relate to the permanence of force. From Galileo to Newton, 
 from Newton to our own time, eager eyes have been scan- 
 ning the heavens, and clear heads have been pondering the 
 phenomena of the solar system. The same eyes and minds 
 have been also observing, experimenting, and reflecting on 
 the action of gravity at the surface of the earth. Nothing 
 has occurred to indicate that the operation of the law has 
 for a moment been suspended ; nothing has ever intimated 
 that Nature has been crossed by spontaneous action, or 
 that a state of things at any time existed which could not 
 be rigorously deduced from the preceding state. Given the 
 distribution of matter and the forces in operation in the 
 time of Galileo, the competent mathematician of that day 
 could predict what is now occurring in our own. We cal- 
 culate eclipses before they have occurred, and find them 
 true to the second. We determine the dates of those that 
 have occurred in the early times of history, and find calcu- 
 lations and history at peace. Anomalies and perturba- 
 tions in the planets have been over and over again observed, 
 but these, instead of demonstrating any inconstancy on the 
 part of natural law, have invariably been reduced to conse- 
 quences of that law. Instead of referring the perturba- 
 tions of Uranus to any interference on the part of the 
 Author of Nature with the law of gravitation, the question 
 which the astronomer proposed to himself was, " How, in 
 accordance with this law, can the perturbation be pro- 
 duced ? " Guided by a principle, he was enabled to fix the 
 point of space in which, if a mass of matter were placed, 
 the observed perturbations would follow. We know the 
 result. The practical astronomer turned his telescope tow- 
 ard the region which the intellect of the theoretic astrono- 
 mer had already explored, and the planet now named 
 Neptune was found in its predicted place. A very re- 
 spectable outcome, it will be admitted, of an impulse which 
 
66 FRAGMENTS OF SCIENCE. 
 
 " rests upon no rational grounds, and can be traced to no 
 rational principle ; " which possesses " no intellectual char- 
 acter;" which "philosophy" has uprooted from "the 
 ground of reason," and fixed in that " large irrational de- 
 partment " discovered for it by Mr. Mozley, in the hitherto 
 unexplored wildernesses of the human mind. 
 
 The proper function of the inductive principle, or the 
 -belief in the order of Nature, says Mr. Mozley, is " to act 
 as a practical basis for the affairs of life, and the carrying 
 on of human society." But what, it may be asked, has the 
 planet Neptune, or the belts of Jupiter, or the whiteness 
 about the poles of Mars, to do with the affairs of society ? 
 How is society affected by the fact that the sun's atmos- 
 phere contains sodium, or that the nebula of Orion contains 
 hydrogen gas ? Nineteen-twentieths of the force employed 
 in the exercise of the inductive principle, which, reiterates 
 Mr. Mozley, is "purely practical," have been expended upon 
 subjects as unpractical as these. What practical interest 
 has society in the fact that the spots on the sun have a 
 decennial period, and that when a magnet is closely 
 watched for half a century, it is found to perform small 
 motions which synchronize with the appearance and disap- 
 pearance of the solar spots ? And yet, I doubt not, Sir 
 Edward Sabine would deem a life of intellectual toil amply 
 rewarded by being privileged to solve, at its close, these 
 infinitesimal motions. 
 
 The inductive principle is founded in man's desire to 
 know a desire arising from his position among phenom- 
 ena which are reducible to order by his intellect. The 
 material universe is the complement of the intellect, and 
 without the study of its laws reason would never have 
 awoke to its higher forms of self-consciousness at all. It 
 is the non-ego, through and by which the ego is endowed 
 with self-discernment. We hold it to be an exercise of 
 reason to explore the meaning of a universe to which we 
 
MIRACLES AND SPECIAL PROVIDENCES. 67 
 
 stand in this relation, and the work we have accomplished 
 is the proper commentary on the methods we have pursued. 
 Before these methods were adopted the unbridled imagi- 
 nation roamed through Nature, putting in the place of 
 law the figments of superstitious dread. For thousands of 
 years witchcraft, and magic, and miracles, and special provi- 
 dences, and Mr. Mozley's " distinctive reason of man," had 
 the world to themselves. They made worse than nothing 
 of it worse, I say, because they let and hindered those 
 who might have made something of it. Hence it is that 
 during a single lifetime of this era of " unintelligent im- 
 pulse," the progress in natural knowledge is all but infinite 
 as compared with that of the ages which preceded ours. 
 
 The believers in magic and miracles of a couple of 
 centuries ago had all the strength of Mr. Mozley's present 
 logic on their side. They had done for themselves what 
 he rejoices in having so effectually done for us cleared the 
 ground of the belief in the order of Nature, and declared 
 magic, miracles, and witchcraft, to be matters for ordinary 
 evidence to decide. "The principle of miracles" thus 
 " befriended " had free scope, and we know the result. 
 Lacking that rock-barrier of natural knowledge which we, 
 laymen of England, now possess, keen jurists and cultivated 
 men were hurried on to deeds, the bare recital of which 
 makes the blood run cold. Skilled in all the rules of human 
 evidence, and versed in all the arts of cross-examination, 
 these men, nevertheless, went systematically astray, and 
 committed the deadliest wrongs against humanity. And 
 why ? Because they could not put Nature into the witness- 
 box, and question her ; of her voiceless " testimony " they 
 knew nothing. In all cases between man and man, their 
 judgment was to be relied on ; but in all cases between 
 man and Nature they were blind leaders of the blind. 1 
 
 1 " In 1664 two women were hung in Suffolk, under a sentence of Sir 
 Matthew Hale, who took the opportunity of declaring that the reality of 
 
68 FRAGMENTS OF SCIENCE. 
 
 Mr. Mozley concedes that it would be no great result 
 for miracles to be accepted by the ignorant and superstitious, 
 " because it is easy to satisfy those who do not inquire." 
 But he does consider it " a great result " that they have 
 been accepted by the educated. In what sense educated ? 
 Like those statesmen, jurists, and church dignitaries whose 
 education was unable to save them from the frightful errors 
 glanced at above? Not even in this sense; for the great 
 mass of Mr. Mozley's educated people had no legal training, 
 and must have been absolutely defenceless against delusions 
 which could set even that training at naught. Like nine- 
 tenths of our clergy at the present day, they were versed in 
 the literature of Greece, Borne, and Judea ; but as regards 
 a knowledge of Nature, which is here the one thing needful, 
 they were " noble savages," and nothing more. In the 
 case of miracles, then, it behooves us to understand the 
 weight of the negative, before we assign a value to the 
 positive ; to comprehend the protest of Nature before we 
 attempt to measure, with it, the assertions of men. We 
 have only to open our eyes to see what honest, and even 
 intellectual, men and women are capable of in the way of 
 evidence in this nineteenth century of the Christian era, 
 and in latitude fifty-two degrees north. The experience 
 thus gained ought, I imagine, to influence our opinion 
 regarding the testimony of people inhabiting a sunnier 
 clime, with a richer imagination, and without a particle of 
 that restraint which the discoveries of physical science have 
 imposed upon mankind. 
 
 witchcraft was unquestionable ; ' for first, the Scriptures had affirmed so 
 much ; and secondly, the wisdom of all nations had provided laws against 
 such persons, which is an argument of their confidence of such a crime.' 
 Sir Thomas Browne, who was a great physician as well as a great writer, 
 was called as a witness, and swore ' that he was clearly of opinion that 
 the persons were bewitched.' " Lecky's History of Rationalism, vol. i. 
 p. 120. 
 
MIRACLES AND SPECIAL PROVIDENCES. 69 
 
 Having thus submitted Mr. Mozley's views to the ex- 
 amination which they challenged at the hands of a student 
 of the order of Nature, I am unwilling to quit his book 
 without expressing my high admiration and respect for his 
 ability. His failure, as I consider it to be, must, I think, 
 await all attempts, however able, to deal with the material 
 universe by logic and imagination, unaided by experiment 
 and observation. With regard to the style of the book, I 
 willingly subscribe to the description with which the Times 
 winds up its able and appreciative review. " It is marked 
 throughout with the most serious and earnest conviction, 
 but is without a single word from first to last of asperity or 
 insinuation against opponents, and this not from any de- 
 ficiency of feeling as to the importance of the issue, but 
 from a deliberate and resolutely maintained self-control, 
 and from an overruling, ever-present sense of the duty, on 
 themes like these, of a more than judicial calmness." l 
 
 [ To the argument regarding the quantity of the mirac- 
 ulous, introduced at page 52, Mr. Mozley has done me the 
 honor of publishing a reply in the seventh volume of the 
 Contemporary Jteview. J. T., 1871.] 
 
 1 See Appendix at the end of the book. 
 
 Library 
 
 California 
 
IV. 
 MATTER AND FORCE. 
 
 A LECTURE TO THE WORKING-MEN OF DUNDEE. 
 
 September 5, 1867. 
 
Heard are the voices, 
 Heard are the sages, 
 The worlds and the ages, 
 * Choose well, your choice is 
 Brief and yet endless. 
 
 ' ' Here eyes do regard you 
 In eternity's stillness ; 
 Here is all fulness 
 Ye brave to reward you, 
 Work and despair not.' " 
 
 GOETHE. 
 
IT. 
 
 MATTER AND FORGE. 
 
 IT is the custom of the Professors in the Royal School 
 of Mines in London to give courses of evening lectures 
 every year to working-men. Each course is duly adver- 
 tised, and at a certain hour the working-men assemble to 
 purchase tickets for the course. The lecture-room holds 
 six hundred people, and tickets to this amount are disposed 
 of as quickly as they can be handed to those who apply for 
 them. So desirous are the working-men of London to 
 attend these lectures, that the persons who fail to obtain 
 tickets always bear a large proportion to those who suc- 
 ceed. Indeed, if the lecture-room could hold two thousand 
 instead of six hundred, I do not doubt that every one of its 
 benches would be occupied on these occasions. It is, 
 moreover, worthy of remark that the lectures are but rarely 
 of a character which could help the working-man in his 
 daily pursuits. The knowledge acquired is hardly ever of 
 a nature which admits of being turned into money. It is a 
 pure desire for knowledge, as a thing good in itself, and 
 without regard to its practical application, which animates 
 these men. They wish to know more of the wonderful 
 universe around them ; their minds desire this knowledge 
 as naturally as their bodies desire food and drink, and to 
 satisfy this intellectual want they come to the School of 
 Mines. 
 
 It is also my privilege to lecture to another audience in 
 4 
 
74 FRAGMENTS OF SCIENCE. 
 
 London, composed in part of the aristocracy of rank, 
 while the audience just referred to is composed wholly of 
 the aristocracy of labor. As regards attention and cour- 
 tesy to the lecturer, neither of these audiences has any 
 thing to learn of the other ; neither can claim superiority 
 over the other. I do not, however, think that it would 
 / be quite correct to take those persons who flock to the 
 School of Mines as average samples of their class ; they 
 are probably picked men the aristocracy of labor, as I have 
 just called them. At all events, their conduct demonstrates 
 that the essential qualities of a gentleman are confined 
 to no class, and they have often raised in my mind the 
 wish that the gentlemen of all classes, artisans as well as 
 lords, could, by some process of selection, be sifted from 
 the general mass of the community, and caused to know 
 each other better. 
 
 When pressed some months ago by the Council of the 
 British Association to give an evening lecture to the work- 
 ing-men of Dundee, my experience of the working-men of 
 London naturally rose to my mind ; and, though heavily 
 weighted with other duties, I could not bring myself to de- 
 cline the request of the Council. Hitherto, the evening 
 discourses of the Association have been delivered before 
 its members and associates alone. But after the meeting 
 at Nottingham, last year, where the working-men, at their 
 own request, were addressed by our late President, Mr. 
 Grove, and by my excellent friend Professor Huxley, the 
 idea rose of incorporating with all subsequent meetings of 
 the Association an address to the working-men of the town 
 in which the meeting is held. A resolution to that effect 
 was sent to the Committee of Recommendations ; the com- 
 mittee supported the resolution ; the Council of the Asso- 
 ciation ratified the decision of the committee ; and here I 
 am to carry out to the best of my ability their united 
 wishes. 
 
MATTER AND FORCE. 75 
 
 Whether it be a consequence of long-continued develop- 
 ment, or an endowment conferred once for all on man at 
 his creation, we find him here gifted with a mind, curious 'Is 
 to know the causes of things, and surrounded by objects 
 which excite its questionings, and raise the desire for an 
 explanation. It is related of a young prince of one of the 
 Pacific Islands, that when he first saw himself in a looking- 
 glass, he ran round the glass to see who was standing at 
 the back. And thus it is with the general human intellect, 
 as regards the phenomena of the external world. It wishes 
 to get behind and learn the causes and connections of these * 
 phenomena. What is the sun, what is the earth, what 
 should we see if we came to the edge of the earth and 
 looked over? What is the meaning of thunder and light- 
 ning, of hail, rain, storm, and snow ? Such questions pre- 
 sented themselves to early men, and by-and-by it was dis- 
 covered, that this desire for knowledge was not implanted 
 in vain. After many trials it became evident that man's 
 capacities were, so to speak, the complement of Nature's - 
 facts, and that, within certain limits, the secret of the uni- 
 verse was open to the human understanding. It was found 
 that the mind of man had the power of penetrating far be- 
 yond the boundaries of his five senses ; that the things 
 which are seen in the material world depend for their action 
 upon things unseen ; in short, that besides the phenomena 
 which address the senses, there are laws and principles and 
 processes which do not address the senses at all, but which 
 must be, and can be, spiritually discerned. 
 
 There are two things which form, so to say, the sub- 
 stance of all scientific thought. The entire play of the 
 scientific intellect is confined to the combination and res- 
 olution of the ideas of matter and force. Newton, it is 
 said, saw an apple fall. To the common mind this pre- 
 sented no difficulty and excited no question. Not so with 
 Newton. He observed the fact ; but one side of his great 
 
76 FRAGMENTS OF SCIENCE. 
 
 intellectual nature was left unsatisfied by the mere act of 
 observation. He sought after the principle which ruled 
 the fact. Whether this anecdote be true or not, it illus- 
 trates how the ordinary operations of Nature, which most 
 people take for granted as perfectly plain and simple, are ^ 
 often those which most puzzle the scientific man. To the 
 conception of the matter of the apple, Newton added that 
 of the force that moved it. The falling of the apple was 
 due to an attraction exerted mutually between it and the 
 earth. He applied the idea of this force to suns, and plan- 
 ets, and moons, and showed that all their motions were 
 necessary consequences of this attraction. 
 
 Newton, you know, was preceded by a grand fellow 
 named John Kepler a true working-man who, by analyz- 
 ing the astronomical observations of his master, Tycho 
 Brahe, had actually found that the planets moved as they are 
 now known to move. As a matter of fact, Kepler knew as 
 much about the motion of the planets as Newton did ; in 
 fact, Kepler taught Newton and the world generally the 
 facts of planetary motion. But this was not enough. The 
 question arose Why should the facts be so ? This was 
 the great question for Newton, and it was the solution of 
 this question which renders his name and fame immortal. 
 He proved that the planetary motions were what observa- 
 tion made them to be, because every particle of matter in 
 the solar system attracts every other particle by a force 
 which varies as the inverse square of the distance between 
 the particles. He showed that the moon fell toward the 
 earth, and that the planets fell toward the sun, through the 
 operation of the same force that pulls an apple from its 
 tree. This all-pervading force, which forms the solder of 
 the material universe, and the conception of which was 
 necessary to Newton's intellectual peace, is called the force " 
 of gravitation. 
 
 All force may be ultimately reduced to a push or a pull in 
 
^ MATTER AND FORCE. 77 
 
 a straight line ;*but its manifestations are various, and some- 
 times so complex as entirely to disguise its elementary con- 
 stituents. Its different manifestations have received differ- 
 ent names. Here, for example, is a magnet freely suspended. 
 I bring the end of a second magnet near one of the ends 
 of the suspended one attraction is the consequence. I re- 
 verse the position of one of the magnets repulsion follows. 
 This display of power is called magnetic force. In the 
 case of gravitation we have a simple attraction, in the case 
 of magnetism attraction and repulsion always go together. 
 Thus magnetism is a double force, or, as it is usually called, 
 a polar force. I present a bit of common iron to the magnet, 
 the iron itself becomes a temporary magnet, and it now 
 possesses the power of attracting other iron. And if sev- 
 eral pieces of iron be presented at the same time, not only 
 will the magnet act on them, but they will also act upon 
 each other. 
 
 This leads me to j&if experiment which will give you 
 some idea of how bodies arrange themselves under the 
 operation of a polar force. Underneath this plate of glass 
 is placed a small magnet, and by an optical arrangement 
 comprising a powerful lamp, a magnified image of the mag- 
 net is now cast upon the screen before you. I scatter iron 
 filings over the glass. You already notice a certain arrange- 
 ment of the particles of iron. Their free action is, how- 
 ever, hampered by friction. I therefore tap the glass, 
 liberate the particles, which, as I tap, arrange themselves in 
 these beautiful curves. This experiment is intended to 
 make clear to you how a definite arrangement of particles 
 a kind of incipient structure may result from, the oper- 
 ation of a polar force. We shall by-and-by see far more 
 wonderful exhibitions of the same structural action when 
 we come to deal with the force of crystallization. 
 
 The magnetic force has here acted upon particles of 
 matter visible to the eye. But, as already stated, there are 
 
78 FKAGMENTS OF SCIENCE. 
 
 numerous processes in Nature which entirely elude the eye 
 of the body, and must be figured by the eye of the mind. 
 The processes of chemistry are examples of these. Long 
 thinking and experimenting on the materials which compose 
 our world have led philosophers to conclude that matter is 
 composed of atoms from which, whether separate or in com- 
 bination, the whole material world is built up. The air we 
 breathe, for example, is mainly a mixture of the atoms of 
 two distinct substances, called oxygen and nitrogen. The 
 water we drink is also composed of two distinct substances, 
 called oxygen and hydrogen. But it differs from the air in 
 this particular, that in water the oxygen and hydrogen are 
 not mechanically mixed, but chemically combined. In fact, 
 the atoms of oxygen and those of hydrogen exert enormous 
 attractions on each other, so that when brought into sufficient 
 proximity they rush together with an almost incredible 
 force to form a chemical compound. But powerful as is the 
 force with which these atoms lock themselves together, we 
 have the means of tearing them asunder, and the agent by 
 which we accomplish this may here receive a few moments' 
 attention. 
 
 Into a vessel containing acidulated water I dip these 
 two strips of metal, the one being zinc and the other plati- 
 num, not permitting them to touch each other in the liquid. 
 I now connect the two upper ends of the strips by a piece 
 of copper wire. The wire is apparently unchanged, but it 
 is not so in reality. It is now the channel of what, for 
 want of a better name, we call an electric current a power 
 generated and maintained by the chemical action going on 
 in the vessel of acidulated water. What the inner change 
 of the wire is we do not know, but we do know that a change 
 has occurred, by the external effects produced by the wire. 
 Let me show you one or two of these effects. And here it 
 is convenient to operate with greater power than can be ob- 
 tained from a single small pair of strips of metal, and a 
 
MATTER AND FORCE. 79 
 
 single vessel of acidulated water. Before you is a series of 
 ten vessels, each with its pair of metals, and I wish to get 
 the added force of all ten. This arrangement is called a 
 voltaic battery. I take a piece of copper wire in my hand, 
 and plunge it among these iron filings ; they refuse to cling 
 to it ; the wire has no power over the filings. J now em- 
 ploy the self-same wire to connect the two ends of the bat- 
 tery, and subject it to the same test. The iron filings now 
 crowd round the wire and cling to it. This is one of the 
 effects of the electric current now traversing the wire. I 
 interrupt the current, and the filings immediately fall ; the 
 power of attraction continues only so long as the wire con- 
 nects the two ends of the battery. 
 
 Here is a piece of similar wire, overspun with cotton, to 
 prevent the contact of its various parts. It is formed into 
 a coil, which at present has no power over these iron nails ; 
 but I now make the coil part of the wire which connects 
 the two ends of the voltaic battery. No visible change has 
 occurred in the coil, but it is no longer what it was. By 
 the attractive force with which it has become suddenly en- 
 dowed, it now empties this tool-box of its nails. I twist a 
 covered copper wire round this common poker. At present 
 the poker is powerless over these iron nails ; but when we 
 connect with the wire surrounding the poker the two ends 
 of the voltaic battery, the poker is instantly transformed 
 into a strong magnet. Here, again, are two flat spirals sus- 
 pended facing each other. They are about six inches 
 apart. By turning this handle in a certain direction a cur- 
 rent is sent through both spirals. When this is done they 
 clash suddenly together, being drawn together by their mu- 
 tual attraction. By turning the handle in another direction, 
 I reverse what is called the direction of the current in one 
 of the spirals, and now they fly asunder, being driven apart 
 by their mutual repulsion. All these effects are due to the 
 power which we name an electric current, and which we 
 
80 FRAGMENTS OF SCIENCE. 
 
 figure as flowing through the wire when the voltaic circuit 
 is complete. 
 
 I have said that no visible change occurs in the wire 
 when the current passes through it. Still a change over 
 and above what you have seen really does take place. Lay 
 hold of those spirals, and you will find them warm. Let 
 me exalt this warmth so as to render it visible to you. In 
 front of the table is a thin platinum wire six feet long. On 
 sending a current from a battery of fifty pairs of plates 
 through this wire it glows, as you see, vividly red. I shorten 
 the wire ; more electricity now flows through it, and its 
 light becomes more intense. It is now bright yellow ; and 
 now it is a dazzling white. This light is so strong that 
 though the wire is not much thicker than a bristle, it ap- 
 pears to those on the nearest benches as thick as a quill ; 
 while to those at a distance it appears as thick as a man's 
 finger. This effect, which we call irradiation, is always pro- 
 duced by a very strong light. It is this same electric cur- 
 rent that furnished us with the powerful light employed in 
 one of our first experiments. The lamp then made use of 
 is provided with these coke rods ; and when the electric 
 current passes between them we obtain a light almost as 
 brilliant as that of the sun. 
 
 And now let us return to the point at which the elec- 
 tric current was introduced the point, namely, where the t 
 tearing asunder of the locked atoms of a chemical com- 
 pound was spoken of. The agent by which we effect this 
 is also the electric current; and I hope to make its action 
 visible to you all. Into this small cell, containing water, 
 dip two thin wires. By means of a solar microscope and 
 the powerful light of our electric lamp, a magnified image 
 of this cell is thrown upon the screen before you. You 
 see plainly the images of the wires. And now I send 
 from a second small battery which rests upon this table an 
 electric current from wire to wire. Bubbles of gas rise 
 
MATTER AND FORCE. 81 
 
 immediately from each of them, and these are the two 
 gases of which the water is composed. The oxygen is 
 always liberated on the one wire, the h}*drogen on the 
 other. The two gases may be collected separately ; in 
 fact, they have been thus collected in these jars. A lighted 
 taper'placed in one jar inflames the gas, which proves it to 
 be hydrogen ; a burning ember of wood placed in the 
 other jar instantly bursts into vivid combustion, which 
 proves the gas in the jar to be oxygen. I place upon my 
 hand a soap-bubble filled with a mixture of both gases in 
 the exact proportions in which they exist in water. Apply- 
 ing a taper to the bubble, a loud explosion is heard. The 
 gases have rushed together with detonation, but without 
 injury to my hand, and the water from which they were 
 extracted is the result of the reunion. 
 
 I wish you to see with the utmost possible clearness " 
 what has here taken place^ First, then, you are to re- 
 member that to form water the proportions by weight of 
 oxygen and hydrogen are as eight to one. Eight ounces 
 of oxygen, for example, unite with one of hydrogen to 
 form nine ounces of water. But if, instead of comparing 
 weights, we compare volumes, two volumes of hydrogen 
 unite with one of oxygen to form water. Now, these vol- 
 umes, and not the weights, express the proportions in 
 which the atoms of hydrogen unite with those of oxygen. 
 In the act of combination two atoms of hydrogen combine 
 with one of oxygen to form what we call the molecule of 
 water. Every such molecule is a group of three atoms, 
 two of which are hydrogen and one oxygen. 
 
 One consequence of the rushing together of the atoms 
 is the development of heat. What is this heat ? How 
 are we to figure it before our minds ? I do not despair of 
 being able to give you a tolerably distinct answer to this 
 question. Here are two ivory balls suspended from the 
 same point of support by two short strings. I draw them 
 
82 FRAGMENTS OF SCIENCE. 
 
 thus apart and then liberate them. They clash together, 
 but, by virtue of their elasticity, they quickly recoil from 
 each other, and a sharp vibratory rattle succeeds their col- 
 lision. This experiment will enable you to figure to your 
 mind a pair of clashing atoms. We have, in the first place, 
 a motion of the one atom toward the other a motion of 
 translation, as it is usually called. But when the atoms 
 come sufficiently near each other, elastic repulsion sets in, 
 the motion of translation is stopped and converted into a 
 motion of vibration. To this vibratory motion we give the 
 name of heat. Thus, three things are to be kept before 
 the mind first, the atoms themselves ; secondly, the force 
 Avith which they attract each other; and thirdly, the mo- 
 tion consequent upon the exertion of that force. This mo- 
 tion must be figured first as a motion of translation, and 
 then as a motion of vibration ; and it is not until the mo- 
 tion reaches the vibratory stage that we give it the name 
 of heat. It is this motion imparted to the nerves that pro- 
 duces the sensation of heat. 
 
 It would be useless to attempt a more detailed 'descrip- 
 tion of this molecular motion. After the atoms have 
 been thrown into this state of agitation, very complicated 
 motions must ensue from their incessant collision. There 
 must be a wild whirling about among the molecules. For 
 some time after the act of combination this action is so 
 violent as to prevent the molecules from coming together. 
 The water is maintained for a time in a state of vapor. 
 But as the vapor cools, or in other words loses its mo- 
 tion, the water molecules coalesce to form a liquid. And 
 now we are approaching a new and wonderful display of 
 force. No one who had only seen water in its vaporous or 
 liquid form could imagine the existence of the forces now 
 to be referred to ; for as long as the substance remains in 
 a liquid or vaporous condition, the play of these forces is 
 altogether masked and hidden. But let the heat be gradu- 
 
MATTER AND FORCE. 83 
 
 ally withdrawn, the antagonist to their union being re- 
 moved, the molecules prepare for new arrangements and 
 combinations. Like the particles of iron in our magnetic 
 experiment, the water molecules are endowed with attractive 
 and repulsive poles, and they arrange themselves together 
 in accordance with these attractions and repulsions. Solid 
 crystals of water are thus formed, to which we give the fa- 
 miliar name of ice. To the eye of science these ice-crystals 
 are as precious as the diamond as purely formed, as deli- 
 cately built. Where no disturbing causes intervene, there 
 is no disorder in this crystalline architecture. By their own 
 constructive power molecule builds itself on to molecule 
 with a precision far greater than that attainable by the 
 hands of man. We are apt to overlook the wonderful when 
 it becomes common. Imagine the bricks and stones of 
 this town of Dundee endowed with locomotive power. Im- 
 agine them attracting and repelling each other, and arrang- 
 ing themselves in consequence of these attractions and re- 
 pulsions to form streets and houses and Kinnaird Halls ; 
 would not that be wonderful ? Hardly less wonderful is 
 the play of force by which the molecules of water build 
 themselves into the sheets of crystal which every winter 
 roof your ponds and lakes. 
 
 If I could show you the actual progress of this molecu- 
 lar architecture, its beauty would delight and astonish you. 
 A reversal of the process may be actually shown. The 
 molecules of a piece of ice may be taken asunder before 
 your eyes, and from the manner in which they separate, you 
 may to some extent infer the manner in which they aggre- 
 gate. When a beam is sent from our electric lamp through 
 a plate of glass, a portion of the beam is intercepted, and 
 the glass is warmed by the portion thus retained within it. 
 When the beam is sent through a plate of ice, a portion 
 of the beam is also absorbed ; but instead of warming the 
 ice, the intercepted heat melts it internally. It is to the 
 
84 FRAGMENTS OF SCIENCE. 
 
 delicate, silent action of this beam within the ice that I 
 now wish to direct your attention. Upon the screen is 
 thrown a magnified image of the slab of ice : the light of 
 the beam passes freely through the ice without melting it, 
 and enables us to form the image, but the heat of the 
 beam is in great part intercepted by the ice, and that heat 
 now applies itself to the work of internal liquefaction. 
 Observe those stars breaking out over the white surface, 
 and expanding in size as the action of the beam continues. 
 These stars are liquefied ice, and each of them, you ob- 
 serve, has six rays. They still more closely resemble 
 flowers, each of six petals. Under the action of the heat 
 the molecules of the ice fall asunder, so as to leave be- 
 hind them these exquisite forms. We have here the pro- 
 cess of crystallization reversed. In this fashion, and in 
 strict accordance with this hexangular type every ice mole- 
 cule takes its place upon our ponds and lakes during the 
 frosts of winter. To use the language of an American 
 poet, " the atoms march in tune," moving to the music of 
 law, which thus renders the commonest substance in Na- 
 ture a miracle of beauty. 
 
 It is the function of science, not as some think to divest^ 
 this universe of its wonder and its mystery, but, as in the 
 case here before us, to point out the wonder and the 
 mystery of common things. Those fern-like forms, which 
 on a frosty morning overspread your window-panes, illus- 
 trate the action of the same force. Breathe upon such a 
 pane before the fires are lighted, and reduce the solid crys- 
 talline film to the liquid condition, then watch its subse- 
 quent appearance. You will see it all the better if you 
 look at it through a common magnifying-glass. After you 
 have ceased breathing, the film, abandoned to the action of 
 its own forces, appears for a moment to be alive. Lines of 
 motion run through it ; molecule closes with molecule, until 
 finally the whole film passes from the state of liquidity, 
 
MATTER AND FORCE. 85 
 
 through this state of motion, to its final crystalline re- 
 pose. 
 
 [ ft I can show you something similar. Over a piece of 
 perfectly clean glass I pour a little water in which a crystal 
 has been dissolved. A film of the solution clings to the 
 glass, and this film will now be caused to crystallize before 
 your eyes. By means of a microscope and a lamp, an image 
 of the plate of glass is thrown upon the screen. The beam 
 of the lamp, besides illuminating the glass, also heats it ; 
 evaporation sets in, and, at a certain moment, when the 
 solution has become supersaturated, splendid branches of 
 crystals shoot out over the screen. A dozen square feet of 
 surface are now covered by those beautiful forms. With 
 another solution we obtain crystalline spears, feathered 
 right and left by other spears. From distant nuclei in 
 the middle of the field of view the spears shoot with magical 
 rapidity in all directions. The film of water on a window- 
 pane on a frosty morning exhibits effects quite as wonderful 
 as these. Latent in this formless solution, latent in every 
 drop of water, lies this marvellous structural power, which 
 only requires the withdrawal of opposing forces to bring it 
 into action. 
 
 Our next experiment on crystallization you will probably 
 consider more startling even than these. The clear liquid 
 now held up before you is a solution of nitrate of silver a 
 compound of silver and nitric acid. When an electric cur- 
 rent is sent through this liquid the silver is severed from 
 the acid, as the hydrogen was separated from the oxygen 
 in a former experiment ; and I would ask you to observe 
 how the metal behaves when its molecules are thus succes- 
 sively set free. The image of the cell, and of the two wires 
 which dip into the liquid of the cell, are now clearly shown 
 upon the screen. Let us close the circuit, and send the 
 current through the liquid. From one of the wires a beau- 
 tiful silver tree commences immediately to sprout. Branches 
 
86 FRAGMENTS OF SCIENCE. 
 
 of the metal are thrown out, and umbrageous foliage loads 
 the branches. You have here a growth apparently as won- 
 derful as that of any vegetable perfected in a minute before 
 your eyes. Substituting for the nitrate of silver acetate of 
 lead, which is a compound of lead and acetic acid, the 
 electric current severs the lead from the acid, and there you 
 see the metal slowly branching into these exquisite metallic 
 ferns, the fronds of which, as they become too heavy, break 
 from their roots and fall to the bottom of the cell. 
 
 These experiments show that the common matter of our 
 earth " brute matter," as Dr. Young pleases to call it 
 when its atoms and molecules are permitted to bring their 
 forces into free play, arranges itself, under the operation of 
 these forces, into forms which rival in beauty those of the 
 vegetable world. And what is the vegetable world itself 
 but the result of the complex play of these molecular forces ? 
 Here, as elsewhere throughout Nature, if matter moves, it 
 is force that moves it ; and if a certain structure, vegetable 
 or mineral, is produced, it is through the operation of the 
 forces exerted between the atoms and molecules. These 
 atoms and molecules resemble little magnets with mutually 
 attractive and mutually repellant poles. The attracting 
 poles unite, the repellant poles retreat, and vegetable as 
 well as mineral forms are the final expression of this com- 
 plicated molecular action. 
 
 In the formation of our lead and silver trees, we needed 
 an agent to wrest the lead and the silver from the acids 
 with which they were combined. A similar agent is re- 
 quired in the vegetable world. The solid matter of which 
 our lead and silver trees were formed was, in the first in- 
 stance, disguised in a transparent liquid ; the solid matter 
 of which our woods and forests are composed is also, for 
 the most part, disguised in a transparent gas, which is 
 mixed in small quantities with the air of our atmosphere. 
 This gas is formed by the union of carbon and oxygen, and 
 
MATTER AND FORCE. 87 
 
 is called carbonic acid gas. Two atoms of oxygen and one 
 of carbon unite to fonn the molecule of carbonic acid which, 
 as I have said, is the material from which wood and vege- 
 table tissues are mainly derived. The carbonic acid of the 
 air being subjected to an action somewhat analogous to 
 that of the electric current in the case of our lead and silver 
 solutions, has its carbon liberated and deposited as woody 
 fibre. The watery vapor of the air is subjected to similar 
 action ; its hydrogen is liberated from its oxygen, and lies 
 down side by side with the carbon in the tissues of the 
 tree. The oxygen in both cases is permitted to wander 
 away into the atmosphere. But what is it which thus tears 
 the carbon and the hydrogen from the strong embrace of 
 the oxygen ? What is it in Nature that plays the part of 
 the electric current in our experiments ? The rays of the,//' 
 sun. The leaves of the plants absorb both the carbonic 
 acid and the aqueous vapor of the air; these leaves an- 
 swer to the cells in which our decompositions by the electric 
 current took place. In the leaves the solar rays decompose 
 both the carbonic acid and the water, permitting the oxygen 
 in both cases to escape into the air, and allowing the carbon 
 and the hydrogen to follow the bent of their own forces. 
 And just as the molecular attractions of the silver and the 
 lead found expression in the production of those beautiful 
 branching forms seen in our experiments, so do the molecular 
 attractions of the liberated carbon and hydrogen find ex- 
 pression in the architecture of grasses, plants, and trees. 
 
 In the fall of a cataract and the rush of the wind we 
 have examples of mechanical power. In the combinations 
 of chemistry and in the formation of crystals and vegetables 
 we have examples of molecular power. But before pro- 
 ceeding further I should like to make clear to you the 
 present condition of the surface of our globe with reference 
 to power generally. You have learned how the atoms of 
 oxygen and hydrogen rush together to form water. I have 
 
88 FRAGMENTS OF SCIENCE. 
 
 not thought it necessary to dwell upon the mighty mechani- 
 cal energy of their act of combination, but, in passing, I 
 would say that the clashing together of 1 Ib. of hydrogen 
 and 8 Ibs. of oxygen to form 9 Ibs. of aqueous vapor, is 
 greater than the clash of a weight of 1,000 tons falling 
 from a height of 20 feet against the earth. Now, in order 
 that the atoms of oxygen arid hydrogen should rise by their 
 mutual attractions to the velocity corresponding to this 
 enormous mechanical effect, a certain distance must exist 
 between the particles. It is in rushing over this that the 
 velocity is attained. 
 
 This idea of distance between the attracting atoms is of 
 the highest importance in our conception of the system, of 
 the world. For the world may be divided into two kinds 
 of matter ; or rather the matter of the world may be classi- 
 fied under two distinct heads namely, of atoms and mole- 
 cules which have already rushed together and thus satisfied 
 their mutual attractions, and of atoms and molecules which 
 have not yet rushed together, and whose mutual attractions 
 are, therefore, as yet unsatisfied. Now, as regards motive 
 power, the working of machinery, or the performance of 
 mechanical work generally, by means of the materials of 
 the earth's crust, we are entirely dependent on those atoms/' 
 and molecules whose attractions are as yet unsatisfied. Those 
 attractions can produce motion, because sufficient distance 
 intervenes between the attracting molecules, and it is this 
 molecular motion that we utilize in our machines. Thus 
 we can get power out of oxygen and hydrogen by the act 
 of their union, but once they are combined, and once the 
 motion consequent on their combination has been expended, 
 no further power can be got out of the mutual attraction 
 of oxygen and hydrogen. As dynamic agents they are 
 dead. If we examine the materials of which the earth's 
 crust is composed, we find them to consist for the most part 
 of substances whose atoms have already closed in chemical 
 
MATTER AND FORCE. 89 
 
 union whose mutual attractions are satisfied. Granite, 
 for instance, is a widely-diffused substance, but granite 
 consists, in great part, of silicon, oxygen, potassium, cal- 
 cium, and aluminum, the atoms of which substances met 
 long ago in chemical combination, and are therefore dead. 
 Limestone is also a widely-diffused substance. It is com- 
 posed of carbon, oxygen, and a metal called calcium. But 
 the atoms of those substances closed long ago in chemical 
 union, and are therefore dead. And in this way we might 
 go over the whole of the materials of the earth's crust, and 
 satisfy ourselves that though they were sources of power in 
 ages past, and long before any being appeared on the 
 surface of the earth capable of turning their power to 
 account, they are sources of power no longer. And here 
 we might halt for a moment to remark on that tendency, 
 so prevalent in the world, to regard every thing as made for 
 human use. Those who entertain this notion hold, I think, 
 an overweening opinion of their own importance in the 
 system of Nature. Flowers bloomed before men saw them, 
 and the quantity of power wasted before man could utilize 
 it is all but infinite compared with what now remains to be 
 applied. The healthy attitude of mind with reference to 
 this subject is that of the poet, who, when asked whence 
 came the rhodora, replied : 
 
 " Why thou wert there, rival of the rose ! 
 
 I never thought to ask, I never knew, 
 But in my simple ignorance supposed 
 
 The self-same power that brought me there brought you." 1 
 
 A few exceptions to this general state of union of the 
 particles of the earth's crust all-important to us, but trivial 
 in comparison to the total store of which they are the resi- 
 due still remain. They constitute our main sources of 
 motive power. By far the most important of these are our 
 1 Emerson. 
 
90 FRAGMENTS OF SCIENCE. 
 
 beds of coal, composed chiefly of carbon, which has not yet 
 closed in chemical union with oxygen. Distance still inter- 
 venes between the atoms of carbon and those of oxygen, 
 across which the atoms may be impelled by their mutual 
 attractions, and we can do nothing more than utilize the 
 motion produced by this attraction. Once the carbon and 
 the oxygen have rushed together, so as to form carbonic 
 acid, their mutual attractions are satisfied, and, while they 
 continue in this condition, as dynamic agents they are dead. 
 A pound of coal produces by its combination with oxygen 
 an amount of heat which, if mechanically applied, would 
 raise a weight of 100 Ibs. to a height of twenty miles above 
 the earth's surface. Conversely, 100 Ibs. falling from a 
 height of twenty miles, and striking against the earth, 
 would generate an amount of heat equal to that devel- 
 oped by the combustion of a pound of coal. Wherever 
 work is done by heat, heat disappears. A gun which fires 
 a ball is less heated than one which fires blank cartridge. 
 The quantity of heat communicated to the boiler of a 
 working steam-engine is greater than that which could be 
 obtained from the recondensation of the steam after it had 
 done its work ; and the amount of work performed is the 
 exact equivalent of the amount of heat missing. We dig 
 annually nearly 100 millions of tons of coal from our pits. 
 The amount of mechanical force represented by this quantity 
 of coal seems perfectly fabulous. The combustion of a 
 single pound of coal, supposing it to take place in a minute, 
 would be equivalent to the work of 300 horses; and if we 
 suppose 120 millions of horses working day and night with 
 unimpaired strength, for a year, their united energies would 
 enable them to perform an amount of work just equivalent 
 to the heat to be derived from the annual produce of our 
 coal-fields. Our woods and forests are also sources of 
 mechanical energy, because they also have the power of 
 uniting with the atmospheric oxygen, and the molecular 
 
MATTER AND FORCE. 91 
 
 motion produced in the act of union may be turned to 
 mechanical account. Passing from dead matter to living ^ 
 matter, we find that the source of motive power here re- 
 ferred to is also the source of muscular power. A horse 
 can perform work, and so can a man, but this work is at 
 bottom the molecular work of the elements of the food and 
 the oxygen of the air. We inhale this vital gas, and bring 
 it into sufficiently close proximity with the carbon and the 
 hydrogen of the food. They unite in obedience to their 
 mutual attractions, and their motion toward each other, 
 properly turned to account by the wonderful mechanism of 
 the body, becomes muscular motion. 
 
 One fundamental thought pervades all these statements : / 
 there is one-tap root from which they all spring. This is 
 the ancient maxim that out of nothing nothing comes ; that 
 neither in the organic world nor in the inorganic is power 
 produced without the expenditure of other power; that 
 neither in the plant nor in the animal is there a creation of 
 force or motion. Trees grow, and so do men and horses ;^ 
 and here we have new power incessantly introduced upon 
 the earth. But its source, as I have already stated, is the 
 sun. For he it is who separates the carbon from the oxy- 
 gen of the carbonic acid, and thus enables them to recom- 
 bine. Whether they recombine in the furnace of the 
 steam-engine or in the animal body, the origin of the power 
 they produce is the same. In this sense we are all " souls 
 of fire and children of the sun." But, as remarked by 
 Helmholtz, we must be content to share our celestial 
 pedigree with the meanest living things. The frog, and 
 the toad, and those terrible creatures, the monkey and {/ 
 the gorilla, draw their power from the same source as 
 man. . 
 
 Some estimable persons, here present, very possibly 
 shrink from accepting these statements ; they may be 
 frightened by their apparent tendency toward what is called 
 
92 FRAGMENTS OF SCIENCE. 
 
 materialism a word which, to many minds, expresses some- 
 thing very dreadful. But it ought to be known and avowed 
 that the physical philosopher, as such, must be a pure ma- 
 terialist. His inquiries deal with matter and force, and 
 with them alone. The action which he has to investigate 
 is necessary action ; not spontaneous action the transfor- 
 mation, not the creation, of matter and force. And what- 
 ever be the forms which matter and force may assume, 
 whether in the organic world or in the inorganic, whether 
 in the coal-beds and forests of the earth, or in the brains 
 and muscles of men, the physical philosopher will make 
 good his right to investigate them. It is perfectly vain to 
 attempt to stop inquiry as to the actual and possible actions 
 of matter and force. Depend upon it, if a chemist by 
 bringing the proper materials together, in a retort or 
 crucible, could make a baby, he would do it. There is no 
 law, moral or physical, forbidding him to do it his in- 
 quiries in this direction are limited solely by his own ca- 
 pacity and the laws of matter and force. At the present 
 moment there are, no doubt, persons experimenting on 
 the possibility of producing what we call life out of in- 
 organic materials. Let them pursue their studies in peace ; 
 it is only by such trials that they will learn the limits of 
 their powers. 
 
 But while I thus make the largest demand for freedom 
 of investigation while I as a man of science feel a natural 
 pride in scientific achievement, while I regard science as the 
 most powerful instrument of intellectual culture, as well as 
 the most powerful ministrant to the material wants of men ; 
 if you ask me whether science has solved, or is likely in our 
 day to solve, the problem of this universe, I must shake my 
 head in doubt. You remember the first Napoleon's ques- 
 tion, when the savans who accompanied him to Egypt dis- 
 cussed in his presence the origin of the universe, and solved 
 it to their own apparent satisfaction. He looked aloft to 
 
MATTER AND FORCE. 93 
 
 the starry heavens, and said, " It is all very well, gentle- \ 
 men ; but who made all these ? " That question still re- j 
 mains unanswered, and science makes no attempt to answer j 
 it. As far as I can see, there is no quality in the human 
 intellect which is fit to be applied to the solution of the 
 problem. It entirely transcends us. The mind of man may 
 be compared to a musical instrument with a certain range 
 of notes, beyond which in both directions we have an 
 infinitude of silence. The phenomena of matter and force 
 lie within our intellectual range, and as far as they reach 
 we will at all hazards push our inquiries. But behind, and 
 above, and around all, the real mystery of this universe lies 
 unsolved, and, as far as we are concerned, is incapable of 
 solution. Fashion this mystery as you will, with that I 
 have nothing to do. But be careful that your conception 
 of it be not an unworthy one. Invest that conception with 
 your highest and holiest thought, but be careful of pre- 
 tending to know more about it than is given to man to 
 know. Be careful, above all things, of professing to see in 
 the phenomena of the material world the evidences of Di- 
 vine pleasure or displeasure. Doubt those who would 
 deduce from the fall of the tower of Siloam the anger of the 
 Lord against those who were crushed. Doubt those equally 
 who pretend to see in cholera, cattle-plague, and bad har- 
 vests, evidences of Divine' anger. Doubt those spiritual 
 guides who in Scotland have lately propounded the mon- 
 strous theory that the depreciation of railway scrip is a con- 
 sequence of railway travelling on a Sunday. Let them not, 
 as far as you are concerned, label and libel the system of 
 Nature with their ignorant hypotheses. Well might the 
 mightiest of living Scotchmen, that hero of the intellect 
 who might have been a hero in the field, that strong and 
 earnest soul who has made every soul of like nature in 
 these islands his debtor looking from the solitudes of 
 thought into this highest of questions, well, I say, might 
 
94 FRAGMENTS OF SCIENCE. 
 
 your noble old Carlyle scornfully retort on such interpreters 
 of the ways of God to men : 
 
 The Builder of this universe was wise, 
 He formed all souls, all systems, planets, particles ; 
 
 The plan he formed his worlds and JEons by, 
 Was Heavens ! was thy small nine-and-thirty articles ! 
 
V. 
 
 ADDRESS TO THE STUDENTS OF UNIVER- 
 SITY COLLEGE, LONDON, 
 
 ON THE DISTRIBUTION OF PEIZES IN THE FACULTY OF AETS. 
 
 Session 1868-'69. 
 
" Self-reverence, self-knowledge, self-control, 
 These three alone lead life to sovereign power, 
 Yet not for power (power of herself 
 Would come uncalled for), but to live by law, 
 Acting the law we live by without fear ; 
 And, because right is right, to follow right 
 Were wisdom in the scorn of consequence." 
 
 TENNYSON. 
 
Y. 
 
 AN ADDRESS TO STUDENTS. 
 
 THEKE is an idea regarding the nature of man which 
 modern philosophy has sought, and is still seeking, to raise 
 into clearness, the idea, namely, of secular growth. Man 
 is not a thing of yesterday ; nor do I imagine that the 
 slightest controversial tinge is imported into this address 
 when I say that he is not a thing of 6,000 years ago. 
 Whether he came originally from stocks or stones, from 
 nebulous gas or solar fire, I know not ; if he had any such 
 origin the process of his transformation is as inscrutable to 
 you and to me as that of the grand old legend, according 
 to which " the Lord God formed man of the dust of the 
 ground, and breathed into his nostrils the breath of life ; 
 and man became a living soul." But, however obscure 
 man's origin may be, his growth is not to be denied. Here 
 a little and there a little added through the ages have 
 slowly transformed him from what he was into what he is. 
 The doctrine has been held that the mind of the child is like 
 a sheet of white paper, on which by education we can write 
 what characters we please. This doctrine assuredly needs 
 qualification and correction. In physics, when an external 
 force is applied to a body with a view of affecting its inner 
 texture, if we wish to predict the result, we must know 
 whether the external force conspires with or opposes the 
 internal forces of the body itself; and in bringing the influ- 
 ence of education to bear upon the new-born man his inner 
 5 
 
98 FRAGMENTS OF SCIENCE. 
 
 powers must be also taken into account. He conies to us 
 as a bundle of inherited capacities and tendencies, labelled 
 " from the indefinite past to the indefinite future ; " and he 
 makes his transit from the one to the other through the 
 education of the present time. The object of that educa- 
 tion is, or ought to be, to provide wise exercise for his ca- 
 pacities, wise direction for his tendencies, and through this 
 exercise and this direction to furnish his mind with such 
 knowledge as may contribute to the usefulness, the beauty, 
 and the nobleness of his life. 
 
 How is this discipline to be secured, this knowledge im- 
 parted ? Two rival methods now solicit attention the one 
 organized and equipped, the labor of centuries having been 
 expended in bringing it to its present state of perfection ; 
 the other, more or less chaotic, but becoming daily less so, 
 and giving signs of enormous power, both as a source of 
 knowledge and as a means of discipline. These two 
 methods are the classical and the scientific method. I wish 
 they were not rivals ; it is only bigotry and short-sighted- 
 ness that make them so ; for assuredly it is possible to give 
 both of them fair play. Though hardly authorized to ex- 
 press any opinion whatever upon the subject, I nevertheless 
 hold the opinion that the proper study of a language is an 
 intellectual discipline of the highest kind. If I except dis- 
 cussions on the comparative merits of popery and Protes- 
 tantism, English grammar was the most important discipline 
 of my boyhood. The piercing through the involved and 
 inverted sentences of " Paradise Lost ; " the linking of the 
 verb to its often distant nominative, of the relative to its 
 distant antecedent, of the agent to the object of the transi- 
 tive verb, of the preposition to the noun or pronoun which 
 it governed the study of variations in mood and tense, the 
 transformations often necessary to bring out the true gram- 
 matical structure of a sentence all this was to my young 
 mind a discipline of the highest value, and, indeed, a source 
 
AN ADDRESS TO STUDENTS. 99 
 
 of unflagging delight. How I rejoiced when I found a 
 great author tripping, and was fairly able to pin him to a 
 corner from which there was no escape ! As I speak, some 
 of the sentences which exercised me when a boy rise to my 
 recollection. " He that hath ears to hear let him hear." 
 That was one of them, where the " He " is left, as it were, 
 floating in mid air without any verb to support it. I speak 
 thus of English because it was of real value to me. I do 
 not speak of other languages because their educational value 
 for me was almost insensible. But, knowing the value of 
 English so well, I should be the last to deny, or even to 
 doubt, the high discipline involved in the proper study of 
 Latin and Greek. 
 
 That study, moreover, has other merits and recommen- 
 dations which have been already slightly touched upon. 
 It is organized and systematized by long-continued use. 
 It is an instrument wielded -by some of the best intellects 
 of the country in the education of youth ; and it can point 
 to results in the achievements of our foremost men. What, 
 then, has science to offer which is in the least degree likely S 
 to compete with such a system ? I cannot better reply 
 than by recurring to the grand old story from which I have / 
 already quoted. Speaking of the world and all that therein 
 is, of the sky and the stars around it, the ancient writer 
 says, " And God saw all that he had made, and behold it 
 was very good." It is the body of things thus described y 
 which science offers to the study of man. There is a very 
 renowned argument much prized and much quoted by 
 theologians, in which the universe is compared to a watch. 
 Let us deal practically with this comparison. Supposing a 
 watchmaker, having completed his instrument, to be so 
 satisfied with his work as to call it very good, what would 
 you understand him to mean? You would not suppose 
 that he referred to the dial-plate in front and the chasing 
 of the case behind, so much as to the wheels and pinions, 
 
100 FRAGMENTS OF SCIENCE. 
 
 the springs and jewelled pivots of the works within, those 
 qualities and powers, in short, which enable the watch to 
 perform accurately its work as a keeper of time. With re- 
 gard to the knowledge of such a watch he would be a mere 
 ignoramus who would content himself with outward inspec- 
 tion. I do not wish to say one severe word here to-day, 
 but I fear that many of those who are very loud in their 
 praise of the works of the Lord know them only in this out- 
 side and superficial way. It is the inner works of the uni- 
 ' verse which science reverently uncovers ; it is the study of 
 these that she recommends as a discipline worthy of all 
 acceptation. 
 
 The ultimate problem of physics is to reduce matter by 
 analysis to its lowest condition of divisibility, and force to 
 its simplest manifestations, and then by synthesis to con- 
 struct from these elements the world as it stands. We are 
 still a long way from the final solution of this problem ; 
 and when the solution comes, it will be one more of spir- 
 itual insight than of actual observation. But though we 
 are still a long way from this complete intellectual mastery 
 of Nature, we have conquered vast regions of it, have 
 learned their polities and the play of their powers. We 
 live upon a ball of matter eight thousand miles in diameter, 
 swathed by an atmosphere of unknown height. This ball 
 has been molten by heat, chilled to a solid, and sculptured 
 by water ; it is made up of substances possessing distinctive 
 properties and modes of action, properties which have an 
 immediate bearing upon the continuance of man in health, 
 and on his recovery from disease, on which moreover de- 
 pend all the arts of industrial life. These properties and 
 modes of action offer problems to the intellect, some profit- 
 able to the child, and others sufficient to tax the highest 
 powers of the philosopher. Our native sphere turns on its 
 axis and revolves in space. It is one of a band which do 
 the same. It is illuminated by a sun which, though nearly 
 
AN ADDRESS TO STUDENTS. 101 
 
 a hundred millions of miles distant, can be brought virtually 
 into our closets and there subjected to examination. It 
 has its winds and clouds, its rain and frost, its light, heat, 
 sound, electricity, and magnetism. And it has its vast 
 kingdoms of animals and vegetables. To a most amazing 
 extent the human mind has conquered these things, and 
 revealed the logic which runs through them. Were they 
 facts only, without logical relationship, science might, as a 
 means of discipline, suffer in comparison with language. 
 But the whole body of phenomena is instinct with law; 
 the facts are hung on principles, and the value of physical 
 science as a means of discipline consists in the motion of 
 the intellect, both inductively and deductively, along the 
 lines of law marked out by phenomena. As regards that 
 discipline to which I have already referred as derivable 
 from the study of languages that, and more, are involved 
 in the study of physical science. Indeed, I believe it would 
 be possible so to limit and arrange the study of a portion 
 of physics as to render the mental exercise involved in it 
 almost qualitatively the same as that involved in the un- 
 ravelling of a language. 
 
 I have thus far limited myself to the purely intellectual 
 side of this question. But man is not all intellect. If he 
 were so, science would, I believe, be his proper nutriment. 
 But he feels as well as thinks ; he is receptive of the sub- 
 lime and the beautiful as well as of the true. Indeed, I be- 
 lieve that even the intellectual action of a complete man is, 
 consciously or unconsciously, sustained by an under-current 
 of the emotions. It is vain, I think, to attempt to separate 
 moral and emotional nature from intellectual nature. Let 
 a man but observe himself, and her will, if I mistake not, 
 find that in nine cases out of ten, moral or immoral consid- 
 erations, as the case may be, are the motive force which 
 pushes his intellect into action. The reading of the works 
 of two men, neither of them imbued with the spirit of 
 
102 FRAGMENTS OF SCIENCE. 
 
 modern science, neither of them, indeed, friendly to that 
 spirit, has placed me here to-day. These men are the Eng- 
 lish Carlyle and the American Emerson. I must ever re- 
 member with gratitude that through three long, cold Ger- 
 man winters Carlyle placed me in my tub, even when ice 
 was on its surface, at five o'clock every morning ; not 
 slavishly, but cheerfully, meeting each day's studies with a 
 resolute will, determined whether victor or vanquished not 
 to shrink from difficulty. I never should have gone through 
 Analytical Geometry and the Calculus had it not been for 
 those men. I never should have become a physical inves- 
 tigator, and hence without them I should not have been 
 here to-day. They told me what I ought to do in a way 
 that caused me to do it, and all my consequent intellectual 
 action is to be traced to this purely moral source. To Car- 
 lyle and Emerson I ought to add Fichte, the greatest rep- 
 resentative of pure idealism. These three unscientific men 
 made me a practical scientific worker. They called out, 
 " Act ! " I hearkened to the summons, taking the liberty, 
 however, of determining for myself the direction which 
 effort was to take. 
 
 And I may now cry, " Act ! " but the potency of action 
 must be yours. I may pull the trigger, but if the gun be 
 not charged there is no result. We are creators in the 
 intellectual world as little as in the physical. "We may 
 remove obstacles, and render latent capacities active, but 
 we cannot suddenly change the nature of man. The " new 
 birth " itself implies the preexistence of the new character 
 which requires not to be created but brought forth. You 
 cannot by any amount of missionary labor suddenly trans- 
 form the savage into the civilized Christian. The improve- 
 ment of man is secular not the work of an hour or of a 
 day. But though indubitably bound by our organizations, 
 no man knows what the potentialities of any human mind 
 may be, which require only release to be brought into ac- 
 
AN ADDRESS TO STUDENTS. 103 
 
 tion. Let me illustrate this point. There are in the min- 
 eral world certain crystals, certain forms, for instance, of 
 fluor-spar, which have lain darkly in the earth for ages, but 
 which nevertheless have a potency of light locked up within 
 them. In their case the potential has never become actual 
 the light is in fact held back by a molecular detent. When 
 these crystals are warmed, the detent is lifted, and an out- 
 flow of light immediately begins. I know not how many 
 of you may be in the condition of this fluor-spar. For aught 
 I know, every one of you may be in this condition, requiring 
 but the proper agent to be applied the proper word to be 
 spoken to remove a detent, and to render you conscious 
 of light within yourselves and sources of light to others. 
 
 The circle of human nature, then, is not complete with- 
 out the arc of feeling and emotion. The lilies of the field 
 have a value for us beyond their botanical ones a certain 
 lightening of the heart accompanies the declaration that 
 " Solomon in all his glory was not arrayed like one of these." 
 The sound of the village bell which comes mellowed from 
 the valley to the traveller upon the hill, has a value beyond 
 its acoustical one. The setting sun when it mantles with 
 the bloom of roses the alpine snows, has a value beyond its 
 optical one. The starry heavens, as you know, had for Im- 
 manuel Kant a value beyond their astronomical one. Round 
 about the intellect sweeps the horizon of emotions from 
 which all our noblest impulses are derived. I think it very 
 desirable to keep this horizon open ; not to permit either 
 priest or philosopher to draw down his shutters between 
 you and it. And here the dead languages, which are sure 
 to be beaten by science in the purely intellectual fight, have 
 an irresistible claim. They supplement the work of science 
 by exalting and refining the aesthetic faculty, and must on 
 this account be cherished by all who desire to see human 
 culture complete. There must be a reason for the fascina- 
 tion which these languages have so long exercised upon 
 
104 FRAGMENTS OF SCIENCE. 
 
 the most powerful and elevated minds a fascination which 
 will probably continue for men of Greek and Romtta mould 
 to the end of time. 
 
 In connection with this question of the emotions one 
 very obvious danger besets many of the more earnest spirits 
 of our day the danger of haste in endeavoring to give the 
 feelings repose. We are distracted by systems of theology 
 and philosophy which were taught to us when young, and 
 which now excite in us -a hunger and a thirst for knowledge 
 not proved to be attainable. There are periods when the 
 judgment ought to remain in suspense, the data on which 
 a decision might be based being absent. This discipline 
 of suspending the judgment is a common one in science, 
 but not so common as it ought to be elsewhere. I walked 
 down Regent Street some time ago with a man of great 
 gifts and acquirements, discussing with him various theo- 
 logical questions. I could not accept his views of the origin 
 and destiny of the universe, nor was I prepared to enun- 
 ciate any definite view r s of my own. He turned to me at 
 length and said, " You surely must have a theory of the 
 universe." That I should in one way or another have solved 
 this mystery of mysteries seemed to my friend a matter of 
 course. " I have not even a theory of magnetism," was my 
 reply. We ought to learn to wait, and pause before closing 
 with the advances of those expounders of the ways of God 
 to men, who offer us intellectual peace at the modest cost 
 of intellectual life. 
 
 The teachers of the world ought to be its best men, and 
 for the present at all events such men must learn self-trust. 
 They must learn more and more to do without external aid ; 
 save such aid as comes from the contemplation of a uni- 
 verse, which, though it baffles the intellect, can elevate the 
 heart. But they must learn to feel the mystery of that 
 universe without attempting to give it a rigid form, per- 
 sonal or otherwise. By the fulness and freshness of their 
 
AN ADDRESS TO STUDENTS. 105 
 
 own lives and utterances they must awaken life in othersX 
 The position of science is already assured, but I think the \ 
 poet also will have a great part to play in the future of the/ 
 world. To him it is given for a long time to come to fill 
 those shores which the recession of the theologic tide has 
 left exposed ; to him, when he rightly understands his mis- 
 sion, and does not flinch from the tonic discipline which it 
 assuredly demands, we have a right to look for that height- 
 ening and brightening of life which so many of us need. 
 He ought to be the interpreter of that power which as 
 
 " Jehovah, Jove, or Lord," 
 
 has hitherto filled and strengthened the human heart. 
 
 Let me utter one practical word in conclusion take 
 care of your health. There have been men who by wise 
 attention to this point might have risen to any eminence 
 might have made great discoveries, written great poems, 
 commanded armies, or ruled states, but who by unwise 
 neglect of this point have come to nothing. Imagine Her- 
 cules as oarsman in a rotten boat ; what can he do there 
 but by the very force of his stroke expedite the ruin of his 
 craft. Take care then of the timbers of your boat, and 
 avoid all practices likely to introduce either wet or dry rot 
 among them. And this is not to be accomplished by desul- 
 tory or intermittent efforts of the will, but by the formation 
 of habits. The will no doubt has sometimes to put forth 
 its strength in order to strangle or crush the special tempta- 
 tion. But the formation of right habits is essential to your 
 permanent security. They diminish your chance of falling 
 when assailed, and they augment your chance of recovery 
 when overthrown. 
 
VI. 
 
 SCOPE AND LIMIT OF SCIENTIFIC 
 MATERIALISM, 
 
 AN ADDRESS. 
 
 DELIVERED IN THE MATHEMATICAL AND PHYSICAL SECTION OF 
 THE BRITISH ASSOCIATION IN NORWICH. 
 
 August 19, 1868. 
 
"As I proceeded I found my philosopher altogether forsaking mind 
 or any other principle of order, and having recourse to air and ether, and 
 water, and other eccentricities. I might compare him to a person who 
 began by maintaining generally that mind is the cause of the actions of 
 Socrates, but who, when he endeavored to explain the cause of my several 
 actions in detail, went on to show that I sit here because my body is 
 made up of bones and muscles ; and the bones he would say are hard 
 and have ligaments which divide them, and the muscles are elastic, and 
 they cover the bones, which have also a covering or environment of flesh 
 and skin which contains them ; and as the bones are lifted at their joints 
 by the contraction or relaxation of the muscles, I am able to bend my 
 limbs, and this is why I am sitting here in a curved posture ; that is 
 what he would say, and he would have a similar explanation of my talk- 
 ing to you, which he would attribute to sound, and air, and hearing, and 
 he would assign ten thousand other causes of the same sort, forgetting to 
 mention the true cause, which is that the Athenians have thought fit to 
 condemn me, and accordingly I have thought it better and more right to 
 remain here and undergo my sentence ; for I am inclined to think that 
 these muscles and bones of mine would have gone off to Megara or 
 Boaotia by the dog of Egypt they would, if they had been guided by 
 their own idea of what was best, and if I had not chosen as the better and 
 nobler part, instead of playing truant and running away, to undergo any 
 punishment which the State inflicts." PLATO, Jowctfs Translation. 
 
VI. 
 
 SCIENTIFIC MATERIALISM. 
 
 THE CELEBRATED FiCHTE, in his lectures on the "Vo- 
 cation of the Scholar," insisted on a culture which should 
 not be one-sided, but all-sided. The scholar's intellect 
 was to expand spherically and not in a single direction 
 only. In one direction, however, Fichte required that 
 the scholar should apply himself directly to Nature, be- 
 come a creator of knowledge, and thus repay by original 
 labors of his own the immense debt he owed to the 
 labors of others. It was these which enabled him to sup- 
 plement the knowledge derived from his own researches, 
 so as to render his culture rounded and not one-sided. 
 
 As regards science Fichte's idea is to some extent 
 illustrated by the constitution and the labors of the British 
 Association. We have a body of men engaged in the 
 pursuit of Natural Knowledge, but variously engaged^ 
 While sympathizing with each of its departments, and 
 supplementing his culture by knowledge drawn from all 
 of them, each student among us selects one subject for the 
 exercise of his own original faculty one line along which 
 he may carry the light of his private intelligence a little 
 way into the darkness by which all knowledge is sur- 
 rounded. Thus, the geologist deals with the rocks ; the 
 biologist with the conditions and phenomena of life ; the 
 astronomer with stellar masses and motions ; the mathe- 
 matician with the relations of space and number; the 
 
110 FRAGMENTS OF SCIENCE. 
 
 chemist pursues his atoms, while the physical investigator 
 has his own large field in optical, thermal, electrical, 
 acoustical, and other phenomena. The British Associa- 
 tion then, as a whole, faces physical Nature on all sides 
 and pushes knowledge centrifugally outward, the sum of 
 its labors constituting what Fichte might call the sphere of 
 natural knowledge. In the meetings of the Association it 
 is found necessary to resolve this sphere into its component 
 parts, which take concrete form under the respective letters 
 of our Sections. 
 
 This is the Mathematical and Physical Section. Mathe- 
 matics and physics have been long accustomed to coalesce. 
 For, no matter how subtle a natural phenomenon may be, 
 whether we observe it in the region of sense, or follow it 
 into that of imagination, it is in the long-run reducible to 
 mechanical laws. But the mechanical data once guessed 
 or given, mathematics become all-powerful as an instru- 
 ment of deduction. The command of geometry over the 
 relations of space, the far-reaching power which organized 
 symbolic reasoning confers, are potent both as means of 
 physical discovery, and of reaping the entire fruits of dis- 
 covery. Indeed, without mathematics, expressed or im- 
 plied, our knowledge of physical science would be friable 
 in the extreme. 
 
 Side by side with the mathematical method we have 
 the method of experiment. Here, from a starting-point 
 furnished by his own researches, or those of others, the in- 
 vestigator proceeds by combining intuition and verification. 
 He ponders the knowledge he possesses and tries to push 
 it further, he guesses and checks his guess, he conjectures 
 and confirms or explodes his conjecture. These guesses 
 and conjectures are by no means leaps in the dark ; for 
 knowledge once gained casts a faint light beyond its own 
 immediate boundaries. There is no discovery so limited 
 as not to illuminate something beyond itself. The force 
 
SCIENTIFIC MATERIALISM. Ill 
 
 of intellectual penetration into this penumbral region which 
 surrounds actual knowledge is not, as some seem to think, 
 dependent upon method, but upon the genius of the in- 
 vestigator. There is, however, no genius so gifted as not 
 to need control and verification. The profoundest minds 
 know best that Nature's ways are not at all times their 
 ways, and that the brightest flashes in the world of 
 thought are incomplete until they have been proved to 
 have their counterparts in the world of fact. Thus the^ 
 vocation of the true experimentalist may be defined as 
 the continued exercise of spiritual insight, and its inces- 
 sant correction and realization. His experiments consti- 
 tute a body, of which his purified intuitions are, as it were, 
 the soul. 
 
 Partly through methematical and partly through ex- 
 perimental research, physical science has of late years as- 
 sumed a momentous position in the world. Both in a 
 material and in an intellectual point of view it has pro- 
 duced, and it is destined to produce, immense changes 
 vast social ameliorations, and vast alterations in the popu- 
 lar conception of the origin, rule, and governance of natural 
 things. By science, in the physical world, miracles are 
 wrought, while philosophy is forsaking its ancient meta- 
 physical channels and pursuing others which have been 
 opened or indicated by scientific research. This must be- 
 come more and more the case as philosophical writers 
 become more deeply imbued with the methods of science, 
 better acquainted with the facts which scientific men have 
 won, and with the great theories which they have elaborated. 
 
 If you look at the face of a watch, you see the hour 
 and minute hands, and possibly also a second-hand, moving 
 over the graduated dial. Why do these hands move ? and 
 why are their relative motions such as they are observed 
 to be ? These questions cannot be answered without open- 
 ing the watch, mastering its various parts, and ascertaining 
 
112 FRAGMENTS OF SCIENCE. 
 
 their relationship to each other. When this is done, we 
 find that the observed motion of the hands follows of ne- 
 cessity from the inner mechanism of the watch, when acted 
 upon by the force invested in the spring. 
 
 The motion of the hands may be called a phenomenon 
 of art, but the case is similar with the phenomena of Nature. 
 These also have their inner mechanism, and their store of 
 force to set that mechanism going. The ultimate problem 
 of physical science is to reveal this mechanism, to discern 
 this store, and to show that from the combined action of 
 both the phenomena of which they constitute the basis 
 must of necessity flow. 
 
 I thought an attempt to give you even a brief and 
 sketchy illustration of the manner in which scientific think- 
 ers regard this problem would not be uninteresting to you 
 on the present occasion ; more especially as it will give me 
 occasion to say a word or two on the tendencies and limits 
 of modern science ; to point out the region which men of 
 science claim as their own, and where it is mere waste of 
 time to oppose their advance, and also to define, if possible, 
 the bourne between this and that other region to which 
 the questionings and yearnings of the scientific intellect 
 are directed in vain. 
 
 But here your tolerance will be needed. It was the 
 American Emerson, I think, who said that it is hardly pos- 
 sible to state any truth strongly without apparent injustice 
 to some other truth. Truth is often of a dual character, 
 taking the form of a magnet with two poles ; and many of 
 the differences which agitate the thinking part of mankind 
 are to be traced to the exclusiveness with which partisan 
 reasoners dwell upon one-half of the duality in forgetfulness 
 of the other. The proper course appears to be to state 
 both halves strongly, and allow each its fair share in the 
 formation of the resultant conviction. But this waiting for 
 the statement of the two sides of a question implies pa- 
 
SCIENTIFIC MATERIALISM. 113 
 
 tience. It implies a resolution to suppress indignation if 
 the statement of the one-half should clash with our convic- 
 tions, and to repress equally undue elation if the half-state- 
 ment should happen to chime in with our views. It implies 
 a determination to wait calmly for the statement of the 
 whole, before we pronounce judgment in the form of either 
 acquiescence or dissent. 
 
 This premised, and, I trust, accepted, let us enter upon 
 our task. There have been writers who affirmed that the 
 pyramids of Egypt were the productions of Nature ; and in 
 his early youth Alexander von Humboldt wrote a learned 
 essay with the express object of refuting this notion. We 
 now regard the pyramids as the work of men's hands, aided 
 probably by machinery of which no record remains. We 
 picture to ourselves the swarming workers toiling at those 
 vast erections, lifting the inert stones, and, guided by the 
 volition, the skill, and possibly at times by the whip of the 
 architect, placing them in their proper positions. The 
 blocks in this case were moved and posited by a power 
 external to themselves, and the final form of the pyramid 
 expressed the thought of its human builder. 
 
 Let us pass from this illustration of constructive power 
 to another of a different kind. When a solution of common 
 salt is slowly evaporated, the water which holds the salt 
 in solution diappears, but the salt itself remains behind. At 
 a certain stage of concentration the salt can no longer retain 
 the liquid form ; its particles, or molecules, as they are 
 called, begin to deposit themselves as minute solids, so 
 minute, indeed, as to defy all microscopic power. As evapo- 
 ration continues solidification goes on, and we finally obtain, 
 through the clustering together of innumerable molecules, 
 a finite crystalline mass of a definite form. What is this 
 form ? It sometimes seems a mimicry of the architecture 
 of Egypt. We have little pyramids built by the salt, 
 terrace above terrace from base to apex, forming a series of 
 
114 FRAGMENTS OF SCIENCE. 
 
 steps resembling those up which the Egyptian traveller is 
 dragged by his guides. The human mind is as little dis- 
 posed to look unquestioning at these pyramidal salt-crys- 
 tals as to look at the pyramids of Egypt without inquiring 
 whence they came. How, then, are those salt-pyramids 
 built up ? 
 
 Guided by analogy, you may, if you like, suppose that 
 swarming among the constituent molecules of the salt, 
 there is an invisible population, controlled and coerced by 
 some invisible master, and placing the atomic blocks in 
 their positions. This, however, is not the scientific idea, 
 nor do I think your good sense will accept it as a likely 
 one. The scientific idea is that the molecules act upon each 
 other without the intervention of slave labor ; that they 
 attract each other and repel each other at certain definite 
 points, or poles, and in certain definite directions ; and that 
 the pyramidal form is the result of this play of attraction 
 and repulsion. While, then, the blocks of Egypt were laid 
 down by a power external to themselves, these molecular 
 blocks of salt are self-posited, being fixed in their places by 
 the forces with which they act upon each other. 
 
 I take common salt as an illustration because it is so 
 familiar to us all ; but any other crystalline substance would 
 answer my purpose equally well. Everywhere, in fact, 
 throughout inorganic Nature, we have this formative power, 
 as Fichte would call it this structural energy ready to 
 come into play, and build the ultimate particles of matter 
 into definite shapes. The ice of our winters and of our 
 polar regions is its handywork, and^ so equally are the 
 quartz, felspar, and mica of our rocks.\ Our chalk-beds are 
 for the most part composed of minute shells, which are also 
 the product of structural energy ; but, behind the shell, as 
 a whole, lies a more remote and subtle formative act. These 
 shells are built up of little crystals of calc-spar, and to form 
 these crystals the structural force had to deal with the 
 
SCIENTIFIC MATERIALISM. 115 
 
 intangible molecules of carbonate of lime. This tendency 
 on the part of matter to organize itself, to grow into shape, 
 to assume definite forms in obedience to the definite action 
 of force, is, as I have said, all-pervading. It is in the 
 ground on which you tread, in the water you drink, in the 
 air you breathe. Incipient life, as it were, manifests itself 
 throughout the whole of what we call inorganic Nature. 
 
 The forms of the minerals resulting from this play of 
 polar forces are various, and exhibit different degrees of 
 complexity. Men of science avail themselves of all possible 
 means of exploring their molecular architecture. For this 
 purpose they employ in turn as agents of exploration, light, 
 heat, magnetism, electricity, and sound. Polarized light is 
 especially useful and powerful here. A beam of such light, 
 when sent in among the molecules of a crystal, is acted on 
 by them, and from this action we infer with more or less of 
 clearness the manner in which the molecules are arranged. 
 That differences, for example, exist between the inner 
 structure of rock-salt and crystallized sugar or sugar-candy, 
 is thus strikingly revealed. These actions often display 
 themselves in chromatic phenomena of great splendor, the 
 play of molecular force being so regulated as to remove 
 some of the colored constituents of white light, and to leave 
 others with increased intensity behind. 
 
 And now let us pass from what we are accustomed to 
 regard as a dead mineral to a living grain of corn. When 
 it is examined by polarized light, chromatic phenomena 
 similar to those noticed in crystals are observed. And 
 why ? Because the architecture of the grain resembles the 
 architecture of the crystal. In the grain also the molecules 
 are set in definite positions, and in accordance with their 
 arrangement they act upon the light. But what has built 
 together the molecules of the corn ? I have already said 
 regarding crystalline architecture that you may, if you 
 please, consider the atoms and molecules to be placed in 
 
116 FRAGMENTS OF SCIENCE. 
 
 position by a power external to themselves. The same 
 hypothesis is open to you now. But if in the case of crys- 
 tals you have rejected this notion of an external architect, 
 I think you are bound to reject it now, and to conclude 
 that the molecules of the corn are self-posited by the forces 
 with which they act upon each other. It would be poor 
 philosophy to invoke an external agent in the one case and 
 to reject it in the other. 
 
 Instead of cutting our grain of corn into slices and sub- 
 jecting it to the action of polarized light, let us place it in 
 the earth and subject it to a certain degree of warmth. In 
 other words, let the molecules, both of the corn and of the 
 surrounding earth, be kept in that state of agitation which 
 we call warmth. Under these circumstances, the grain and 
 the substances which surround it interact, and a definite 
 molecular architecture is the result. A bud is formed ; this 
 bud reaches the surface, where it is exposed to the sun's 
 rays, which are also to be regarded. as a kind of vibratory 
 motion. And as the motion of common heat with which 
 the grain and the substances surrounding it were first 
 endowed, enabled the grain and these substances to exer- 
 cise their attractions and repulsions, and thus to coalesce 
 in definite forms, so the specific motion of the sun's rays 
 now enables the green bud to feed upon the carbonic acid 
 and the aqueous vapor of the air. The bud appropriates 
 those constituents of both for which it has an elective 
 attraction, and permits the other constituent to resume its 
 place in the air. Thus the architecture is carried on. 
 Forces are active at the root, forces are active in the blade, 
 the matter of the earth and the matter of the atmosphere 
 are drawn toward the root and blade, and the plant aug- 
 ments in size. We have in succession the bud, the stalk, 
 the ear, the full corn in the ear; the cycle of molecular 
 action being completed by the production of grains similar 
 to that with which the process began. 
 
SCIENTIFIC MATERIALISM. 117 
 
 Now there is nothing in this process which necessarily 
 eludes the conceptive or imagining power of the purely 
 human mind. An intellect the same in kind as our own 
 would, if only sufficiently expanded, be able to follow the 
 whole process from beginning to end. It would see every 
 molecule placed in its position by the specific attractions 
 and repulsions exerted between it and other molecules, the 
 whole process and its consummation being an instance of 
 the play of molecular force. Given the grain and its envi- 
 ronment, the purely human intellect might, if sufficiently 
 expanded, trace out a priori every step of the process of 
 growth, and by the application of purely mechanical prin- 
 ciples demonstrate that the cycle must end, as it is seen to 
 end, in the reproduction of forms like that with which it 
 began. A similar necessity rules here to that which rules 
 the planets in their circuits round the sun. 
 
 You will notice that I am stating my truth strongly, as 
 at the beginning we agreed it should be stated. But I 
 must go still further, and affirm that in the eye of science 
 the animal body is just as much the product of molecular 
 force as the stalk and ear of corn, or as the crystal of salt 
 or sugar. Many of the parts of the body are obviously 
 mechanical. Take the human heart, for example, with its 
 system of valves, or take the exquisite mechanism of the 
 eye or hand. Animal heat, moreover, is the same in kind 
 as the heat of a fire, being produced by the same chemical 
 process. Animal motion, too, is as directly derived from 
 the food of the animal, as the motion of Trevethyck's walk- 
 ing-engine from the fuel in its furnace. As regards matter, 
 the animal body creates nothing ; as regards force, it creates 
 nothing. Which of you by taking thought can add one 
 cubit to his stature ? All that has been said, then, regard- 
 ing the plant may be restated with regard to the animal. 
 Every particle that enters into the composition of a muscle, 
 a nerve, or a bone, has been placed in its position by mo- 
 
118 FRAGMENTS OF SCIENCE. 
 
 lecular force. And unless the existence of law in these 
 matters be denied, and the element of caprice introduced, 
 we must conclude that, given the relation of any molecule 
 of the body to its environment, its position in the body 
 might be determined mathematically. Our difficulty is not 
 with the quality of the problem, but with its complexity 
 and this difficulty might be met by the simple expansion 
 of the faculties which we now possess. Given this expan- 
 sion, with the necessary molecular data, and the chick 
 might be deduced as rigorously and as logically from the 
 egg as the existence of Neptune from the disturbances of 
 Uranus, or as conical refraction from the undulatory theory 
 L of light. 
 
 You see I am not mincing matters, but avowing nakedly 
 what many scientific thinkers more or less distinctly be- 
 lieve. The formation of a crystal, a plant, or an animal, is 
 in their eyes a purely mechanical problem, which differs 
 from the problems of ordinary mechanics in the smallness 
 of the masses and the complexity of the processes involved. 
 Here you have one half of our dual truth ; let us now glance 
 at the other half. Associated with this wonderful mechan- 
 ism of the animal body we have phenomena no less certain 
 than those of physics, but between which and the mechan- 
 ism we discern no necessary connection. A man, for ex- 
 ample, can say, I feel, I thinJc, I love ; but how does 
 consciousness infuse itself into the problem ? The human 
 brain is said to be the organ of thought and feeling ; when 
 we are hurt the brain feels it, when we ponder it is the 
 brain that thinks, when our passions or affections are ex- 
 cited it is through the instrumentality of the brain. Let us 
 endeavor to be a little more precise here. I hardly imagine 
 there exists a profound scientific thinker, who has reflected 
 
 I upon the subject, unwilling to admit the extreme proba- 
 bility of the hypothesis that, for every fact of consciousness, 
 whether in the domain of sense, of thought, or of emotion, 
 
SCIENTIFIC MATERIALISM. 119 
 
 a definite molecular condition of motion or structure is set 
 up in the brain ; or who would be disposed even to deny 
 that if the motion or structure be induced by internal 
 causes instead of external, the effect on consciousness will 
 be the same ? Let any nerve, for example, be thrown by 
 morbid action into the precise state of motion which would 
 be communicated to it by the pulses of a heated body, 
 surely that nerve will declare itself hot the mind will 
 accept the subjective intimation exactly as if it were ob- 
 jective. The retina may be excited by purely mechanical 
 means. A blow on the eye causes a luminous flash, and 
 the mere pressure of the finger on the external ball pro- 
 duces a star of light, which Newton compared to the circles 
 on a peacock's tail. Disease makes people see visions and 
 dream dreams ; but, in all such cases, could we examine 
 the organs implicated, we should, on philosophical grounds, 
 expect to find them in that precise molecular condition 
 which the real objects, if present, would superinduce. 
 
 The relation of physics to consciousness being thus 
 invariable, it follows that, given the state of the brain, the 
 corresponding thought or feeling might be inferred ; or 
 given the thought or feeling, the corresponding state of the 
 brain might be inferred. But how inferred ? It would be 
 at bottom not a case of logical inference at all, but of 
 empirical association. You may reply that many of the 
 inferences of science are of this character ; the inference, 
 for example, that an electric current of a given direction 
 will deflect a magnetic needle in a definite way ; but the 
 cases differ in this, that the passage from the current to the 
 needle, if not demonstrable, is thinkable, and that we enter- 
 tain no doubt as to the final mechanical solution of the 
 problem. But the passage from the physics of the brain 
 to the corresponding facts of consciousness is unthinkable. 
 Granted that a definite thought, and a definite molecular 7 
 action in the brain occur simultaneously ; we do not possess 
 
120 FRAGMENTS OF SCIENCE. 
 
 the intellectual organ, nor apparently any rudiment of the 
 organ, which would enable us to pass, by a process of rea- 
 i soning, from the one to the other. They appear together, 
 I but we do not know why. Were our minds and senses so 
 expanded, strengthened, and illuminated as to enable us to 
 see and feel the very molecules of the brain ; were we 
 capable of following all their motions, all their groupings, 
 all their electric discharges, if such there be ; and were we 
 intimately acquainted with the corresponding states of 
 thought and feeling, we should be as far as ever from the 
 solution of the problem, " How are these physical processes 
 I connected with the_facts of consciousness ? " The chasm 
 between the two classes of phenomena would still remain 
 intellectually impassable. Let the consciousness of love, 
 for example, be associated with a right-handed spiral 
 motion of the molecules of the brain, and the consciousness 
 of hate with a left-handed spiral motion. "We should then 
 know when we love that the motion is in one direction, 
 and when we hate that the motion is in the other ; but the 
 " WHY ? " would remain as unanswerable as before. ' 
 
 ' r ""ln affirming that the growth of the body is mechanical, 
 and that thought, as exercised by us, has its correlative in 
 the physics of the brain, I think the position of the " Ma- 
 terialist" is stated, as far as that position is a tenable 
 one. I think the materialist will be able finally to main- 
 tain this position against all attacks ; but I do not think, 
 in the present condition of the human mind, that he can 
 pass beyond this position. I do not think he is entitled 
 to say that his molecular groupings and his molecular 
 motions explain every thing. In reality, they explain 
 nothing. The utmost he can affirm is the association of 
 two classes of phenomena, of whose real bond of union 
 he is in absolute ignorance. The problem of the con- 
 nection of body and soul is as insoluble in its modern 
 form as it was in the prescientific ages. Phosphorus is 
 
SCIENTIFIC MATERIALISM. 121 
 
 known to enter into the composition of the human brain, 
 and a trenchant German writer has exclaimed, -^-Ohrre 
 Phespht3iy^eift-Gedaiikez" That may or may not be the 
 case ; but even if we knew it to be the case, the knowledge 
 would not lighten our darkness. On both sides of the zone 
 here assigned to the materialist he is equally helpless. If 
 you ask him whence is this " Matter " of which we have 
 been discoursing, who or what divided it into molecules, 
 who or what impressed upon them this necessity of running 
 into organic forms, he has no answer. Science is mute in 
 reply to these questions. But if the materialist is con- 
 founded and science rendered dumb, who else is prepared 
 with a solution ? To whom has this arm of the Lord been 
 revealed? Let us lower our heads and acknowledge our 
 ignorance, priest and philosopher, one and all. 
 
 Perhaps the mystery may resolve itself into knowledge 
 nt some future day. The process of things upon this earth 
 has been one of amelioration. It is a long way from the 
 Iguanodon and his contemporaries to the President and 
 members of the British Association. And whether we re- 
 gard the improvement from the scientific or from the theo- 
 logical point of view, as the result of progressive develop- 
 ment, or as the result of successive exhibitions of creative 
 energy, neither view entitles us to assume that man's present 
 faculties end the series that the process of amelioration 
 stops at him. A time may therefore come when this ultra- 
 scientific region by which we are now enfolded may offer it- 
 self to terrestrial, if not to human investigation. Two-thirds 
 of the rays emitted by the sun fail to arouse in the eye the 
 sense of vision. The rays exist, but the visual organ requi- 
 site for their translation into light does not exist. And so 
 from this region of darkness and mystery which surrounds 
 us, rays may now be darting which require but the develop- 
 ment of the proper intellectual organs to translate them 
 into knowledge as far surpassing ours as ours surpasses 
 G 
 
122 FRAGMENTS OF SCIENCE. 
 
 that of the wallowing reptiles which once held possession 
 gf this planet. Meanwhile the mystery is not without its 
 uses. It certainly may be made a power in the human soul ; 
 but it is a power which has feeling, not knowledge, for its 
 base. It may be, and will be, and I hope is, turned to 
 account, both in studying and strengthening the intellect, 
 and in rescuing man from that littleness to which, in the 
 struggle for existence, or for precedence in the world, he 
 is continually prone. 
 
Musings on the Matterliorn, .July 27, 1868. 
 
 " HACKED and hurt by time, the aspect of the mountain from its 
 higher crags saddened me. Hitherto the impression it made was that of 
 savage strength ; here we had inexorable decay. But this notion of decay 
 implied a reference to a period when the Matterhora was in the full 
 strength of mountainhood. Thought naturally ran back to its remoter 
 origin and sculpture. Nor did thought halt there, but wandered on 
 through molten worlds to that nebulous haze which philosophers have 
 regarded, and with good reason, as the proximate source of all material 
 things. I tried to look at this universal cloud, containing within itself the 
 prediction of all that has since occurred ; I tried to imagine it as the seat 
 of those forces whose action was to issue in solar and stellar systems, 
 and all that they involve. Did that formless fog contain potentially the 
 sadness with which I regarded the Matterhorn ? Did the thought which 
 now ran back to it simply return to its primeval home ? If so, had we 
 not better recast our definitions of matter and force ; for if life and 
 thought be the very flower of both, any definition which omits life and 
 thought must be inadequate, if not untrue. Are questions like these 
 warranted ? Why not ? If the final goal of man has not been yet 
 attained ; if his development has not been yet arrested, who can say that 
 such yearnings and questionings are not necessary to the opening of a 
 finer vision, to the budding and the growth of diviner powers ? When I 
 look at the heavens and the earth, at my own body, at my strength and 
 weakness of mind, even at these ponderings, and ask myself, is there no 
 being or thing in the universe that knows more about these matters than 
 I do ; what is my answer ? Supposing our theologic schemes of crea- 
 tion, condemnation, and redemption, to be dissipated ; and the warmth 
 of denial which they excite, and which, as a motive force, can match the 
 warmth of affirmation dissipated at the same time ; would the undeflected 
 human mind return to the meridian of absolute neutrality as regards these 
 ultra-physical questions ? Is such a position one of stable equilibrium ? 
 The channels of thought being already formed, such are the questions 
 without replies, which could run athwart consciousness during a ten- 
 rninutes, halt upon the weathered point of the Matterhorn." 
 
VII. 
 
 ON THE 
 
 SCIENTIFIC USE OF THE IMAGINATION, 
 
 A DISCOUKSE. 
 
 DELIVERED BEFORE THE BRITISH ASSOCIATION AT LITERPOOL. 
 
 September 16, 1870. 
 
" If thou wouldst know the mystic song 
 Chanted when the sphere was young, 
 Aloft, abroad, the paean swells, 
 wise man, hear'st thou half it tells ? 
 To the open ear it sings 
 The early genesis of things ; 
 Of tendency through endless ages 
 Of star-dust and star-pilgrimages, 
 Of rounded worlds, of space and time, 
 Of the old floods' subsiding slime, 
 Of chemic matter, force and form, 
 Of poles and powers, cold, wet, and warm. 
 The rushing metamorphosis 
 Dissolving all that fixture is, 
 Melts things that be to things that seem, 
 And solid Nature to a dream." 
 
 EMERSON. 
 
 " Was war' ein Gott der nur von aussen stiesse 
 Im Kreis das All am Finger laufen Hesse ! 
 Ihm ziemt's, die Welt im Innern zu bewegen, 
 Natur in Sich, Sich in Natur zu hegen." 
 
 GOETHE. 
 
SCIENTIFIC USE OF THE IMAGINATION. 
 
 " Lastly, physical investigation more than any thing besides helps to teach 
 us the actual value and rigid use of the Imagination of that wondrous 
 faculty, which, left to ramble uncontrolled, leads us astray into a wilderness of 
 perplexities and errors, a land of mists and shadows ; but which properly con- 
 trolled by experience and reflection, becomes the noblest attribute of man : the 
 source of poetic genius, the instrument of discovery in Science, without tlie aid 
 of which Newton would never have invented jluxions, nor Davy have decom- 
 posed the earths and alkalies, nor would Columbus have found another Con- 
 tinent." Address to the Royal Society by its President, Sir Benjamin 
 Brodie, November 30, 1859. 
 
 I CARRIED with me to the Alps this year the heavy 
 burden of this evening's work. In the way of new inves- 
 tigation I had nothing complete enough to be brought 
 before you ; so all that remained to me was to fall back 
 upon such residues as I could find in the depths of con- 
 sciousness, and out of them to spin the fibre and weave the 
 web of this discourse. Save from memory I had no direct 
 aid upon the mountains ; but to spur up the emotions, on 
 which so much depends, as well as to nourish indirectly the 
 intellect and will, I took with me two volumes of poetry, 
 Goethe's " Farbenlehre," and the work on " Logic " recently 
 published by Mr. Alexander Bain. 1 The spur, I am sorry 
 to say, was no match for the integument of dulness it had 
 
 1 One of my critics remarks, that he does not see the wit of calling 
 Goethe's " Farbenlehre" and Bain's " Logic," " two volumes of poetry." 
 Nor do I. 
 
128 FRAGMENTS OF SCIENCE. 
 
 to pierce. In Goethe, so glorious otherwise, I chiefly 
 noticed the self-inflicted hurts of genius, as it broke itself 
 in vain against the philosophy of Newton. For a time, 
 Mr. Bain became my principal companion. I found him 
 learned and practical, shining generally with a dry light, 
 but exhibiting at times a flush of emotional strength, which 
 proved that even logicians share the common fire of hu- 
 manity. He interested me most when he became the 
 mirror of my own condition. Neither intellectually nor 
 socially is it good for man to be alone, and the griefs of 
 thought are more patiently borne when we find that they 
 have been experienced by another. From certain passages 
 in his book I could infer that Mr. Bain was no stranger to 
 such sorrows. Take this passage as an illustration. Speak- 
 ing of the ebb of intellectual force, which we all from time 
 to time experience, Mr. Bain says, " The uncertainty where 
 to look for the next opening of discovery brings the pain of 
 conflict and the debility of indecision." These words have 
 in them the true ring of personal experience. The action 
 of the investigator is periodic. He grapples with a subject 
 of inquiry, wrestles with it, overcomes it, exhausts, it may 
 be, both himself and it for the time being. He breathes a 
 space, and then renews the struggle in another field. Now 
 this period of halting between two investigations is not 
 always one of pure repose. It is often a period of doubt 
 and discomfort, of gloom and ennui. "The uncertainty 
 where to look for the next opening of discovery brings the 
 pain of conflict and the debility of indecision." Such was 
 my precise condition in the Alps this year ; in a score of 
 words Mr. Bain has here sketched my mental diagnosis ; 
 and it was under these evil circumstances that I had to 
 equip myself for the hour and the ordeal that are now 
 come. 
 
 / Gladly, however, as I should have seen this duty in 
 other hands, I could by no means shrink from it. Disloy- 
 
SCIENTIFIC USE OF THE IMAGINATION. 129 
 
 alty would have been worse than failure. In some fashion 
 or other feebly or strongly, meanly or manfully, on the 
 higher levels of thought, or on the flats of commonplace 
 the task had to be accomplished. I looked in various direc- 
 tions for help and furtherance ; but without me for a time 
 I saw only " antres vast," and within me "deserts idle." 
 >My case resembled that of a sick doctor who had forgotten 
 his art and sorely needed the prescription of a friend. Mr. 
 Bain wrote one for me. He said, " Your present knowl- 
 edge must forge the links of connection between what has 
 been already achieved and what is now required." 3 In 
 these words he admonished me to review the past and re- 
 cover from it the broken ends of former investigations. I 
 tried to do so. Previous to going to Switzerland I had 
 been thinking much of light and heat, of magnetism and 
 electricity, of organic germs, atoms, molecules, spontaneous 
 generation, comets, and skies. With one or another of 
 these I now sought to reform an alliance, and finally suc- 
 ceeded in establishing a kind of cohesion between Thought 
 and Light. The wish grew within me to trace, and to en- 
 able you to trace, some of the more occult operations of 
 this agent. I wished, if possible, to take you behind the 
 drop-scene of the senses, and to show you the hidden mech- 
 anism of optical action. For I take it to be well worth 
 the while of the scientific teacher to take some pains, and 
 even great pains, to make those whom he addresses copart- 
 ners of his thoughts. To clear his own mind in the first place 
 of all haze and vagueness, and then to project into lan- 
 guage which shall leave no mistake as to his meaning 
 which shall leave even his errors naked the definite ideas 
 he has shaped. A great deal is, I think, possible to scien- 
 tific exposition conducted in this way. It is possible, I 
 believe, even before an audience like the present, to un- 
 cover to some extent the unseen things of Nature; and 
 
 1 Induction, p. 422. 
 
130 FRAGMENTS OF SCIENCE. 
 
 thus to give not only to professed students, but to others 
 with the necessary bias, industry, and capacity, an intelli- 
 gent interest in the operations of science. Time and labor 
 are necessary to this result, but science is the gainer from 
 the public sympathy thus created. 
 
 How, then, are those hidden things to be revealed ? 
 How, for example, are we to lay hold of the physical basis 
 of light, since, like that of life itself, it lies entirely without 
 the domain of the senses ? Philosophers may be right in 
 affirming that we cannot transcend experience ; but we can 
 at all events carry it a long way from its origin. We can also 
 magnify, diminish, qualify, and combine experiences, so as to 
 render them fit for purposes entirely new. We are gifted 
 with the power of imagination combining what the Ger- 
 mans call Anschauungsgabe and Einbildungskraft and by 
 this power we can lighten the darkness which surrounds the 
 world of the senses. There are tories even in science who 
 regard imagination as a faculty to be feared and avoided 
 rather than employed. They had observed its action in 
 weak vessels, and were unduly impressed by its disasters. 
 But they might with equal justice point to exploded boil- 
 ers as an argument against the use of steam. Bounded and 
 conditioned by cooperant Reason, imagination becomes the 
 mightiest instrument of the physical discoverer. Newton's 
 passage from a falling apple to a falling moon was, at the 
 outset, a leap of the imagination. When William Thom- 
 son tries to place the ultimate particles of matter between 
 his compass-points, and to apply to them a scale of milli- 
 metres, he is powerfully aided by this faculty. And in 
 much that has been recently said about protoplasm and 
 life, we have the outgoings of the imagination guided and 
 controlled by the known analogies of science. In fact, 
 without this power, our knowledge of Nature would be a 
 mere tabulation of coexistences and sequences. We should 
 still believe in the succession of day and night, of summer 
 
SCIENTIFIC USE OF THE IMAGINATION. 131 
 
 and winter ; but the soul of Force would be dislodged from 
 our universe ; causal relations would disappear, and with 
 them that science which is now binding the parts of Nature 
 to an organic whole. 
 
 / I should like to illustrate by a few simple instances the 
 use that scientific men have already made of this power of 
 imagination, and to indicate afterward some of the further 
 uses that they are likely to make of it. Let us begin with 
 the rudimentary experiences. Observe the falling of heavy 
 rain-drops into a tranquil pond. Each drop as it strikes the 
 water becomes a centre of disturbance, from which a series 
 of ring-ripples expand outwards. Gravity and inertia are 
 the agents by which this wave-motion is produced, and a 
 rough experiment will suffice to show that the rate of 
 propagation does not amount to a foot a second. A series 
 of slight mechanical shocks is experienced by a body 
 plunged in the water as the wavelets reach it in succes- 
 sion. But a finer motion is at the same time set up and 
 propagated. If the head and ears be immersed in the wa- 
 ter, as in an experiment of Franklin's, the shock of the 
 drop is communicated to the auditory nerve the tick of 
 the drop is heard. Now this sonorous impulse is propa- 
 gated, not at the rate of a foot a second, but at the rate of 
 forty-seven hundred feet a second. In this case it is not 
 the gravity, but the elasticity of the water that is the ur- 
 ging force. Every liquid particle pushed against its neigh- 
 bor delivers up its motion with extreme rapidity, and the 
 pulse is propagated as a thrill. The incompressibility of 
 water, as illustrated by the famous Florentine experiment, 
 is a measure of its elasticity, and to the possession of this 
 property in so high a degree the rapid transmission of a 
 sound-pulse through water is to be ascribed. 
 
 But water, as youiknow, is not necessary to the conduc- 
 tion of sound ; air is its most common vehicle. And you 
 know that when the air possesses the particular density 
 
132 FRAGMENTS OF SCIENCE. 
 
 and elasticity corresponding to the temperature of freezing- 
 water the velocity of sound in it is ten hundred and ninety 
 feet a second. It is almost exactly one-fourth of the ve- 
 locity in water ; the reason being that though the greater 
 weight of the water tends to diminish the velocity, the 
 enormous molecular elasticity of the liquid far more than 
 atones for the disadvantage due to weight. By various 
 contrivances we can compel the vibrations of the air to 
 declare themselves ; we know the length and frequency of 
 sonorous waves, and we have also obtained great mastery 
 over the various methods by which the air is thrown into 
 vibration. We know the phenomena and laws of vibrating 
 rods, of organ-pipes, strings, membranes, plates, and bells. 
 We can abolish one sound by another. We know the 
 physical meaning of music and noise, of harmony and dis- 
 cord. In short, as regards sounds we have a very clear 
 notion of the external physical processes which corre- 
 spond to our sensations. 
 
 In these phenomena of sound we travel a very little 
 way from downright sensible experience. Still the imagi- 
 nation is to some extent exercised. The bodily eye, for 
 example, cannot see the condensations and rarefactions of 
 the waves of sound. We construct them in thought, and 
 we believe as firmly in their existence as in that of the air 
 itself. But now our experience has to be carried into a 
 new region, where a new use is to be made of it. Having 
 mastered the cause and mechanism of sound, we desire to 
 know the cause and mechanism of light. We wish to ex- 
 tend our inquiries from the auditory nerve to the optic nerve. 
 There is in the human intellect a power of expansion^ I 
 might almost call it a power of creation^-which is brought 
 into play by the simple brooding upon facts. The legend 
 of the Spirit brooding over chaos may have originated in a 
 knowledge of this power. In the case now before us it has 
 manifested itself by transplanting into space, for the pur- 
 
SCIENTIFIC USE OF THE IMAGINATION. 133 
 
 poses of light, an adequately modified form of the mechan- 
 ism of sound. We know intimately whereon the velocity 
 of sound depends. When we lessen the density of a medium 
 and preserve its elasticity constant we augment the velocity. 
 When we heighten the elasticity and keep the density con- 
 stant we also augment the velocity. A small density, 
 therefore, and a great elasticity, are the two things neces- 
 sary to rapid propagation. Now light is known to move 
 with the astounding velocity of 185,000 miles a second. 
 How is such a velocity to be obtained? By boldly dif- 
 fusing in space a medium of the requisite tenuity and 
 elasticity. 
 
 Let us make such a medium our starting-point, endow- 
 ing it with one or two other necessary qualities ; let us 
 handle it in accordance with strict mechanical laws ; let us 
 give to every step of our deduction the surety of the syl- 
 logism ; let us carry it thus forth from the world of imagi- 
 nation into the world of sense, and see whether the final 
 outcrop of the deduction be not the very phenomena of 
 light which ordinary knowledge and skilled experiment re- 
 veal. If in all the multiplied varieties of these phenomena, 
 including those of the most remote and entangled descrip- 
 tion, this fundamental conception always brings us face to 
 face with the truth ; if no contradiction to our deductions 
 from it be found in external Nature, but on all sides agree- 
 ment and verification ; if, moreover, as in the case of Coni- 
 cal Refraction and in other cases, it has actually forced 
 upon our attention phenomena which no eye had previously 
 seen, and which no mind had previously imagined, such a 
 conception, which never disappoints us, but always lands 
 us on the solid shores of fact, must, we think, be something 
 more than a mere figment of the scientific fancy. In form- 
 ing it that composite and creative unity in which reason 
 and imagination are together blent, has, we believe, led us 
 into a world not less real than that of the senses, and of 
 
134 FRAGMENTS OF SCIENCE. 
 
 which the world of sense itself is the suggestion and justi- 
 fication. 
 
 Far be it from me, however, to wish to fix you immov- 
 ably in this or in any other theoretic conception. With 
 all our belief of it, it will be well to keep the theory plastic 
 and capable of change. You may, moreover, urge that 
 although the phenomena occur as if the medium existed, 
 the absolute demonstration of its existence is still wanting. 
 Far be it from me to deny to this reasoning such validity 
 as it may fairly claim. Let us endeavor by means of anal- 
 ogy to form a fair estimate of its force. You believe that 
 in society you are surrounded by reasonable beings like 
 yourself. You are perhaps as firmly convinced of this as 
 of any thing. What is your warrant for this conviction ? 
 Simply and solely this, your fellow-creatures behave as if 
 they were reasonable ; the hypothesis, for it is nothing 
 more, accounts for the facts. To take an eminent example : 
 you believe that our President is a reasonable being. Why ? 
 There is no known method of superposition by which any 
 one of us can apply himself intellectually to another so as 
 to demonstrate coincidence as regards the possession of 
 reason. If, therefore, you hold our President to be reason- 
 able, it is because he behaves as if he were reasonable. As 
 in the case of the ether, beyond the " as if" you cannot go. 
 Nay I should not wonder if a close comparison of the data 
 on which both inferences rest, caused many respectable 
 persons to conclude that the ether had the best of it. 
 
 This universal medium, this light-ether as it is called, is 
 a vehicle, not an origin of wave-motion. It receives and 
 transmits, but it does not create. Whence does it derive 
 the motions it conveys ? For the most part from luminous 
 bodies. By this motion of a luminous body I do not mean 
 its sensible motion, such as the flicker of a candle, or the 
 shooting out of red prominences from the limb of the sun. 
 I mean an intestine motion of the atoms or molecules of 
 
SCIENTIFIC USE OF THE IMAGINATION. 135 
 
 the luminous body. But here a certain reserve is necessary. 
 Many chemists of the present day refuse to speak of atoms 
 and molecules as real things. Their caution leads them to 
 stop short of the clear, sharp, mechanically intelligible 
 atomic theory enunciated by Dalton, or any form of that 
 theory, and to make the doctrine of multiple proportions 
 their intellectual bourne. I respect the caution, though I 
 think it is here misplaced. The chemists who recoil from 
 these notions of atoms and molecules accept without hesita- 
 tion the Undulatory Theory of Light. Like you and me, 
 they one and all believe in an ether and its light-producing 
 waves. Let us consider what this belief involves. Bring 
 your imaginations once more into play and figure a series 
 of sound-waves passing through air. Follow them up to 
 their origin, and what do you there find ? A definite, tan- 
 gible, vibrating body. It may be the vocal chords of a 
 human being, it may be an organ-pipe, or it may be a 
 stretched string. Follow in the same manner a train of 
 ether-waves to their source ; remembering at the same time 
 that your ether is matter, dense, elastic, and capable of 
 motions subject to and determined by mechanical laws. 
 What then do you expect to find as the source of a series 
 of ether-waves ? Ask your imagination if it will accept a 
 vibrating multiple proportion a numerical ratio in a state 
 of oscillation ? I do not think it will. You cannot crown 
 the edifice by this abstraction. The scientific imagination, 
 which is here authoritative, demands as the origin and cause 
 of a series of ether-waves a particle of vibrating matter 
 quite as definite, though it may be excessively minute, as 
 that which gives origin to a musical sound. Such a particle 
 we name an atom or a molecule. I think the seeking intel- 
 lect when focussed so as to give definition without penum- 
 bral haze, is sure to realize this image at the last. 
 
 With a view of preserving thought continuous through- 
 out this discourse, and of preventing either failure of knowl- 
 
136 FRAGMENTS OF SCIENCE. 
 
 edge or of memory from causing any rent in our picture, I 
 here propose to run rapidly over a bit of ground which is 
 probably familiar to most of you, but which I am anxious to 
 make familiar to you all. The waves generated in the ether 
 by the swinging atoms of luminous bodies are of different 
 lengths and amplitudes. The amplitude is the width of 
 swing of the individual particles of the wave. In water- 
 waves it is the height of the crest above the trough, while 
 the length of the wave is the distance between two con- 
 secutive crests. The aggregate of waves emitted by the sun 
 may be broadly divided into two classes : the one class com- 
 petent, the other incompetent, to excite vision. But the 
 light-producing waves differ markedly among themselves 
 in size, form, and force. The length of the largest of these 
 waves is about twice that of the smallest, but the amplitude 
 of the largest is probably a hundred times that of the 
 smallest. Now the force or energy of the wave, which, ex- 
 pressed with reference to sensation, means the intensity of 
 the light, is proportional to the square of the amplitude. 
 Hence the amplitude being one-hundred-fold, the energy of 
 the largest light-giving waves would be ten-thousand-fold 
 that of the smallest. This is not improbable. I use these 
 figures not with a view to numerical accuracy, but to give 
 you definite ideas of the differences that probably exist 
 among the light-giving waves. And if we take the whole 
 range of solar radiation into account its non-visual as well 
 as its, visual waves I think it probable that the force or 
 eo^fgy of the largest wave is a million times that of the 
 smallest. 
 
 Turned into their equivalents of sensation, the different 
 light-waves produce different colors. Red, for example, is 
 roduced by the largest waves, violet by the smallest, while 
 green is produced by a wave of intermediate length and 
 amplitude. On entering from air into more highly refract- 
 ing substances, such as glass or water, or the sulphide of 
 
SCIENTIFIC USE OF THE IMAGINATION. 137 
 
 carbon, all the waves are retarded, but the smallest ones 
 most. This furnishes a means of separating the different 
 classes of waves from each other ; in other words, of ana- 
 lyzing the light. Sent through a refracting prism, the waves 
 of the sun are turned aside in different degrees from their 
 direct course, the red least, the violet most. They are vir- 
 tually pulled asunder, and they paint upon a white screen 
 placed to receive them "the solar spectrum." Strictly 
 speaking, the spectrum embraces an infinity of colors, but 
 the limits of language and of our powers of distinction cause 
 it to be divided into seven segments : red, orange, yellow, 
 green, blue, indigo, violet. These are the seven primary or 
 prismatic colors. 
 
 Separately, or mixed in, various proportions, the solar 
 waves yield all the colors observed in nature and employed 
 in art. Collectively, they give us the impression of white- 
 ness. Pure unsifted solar light is white ; and if all the 
 wave-constituents of such light be reduced in the same pro- 
 portion the light, though diminished in intensity, will still 
 be white. The whiteness of Alpine snow with the sun 
 shining upon it, is barely tolerable to the eye. The same 
 snow under an overcast firmament is still white. Such a 
 firmament enfeebles the light by reflection, and when we lift 
 ourselves above a cloud-field to an Alpine summit, for in- 
 stance, or to the top of Snowdon and see, in the proper 
 direction, the sun shining on the clouds, they appear daz- 
 zlingly white. Ordinary clouds, in fact, divide the solar 
 light impinging on them into two parts a reflected part 
 and a transmitted part, in each of which the proportions of 
 wave-motion which produce the impression of whiteness 
 are sensibly preserved. 
 
 It will be understood that the conditions of whiteness 
 would fail if all the waves were diminished equally r , or by 
 the same absolute quantity. They must be reduced pro- 
 portionately, instead of equally. If by the act of reflection 
 
138 FRAGMENTS OF SCIENCE. 
 
 the waves of red light are split into exact halves, then, to 
 preserve the light white, the waves of yellow, orange, green, 
 and blue must also be split into exact halves. In short, the 
 reduction must take place, not by absolutely equal quanti- 
 ties, but by equal fractional parts. In white light the pre- 
 ponderance as regards energy of the latter over the smaller 
 waves must always be immense. Were the case otherwise, 
 the physiological correlative, blue, of the smaller waves 
 would have the upper hand in our sensations. 
 
 My wish to render our mental images complete, causes 
 me to dwell briefly upon these known points, and the 
 same wish will cause me to linger a little longer among 
 others. But here I am disturbed by my reflections. When 
 /I consider the effect of dinner upon the nervous system, and 
 the relation of that system to the intellectual powers I am 
 now invoking when I remember that the universal expe- 
 rience of mankind has fixed upon certain definite elements 
 of perfection in an after-dinner speech, and when I think 
 how conspicuous by their absence these elements are on the 
 present occasion, the thought is not comforting to a man 
 who wishes to stand well with his fellow-creatures in gen- 
 eral, and with the members of the British Association in 
 particular. My condition might well resemble that of the 
 ether, which is scientifically defined as an assemblage of 
 vibrations. And the worst of it is, that unless you reverse 
 the general verdict regarding the effect of dinner, and prove 
 in your own persons that a uniform experience need not con- 
 tinue uniform which will be a great point gained for some 
 people these tremors of mine are likely to become more 
 and more painful. But I call to m jpd the comforting words 
 of an inspired though uncanonical writer, who admonishes 
 us in the Apocrypha that fear is a bad counsellor. Let me 
 then cast him out, and let me trustfully assume that you 
 will one and all postpone that balmy sleep, of which dinner, 
 might under the circumstances be regarded as the indis- 
 
SCIENTIFIC USE OF THE IMAGINATION. 139 
 
 soluble antecedent, and that you will manfully and woman- 
 fully prolong your investigations of the ether and its waves 
 into regions which have been hitherto crossed by the 
 pioneers of science alone. 
 
 Not only are the waves of ether reflected by clouds, 
 by solids, and by liquids, but when they pass from light air 
 to dense, or from dense air to light, a portion of the wave- 
 motion is always reflected. Now our atmosphere changes 
 continually in density from top to bottom. It will help 
 our conceptions if we regard it as made up of a series of thin 
 concentric layers, or shells of air, each shell being of the 
 same density throughout, and a small and sudden change 
 of density occurring in passing from shell to shell. Light 
 would be reflected at the limiting surfaces of all these shells, 
 and their action would be practically the same as that of the 
 real atmosphere. And now I would ask your imagination 
 to picture this act of reflection. What must become of the 
 reflected light ? The atmospheric layers turn their convex 
 surfaces toward the sun ; they are so many convex mirrors 
 of feeble power, and you will immediately perceive fhat the 
 light regularly reflected from these surfaces cannot reach 
 the earth at all, but is dispersed in space. 
 
 But though the sun's light is not reflected in this fashion 
 from the aerial layers to the earth, there is indubitable evi- 
 dence to show that the light of our firmament is reflected 
 light. Proofs of the most cogent description could be here 
 adduced ; but we need only consider that we receive light 
 at the same time from all parts of the hemisphere of heav- 
 en. The light of the firmament comes to us across the di- 
 rection of the solar rays, and even against the direction of 
 the solar rays ; and this lateral and opposing rush of wave- 
 motion can only be due to the rebound of the waves from 
 the air itself, or from something suspended in the air. It is 
 also evident that, unlike the action of clouds, the solar light 
 is not reflected by the sky in the proportions which produce 
 
140 FRAGMENTS OF SCIENCE. 
 
 white. The sky is blue, which indicates a deficiency on part 
 of the larger waves. In accounting for the color of the sky, 
 the first question suggested by the analogy would undoubt- 
 edly be, Is not the air blue ? The blueness of the air has 
 in fact been given as a solution of the blueness of the sky. 
 But reason, basing itself on observation, asks in reply, How, 
 if the air be blue, can the light of sunrise and sunset, which 
 travels through vast distances of air, be yellow, orange, or 
 even red ? The passage of white solar light through a blue 
 medium could by no possibility redden the light. The hy- 
 pothesis of a blue air is therefore untenable. In fact, the 
 agent, whatever it is, which sends us the light of the sky, 
 exercises in so doing a dichroitic action. The light reflected 
 is blue, the light transmitted is orange or red. A marked 
 distinction is thus exhibited between the matter of the sky 
 and that of an ordinary cloud, which exercises no such di- 
 chroitic action. 
 
 By the force of imagination and reason combined we 
 may penetrate this mystery also. The cloud takes no note 
 of size on the part of the waves of ether, but reflects them 
 ah 1 alike. It exercises no selective action. Now, the cause 
 of this may be that the cloud-particles are so large in com- 
 parison with the size of the waves of ether as to reflect them 
 all indifferently. A broad cliff reflects an Atlantic roller as 
 easily as a ripple produced by a sea-bird's wing ; and in the 
 presence of large reflecting surfaces, the existing differences 
 of magnitude among the waves of ether may disappear. 
 But supposing the reflecting particles, instead of being very 
 large, to be very small, in comparison with the size of the 
 waves. In this case, instead of the whole wave being 
 fronted and in great part thrown back, a small portion only 
 is shivered off. The great mass of the wave passes over 
 such a particle without reflection. Scatter, then, a handful 
 of such minute foreign particles in our atmosphere, and set 
 imagination to watch their action upon the solar waves. 
 
SCIENTIFIC USE OF THE IMAGINATION. 141 
 
 Waves of all sizes impinge upon the particles, and you see 
 at every collision a portion of the impinging wave struck 
 off. All the waves of the spectrum, from the extreme red 
 to the extreme violet, are thus acted upon. But in what 
 proportions will the waves be scattered ? A clear picture 
 will enable us to anticipate the experimental answer. Re- 
 membering that the red waves are to the blue much in the 
 relation of billows to ripples, let us consider whether those 
 extremely small particles are competent to scatter all the 
 waves in the same proportion. If they be not and a little 
 reflection will make it clear to you that they are not the 
 production of color must be an incident of the scattering. 
 Largeness is a thing of relation ; and the smaller the wave, 
 the greater is the relative size of any particle on which the 
 wave impinges, and the greater also the ratio of the scat- 
 tered portion to the total wave. A pebble placed in the 
 way of the ring-ripples produced by our heavy rain-drops 
 on a tranquil pond will throw back a large fraction of the 
 ripple incident upon it, while the fractional part of a larger 
 wave thrown back by the same pebble might be infinitesi- 
 mal. - Now we have already made it clear to our minds 
 that to preserve the solar light white, its constituent pro- 
 portions must not be altered ; but in the act of division 
 performed by these very small particles we see that the 
 proportions are altered ; an undue fraction of the smaller 
 waves is scattered by the particles, and, as a consequence, 
 in the scattered light, blue will be the predominant color. 
 The other colors of the spectrum must, to some extent, be 
 associated with the blue. They are not absent but deficient. 
 We ought, in fact, to have them all, but in diminishing pro- 
 portions, from the violet to the red. 
 
 We have here presented a case to the imagination, and, 
 assuming the undulatory theory to be a reality, we have, I 
 think, fairly reasoned our way to the conclusion that, were 
 particles, small in comparison to the size of the ether-waves. 
 
142 FRAGMENTS OF SCIENCE. 
 
 sown in our atmosphere, the light scattered by those parti- 
 cles would be exactly such as we observe in our azure skies. 
 When this light is analyzed, all the colors of the spectrum 
 are found ; but they are found in the proportions indicated 
 by our conclusion. 
 
 Let us now turn our attention to the light which passes 
 unscattered among the particles. How must it be finally 
 affected ? By its successive collisions with the particles, 
 the white light is more and more robbed of its shorter 
 waves ; it therefore loses more and more of its due propor- 
 tion of blue. The result may be anticipated. The trans- 
 mitted light, where short distances are involved, will appear 
 yellowish. But as the sun sinks toward the horizon the 
 atmospheric distances increase, and consequently the num- 
 ber of the scattering particles. They abstract in succession 
 the violet, the indigo, the blue, and even disturb the pro- 
 portions of green. The transmitted light under such cir- 
 cumstances must pass from yellow through orange to red. 
 This also is exactly what we find in Nature. Thus, while 
 the reflected light gives us at noon the deep azure of the 
 Alpine skies, the transmitted light gives us at sunset the 
 warm crimson of the Alpine snows. The phenomena cer- 
 tainly occur as if our atmosphere were a medium rendered 
 slightly turbid by the mechanical suspension of exceedingly 
 small foreign particles. 
 
 Here, as before, we encounter our skeptical " as if" It 
 is one of the parasites of science, ever at hand, and ready 
 to plant itself and sprout, if it can, on the weak points of 
 our philosophy. But a strong constitution defies the para- 
 site, and in our case, as we question the phenomena, proba- 
 bility grows like growing health, until in the end the malady 
 of doubt is completely extirpated. The first question that 
 naturally arises is, Can small particles be really proved to 
 act in the manner indicated ? No doubt of it. Each one 
 of you can submit the question to an experimental test. 
 
SCIENTIFIC USE OF THE IMAGINATION. 143 
 
 Water will not dissolve resin, but spirit will; and when 
 spirit which holds resin in solution is dropped into water, 
 the resin immediately separates in solid particles, which 
 render the water milky. The coarseness of this precipitate 
 depends on the quantity of the dissolved resin. You can 
 cause it to separate in thick clots or in exceedingly fine 
 particles. Professor Brlicke has given us the proportions 
 which produce particles particularly suited to our present 
 purpose. One gramme of clean mastic is dissolved in 
 eighty-seven grammes of absolute alcohol, and the trans- 
 parent solution is allowed to drop into a beaker containing 
 clear water kept briskly stirred. An exceedingly fine 
 precipitate is thus formed, which declares its presence by 
 its action upon light. Placing a dark surface behind the 
 beaker, and permitting the light to fall into it from the top 
 or front, the medium is seen to be distinctly blue. It is not 
 perhaps so perfect a blue as I have seen on exceptional 
 days, this year, among the Alps, but it is a very fair sky- 
 blue. A trace of soap in water gives a tint of blue. Lon- 
 /don, and I fear Liverpool milk, makes an approximation to 
 the same color through the operation of the same cause ; 
 and Helmholtz has irreverently disclosed the fact that the 
 deepest blue eye is simply a turbid medium. 
 
 The action of turbid media upon light was illustrated 
 by Goethe, who, though unacquainted with the undula- 
 tory theory, was led by his experiments to regard the 
 firmament as an illuminated turbid medium with the dark- 
 ness of space behind it. He describes glasses showing a 
 bright yellow by transmitted, and a beautiful blue by re- 
 flected light. Professor Stokes, who was probably the first 
 to discern the real nature of the action of small particles 
 on the waves of ether, describes a glass of a similar kind. 1 
 
 1 This glass, by reflected light, had a color " strongly resembling that 
 of a decoction of a horse-chestnut bark." Curiously enough, Goethe 
 refers to this very decoction : " Man nehme "einen Streifcn frischer Rinde 
 
144 FRAGMENTS OF SCIENCE. 
 
 Capital specimens of such glass are to be found at Salviati's 
 in St. James's Street. What artists call "chill" is no 
 doubt an effect of this description. Through the action of 
 minute particles, the browns of a picture often present the 
 appearance of the bloom of a plum. By rubbing the var- 
 nish with a silk handkerchief optical continuity is estab- 
 lished, and the chill disappears. Some years ago I wit- 
 nessed Mr. Hirst experimenting at Zermatt on the turbid 
 water of the Visp, which was charged with the finely-divided 
 matter ground down by the glaciers. When kept still for 
 a day or so, the grosser matter sank, but the finer matter 
 remained suspended, and gave a distinctly blue tinge to 
 the water. The blueness of certain Alpine lakes has been 
 shown to be in part due to this cause. Professor Roscoe has 
 noticed several striking cases of a similar kind. In a very 
 remarkable paper the late Principal Forbes showed that 
 steam issuing from the safety-valve of a locomotive, when 
 favorably observed, exhibits at a certain stage of its con- 
 densation the colors of the sky. It is blue by reflected 
 light, and orange or red by transmitted light. The same 
 effect, as pointed out by Goethe, is to some extent ex- 
 hibited by peat-smoke. More than ten years ago I amused 
 myself at Killarney by observing on a calm day the straight 
 smoke-columns rising from the cabin chimneys. It was 
 easy to project the lower portion of a column against a 
 dark pine, and its upper portion against a bright cloud. 
 The smoke in the former case was blue, being seen mainly 
 by reflected light ; in the latter case it was reddish, being 
 seen mainly by transmitted light. Such smoke was not in 
 exactly the condition to give us the glow of the Alps, 
 but it was a step in this direction. Brticke's fine pre- 
 cipitate above referred to koks yellowish by transmitted 
 
 von der Rosskastanie, man stecke denselben in ein Glas Wasser, und in 
 der kiirzesten Zeit werden wir das vollkommenste Himmelblau entstchcn 
 sehen." Goethe's Werlce, b. xxix., p. 24. 
 
SCIENTIFIC USE OF THE IMAGINATION. 145 
 
 light, but by duly strengthening the precipitate you may 
 render the white light of noon as ruby-colored as the sun 
 when seen through Liverpool smoke, or upon Alpine hori- 
 zons. I do not, however, point to the gross smoke arising 
 from coal as an illustration of the action of small particles, 
 because such smoke soon absorbs and destroys the waves 
 of blue instead of sending them to the eyes of the observer. 
 These multifarious facts, and numberless others which 
 cannot now be referred to, are explained by reference to 
 the single principle, that where the scattering particles are 
 small in comparison to the size of the waves w^e have in 
 the reflected light a greater proportion of the smaller 
 waves, and in the transmitted light a greater proportion 
 of the larger waves, than existed in the original white 
 light. The physiological consequence is that in the one 
 light blue is predominant, and in the other light orange 
 or red. And now let us push our inquiries forward. 
 Our best microscopes can readily reveal objects not more 
 than -^-g-o-oth of an inch in diameter. This is less than 
 the length of a wave of red light. Indeed, a first-rate 
 microscope would enable us to discern objects not exceed- 
 ing in diameter the length of the smallest waves of the 
 visible spectrum. By the microscope, therefore, we can 
 submit our particles to an experimental test. If they are 
 as large as the light-waves they will infallibly be seen : 
 and if they are not seen it is because they are smaller. I 
 placed in the hands of our President a bottle containing 
 Briicke's particles in greater number and coarseness than 
 those examined by Brticke himself. The liquid was a 
 milky blue, and Mr. Huxley applied to it his highest 
 microscopic power. He satisfied me at the time that had 
 particles of even I0o 1 o0o th of an inch in diameter existed 
 in the liquid they could riot have escaped detection. But 
 no particles were seen. Under the microscope the turbid 
 liquid was not to be distinguished from distilled water. 
 7 
 
146 FRAGMENTS OF SCIENCE. 
 
 Briicke, I may say, also found the particles to be of ultra- 
 microscopic magnitude. 
 
 But we have it in our power to imitate far more closely 
 than we have hitherto done the natural conditions of this 
 problem. "We can generate in air, as many of you know, 
 artificial skies, and prove their perfect identity with the 
 natural one, as regards the exhibition of a number of 
 wholly unexpected phenomena. By a continuous process 
 of growth, moreover, we are able to connect sky-matter, 
 if I may use the term, with molecular matter on the one 
 side, and with molar matter, or matter in sensible masses, 
 on the other. In illustration of this, I will take an ex- 
 periment described by M. Morren of Marseilles at the last 
 meeting of the British Association. Sulphur and oxygen 
 combine , to form sulphurous acid gas. It is this choking 
 gas that is smelt when a sulphur-match is burnt in air. 
 Two atoms of oxygen and one of sulphur constitute the 
 molecule of sulphurous acid. Now it has been recently 
 shown in a great number of instances that waves of ether 
 issuing from a strong source, such as the sun or the 
 electric light, are competent to shake asunder the atoms 
 of gaseous molecules. A chemist would call this " decom- 
 position" by light; but it behooves us, who are examin- 
 ing the power and function of the imagination, to keep 
 constantly before us the physical images which underlie 
 our terms. Therefore, I say, sharply and definitely, that 
 the components of the molecules of sulphurous acid are 
 shaken asunder by the ether-waves. Enclosing the sub- 
 stance in a suitable vessel, placing it in a dark room, and 
 sending through it a powerful beam of light, we at first 
 see nothing : the vessel containing the gas is as empty 
 as a vacuum. Soon, however, along the track of the 
 beam a beautiful sky-blue color is observed, which is 
 due to the liberated particles of sulphur. For a time 
 the blue grows more intense ; it then becomes whitish ; 
 
SCIENTIFIC USE OF THE IMAGINATION. 147 
 
 and from a whitish blue it passes to a more or less perfect 
 white. If the action be continued long enough, we end 
 by filling the tube with a dense cloud of sulphur-particles, 
 which by the application of proper means may be rendered 
 visible. 
 
 Here, then, our ether-waves untie the bond of chemical 
 affinity, and liberate a body sulphur which at ordinary 
 temperatures is a solid, and which therefore soon becomes 
 an object of the senses. We have first of all the free 
 atoms of sulphur, which are both invisible and incompetent 
 to stir the retina sensibly with scattered light. But these 
 atoms gradually coalesce and form particles, which grow 
 larger by continual accretion, until after a minute or two 
 they appear as sky-matter. In this condition they are in- 
 visible themselves, but competent to send an amount of 
 wave-motion to the retina sufficient to produce the fir- 
 mamental blue. The particles continue, or may be caused 
 to continue, in this condition for a considerable time, 
 during which no microscope can cope with them. But 
 they continually grow larger, and pass by insensible grada- 
 tions into the state of cloud, when they can no longer elude 
 the armed eye. Thus without solution of continuity we 
 start with matter in the molecule, and end with matter in 
 the mass, sky-matter being the middle term of the series of 
 transformations. 
 
 Instead of sulphurous acid, we might choose from a 
 dozen other substances, and produce the same effect with 
 any of them. In the case of some probably in the case 
 of all it is possible to preserve matter in the skyey con- 
 dition for fifteen or twenty minutes under the continual 
 operation of the light. During these fifteen or twenty 
 minutes the particles are constantly growing larger, with- 
 out ever exceeding the size requisite to the production of 
 the celestial blue. Now, when two vessels are placed be- 
 fore you, each containing sky-matter, it is possible to state 
 
148 FRAGMENTS OF SCIENCE. 
 
 with great distinctness which vessel contains the largest 
 particles. The retina is very sensitive to differences of 
 light, when, as here, the eye is in comparative darkness, 
 and when the quantities of wave-motion thrown against 
 the retina are small. The larger particles declare them- 
 selves by the greater whiteness of their scattered light. 
 Call now to mind the observation, or effort at observation, 
 made by our President, when he failed to distinguish the 
 particles of mastic in Brticke's medium, and when you have 
 done so follow me. I permitted a beam of light to act 
 upon a certain vapor. In two minutes the azure appeared, 
 but at the end of fifteen minutes it had not ceased to be 
 azure. After fifteen minutes, for example, its color, and 
 some other phenomena, pronounced it to be a blue of dis- 
 tinctly smaller particles than those sought for in vain by 
 Mr. Huxley. These particles, as already stated, must have 
 been less than 1 ^ O th of an inch in diameter. And now 
 I want you to submit to your imagination the following 
 question: Here are particles which have been growing 
 continually for fifteen minutes, and at the end of that time 
 are demonstrably smaller than those which defied the mi- 
 croscope of Mr. Huxley : what must have been the size of 
 these particles at the beginning of their growth ? What 
 notion can you form of the magnitude of such particles ? 
 The distances of stellar space give us simply a bewildering 
 sense of vastness without leaving any distinct impression 
 on the mind, and the magnitudes with which we have here 
 to do bewilder us equally in the opposite direction. We 
 are dealing with infinitesimals compared with which the 
 test objects of the microscope are literally immense. 
 
 From their perviousness to stellar light and other con- 
 siderations, Sir John Herschel drew some startling conclu- 
 sions regarding the density and weight of comets. You 
 know that these extraordinary and mysterious bodies some- 
 times throw out tails 100,000,000 of miles in length, and 
 
SCIENTIFIC USE OF THE IMAGINATION. 149 
 
 50,000 miles in diameter. The diameter of our earth is 
 8,000 miles. Both it and the sky, and a good portion of 
 space beyond the sky, would certainly be included in a 
 sphere 10,000 miles across. Let us fill a hollow sphere of 
 this diameter with cometary matter, and make it our unit 
 of measure. To produce a comet's tail of the size just men- 
 tioned, about 300,000 such measures would have to be 
 emptied into space. Now, suppose the whole of this stuff 
 to be swept together and suitably compressed, what do you 
 suppose its volume would be ? Sir John Herschel would 
 probably tell you that the whole mass might be carted 
 away at a single effort by one of your dray-horses. In fact, 
 I do not know that he would require more than a small 
 fraction of a horse-power to remove the cometary dust. 
 After this you will hardly regard as monstrous a notion I 
 have sometimes entertained concerning the quantity of 
 matter in our sky. Suppose a shell to surround the earth 
 at a height above the surface which would place it beyond 
 the grosser matter that hangs in the lower regions of the 
 air say at the height of the Matterhorn or Mont Blanc. 
 Outside this shell we have the deep-blue firmament. Let 
 the atmospheric space beyond the shell be swept clean, and 
 let the sky-matter be properly gathered up. What is its 
 probable amount ? I have sometimes thought that a lady's 
 portmanteau would contain it all. I have thought that 
 even a gentleman's portmanteau possibly his snuff-box 
 might take it in. And whether the actual sky be capable 
 of this amount of condensation or not, I entertain no doubt 
 that a sky quite as vast as ours, and as good in appearance, 
 could be formed from a quantity of matter which might be 
 held in the hollow of the hand. 
 
 Small in mass, the vastness in point of number of the 
 particles of our sky may be inferred from the continuity of 
 its light. It is not in broken patches, nor at scattered points 
 that the heavenly azure is revealed. To the observer on 
 
150 FRAGMENTS OF SCIENCE. 
 
 the summit of Mont Blanc the blue is as uniform and co- 
 herent as if it formed the surface of the most close-grained 
 solid. A marble dome would not exhibit a stricter con- 
 tinuity. And Mr. Glaisher will inform you that if our hy- 
 pothetical shell were lifted to twice the height of Mont 
 Blanc above the earth's surface, we should still have the 
 azure overhead. Everywhere through the atmosphere those 
 sky-particles are strewn. They fill the Alpine valleys, 
 spreading like a delicate gauze in front of the slopes of 
 pine. They sometimes so swathe the peaks with light as 
 to abolish their definition. This year I have seen the 
 Weisshorn thus dissolved in opalescent air. By proper 
 instruments the glare thrown from the sky-particles against 
 the retina may be quenched, and then the mountain which 
 it obliterated starts into sudden definition. Its extinction 
 in front of a dark mountain resembles exactly the with- 
 drawal of a veil. It is the light then taking possession of 
 the eye, and not the particles acting as opaque bodies, that 
 interferes with the definition. By day this light quenches 
 the stars ; even by moonlight it is able to exclude from 
 vision all stars between the fifth and the eleventh magni- 
 .,- tude. It may be likened to a noise, and the stellar radiance 
 to a whisper drowned by the noise. 
 
 What is tl^ nature of the particles which shed this 
 light ? The celebrated De la Rive ascribes the haze of the 
 Alps in fine weather to floating organic germs. Now, the 
 possible existence of germs in such profusion has been held 
 up as an absurdity. It has been affirmed that they would 
 darken the air, and on the assumed impossibility of their 
 existence in the requisite numbers, without invasion of the 
 solar light, a powerful argument has been based by be- 
 lievers in spontaneous generation. Similar arguments 
 have been used by the opponents of the germ theory of 
 epidemic disease, who have triumphantly challeged an ap- 
 peal to the microscope and the chemist's balance to decide 
 
SCIENTIFIC USE OF THE IMAGINATION. 151 
 
 the question. Such arguments are absolutely valueless. 
 Without committing myself in the least to De la Rive's 
 notion, without offering any objection here to the doctrine 
 of spontaneous generation, without expressing any adhe- 
 rence to the germ theory of disease, I would simply draw 
 attention to the fact that in the atmosphere we have parti- 
 cles which defy both the microscope and the balance, which 
 do not darken the air, and which exist, nevertheless, in 
 multitudes sufficient to reduce to insignificance the Israel- 
 itish hyperbole regarding the sands upon the sea-shore. 
 
 The varying judgments of men on these and other ques- 
 tions may perhaps be, to some extent, accounted for by 
 that doctrine of relativity which plays so important a part 
 in philosophy. This doctrine affirms that the impressions 
 made upon us by any circumstance, or combination of cir- 
 cumstances, depend upon our previous state. Two travellers 
 upon the same peak, the one having ascended to it from 
 the plain, the other having descended to it from a higher 
 elevation, will be differently affected by the scene around 
 them. To the one Nature is expanding, to the other it is 
 contracting, and feelings are sure to differ which have two 
 such different antecedent states. In our scientific judg- 
 ments the law of relativity may also play an important part. 
 To two men, one educated in the school of the senses, who 
 has mainly occupied himself with observation, and the other 
 educated in the school of imagination as well, and exercised 
 in the conceptions of atoms and molecules, to which we 
 have so frequently referred, a bit of matter, say -^^th of 
 of an inch in diameter, will present itself differently. The 
 one descends to it from his molar heights, the other climbs 
 to it from his molecular low-lands. To the one it appears 
 small, to the other large. So also as regards the apprecia- 
 tion of the most minute forms of life revealed by the micro- 
 scope. To one of these men they naturally appear conter- 
 minous with the ultimate particles of matter, and he readily 
 
152 FRAGMENTS OF SCIENCE. 
 
 figures the molecules from which they directly spring ; with 
 him there is but a step from the atom to the organism. 
 The other discerns numberless organic gradations between 
 both. Compared with his atoms, the smallest vibrios and 
 bacteria of the microscopic fielcl are as behemoth and levia- 
 than. The law of relativity may to some extent explain 
 the different attitudes of these two men with regard to the 
 question of spontaneous generation. An amount of evi- 
 dence which satisfies the one entirely fails to satisfy the 
 other ; and while to the one the last bold defence and start- 
 ling expansion of the doctrine will appear perfectly conclu- 
 sive, to the other it will present itself as imposing a profit- 
 less labor of demolition on subsequent investigators. 1 
 
 I trust, Mr. President, that you whom untoward circum- 
 stances have made a biologist, but who still keep alive your 
 sympathy with that class of inquiries which Nature intend- 
 ed you to pursue and adorn will excuse me to your breth- 
 ren if I say that some of them seem to form an inadequate 
 estimate of the distance which separates the microscopic 
 from the molecular limit, and that, as a consequence, they 
 sometimes employ a phraseology which is calculated to mis- 
 lead. When, for example, the contents of a cell are de- 
 scribed as perfectly homogeneous, as absolutely structure- 
 less, because the microscope fails to distinguish any struct- 
 ure, then I think the microscope begins to play a mischiev^ 
 ous part. A little consideration will make it plain to all 
 of you that the microscope can have no voice in the real 
 question of germ-structure. Distilled water is more per- 
 fectly homogeneous than the contents of any possible or- 
 ganic germ. What causes the liquid to cease contracting 
 at 39 Fahrenheit, and to expand until it freezes ? It is a 
 structural process of which the microscope can take no 
 note, nor is it likely to do so by any conceivable extension 
 
 1 A resolute scrutiny of the experiments, recently executed with 
 reference to this question, is sure to yield i instructive results. 
 
SCIENTIFIC USE OF THE IMAGINATION. 153 
 
 of its powers. Place this distilled water in the field of an 
 electro-magnet, and bring a microscope to bear upon it. 
 Will any change be observed when the magnet is excited ? 
 Absolutely none ; and still profound and complex changes 
 have occurred. First of all, the particles of water are ren- 
 dered diamagneticallj polar ; and secondly, in virtue of the 
 structure impressed upon it by the magnetic strain of its 
 molecules, the liquid twists a ray of light in a fashion per- 
 fectly determinate both as to quantity and direction. It 
 would be immensely interesting to both you and me if one 
 whom I hoped to see here present, 1 who has brought his 
 brilliant imagination to bear upon this subject, could make 
 us see as he sees the entangled molecular processes involved 
 in the rotation of the plane of polarization by magnetic 
 force. While dealing with this question, he lived in a world 
 of matter and of motion, to which the microscope has no 
 passport, and in which it can offer no aid. The cases in 
 which similar conditions hold are simply numberless. Have 
 the diamond, the amethyst, and the countless other crystals 
 formed in the laboratories of Nature and of man no struct- 
 ure ? Assuredly they have ; but what can the microscope 
 make of it ? Nothing. It cannot be too distinctly borne 
 in mind that between the microscope limit and the true 
 molecular limit there is room for infinite permutations and 
 combinations. It is in this region that the poles of the 
 atoms are arranged, that tendency is given to their powers, 
 so that when these poles and powers have free action and 
 proper stimulus in a suitable environment, they determine 
 first the germ, and afterward the complete organism. This 
 first marshalling of the atoms on which all subsequent ac- 
 tion depends baffles a keener power than that of the micro- 
 scope. Through pure excess of complexity, and long be- 
 fore observation can have any voice in the matter, the most 
 highly-trained intellect, the most refined and disciplined 
 
 1 Sir William Thomson 
 
154 FKAGMENTS OF SCIENCE. 
 
 imagination, retires in bewilderment from the contempla- 
 tion of the problem. "We are struck dumb by an astonish- 
 ment which no microscope can relieve, doubting not only 
 the power of our instrument, but even whether we ourselves 
 possess the intellectual elements which will ever enable us 
 to grapple with the ultimate structural energies of Nature. 
 But the speculative faculty, of which imagination forms 
 so large a part, will nevertheless wander into regions where 
 the hope of certainty would seem to be entirely shut out. 
 We think that though the detailed analysis may be, and 
 may forever remain, beyond us, general notions may be at- 
 tainable. At all events, it is plain that beyond the present 
 outposts of microscopic inquiry lies an immense field for 
 the exercise of the speculative power. It is only, however, 
 the privileged spirits who know how to use their liberty 
 without abusing it, who are able to surround imagination 
 by the firm frontiers of reason, that are likely to work with 
 any profit here. But freedom to them is of such paramount 
 importance that, for the sake of securing it, a good deal of 
 wildness on the part of weaker brethren may be overlooked. 
 In more senses than one Mr. Darwin has drawn heavily 
 upon the scientific tolerance of his age. He has drawn 
 heavily upon time in his development of species, and he has 
 drawn adventurously upon matter in his theory of pangen- 
 esis. According to this theory, a germ already microscopic 
 is a world of minor germs. Not only is the organism as a 
 whole wrapped up in the germ, but every organ of the or- 
 ganism has there its special seed. This, I say, is an adven- 
 turous draft on the power of matter to divide itself and 
 distribute its forces. But, unless we are perfectly sure that 
 he is overstepping the bounds of reason, that he is unwit- 
 tingly sinning against observed fact or demonstrated law 
 for a mind like that of Darwin can never sin wittingly 
 against either fact or law we ought, I think, to be cautious 
 in limiting his intellectual horizon. If there be the least 
 
SCIENTIFIC USE OF THE IMAGINATION. 155 
 
 doubt in the matter, it ought to be given in favor of the 
 freedom of such a mind. To it a vast possibility is in it- 
 self a dynamic power, though the possibility may never 
 be drawn upon. It gives me pleasure to think that the 
 facts and reasonings of this discourse tend rather toward 
 the justification of Mr. Darwin than toward his condemna- 
 tion, that they tend rather to augment than to diminish the 
 cubic space demanded by this soaring speculator ; for they 
 seem to show the perfect competence of matter and force, 
 as regards divisibility and distribution, to bear the heaviest 
 strain that he has hitherto imposed upon them. 
 
 In the case of Mr. Darwin, observation, imagination, and 
 reason combined, have run back W 7 ith wonderful sagacity 
 and success over a certain length of the line of biological 
 succession. Guided by analogy, in his " Origin of Species," 
 he placed at the root of life a primordial germ, from which 
 he conceived the amazing richness and variety of the life 
 that now is upon the earth's surface might be deduced. If 
 this hypothesis were true, it would not be final. The hu- 
 man imagination would infallibly look behind the germ, 
 and, however hopeless the attempt, would inquire into the 
 history of its genesis. In this dim twilight of conjecture 
 the searcher welcomes every gleam, and seeks to augment 
 his light by indirect incidences. He studies the methods 
 of Nature in the ages and the worlds within his reach, in 
 order to shape the course of speculation in the antecedent 
 ages and worlds. And though the certainty possessed by 
 experimental inquiry is here shut out, the imagination is 
 not left entirely without guidance. From the examination 
 of the solar system, Kant and Laplace came to the conclu- 
 sion that its various bodies once formed parts of the same 
 undislocated mass ; that matter in a nebulous form preceded 
 matter in a dense form ; that as the ages rolled away, heat 
 was wasted, condensation followed, planets were detached, 
 and that finally the chief portion of the fiery cloud reached, 
 
156 FRAGMENTS OF SCIENCE. 
 
 by self-compression, the magnitude and density of our sun. 
 Tlie earth itself offers evidence of a fiery origin ; and in our 
 day the hypothesis of Kant and Laplace receives the inde- 
 pendent countenance of spectrum analysis, which proves 
 the same substances to be common to the earth and sun. 
 
 Accepting some such view of the construction of our 
 system as probable, a desire immediately arises to connect 
 the present life of our planet with the past. "We wish to 
 know something of our remotest ancestry. On its first de- 
 tachment from the central mass, life, as we understand it, 
 could hardly have been present on the earth. How, then, 
 did it come there ? The thing to be encouraged here is 
 a reverent freedom a freedom preceded by the hard disci- 
 pline which checks licentiousness in speculation while the 
 thing to be repressed, both in science and out of it, is dog- 
 matism. And here I am in the hands of the meeting 
 willing to end, but ready to go on. I have no right to in- 
 trude upon you, unasked, the unformed notions which are 
 floating like clouds, or gathering to more solid consistency 
 in the modern speculative scientific mind. But if you wish 
 me to speak plainly, honestly, and undisputatiously, I am 
 willing to do so. On the present occasion 
 
 " You are ordained to call, and I to come." 
 
 Two views, then, offer themselves to us. Life was pres- 
 ent potentially in matter when in the nebulous form, and 
 was unfolded from it by the way of natural development, 
 or it is a principle inserted into matter at a later date. With 
 regard to the question of time, the views of men have 
 changed remarkably in our day and generation ; and I 
 must say as regards courage also, and a manful willingness 
 to engage in open contest, with fair weapons, a great 
 
 change has also occurred. The clergy of England at all 
 events the clergy of London have nerve enough to listen 
 to the strongest views which any one among us would care 
 
SCIENTIFIC USE OF THE IMAGINATION. 157 
 
 to utter ; and they invite, if they do not challenge, men of 
 the most decided opinions to state and stand by those opin- 
 ions in open court. Let the hardiest theory be stated only 
 in the language current among gentlemen, and they look it 
 in the face ; smiting the theory, if they do not like it, not 
 with theologicfulrnination, but with honest secular strength. 
 With the country clergy I am told the case is different. It 
 is right that I should say this, because the clergy of Lon- 
 don have more than once offered me the chance of meeting 
 them in open, honorable discussion. 
 
 Two or three years ago, in an ancient London College, 
 I listened to such a discussion at the end of a remarkable 
 lecture by a very remarkable man. Three or four hundred 
 clergymen were present at the lecture. The orator began 
 with the civilization of Egypt in the time of Joseph ; 
 pointing out that the very perfect organization of the 
 kingdom, and the possession of chariots, in one of which 
 Joseph rode, indicated a long antecedent period of civili- 
 zation. He then passed on to the mud of the Nile, its 
 rate of augmentation, its present thickness, and the re- 
 mains of human handiwork found therein ; thence to the 
 rocks which bound the Nile valley, and which teem with 
 organic remains. Thus in his own clear and admirable 
 way he caused the idea of the world's age to expand itself 
 indefinitely before the mind of his audience, and he con- 
 trasted this with the age usually assigned to the world. 
 During his discourse he seemed to be swimming against 
 the stream ; he manifestly thought that he was opposing 
 a general conviction. He expected resistance ; so did I. 
 But it was all a mistake : there was no adverse current, 
 o opposing conviction, no resistance, merely here and 
 there a half-humorous, but unsuccessful attempt to entan- 
 gle him in his talk. The meeting agreed with all that 
 had been said regarding the antiquity of the earth and of 
 its life. They had, indeed, known it all long ago, and 
 
158 FRAGMENTS OF SCIENCE. 
 
 they good-humoredly rallied the lecturer for coming among 
 them with so stale a story. It was quite plain that this 
 large body of clergymen, who were, I should say, the finest 
 samples of their class, had entirely given up the ancient 
 landmarks, and transported the conception of life's origin 
 to an indefinitely distant past. 
 
 ^- This leads us to the gist of our present inquiry, which 
 is this : Does life belong to what we call matter, or is it an 
 independent principle inserted into matter at some suitable 
 epoch say when the physical conditions became such as to 
 permit of the development of life ? Let us put the ques- 
 tion with all the reverence due to a faith and culture in 
 which we all were cradled a faith and culture, moreover, 
 which are the undeniable historic antecedents of our pres- 
 ent enlightenment. I say, let us put the question rever- 
 ently, but let us also put it clearly and definitely. There 
 are the strongest grounds for believing that during a cer- 
 tain period of its history the earth was not, nor was it fit 
 to be, the theatre of life. Whether this was ever a nebu- 
 lous period, or merely a molten period, does not much 
 matter ; and if we revert to the nebulous condition, it is 
 because the probabilities are really on its side. Our ques- 
 tion is this : Did creative energy pause until the nebulous 
 matter had condensed, until the earth had been detached, 
 until the solar fire had so far withdrawn from the earth's 
 / vicinity as to permit a crust to gather round the planet ? 
 
 . Did it wait until the air was isolated, until the seas were 
 formed, until evaporation, condensation, and the descent of 
 rain had begun, until the eroding forces of the atmosphere 
 had weathered and decomposed the molten rocks so as to 
 form soils, until the sun's rays had become so tempered by 
 distance and by waste as to be chemically fit for the de- 
 compositions necessary to vegetable life ? Having waited 
 through those MOILS until the proper conditions had set in, 
 did it send the fiat forth, " Let Life be ! " ? These ques- 
 
SCIENTIFIC USE OF THE IMAGINATION. 159 
 
 tions define a hypothesis not without its difficulties, but 
 the dignity of which was demonstrated by the nobleness 
 of the men whom it sustained. 
 
 Modern scientific thought is called upon to decide be- 
 tween this hypothesis and another: and public thought 
 generally will afterward be called upon to do the same. 
 You may, however, rest secure in the belief that the hy- 
 pothesis just sketched can never be stormed, and that it is 
 sure, if it yield at all, to yield to a prolonged siege. To 
 gain new territory modern argument requires more time 
 than modern arms, though both of them move with greater 
 rapidity than of yore. But however the convictions of indi- 
 viduals here and there may be influenced, the process must 
 be slow and secular which commends the rival hypothesis 
 of Natural Evolution to the public mind. For what are the 
 core and essence of this hypothesis ? Strip it naked and 
 you stand face to face with the notion that not alone the 
 more ignoble forms of animalcular or animal life, not alone 
 the nobler forms of the horse and lion, not alone the exqui- 
 site and wonderful mechanism of the human body, but that 
 the human mind itself emotion, intellect, will, and all their 
 phenomena were once latent in a fiery cloud. Surely 
 the mere statement of such a notion is more than a refu- 
 tation. But the hypothesis would probably go even further 
 than this. Many who hold it would probably assent to the 
 position that at the present moment all our philosophy, all 
 our poetry, all our science, and all our art Plato, Shake- 
 speare, Newton, and Raphael are potential in the fires of 
 the sun. We long to learn something of our origin. If 
 the Evolution hypothesis be correct, even this unsatisfied 
 yearning must have come to us across the ages which sepa- 
 rate the unconscious primeval mist from the consciousness 
 of to-day. I do not think that any holder of the Evolution 
 hypothesis would say that I overstate it or overstrain it in 
 any way. I merely strip it of all vagueness, and bring 
 
160 FRAGMENTS OF SCIENCE. 
 
 before you unclothed and unvarnished the notions by which 
 it must stand or fall. 
 
 Surely these notions represent an absurdity too mon- 
 strous to be entertained by any sane mind. Let us, how- 
 ever, give them fair play. Let us steady ourselves in front 
 of the hypothesis, and, dismissing all terror and excitement 
 from our minds, let us look firmly into it with the hard sharp 
 eye of intellect alone. Why are these notions absurd, and 
 why should sanity reject them ? The law of Relativity, of 
 which we have previously spoken, may find its application 
 here. These Evolution notions are absurd, monstrous, and 
 fit only for the intellectual gibbet, in relation to the ideas 
 concerning matter which were drilled into us when young. 
 Spirit and matter have ever been presented to us in the 
 rudest contrast, the one as all-noble, the other as all-vile. 
 But is this correct ? Does it represent what our mightiest 
 spiritual teacher would call the Eternal Fact of the Uni- 
 verse ? Upon the answer to this question all depends. 
 Supposing, instead of having the foregoing antithesis of 
 spirit and matter presented to our youthful minds, we had 
 been taught to regard them as equally worthy and equally 
 wonderful ; to consider them in fact as two opposite faces 
 of the self-same mystery. Supposing that in youth we had 
 been impregnated with the notion of the poet Goethe, in- 
 stead of the notion of the poet Young, looking at matter, 
 not as brute matter, but as " the living garment of God ; " 
 do you not think that under these altered circumstances 
 the law of Relativity might have had an outcome different 
 from its present one ? Is it not probable that our repug- 
 nance to the idea of primeval union between spirit and 
 matter might be considerably abated ? Without this total 
 revolution of the notions now prevalent, the Evolution hy- 
 pothesis must stand condemned ; but in many profoundly 
 thoughtful minds such a revolution has already taken place. 
 They degrade neither member of the mysterious duality 
 
SCIENTIFIC USE OF THE IMAGINATION. 161 
 
 referred to ; but they exalt one of them from its abasement, 
 and repeal the divorce hitherto existing between both. In 
 substance, if not in words, their position as regards the 
 relation of spirit and matter is : " What God hath joined 
 together let not man put asunder." And with regard to 
 the ages of forge tfulness which lie between the unconscious 
 life of the nebula and the conscious life of the earth, it is, 
 they would urge, but an extension of that forgetfulness 
 which preceded the birth of us all. 
 
 I have thus led you to the outer rim of speculative 
 science, for beyond the nebulas scientific thought has never 
 ventured hitherto, and have tried to state that which I con- 
 sidered ought, in fairness, to be outspoken. I do not think 
 this Evolution hypothesis is to be flouted away contempt- 
 "ously ; I do not think it is to be denounced as wicked. It 
 is to be brought before the bar of disciplined reason, and 
 there justified or condemned. Let us hearken to those who 
 wisely support it, and to those who wisely oppose it ; and 
 let us tolerate those, and they are many, who foolishly try 
 to do either of these things. The only thing out of place 
 in the discussion is dogmatism on either side. Fear not 
 the Evolution hypothesis. Steady yourselves in its presence 
 upon that faith in the ultimate triumph of truth which was 
 expressed by old Gamaliel when he said : " If it be of God, 
 ye cannot overthrow it ; if it be of man, it will come to 
 naught." Under the fierce light of scientific inquiry, this 
 hypothesis is sure to be dissipated if it possess not a core 
 of truth. Trust me, its existence as a hypothesis in the 
 mind is quite compatible with the simultaneous existence 
 of all those virtues to which the term Christian has been 
 applied. It does not solve it does not profess to solve 
 the ultimate mystery of this universe. It leaves in fact 
 that mystery untouched. For granting the nebula and its 
 potential life, the question, whence came they ? would still 
 remain to baffle and bewilder us. At bottom, the hypothe- 
 
162 FRAGMENTS OF SCIENCE. 
 
 sis does nothing more than " transport the conception of 
 life's origin to an indefinitely distant past." 
 
 Those who hold the doctrine of Evolution are by no 
 means ignorant of the uncertainty of their data, and they 
 yield no more to it than a provisional assent. They regard 
 the nebular hypothesis as probable, and in the utter absence 
 of any evidence to prove the act illegal, they extend the 
 method of Nature from t the present into the past. Here the 
 observed uniformity of Nature is their only guide. Within 
 the long range of physical inquiry, they have never dis- 
 cerned in Nature the insertion of caprice. Throughout 
 this range the laws of physical and intellectual continuity 
 have run side by side. Having thus determined the ele- 
 ments of their curve in a world of observation and experi- 
 ment, they prolong that curve into an antecedent world, 
 and accept as probable the unbroken sequence of develop- 
 ment from the nebula to the present time. You never hear 
 the really philosophical defenders of the doctrine of Uni- 
 formity speaking of impossibilities in Nature. They never 
 say, what they are constantly charged with saying, that it 
 is impossible for the Builder of the universe to alter His 
 work. Their business is not with the possible, but the 
 actual not with a world which might be, but with a world 
 that is. This they explore with a courage not unmixed 
 with reverence, and according to methods which, like the 
 'quality of a tree, are tested by their fruits. They have but 
 jine desire to know the truth. They have but one fear 
 lo believe a lie. And if they know the strength of science, 
 and rely upon it with unswerving trust, they also know the 
 limits beyond which science ceases to be strong. They best 
 know that questions offer themselves to thought which 
 science, as now prosecuted, has not even the tendency to 
 solve. They keep such questions open, and will not toler- 
 ate any unnecessary limitation of the horizon of their souls. 
 They have as little fellowship with the atheist who says 
 
SCIENTIFIC USE OF THE IMAGINATION. 163 
 
 there is no God, as with the theist who professes to know 
 the mind of God. " Two things," said Immanuel Kant, 
 " fill me with awe : the starry heavens and the sense of 
 moral responsibility in man." And in his hours of health 
 and strength and sanity, when the stroke of action has 
 ceased and the pause of reflection has set in, the scientific 
 investigator finds himself overshadowed by the same awe. 
 Breaking contact with the hampering details of earth, it 
 associates him with a power which gives fulness and tone 
 to his existence, but which he can neither analyze nor com- 
 prehend. 
 
A TRANSLATION 
 
 OF 
 
 GOETHE'S PROEMIUM TO " GOTT UND WELT.' 
 
 To Him who from eternity, self-stirred, 
 Himself hath made by His creative word ! 
 To Him, Supreme, who causeth faith to be, 
 Trust, hope, love, power, and endless energy ! 
 To Him, who, seek to name Him as we will, 
 UNKNOWN within Himself abideth still ! 
 
 Strain ear and eye, till sight and sense be dim ; 
 
 Thou' It find but faint similitudes of Him : 
 
 Yea, and thy spirit in her flight of flame 
 
 Still strives to gauge the symbol and the name : 
 
 Charmed and compelled thou climb'st from height to height, 
 
 And' round thy path the world shines wondrous bright ; 
 
 Time, space, and size, and distance cease to be, 
 
 And every step is fresh infinity. 
 
 What were the God who sat outside to scan 
 
 The spheres that 'neath His finger circling ran ? 
 
 God dwells within, and moves the world and moulds, 
 
 Himself and Nature in one form enfolds : 
 
 Thus all that lives in Him, and breathes, and is, 
 
 Shall ne'er His puissance, ne'er His spirit miss. 
 
 The soul of man, too, is a universe ; 
 
 Whence follows it that race with race concurs 
 
 In framing all it knows of good and true 
 
 God ? yea, its own God ; and, with homage due, 
 
 Surrenders to His sway both earth and heaven ; 
 
 Fears Him, and loves, where place for love is given. 
 
 J. A. S. 
 Spectator, September 24, 1870. 
 
VIII. 
 ON RADIATION. 
 
 THE "REDE" LECTURE. 
 
 DELIVERED IN THE SENATE-HOUSE BEFORE THE UNIVERSITY OF 
 CAMBRIDGE. 
 
 On Tuesday, May 16, 1865. 
 
" Forsitan et rosea Sol alte lampade lucens, 
 Possideat multum caecis fervoribus ignem 
 Circum se, qui sit fulgore notatus, 
 -^stifer ut tantum radiorum exaugeat ictum." 
 
 Lucretius, v. 610. 
 
 " Perhaps too the sun as he shines aloft with rosy lamp has round 
 about him much fire with heats that are not visible, and thus the fire may 
 be marked by no radiance, so that fraught with heat it increases to such 
 a degree the stroke of the rays." Monro's Translation. 
 
 My attention was drawn to this remarkable passage by the late ex- 
 cellent and accomplished Sir Edmund Head, Bart. 
 

 VIII. 
 
 RADIATION. 
 
 1. Visible and Invisible Radiation. 
 
 BETWEEN the mind of man and the outer world are in- 
 terposed the nerves of the human body, which translate, 
 or enable the mind to translate, the impressions of that 
 world into facts of consciousness and thought. 
 
 Different nerves are suited to the perception of different 
 impressions. We do not see with the ear, nor hear with 
 the eye, nor are we rendered sensible of sound by the 
 nerves of the tongue. Out of the general assemblage of 
 physical actions, each nerve, or group of nerves, selects and 
 responds to those for the perception of which it is specially 
 organized. 
 
 The optic nerve passes from the brain to the back of 
 the eyeball and there spreads out, to form the retina, a web 
 of nerve filaments, on which the images of external objects 
 are projected by the optical portion of the eye. This nerve 
 is limited to the apprehension of the phenomena of radia- 
 tion, and, notwithstanding its marvellous sensibility to 
 certain impressions of this class, it is singularly obtuse to 
 other impressions. 
 
 Nor does the optic nerve embrace the entire range even 
 of radiation. Some rays, when they reach it, are incom- 
 petent to evoke its power, while others never reach it at 
 all, being absorbed by the humors of the eye. To all rays 
 
168 FRAGMENTS OF SCIENCE. 
 
 which, whether they reach the retina or not, fail to excite 
 vision, we give the name of invisible or obscure rays. All 
 non-luminous bodies emit such rays. There is no body in 
 Nature absolutely cold, and every body not absolutely cold 
 emits rays of heat. But to render radiant heat fit to affect 
 the optic nerve a certain temperature is necessary. A cool 
 poker thrust into a fire remains dark for a time, but when 
 its temperature has become equal to that of the surrounding 
 coals it glows like them. In like manner, if a current of 
 electricity of gradually increasing strength be sent through 
 a wire of the refractory metal platinum, the wire first be- 
 comes sensibly warm to the touch ; for a time its heat aug- 
 ments, still, however, remaining obscure ; at length we can 
 no longer touch the metal with impunity ; and at a certain 
 definite temperature it emits a feeble red light. As the 
 current augments in power the light augments in brilliancy, 
 until finally the wire appears of a dazzling white. The 
 lio-ht which it now emits is similar to that of the sun. 
 
 O 
 
 By means of a prism Sir Isaac Newton unravelled the 
 'texture of solar light, and by the same simple instrument 
 we can investigate the luminous changes of our platinum 
 wire. In passing through the prism all its rays (and they 
 are infinite in variety) are bent or refracted from their 
 straight course ; and as different rays are differently re- 
 fracted by the prism, we are by it enabled to separate one 
 class of rays from another. By such prismatic analysis Dr. 
 Draper has shown that, when the platinum wire first begins 
 to glow, the light emitted is a pure red. As the glow 
 augments the red becomes more brilliant, but at the same 
 time orange rays are added to the emission. Augmenting 
 the temperature still further, yellow rays appear beside the 
 orange, after the yellow green rays are emitted, and after 
 the green come, in succession, blue, indigo and violet rays. 
 To display all these colors at the same time the platinum 
 wire must be white-hot : the impression of whiteness being 
 
RADIATION. 169 
 
 in fact produced by the simultaneous action of all these 
 colors on the optic nerve. 
 
 In the experiment just described we began with a plat- 
 inum wire at an ordinary temperature, and gradually 
 raised it to a white heat. At the beginning, and even 
 before the electric current had acted at all upon the wire, 
 it emitted invisible rays. For some time after the action 
 of the current had commenced, and even for a time after 
 the wire had become intolerable to the touch, its radiation 
 was still invisible. The question now arises, what becomes 
 of these invisible rays when the visible ones make their 
 appearance? It will be proved in the sequel that they 
 maintain themselves in the radiation ; that a ray once 
 emitted continues to be emitted when the temperature 
 is increased, and hence the emission from our platinum 
 wire, even when it has attained its maximum brilliancy, 
 consists of a mixture of visible and invisible rays. If, 
 instead of the platinum wire, the earth itself were raised to 
 incandescence, the obscure radiation which it now emits 
 would continue to be emitted. To reach incandescence the 
 planet would have to pass through all the stages of non- 
 luminous radiation, and the final emission would embrace 
 the rays of all these stages. There can hardly be a doubt 
 that from the sun itself, rays proceed similar in kind to 
 those which the dark earth pours nightly into space. In 
 fact, the various kinds of obscure rays emitted by all the 
 planets of our system are included in the present radiation 
 of the sun. 
 
 The great pioneer in this domain of science was Sir 
 William Herschel. Causing a beam of solar light to pass 
 through a prism he resolved it into its colored constituents ; 
 he formed what is technically called the solar spectrum. 
 Exposing thermometers to the successive colors he deter- 
 mined their heating power, and found it to augment from 
 the violet or most refracted end, to the red or least refracted 
 8 
 
170 FRAGMENTS OF SCIENCE. 
 
 end of the spectrum. But he did not stop here. Pushing 
 his thermometers into the dark space beyond the red he 
 found that, though the light had disappeared, the radiant 
 heat falling on the instruments was more intense than that 
 at any visible part of the spectrum. In fact, Sir William 
 Herschel showed, and his results have been verified by vari- 
 ous philosophers since his time, that besides its luminous 
 rays, the sun pours forth a multitude of other rays more 
 powerfully calorific than the luminous ones, but entirely 
 unsuited to the purposes of vision. 
 
 At the less refrangible end of the solar spectrum, then, 
 the range of the sun's radiation is not limited by that of 
 the eye. The same statement applies to the more refran- 
 gible end. Ritter discovered the extension of the spectrum 
 into the invisible region beyond the violet ; and, in recent 
 times, this ultra-violet emission has had peculiar interest 
 conferred upon it by the admirable researches of Professor 
 Stokes. The complete spectrum of the sun consists, there- 
 fore, of three distinct parts : first, of ultra-red rays of high 
 heating power, but unsuited to the purposes of vision ; 
 secondly, of luminous rays which display the succession of 
 colors, red, orange, yellow, green, blue, indigo, violet; 
 thirdly, of ultra-violet rays which, like the ultra-red ones, 
 are incompetent to excite vision, but which, unlike the 
 ultra-red rays, possess a very feeble heating power. In 
 consequence, however, of their chemical energy these ultra- 
 violet rays are of the utmost importance to the organic 
 world. 
 
 2. Origin and Character of Radiation. The Ether. 
 
 "When we see a platinum wire raised gradually to a 
 white heat, and emitting in succession all the colors of the 
 spectrum, we are simply conscious of a series of changes in 
 the condition of our own eyes. We do not see the actions 
 in which these successive colors originate, but the mind 
 
EADIATION. 171 
 
 irresistibly infers that the appearance of the colors corre- 
 sponds to certain contemporaneous changes in the wire. 
 What is the nature of these changes ? In virtue of what 
 condition does the wire radiate at all? We must now 
 look from the wire as a whole to its constituent atoms. 
 Could we see those atoms, even before the electric current 
 has begun to act upon them, we should find them in a 
 state of vibration. In this vibration, indeed, consists such 
 warmth as the wire then possesses. Locke enunciated this 
 idea with great precision, and it seems placed beyond the 
 pale of doubt by the excellent quantitative researches of 
 Mr. Joule. "Heat," says Locke, "is a very brisk agitation 
 of the insensible parts of the object, which produce in us 
 that sensation from which we denominate the object hot : 
 so what in our sensation is heat in the object is nothing 
 but motion" When the electric current, still feeble, begins 
 to pass through the wire, its first act is to intensify the 
 vibrations already existing, by causing the atoms to swing 
 through wider ranges. Technically speaking, the ampli- 
 tudes of the oscillations are increased. The current does 
 this, however, without altering the periods of the old vi- 
 brations, or the times in which they were executed. But 
 besides intensifying the old vibrations the current gener- 
 ates new and more rapid ones, and when a certain definite 
 rapidity has been attained the wire begins to glow. The 
 color first exhibited is red, which corresponds to the lowest 
 rate of vibration of which the eye is able to take cognizance. 
 By augmenting the strength of the electric current more 
 rapid vibrations are introduced, and orange rays appear. 
 A quicker rate of vibration produces yellow, a still quicker 
 green ; and by further augmenting the rapidity, we pass 
 through blue, indigo, and violet, to the extreme ultra-violet 
 rays. 
 
 Such are the changes which science recognizes in the 
 wire itself, as concurrent with the visual changes taking 
 
172 FRAGMENTS OF SCIENCE. 
 
 place in the eye. But what connects the wire with this 
 organ ? By what means does it send such intelligence of 
 its varying condition to the optic nerve ? Heat being, as 
 defined by Locke, " a very brisk agitation of the insensible 
 parts of an object," it is readily conceivable that on touch- 
 ing a heated body the agitation may communicate itself to 
 the adjacent nerves, and announce itself to them as light 
 or heat. But the optic nerve does not touch the hot plati- 
 num, and hence the pertinence of the question, By what 
 agency are the vibrations of the wire transmitted to the 
 eye? 
 
 The answer to this question involves, perhaps, the most 
 important physical conception that the mind of man has 
 yet achieved : the conception of a medium filling space and 
 fitted mechanically for the transmission of the vibrations 
 of light and heat, as air is fitted for the transmission of 
 sound. This medium is called the luminiferous ether. 
 Every vibration of every atom of our platinum wire raises 
 in this ether a wave, which speeds through it at the rate 
 of 186,000 miles a second. The ether suffers no rupture 
 of continuity at the surface of the eye, the inter-molecular 
 spaces of the various humors are filled with it ; hence the 
 waves generated by the glowing platinum can cross these 
 humors and impinge on the optic nerve at the back of the 
 eye. Thus the sensation of light reduces itself to the com- 
 munication of motion. Up to this point we deal with pure 
 mechanics ; but the subsequent translation of the shock of 
 the ethereal waves into consciousness eludes the analysis 
 of science. As an oar dipping into the Cam generates 
 systems of waves, which, speeding from the centre of dis- 
 turbance, finally stir the sedges on the river's bank, so do 
 the vibrating atoms generate in the surrounding ether un- 
 dulations, which finally stir the filaments of the retina. 
 The motion thus imparted is transmitted with measurable 
 and not very great velocity to the brain, where, by a pro- 
 
RADIATION. 173 
 
 cess which science does not even tend to unravel, the tre- 
 mor of the nervous matter is converted into the conscious 
 impression of light. 
 
 Darkness might then be defined as ether at rest ; light 
 as ether in motion. But in reality the ether is never at 
 rest, for in the absence of light-waves we have heat-waves 
 always speeding through it. In the spaces of the universe 
 both classes of undulations incessantly commingle. Here 
 the waves issuing from uncounted centres cross, coincide, 
 oppose, and pass through each other, without confusion or 
 ultimate extinction. The waves from the zenith do not 
 jostle out of existence those from the horizon, and every 
 star is seen across the entanglement of wave-motions pro- 
 duced by all other stars. It is the ceaseless thrill which 
 those distant orbs collectively create in the ether, which 
 constitutes what we call the temperature of space. As the 
 air of a room accommodates itself to the requirements of an 
 orchestra, transmitting each vibration of every pipe and 
 string, so does the inter-stellar ether accommodate itself to 
 the requirements of light and heat. Its waves mingle in 
 space without disorder, each being endowed with an in- 
 dividuality as indestructible as if it alone had disturbed the 
 universal repose. 
 
 All vagueness with regard to the use of the terms radia- 
 tion and absorption will now disappear. Radiation is the 
 communication of vibratory motion to the ether, and when 
 a body is said to be chilled by radiation, as for example the 
 grass of a meadow on a starlight night, the meaning is, that 
 the molecules of the grass have lost a portion of their mo- 
 tion, by imparting it to the medium in which they vibrate. 
 On the other hand, the waves of ether once generated may 
 so strike against the molecules of a body exposed to their 
 action as to yield up their motion to the latter ; and in this 
 transfer of the motion from the ether to the molecules con- 
 sists the absorption of radiant heat. All the phenomena 
 
174 FRAGMENTS OF SCIENCE. 
 
 of heat are in this way reducible to interchanges of motion ; 
 and it is purely as the recipients or the donors of this mo- 
 tion, that we ourselves become conscious of the action of 
 heat and cold. 
 
 3. The Atomic Theory in reference to the Ether. 
 
 The word " atoms " has been more than once employed 
 in this discourse. Chemists have taught us that all matter 
 is reducible to certain elementary forms to which they give 
 this name. These atoms are endowed with powers of 
 mutual attraction, and under suitable circumstances they 
 coalesce to form compounds. Thus oxygen and hydrogen 
 are elements when separate, or merely mixed, but they may 
 be made to combine so as to form molecules, each consisting 
 of two atoms of hydrogen and one of oxygen. In this con- 
 dition they constitute water. So also chlorine and sodium 
 are elements, the former a pungent gas, the latter a soft 
 metal ; and they unite together to form chloride of sodium 
 or common salt. In the same way the element nitrogen 
 combines with hydrogen, in the proportion of one atom of 
 the former to three of the latter, to form ammonia or spirit 
 of hartshorn. Picturing in imagination the atoms of ele- 
 mentary bodies as little spheres, the molecules of compound 
 bodies must be pictured as groups of such spheres. This is 
 the atomic theory as Dalton conceived it. Now, if this 
 theory have any foundation in fact, and if the theory of an 
 ether pervading space and constituting the vehicle of atomic 
 motion be founded in fact, we may assuredly expect the 
 vibrations of elementary bodies to be profoundly modified 
 by the act of combination. It is on the face of it almost 
 certain that both as regards radiation and absorption, that 
 is to say, both as regards the communication of motion to 
 the ether and the acceptance of motion from it, the deport- 
 ment of the uncombined will be different from that of the 
 combined atoms. 
 
RADIATION. 175 
 
 4. Absorption of Radiant Heat by Gases. 
 
 We have now to submit these considerations to the 
 only test by which they can be tried, namely, that of ex- 
 periment. An experiment is well defined as a question put 
 to Nature ; but to avoid the risk of asking amiss we ought 
 to purify the question from all adjuncts which do not neces- 
 sarily belong to it. Matter has been shown to be composed 
 of elementary constituents, by the compounding of which 
 all its varieties are produced. But besides the chemical 
 unions which they form, both elementary and compound 
 bodies can unite in another and less intimate way. By the 
 attraction of cohesion gases and vapors aggregate to liquids 
 and solids, without any change of their chemical nature. 
 We do not yet know how the transmission of radiant heat 
 may be affected by the entanglement due to cohesion, and 
 as our object now is to examine the influence of chemical 
 union alone, we shall render our experiments more pure by 
 liberating the atoms and molecules entirely from the bonds 
 of cohesion, and employing them in the gaseous or vapor- 
 ous form. 
 
 Let us endeavor to obtain a perfectly clear mental image 
 of the problem now before us. Limiting in the first place 
 our inquiries to the phenomena of absorption, we have to 
 picture a succession of waves issuing from a radiant source 
 and passing through a gas ; some of them striking against 
 the gaseous molecules and yielding up their motion to 
 the latter ; others gliding round the molecules or passing 
 through the inter-molecular spaces without apparent hinder- 
 ance. The problem before us is to determine whether such 
 free molecules have any power whatever to stop the waves 
 of heat, and, if so, whether different molecules possess this 
 power in different degrees. 
 
 The source of waves which I shall choose for these 
 
176 FRAGMENTS OF SCIENCE. 
 
 experiments is a plate of copper, against the back of which 
 a steady sheet of flame is permitted to play. On emerging 
 from the copper, the waves, in the first instance, pass 
 through a space devoid of air, and then enter a hollow 
 glass cylinder, three feet long and three inches wide. The 
 two ends of this cylinder are stopped by two plates of rock- 
 salt, this being the only solid substance which offers a 
 scarcely sensible obstacle to the passage of the calorific 
 waves. After passing through the tube, the radiant heat 
 falls upon the anterior face of a thermo-electric pile, 1 which 
 instantly applies the heat to the generation of an electric 
 current. This current conducted round a magnetic needle 
 deflects it, and the magnitude of the deflection is a measure 
 of the heat falling upon the pile. This famous instrument, 
 and not an ordinary thermometer, is what we shall use in 
 these inquiries, but we shall use it in a somewhat novel 
 way. As long as the two opposite faces of the thermo- 
 electric pile are kept at the same temperature, no matter 
 how high that may be, there is no current generated. The 
 current is a consequence of the difference of temperature 
 between the two opposite faces of the pile. Hence, if after 
 the anterior face has received the heat from our radiating 
 source, a second source, which we may call the compensat- 
 ing source, be permitted to radiate against the posterior 
 face, this latter radiation will tend to neutralize the former. 
 When the neutralization is perfect, the magnetic needle 
 connected with the pile is no longer deflected, but points to 
 the zero of the graduated circle over which it hangs. 
 
 And now let us suppose the glass tube, through which 
 pass, the waves from the heated plate of copper, to be ex- 
 hausted by an air-pump, the two sources of heat acting at 
 the same time on the two opposite faces of the pile. Per- 
 fectly equal quantities of heat being imparted to the two 
 
 1 In the Appendix to the first chapter of " Heat as a Mode of Motion," 
 the construction of the thermo-electric pile is fully explained. 
 
RADIATION. 177 
 
 faces, the needle points to zero. Let any gas be now per- 
 mitted to enter the exhausted tube ; if the molecules pos- 
 sess any power of intercepting the calorific waves, the 
 equilibrium previously existing will be destroyed, the com- 
 pensating source will triumph, and a deflection of the mag- 
 netic needle will be the immediate consequence. From the 
 deflections thus produced by different gases, we can readily 
 deduce the relative amounts of wave-motion which their 
 molecules intercept. 
 
 In this way the substances mentioned in the following 
 table were examined, a small portion only of each being 
 admitted into, the glass tube. The quantity admitted was 
 just sufficient to depress a column of mercury associated 
 with the tube one inch ; in other words, the gases were 
 examined at a pressure of one-thirtieth of an atmosphere. 
 The numbers in the table express the relative amounts of 
 wave-motion absorbed by the respective gases, the quantity 
 intercepted by atmospheric air being taken as unity : 
 
 Radiation through Gases. 
 
 Air ...................................... 1 
 
 Oxygen .................................. 1 
 
 Nitrogen ................................. 1 
 
 Hydrogen ................................ 1 
 
 Carbonic oxide ........................... 750 
 
 Carbonic acid ............................ 972 
 
 Hydrochloric acid ......................... 1,005 
 
 Nitric oxide .............................. 1,590 
 
 Nitrous oxide ............................. 1,860 
 
 Sulphide of hydrogen ...................... 2,100 
 
 Ammonia .......................... *. ...... 5,460 
 
 Olefiant gas .............................. 6,030 
 
 Sulphurous acid ........................... 6,480 
 
 Every gas in this table is perfectly transparent to light, 
 that is to say, all waves within the limits of the visible 
 
178 FRAGMENTS OF SCIENCE. 
 
 spectrum pass through it without obstruction ; but for the 
 waves of slower period, emanating from our heated plate 
 of copper, enormous differences of absorptive power are 
 manifested. These differences illustrate in the most unex- 
 pected manner the influence of chemical combination. Thus 
 the elementary gases, oxygen, hydrogen, and nitrogen, and 
 the mixture atmospheric air, prove to be practical vacua to 
 the rays of heat ; for every ray, or, more strictly speaking, 
 for every unit of wave-motion, which any one of them is 
 competent to intercept, perfectly transparent ammonia in- 
 tercepts 5,460 units, defiant gas 6,030 units, while sulphur- 
 ous acid gas absorbs 6,480 units. What becomes of the 
 wave-motion thus intercepted ? It is applied to the heating 
 of the absorbing gas. Through air. oxygen, hydrogen, and 
 nitrogen, on the contrary, the waves of ether pass without 
 absorption, and these gases are not sensibly changed in 
 temperature by the most powerful calorific rays. The po- 
 sition of nitrous oxide in the foregoing table is worthy of 
 particular notice. In this gas we have the same atoms, in 
 a state of chemical union, that exist uncombined in the 
 atmospheric air; but the absorption of the compound is 
 1,800 times that of air. 
 
 5. Formation of Invisible Foci. 
 
 This extraordinary deportment of the elementary gases 
 naturally directed attention to elementary bodies in another 
 state of aggregation. Some of Melloni's results now at- 
 tained a new significance ; for this celebrated experimenter 
 had found crystals of the element sulphur to be highly per- 
 vious to radiant heat ; he had also proved that lamp-black 
 and black glass (which owes its blackness to the element 
 carbon) were to considerable extent transparent to calorific 
 rays of low refrangibility. These facts, harmonizing so 
 strikingly with the deportment of the simple gases, sug- 
 
RADIATION. 179 
 
 gested further inquiry. Sulphur dissolved in bisulphate of 
 carbon was found almost perfectly transparent. The dense 
 and deeply-colored element bromine was examined, and 
 found competent to cut off the light of our most brilliant 
 flames, while it transmitted the invisible calorific rays with 
 extreme freedom. Iodine, the companion-element of bro- 
 mine, was next thought of, but it was found impracticable 
 to examine the substance in its usual solid condition. It 
 however dissolves freely in bisulphide of carbon. There is 
 no chemical union between the liquid and the iodine ; it is 
 simply a case of solution, in which the uncombined atoms 
 of the element can act upon the radiant heat. When per- 
 mitted to do so, it was found that a layer of dissolved 
 iodine, sufficiently opaque to cut off the light of the mid- 
 day sun, was almost absolutely transparent to the invisible 
 calorific rays. 
 
 By prismatic analysis Sir William Herschel separated 
 the luminous from the non-luminous rays of the sun, and 
 he also sought to render the obscure rays visible by con- 
 centration. Intercepting the luminous portion of his spec- 
 trum he brought, by a converging lens, the ultra-red rays 
 to a focus, but by this condensation he obtained no light. 
 The solution of iodine offers a means of filtering the solar 
 beam, or, failing it, the beam of the electric lamp, which 
 renders attainable far more powerful foci of invisible rays 
 than could possibly be obtained by the method of Sir Wil- 
 liam Herschel. For to form his spectrum he was obliged 
 to operate upon solar light which had passed through 
 a narrow slit or through a small aperture, the amount of 
 the obscure heat being limited by this circumstance. But 
 with our opaque solution we may employ the entire surface 
 of the largest lens, and having thus converged the rays, 
 luminous and non-luminous, we can intercept the former by 
 the iodine, and do what w r e please with the latter. Ex- 
 periments of this character, not only with the iodine solu- 
 
180 FRAGMENTS OF SCIENCE. 
 
 tion, but also with black glass and layers of lamp-black, 
 were publicly performed at the Royal Institution in the 
 early part of 1862, and the effects at the foci of invisible 
 rays then obtained were such as had never been witnessed 
 previously. 
 
 In the experiments here referred to, glass lenses were 
 employed to concentrate the rays. But glass, though 
 highly transparent to the luminous, is in a high degree 
 opaque to the invisible heat-rays of the electric lamp, and 
 hence a large portion of those rays was intercepted by the 
 glass. The obvious remedy here is to employ rock-salt 
 lenses instead of glass ones, or to abandon the use of lenses 
 wholly, and to concentrate the rays by a metallic mirror. 
 Both of these improvements have been introduced, and, as 
 anticipated, the invisible foci have been thereby rendered 
 more intense. The mode of operating remains, however, the 
 same, in principle, as that made known in 1862. It was 
 then found that an instant's exposure of the face of the 
 thermo-electric pile to the focus of invisible rays, dashed 
 the needles of a coarse galvanometer violently aside. It is 
 now found that on substituting for the face of the thermo- 
 electric pile a combustible body, the invisible rays are 
 competent to set that body on fire. 
 
 6. Visible and Invisible J%ays of the Electric Light. 
 
 We have next to examine what proportion the non 
 luminous rays of the electric light bear to the luminous 
 ones. This the opaque solution of iodine enables us to do 
 with an extremely close approximation to the truth. The 
 pure bisulphide of carbon, which is the solvent of the 
 iodine, is perfectly transparent to the luminous, and almost 
 perfectly transparent to the dark rays of the electric lamp. 
 Through the transparent bisulphide the total radiation of 
 the lamp may be considered to pass, while through the 
 
RADIATION. 181 
 
 solution of iodine only the dark rays are transmitted. 
 Determining, then, by means of a thermo-electric pile, the 
 total radiation, and deducting from it the purely obscure, 
 we obtain the amount of the purely luminous emission. 
 Experiments, performed in this way, prove that if all the 
 visible rays of the electric light were converged to a focus 
 of dazzling brilliancy, its heat would only be one-ninth of 
 that produced at the unseen focus of the invisible rays. 
 
 Exposing his thermometers to the successive colors of 
 the solar spectrum, Sir William Herschel determined the 
 heating power of each, and also that of the region beyond 
 the extreme red. Then drawing a straight line to represent 
 the length of the spectrum, he erected, at various points, 
 perpendiculars to represent the calorific intensity existing 
 at those points. Uniting the ends of all his perpendiculars, 
 he obtained a curve which showed at a glance the manner 
 in which the heat was distributed in the solar spectrum. 
 Professor Mliller, of Freiburg, with improved instruments, 
 afterward made similar experiments, and constructed a 
 more accurate diagram of the same kind. We have now to 
 examine the distribution of heat in the spectrum of the 
 electric light ; and for this purpose we shall employ a par- 
 ticular form of the thermo-electric pile, devised by Melloni. 
 Its face is a rectangle, which by means of movable side- 
 pieces can be rendered as narrow as desired. We can, for 
 example, have the face of the pile the tenth, the hundredth, 
 or even the thousandth of an inch in breadth. By means 
 of an endless screw, this linear thermo-electric pile may be 
 moved through the entire spectrum, from the violet to the 
 red, the amount of heat falling upon the pile at every point 
 of its march, being declared by a magnetic needle associated 
 with the pile. 
 
 When this instrument is brought up to the violet end of 
 the spectrum of the electric light, the heat is found to be 
 insensible. As the pile gradually moves from the violet 
 
182 FRAGMENTS OF SCIENCE. 
 
 end toward the red, heat soon manifests itself, augmenting 
 as we approach the red. Of all the colors of the visible 
 spectrum the red possesses the highest heating power. On 
 pushing the pile into the dark region beyond the red, the 
 heat, instead of vanishing, rises suddenly and enormously 
 in intensity, until at some distance beyond the red it 
 attains a maximum. Moving the pile still forward, the 
 thermal power falls, somewhat more rapidly than it rose. 
 It then gradually shades away, but for a distance beyond 
 the red greater than the length of the whole visible spec- 
 trum, signs of heat may be detected. Drawing a datum 
 line, and erecting along it perpendiculars, proportional in 
 length to the thermal intensity at the respective points, we 
 obtain the extraordinary curve, shown on the adjacent page, 
 which exhibits the distribution of heat in the spectrum of 
 the electric light. In the region of dark rays, beyond the 
 red, the curve shoots up to B, in a steep and massive peak 
 a kind of Matterhorn of heat, which dwarfs the portion 
 of the diagram C D E, representing the luminous radiation. 
 Indeed, the idea forced upon the mind by this diagram is 
 that the light-rays are a mere insignificant appendage to 
 the heat-rays represented by the area A B C D, thrown in 
 as it were by Nature for the purposes of vision. 
 
 The diagram drawn by Professor Mliller to represent 
 the distribution of heat in the solar spectrum is not by any 
 means so striking as that just described, and the reason, 
 doubtless, is that prior to reaching the earth the solar rays 
 have to traverse our atmosphere. By the aqueous vapor 
 there diffused, the summit of the peak representing the 
 sun's invisible 'radiation is cut off. A similar lowering of 
 the mountain of invisible heat is observed when the rays 
 from the electric light are permitted to pass through a film 
 of water, which acts upon them as the atmospheric vapor 
 acts upon the rays of the sun. 
 
184 FRAGMENTS OF SCIENCE. 
 
 7. Combustion by Invisible Mays. 
 
 The sun's invisible rays far transcend the visible ones 
 in heating power, so that if the alleged performances of 
 Archimedes during the siege of Syracuse had any founda- 
 tion in fact, the dark solar rays would have been the phi- 
 losopher's chief agents of combustion. On a small scale 
 we can readily produce with the purely invisible rays of 
 the electric light all that Archimedes is said to have per- 
 formed with the sun's total radiation. Placing behind the 
 electric light a small concave mirror, the rays are converged, 
 the cone of reflected rays and their point of convergence 
 being rendered clearly visible by the dust always floating 
 in the air. Placing, between the luminous focus and the 
 source of rays, our solution of iodine, the light of the cone 
 is entirely cut away, but the intolerable heat experienced 
 when the hand is placed, even for a moment, at the dark 
 focus, shows that the calorific rays pass unimpeded through 
 the opaque solution. 
 
 Almost any thing that ordinary fire can effect may be 
 accomplished at the focus of invisible rays ; the air at the 
 focus remaining at the same time perfectly cold, on ac- 
 count of its transparency to the heat-rays. An air-ther- 
 mometer, with a hollow rock-salt bulb, would be unaffected 
 by the heat of the focus : there would be no expansion, 
 and in the open air there is no convection. The ether at 
 the focus, and not the air, is the substance in which the 
 heat is embodied. A block of wood, placed at the focus, 
 absorbs the heat, and dense volumes of smoke rise swiftly 
 upward, showing the manner in which the air itself would 
 rise, if the invisible rays were competent to heat it. At 
 the perfectly dark focus dry paper is instantly inflamed : 
 chips of wood are speedily burnt up : lead, tin, and zinc, 
 are fused : and disks of charred paper are raised to vivid 
 incandescence. It might be supposed that the obscure rays 
 
RADIATION. 185 
 
 would show no preference for black over white ; but they 
 do show a preference, and, to obtain rapid combustion, the 
 body, if not already black, ought to be blackened. When 
 metals are to be burned, it is necessary to blacken or 
 otherwise tarnish them, so as to diminish their reflective 
 power. Blackened zinc-foil, when brought into the focus 
 of invisible rays, is instantly caused to blaze, and burns 
 with its peculiar purple flame. Magnesium wire flattened, 
 or tarnished magnesium ribbon, also bursts into splendid 
 combustion. Pieces of charcoal suspended in a receiver 
 full of oxygen are also set on fire : the dark rays after hav- 
 ing passed through the receiver still possessing sufficient 
 power to ignite the charcoal, and thus initiate the attack 
 of the oxygen. If, instead of being plunged in oxygen, 
 the charcoal be suspended in vacuo, it immediately glows 
 at the place where the focus falls. 
 
 8. Transmutation of Rays : 1 Calorescence. 
 
 Eminent experimenters were long occupied in demon- 
 strating the substantial identity of light and radiant heat, 
 and we have now the means of offering a new and striking 
 proof of this identity. A concave mirror produces beyond 
 the object which it reflects an inverted and magnified image 
 of the object ; withdrawing, for example, our iodine solu- 
 tion, an intensely luminous inverted image of the carbon 
 points of the electric light is formed at the focus of the 
 mirror employed in the foregoing experiments. When the 
 solution is interposed, and the light is cut away, what 
 becomes of this image ? It disappears from sight, but an 
 invisible thermograph remains, and it is only the peculiar 
 constitution of our eyes that disqualifies us from seeing 
 the picture formed by the calorific rays. Falling on 
 white paper, the image chars itself out : falling on black 
 
 1 I borrow this term from Professor Challis, " Philosophical Maga- 
 zine," vol. xii., p. 521. 
 
186 FRAGMENTS OF SCIENCE. 
 
 paper, two holes are pierced in it, corresponding to the 
 images of the two coal points : but falling on a thin plate 
 of carbon in vacuo, or upon a thin sheet of platinized plat- 
 inum, either in vacuo or in air, radiant heat is converted 
 into light, and the image stamps itself in vivid incandes- 
 cence upon both the carbon and the metal. Results similar to 
 those obtained with the electric light have also been obtained 
 with the invisible rays of the lime-light and of the sun. 
 
 Before a Cambridge audience it is hardly necessary to 
 refer to the excellent researches of Professor Stokes at 
 the opposite end of the spectrum. The above results con- 
 stitute a kind of complement to his discoveries. Professor 
 Stokes named the phenomena which he has discovered and 
 investigated Fluorescence for the new phenomena here 
 described I have proposed the term Calorescence. He, by 
 the interposition of a proper medium, so lowered the re- 
 frangibility of the ultra-violet rays of the spectrum as to 
 render them visible ; and here, by the interposition of the 
 platinum-foil, the refrangibility of the ultra-red rays is so 
 exalted as to render them visible. Looking through a 
 prism at the incandescent image of the carbon points, the 
 light of the image is decomposed, and a complete spectrum 
 obtained. The invisible rays of the electric light, remoulded 
 by the atoms of the platinum, shine thus visibly forth ; ultra- 
 red rays being converted into red, orange, yellow, green, 
 blue, indigo, and ultra-violet ones. Could we, moreover, 
 raise the original source of rays to a sufficiently high tem- 
 perature, we might not only obtain from the dark rays of 
 such a source a single incandescent image, but from the 
 dark rays of this image we might obtain a second one, from 
 the dark rays of the second a third, and so on a series of 
 complete images and spectra being thus extracted from the 
 invisible emission of the primitive source. 1 
 
 1 On investigating the calorescence produced by rays transmitted 
 through glasses of various colors, it was found that in the case of certain 
 
RADIATION. 187 
 
 9. Deadness of the Optic Nerve to the Calorific JRays. 
 
 The layer of iodine used in the foregoing experiments 
 intercepted the light of the noonday sun. No trace of 
 light from the electric lamp was visible in the darkest 
 room, even when a white screen was placed at the focus of 
 the mirror employed to concentrate the light. It was 
 thought, however, that if the retina itself were brought 
 into the focus the sensation of light might be experienced. 
 The danger of this experiment was twofold. If the dark 
 rays were absorbed in a high degree by the humors of the 
 eye, the albumen of the humors might coagulate along the 
 line of the rays. If, on the contrary, no such high ab- 
 sorption took place, the rays might reach the retina with a 
 force sufficient to destroy it. To test the likelihood of these 
 results, experiments were made on water and on a solution of 
 alum, and they showed it to be very improbable that in the 
 brief time requisite for an experiment any serious damage 
 
 specimens of blue glass, the platinum-foil glowed with a pink or purplish 
 light. The effect was not subjective, and considerations of obvious in- 
 terest are suggested by it. Different kinds of black glass differ notably 
 as to their power of transmitting radiant heat. In thin plates some de- 
 scriptions tint the sun with a greenish hue : others make it appear a 
 glowing red without any trace of green. The latter are far more diather- 
 mic than the former. In fact, carbon when perfectly dissolved, and in- 
 corporated with a good white glass, is highly transparent to the calorific 
 rays, and by employing it as an absorbent, the phenomena of " calores- 
 cence " may be obtained, though in a less striking form than with the 
 iodine. The black glass chosen for thermometers, and intended to ab- 
 sorb completely the solar heat, may entirely fail in this object, if the 
 glass in which the carbon is incorporated be colorless. To render the 
 bulb of a thermometer a perfect absorbent, the glass ought in the first 
 instance to be green. Soon after the discovery of fluorescence the late 
 Dr. William Allen Miller pointed to the lime-light as an illustration of 
 exalted refrangibility. Direct experiments have since entirely confirmed 
 the view expressed at page 210 of his work on " Chemistry," published 
 in 1855. 
 
188 FRAGMENTS OF SCIENCE. 
 
 could be done. The eye was therefore caused to approach 
 the dark focus, no defence, in the first instance, being pro- 
 vided ; but the heat, acting upon the parts surrounding the 
 pupil, could not be borne. An aperture was, therefore, 
 pierced in a plate of metal, and the eye placed behind the 
 aperture, was caused to approach the point of convergence 
 of invisible rays. The focus was attained, first by the 
 pupil and afterward by the retina. Removing the eye, but 
 permitting the plate of metal to remain, a sheet of platinum- 
 foil was placed in the position occupied by the retina a mo- 
 ment before. The platinum became red hot. No sensible 
 damage was done to the eye by this experiment ; no im- 
 pression of light was produced; the optic nerve was not 
 even conscious of heat. 
 
 But the humors of the eye are known to be highly im- 
 pervious to the invisible calorific rays, and the question 
 therefore arises, " Did the radiation in the foregoing experi- 
 ment reach the retina at all ? " The answer is, that the 
 rays were in part transmitted to the retina, and in part ab- 
 sorbed by the humors. Experiments on the eye of an ox 
 showed that the proportion of obscure rays which reached 
 the retina amounted to 18 per cent, of the total radiation ; 
 while the luminous emission from the electric light amounts 
 to no more than 10 per cent, of the same total. Were the 
 purely luminous rays of the electric lamp converged by our 
 mirror to a focus, there can be no doubt as to the fate of a 
 retina placed there. Its ruin would be inevitable ; and yet 
 this would be accomplished by an amount of wave-motion 
 but little more than half of that which the retina bears, 
 without exciting consciousness, at the focus of invisible 
 rays. 
 
 This subject will repay a moment's further attention. 
 At a common distance of a foot the visible radiation of the 
 electric light is 800 times the light of a candle. At the 
 same distance, the portion of the radiation of the electric 
 
RADIATION. 189 
 
 light which reaches the retina but fails to excite vision, is 
 about 1,500 times the luminous radiation of the candle. 1 
 But a candle on a clear night can readily be seen at a dis- 
 tance of a mile, its light at this distance being less than 
 one 20,000,000th of its light at the distance of a foot. 
 Hence, to make the candle-light a mile off equal in power 
 to the non-luminous radiation received from the electric 
 light at a foot distance, its intensity would have to be mul- 
 tiplied by 1,500 X 20,000,000, or by thirty thousand mill- 
 ions. Thus the thirty thousand millionth part of the in- 
 visible radiation from the electric light, received by the 
 retina at the distance of a foot, would, if slightly changed 
 in character, be amply sufficient to provoke vision. Nothing 
 could more forcibly illustrate that special relationship sup- 
 posed by Melloni and others to subsist between the optic 
 nerve and the oscillating periods of luminous bodies. The 
 optic nerve responds, as it were, to the waves with which 
 it is in consonance, while it refuses to be excited by others 
 of almost infinitely greater energy, whose periods of recur- 
 rence are not in unison with its own. 
 
 10. Persistence of Hays. 
 
 At an early part of this lecture it was affirmed that 
 when a platinum wire was gradually raised to a state of 
 high incandescence, new rays were constantly added, 
 while the intensity of the old ones was increased. Thus 
 in Dr. Draper's experiments the rise of temperature that 
 generated the orange, yellow, green, and blue rays, aug- 
 mented the intensity of the red ones. What is true of the 
 red is true of every other ray of the spectrum, visible and 
 invisible. We cannot indeed see the augmentation of in- 
 
 1 It will be borne in mind that the heat which any ray, luminous or 
 non-luminous, is competent to generate is the true measure of the energy 
 of the ray. 
 
190 ' FRAGMENTS OF SCIENCE. 
 
 tensity in the region beyond the red, but we can measure 
 it and express it numerically. With this view the following 
 experiment was performed. A spiral of platinum wire was 
 surrounded by a small glass globe to protect it from cur- 
 rents of air ; through an orifice in the globe the rays could 
 pass from the spiral and fall afterward upon a thermo-elec- 
 tric pile. Placing in front of the orifice an opaque solution 
 of iodine, the platinum was gradually raised from a low 
 dark heat to the fullest incandescence, with the following 
 results : 
 
 Appearance Energy of 
 
 of spiral. obscure radiation. 
 
 Dark 1 
 
 Dark, but hotter 3 
 
 Dark, but still hotter 5 
 
 Dark, but still hotter 10 
 
 Feeble red 19 
 
 Dull red 25 
 
 Red 37 
 
 Full red 62 
 
 Orange 89 
 
 Bright orange 144 
 
 Yellow 202 
 
 White 276 
 
 Intense white 440 
 
 Thus the augmentation of the electric current, which 
 raises the wire from its primitive dark condition to an in- 
 tense white heat, exalts at the same time the energy of the 
 obscure radiation, until at the end it is fully four hundred 
 and forty times what it was at the beginning. 
 
 What has been here proved true of the totality of the 
 ultra-red rays is true for each of them singly. Placing our 
 linear thermo-electric pile in any part of the ultra-red spec- 
 trum, it may be proved that a ray once emitted continues 
 to be emitted with increased energy as the temperature is 
 augmented. The platinum spiral so often referred to being 
 
RADIATION. 191 
 
 raised to whiteness by an electric current, a brilliant spec- 
 trum was formed from its light. A linear thermo-electric 
 pile was placed in the region of obscure rays beyond the 
 red, and by diminishing the current the spiral was reduced 
 to a low temperature. It was then caused to pass through 
 various degrees of darkness and incandescence, with the 
 following results : 
 
 Appearance Energy of 
 
 of spiral obscure rays. 
 
 Dark 1 
 
 Dark 6 
 
 Faint red 10 
 
 Dull red 13 
 
 Red 18 
 
 Full red ' 27 
 
 Orange 60 
 
 Yellow 93 
 
 White 122 
 
 Here, as in the former case, the dark and bright radia- 
 tions reached their maximum together; as the one aug- 
 mented, the other augmented, until at last the energy of 
 the obscure rays of the particular refrangibility here chosen, 
 became one hundred and twenty-two times what it was at 
 first. To reach a white heat the wire has to pass through 
 all the stages of invisible radiation, and in its most brilliant 
 condition it embraces, in an intensified form, the rays of all 
 those stages. 
 
 And thus it is with all other kinds of matter, as far as 
 they have hitherto been examined. Coke, whether brought 
 to a white heat by the electric current, or by the oxyhydro- 
 gen jet, pours out invisible rays with augmented energy, 
 as its light is increased. The same is true of lime, bricks, 
 and other substances. It is true of all metals which are 
 capable of being heated to incandescence. It also holds 
 good for phosphorus burning in oxygen. Every gush of 
 dazzling light has associated with it a gush of invisible ra- 
 
192 FRAGMENTS OF SCIENCE. 
 
 diant heat, which far transcends the light in energy. This 
 condition of things applies to all bodies capable of being- 
 raised to a white heat, either in the solid or the molten 
 condition. It would doubtless also apply to the luminous 
 fogs formed by the condensation of incandescent vapors. 
 In such cases when the curve representing the radiant en- 
 ergy of the body is constructed, the obscure radiation tow- 
 ers upward like a mountain, the luminous radiation resem- 
 bling a mere spur at its base. From the very brightness 
 of the light of some of the fixed stars we may infer the 
 intensity of the dark radiation, which is the precursor and 
 inseparable associate of their luminous rays. 
 
 We thus find the luminous radiation appearing when 
 the radiant body has attained a certain temperature ; or, 
 in other words, when the vibrating atoms of the body have 
 attained a certain width of swing. In solid and molten 
 bodies a certain amplitude cannot be surpassed without 
 the introduction of periods of vibration, which provoke 
 the sense of vision. How are we to figure this ? If per- 
 mitted to speculate, we might ask, Are not these more 
 rapid vibrations the progeny of the slower ? Is it not really 
 the mutual action of the atoms, when they swing through 
 very wide spaces, and thus encroach upon each other, that 
 causes them to tremble in quicker periods ? If so, what- 
 ever be the agency by which the large swinging space is 
 obtained, we shall have light-giving vibrations associated 
 with it. It matters not whether the large amplitudes be 
 produced by the strokes of a hammer, or by the blows of 
 the molecules of a non-luminous gas, such as the air at some 
 height above a gas-flame ; or by the shock of the ether- 
 particles when transmitting radiant heat. The result in 
 all cases will be incandescence. Thus, the invisible waves 
 of our filtered electric beam may be regarded as generating 
 synchronous vibrations among the atoms of the platinum 
 on which they impinge ; but once these vibrations have at- 
 
RADIATION. 193 
 
 tained a certain amplitude, the mutual jostling of the atoms 
 produces quicker tremors, and the light-giving waves fol- 
 low as the necessary product of the heat-giving ones. 
 
 11. Absorption of Radiant Heat l>y Vapors 
 and Odors. 
 
 We commenced the demonstrations brought forward in 
 this lecture by experiments on permanent gases, and we 
 have now to turn our attention to the vapors of volatile 
 liquids. Here, as in the case .of the gases, vast differences 
 have been proved to exist between various kinds of mole- 
 cules, as regards their power of intercepting the calorific 
 waves. While some vapors allow the waves a compara- 
 tively free passage, the minutest bubble of other vapors, 
 introduced into the tube already employed for gases, causes 
 a deflection of the magnetic needle. Assuming the ab- 
 sorption effected by air at a pressure of one atmosphere to 
 be unity, the followng are the absorptions effected by a se- 
 .ries of vapors at a pressure of one-sixtieth of an atmos- 
 phere : 
 
 Name of vapor. Absorption. 
 
 Bisulphide of carbon 47 
 
 Iodide of methyl 115 
 
 Benzol 136 
 
 Amylene 321 
 
 Sulphuric ether 440 
 
 Formic ether 548 
 
 Acetic ether 612 
 
 Bisulphide of carbon is the most transparent vapor in 
 this list ; and acetic ether the most opaque ; one-sixtieth 
 of an atmosphere of the former, however, produces forty- 
 seven times the effect of a whole atmosphere of air, while 
 one-sixtieth of an atmosphere of the latter produces six 
 hundred and twelve times the effect of a whole atmos- 
 phere of air. Reducing dry air to the pressure of the acetic 
 9 
 
194 FRAGMENTS OF SCIENCE. 
 
 ether here employed, and comparing them then together, 
 the quantity of wave-motion intercepted by the ether would 
 be many thousand times that intercepted by the air. 
 
 Any one of these vapors discharged into the free atmos- 
 phere, in front of a body emitting obscure rays, intercepts 
 more or less of the radiation. A similar effect is produced 
 by perfumes diffused in the air, though their attenuation is 
 known to be almost infinite. Carrying, for example, a cur- 
 rent of dry air over bibulous paper moistened by patchouli, 
 the scent taken up by the current absorbs 30 times the 
 quantity of heat intercepted by the air which carries it ; 
 and yet patchouli acts more feebly on radiant heat than 
 any other perfume yet examined. Here follow the results 
 obtained with various essential oils, the odor, in each case, 
 being carried by a current of dry air into the tube already 
 employed for gases and vapors : 
 
 Name of perfume. Absorption. 
 
 Patchouli 30 
 
 Sandal-wood 32 
 
 Geranium 33 
 
 Oil of cloves 34 
 
 Otto of roses 37 
 
 Bergamot 44 
 
 Neroli 47 
 
 Lavender 60 
 
 Lemon 65 
 
 Portugal 67 
 
 Thyme 68 
 
 Rosemary 74 
 
 Oil of laurel 80 
 
 Camomile-flowers 87 
 
 Cassia 109 
 
 Spikenard 355 
 
 Aniseseed 372 
 
 Thus the absorption by a tube full of dry air being 1, 
 that of the odor of patchouli diffused in it is 30, that of 
 
RADIATION. 195 
 
 lavender 60, that of rosemary 74, while that of aniseseed 
 amounts to 372. It would be idle to speculate on the 
 quantities of matter concerned in these actions. 
 
 12. Aqueous Vapor in relation to the Terrestrial 
 Temperatures. * 
 
 We are now fully prepared for a result which, without 
 such preparation, might appear incredible. Water is, to 
 some extent, a volatile body, and our atmosphere, resting 
 as it does upon the surface of the ocean, receives from it a 
 continual supply of aqueous vapor. It would be an error 
 to confound clouds or fog or any visible mist with the va- 
 por of water : this vapor is a perfectly impalpable gas, dif- 
 fused, even on the clearest days, throughout the atmosphere. 
 Compared with the great body of the air, the aqueous vapor 
 it contains is of almost infinitesimal amount, 99J out of 
 every 100 parts of the atmosphere being composed of oxy- 
 gen and nitrogen. In the absence of experiment, we should 
 never think of ascribing to this scant and varying constitu- 
 ent any important influence on terrestrial radiation ; and 
 yet its influence is far more potent than that of the great 
 body of the air. To say that on a day of average humidity 
 in England, the atmospheric vapor exerts 100 times the 
 action of the air itself, would certainly be an understate- 
 ment of the fact. The peculiar qualities of this vapor, and 
 the circumstance that at ordinary temperatures it is very 
 near its point of condensation, render the results which it 
 yields in the apparatus already described, less than the 
 truth ; and I am not prepared to say that the absorption by 
 this substance is not 200 times that of the air in which it is 
 diffused. Comparing a single molecule of aqueous vapor 
 with an atom of either of the main constituents of our at- 
 mosphere, I am not prepared to say how many thousand 
 times the action of the former exceeds that of the latter. 
 1 See Note at the end of this Lecture. 
 
196 FRAGMENTS OF SCIENCE. 
 
 But it must be borne in mind that these large numbers 
 depend in part upon the extreme feebleness of the air ; the 
 power of aqueous vapor seems vast, because that of the air 
 with which it is compared is infinitesimal. Absolutely con- 
 sidered, however, this substance, notwithstanding its small 
 specific gravity, exercises a very potent action. Probably 
 from 10 to 15 per cent, of the heat radiated from the earth 
 is absorbed within 10 feet of the earth's surface. This 
 must evidently be of the utmost consequence to the life of 
 the world. Imagine the superficial molecules of the earth 
 trembling with the motion of heat, and imparting it to the 
 surrounding ether ; this motion would be carried rapidly 
 away, and lost forever to our planet, if the waves of ether 
 had nothing but the air to contend with in their outward 
 course. But the aqueous vapor takes up the motion of the 
 ethereal waves, and becomes thereby heated, thus wrapping 
 the earth like a warm garment, and protecting its surface 
 from the deadly chill which it would otherwise sustain. 
 Various philosophers have speculated on the influence of 
 an atmospheric envelope. De Saussure, Fourier, M. Pouil- 
 let and Mr. Hopkins have, one and all, enriched scientific 
 literature with contributions on this subject, but the con- 
 siderations which these eminent men have applied to atmos- 
 pheric air, have, if my experiments be correct, to be trans- 
 ferred to the aqueous vapor. 
 
 The observations of meteorologists furnish important, 
 though hitherto unconscious evidence of the influence of 
 this agent. Wherever the air is dry we are liable to daily 
 extremes of temperature. By day, in such places, the sun's 
 heat reaches the earth unimpeded, and renders the maxi- 
 mum high ; by night, on the other hand, the earth's heat 
 escapes unhindered into space, and renders the minimum 
 low. Hence the difference between the maximum and min- 
 imum is greatest where the air is driest. In the plains 
 of India, on the heights of the Himalaya, in central Asia, in 
 
KADIATION. 197 
 
 Australia wherever drought reigns, we have the heat of 
 day forcibly contrasted with the chill of night. In the Sa- 
 hara itself, when the sun's rays cease to impinge on the 
 burning soil, the temperature runs rapidly down to freezing, 
 because there is no vapor overhead to check the calorific 
 drain. And here another instance might be added to the 
 numbers already known, in which Nature tends as it were 
 to check her own excess. By nocturnal refrigeration, the 
 aqueous vapor of the air is condensed to water on the sur- 
 face of the earth, and as only the superficial portions ra- 
 diate, the act of condensation makes water the radiating 
 body. Now experiment proves that to the rays emitted by 
 water, aqueous vapor is especially opaque. Hence the very 
 act of condensation, consequent on terrestrial cooling, be* 
 comes a safeguard to the earth, imparting to its radiation 
 that particular character which renders it most liable to be 
 prevented from escaping into space. 
 
 It might however be urged that, inasmuch as we derive 
 all our he^t from the sun, the self-same covering which pro- 
 tects the earth from chill must also shut out the solar ra- 
 diation. This is partially true, but only partially ; the 
 sun's rays are different in quality from the earth's rays, and 
 it does not at all follow that the substance which absorbs 
 the one must necessarily absorb the other. Through a 
 layer of water, for example, one-tenth of an inch in thick- 
 ness, the sun's rays are transmitted with comparative free- 
 dom ; but through a layer half this thickness, as Melloni has 
 proved, no single ray from the warmed earth could pass. 
 In like manner, the sun's rays pass with comparative free- 
 dom through the aqueous vapor of the air ; the absorbing 
 power of this substance being mainly exerted upon the heat 
 that endeavors to escape from the earth. In consequence 
 of this differential action upon solar and terrestrial heat, 
 the mean temperature of our planet is higher than is due to 
 its distance from the sun. 
 
198 FRAGMENTS OF SCIENCE. 
 
 13. Liquids and their Vapors in relation to Radiant 
 Heat. 
 
 The deportment here assigned to atmospheric vapor has 
 been established by direct experiments on air taken from 
 the streets and parks of London, from the downs of Epsorn, 
 from the hills and sea-beach of the Isle of Wight, and also 
 by experiments on air in the first instance dried, and after- 
 ward rendered artificially humid by pure distilled water. 
 It has also been established in the following way : Ten 
 volatile liquids were taken at random and the power of 
 these liquids, at a common thickness, to intercept the waves 
 of heat was carefully determined. The vapors of the liquids 
 were next taken, in quantities proportional to the quantities 
 of liquid, and the power of the vapors to intercept the waves 
 of heat was also determined. Commencing with the sub- 
 stance which exerted the least absorptive power, and pro- 
 ceeding upward to the most energetic, the following order 
 of absorption was observed : 
 
 Liquids. Vapors. 
 
 Bisulphide of carbon. Bisulphide of carbon. 
 
 Chloroform. Chloroform. 
 
 Iodide of methyl. Iodide of methyl. 
 
 Iodide of ethyl. Iodide of ethyl. 
 
 Benzol. Benzol. 
 
 Amylene. Amylene. 
 
 Sulphuric ether. Sulphuric ether. 
 
 Acetic ether. Acetic ether. 
 
 Formic ether. Formic ether. 
 
 Alcohol. Alcohol. ' 
 Water. 
 
 We here find the order of absorption in both cases to 
 be the same. We have liberated the molecules from the 
 bonds which trammel them more or less in a liquid condi- 
 tion ; but this change in their state of aggregation does not 
 
RADIATION. 199 
 
 change their relative powers of absorption. Nothing could 
 more clearly prove that the act of absorption depends upon 
 the individual molecule, which equally asserts its power in 
 the liquid and the gaseous state. We may assuredly con- 
 clude from the above table that the position of a vapor is 
 determined by that of its liquid. Now, at the very foot of 
 the list of liquids stands water, signalizing itself above all 
 others by its enormous power of absorption. And from 
 this fact, even if no direct experiment on the vapor of water 
 had ever been made, we should be entitled to rank that 
 vapor as the most powerful absorber of radiant heat hitherto 
 discovered. It has been proved by experiment that a shell 
 of air two inches in thickness surrounding our planet, and 
 saturated with the vapor of sulphuric ether, would intercept 
 35 per cent, of the earth's radiation. And though the 
 quantity of aqueous vapor necessary to saturate air is 
 much less than the amount of sulphuric ether vapor which 
 it can sustain, it is still extremely probable that the esti- 
 mate already made of the action of atmospheric vapor 
 within 10 feet of the earth's surface, is altogether under 
 the mark ; and that we are ^indebted to this wonderful sub- 
 stance, to an extent not accurately determined, but certainly 
 far beyond what has hitherto been imagined, for the tem- 
 perature now existing at the surface of the globe. 
 
 14. Reciproctiy of Radiation and Absorption. 
 
 Throughout the reflections which have hitherto occupied 
 us, the image before the mind has been that of a radiant 
 source generating calorific waves, which, on passing among 
 the scattered molecules of a gas or vapor, were intercepted 
 by those molecules in various degrees. In all cases it was 
 the transference of motion from the ether to the compara- 
 tively quiescent molecules of the gas or vapor. We have 
 now to change the form of our conception, and to figure 
 
200 FRAGMENTS OF SCIENCE. 
 
 these molecules not as absorbers, but as radiators, 'not as 
 the recipients, but as the originators of wave motion. That 
 is to say, we must figure them vibrating and generating in 
 the surrounding ether undulations which speed through it 
 with the velocity of light. Our object now is to inquire 
 whether the act of chemical combination, which proves so 
 potent as regards the phenomena of absorption, does not 
 also manifest its power in the phenomena of radiation. For 
 the examination of this question it is necessary, in the first 
 place, to heat our gases and vapors to the same tempera- 
 ture, and then examine their power of discharging the 
 motion thus imparted to them upon the ether in which 
 they swing. 
 
 A heated copper ball was placed above a ring gas- 
 burner possessing a great number of small apertures, the 
 burner being connected by a tube with vessels containing 
 the various gases to be examined. By gentle pressure the 
 gases were forced through the orifices of the burner against 
 the copper ball, where each of them, being heated, rose in 
 an ascending column. A thermo-electric pile, entirely 
 screened off from the hot ball, was exposed to the radiation 
 of the warm gas, and the deflection of a magnetic needle 
 connected with the pile declared the energy of the radia- 
 tion. 
 
 By this mode of experiment it was proved that the self- 
 same molecular arrangement which renders a gas a power- 
 ful absorber, renders it in the same degree a powerful 
 radiator that the atom or molecule which is competent to 
 intercept the calorific waves is in the same degree compe- 
 tent to generate them. Thus, while the atoms of element- 
 ary gases proved themselves unable to emit any sensible 
 amount of radiant heat, the molecules of compound gases 
 were shown to be capable of powerfully disturbing the sur- 
 rounding ether. By special modes of experiment the same 
 was proved to hold good for the vapors of volatile liquids, 
 
RADIATION. 201 
 
 the radiative power of every vapor being found proportional 
 to its absorptive power. 
 
 The method of experiment here pursued, though not of 
 the simplest character, is still within your grasp. When 
 air is permitted to rush into an exhausted tube, the tem- 
 perature of the air is raised to a degree equivalent to the 
 vis viva extinguished. 1 Such air is said to be dynamically 
 heated, and if pure, it shows itself incompetent to radiate, 
 even when a rock-salt window is provided for the passage 
 of its rays. But if instead of being empty the tube contain 
 a small quantity of vapor, then the warmed air will com- 
 municate heat by contact to the vapor, which will be thus 
 enabled to radiate. Thus the molecules of the vapor con- 
 vert into the radiant form the heat imparted dynamically 
 to the atoms of the air. By this process, which has been 
 called dynamic radiation, the radiative power of both vapors 
 and gases has been determined, and the reciprocity of their 
 radiation and absorption proved. 2 
 
 In the excellent researches of Leslie, De la Provostaye 
 and Desains, and Balfour Stewart, the reciprocity of radia- 
 tion and absorption as regards solid bodies has been vari- 
 ously illustrated ; while the labors, theoretical and experi- 
 mental, of Kirchhoff have given this subject a wonderful 
 expansion, and enriched it by applications of the highest 
 kind. To their results are now to be added the foregoing, 
 whereby gases and vapors which have been hitherto thought 
 inaccessible to experiments of this kind are proved to ex- 
 hibit the duality of radiation and absorption, the influence 
 on both of chemical combination being exhibited in the 
 most decisive and extraordinary way. 
 
 1 See page 20 for a definition of vis viva. 
 
 2 When heated, air imparts its motion to another gas or vapor; the 
 transference of heat is accompanied by a change of vibrating period. The 
 dynamic radiation of vapors is rendered possible by the transmutation of 
 vibrations. 
 
202 FRAGMENTS OF SCIENCE. 
 
 15. Influence of Vibrating Period and Molecular Form. 
 Physical Analysis of the Human Breath. 
 
 In the foregoing experiments with gases and vapors we 
 have employed throughout invisible rays ; some of these 
 bodies are so impervious that in lengths of a few feet only 
 they intercept every ray as effectually as a layer of pitch 
 would do. The substances, however, which show themselves 
 thus opaque to radiant heat are perfectly transparent to 
 light. Now the T&ys of light differ from those of invisible 
 heat only in point of period, the former failing to affect the 
 retina because their periods of recurrence are too slow. 
 Hence, in some way or other the transparency of our gases 
 and vapors depends upon the periods of the waves which 
 impinge upon them. What is the nature of this depend- 
 ence ? The admirable researches of Kirchhoff help us to an 
 answer. The atoms and molecules of every gas have cer- 
 tain definite rates of oscillation, and those waves of ether 
 are most copiously absorbed whose periods of recurrence 
 synchronize with the periods of the molecules among which 
 they pass. Thus, when we find the invisible rays absorbed 
 and the visible ones transmitted by a layer of gas, we con- 
 clude that the oscillating periods of the gaseous molecules 
 coincide with those of the invisible, and not with those of 
 the visible spectrum. 
 
 It requires some discipline of the imagination to form a 
 clear picture of this process. Such a picture is, however, 
 possible, and ought to be obtained. When the waves of 
 ether impinge upon molecules whose periods of vibration 
 coincide with the recurrence of the undulations, the timed 
 strokes of the waves, the vibration of the molecules aug- 
 ments, as a heavy pendulum is set in motion by well-timed 
 puffs of breath. Millions of millions of shocks are received 
 every second from the calorific waves, and it is not difficult 
 
RADIATION. 203 
 
 to see that, as every wave arrives just in time to repeat the 
 action of its predecessor, the molecules must finally be 
 caused to swing through wider spaces than if the arrivals 
 were not so timed. In fact, it is not difficult to see that an 
 assemblage of molecules, operated upon by contending 
 waves, might remain practically quiescent, and this is act- 
 ually the case when the waves of the visible spectrum pass 
 through a transparent gas or vapor. There is here no sen- 
 sible transference of motion from the ether to the molecules ; 
 in other words, there is no sensible absorption of heat. 
 
 One striking example of the influence of period may here 
 be recorded. Carbonic-acid gas is one of the feeblest of 
 absorbers of the radiant heat emitted by solid sources. It 
 is, for example, to a great extent transparent to the rays 
 emitted by the heated copper-plate already referred to. 
 There are, however, certain rays, comparatively few in num- 
 ber, emitted by the copper, to which the carbonic acid is 
 impervious ; and could, we obtain a source of heat emitting 
 such rays only, we should find carbonic acid more opaque to 
 the radiation from that source than any other gas. Such a 
 source is actually found in the flame of carbonic oxide, 
 where hot carbonic acid constitutes the main radiating body. 
 Of the rays emitted by our heated plate of copper, olefiant 
 gas absorbs ten times the quantity absorbed by carbonic 
 acid. Of the rays emitted by a carbonic-oxide flame, car- 
 bonic acid absorbs twice as much as olefiant gas. This won- 
 derful change in the power of the former as an absorber is 
 simply due to the fact that the periods of the hot and cold 
 carbonic acid are identical, and that the waves from the 
 flame freely transfer their motion to the molecules which 
 synchronize with them. Thus it is that the tenth of an at- 
 mosphere of carbonic acid, enclosed in a tube four feet long, 
 absorbs 60 per cent, of the radiation from a carbonic-oxide 
 flame, while one-thirtieth of an atmosphere absorbs 48 per 
 cent, of the heat from the same origin. 
 
204 FRAGMENTS OF SCIENCE. 
 
 In fact, the presence of the minutest quantity of car- 
 bonic acid may be detected by its action on the rays from 
 the carbonic-oxide flame. Carrying, for example, the dried 
 human breath into a tube four feet long, the absorption 
 there effected by the carbonic acid of the breath amounts 
 to 50 per cent, of the entire radiation. Radiant heat may 
 indeed be employed as a means of determining practically 
 the amount of carbonic acid expired from the lungs. My 
 late assistant, Mr. Barrett, has made this determination. 
 The absorption produced by the breath freed from its moist- 
 ure, but retaining its carbonic acid, was first determined. 
 Carbonic acid artificially prepared was then mixed with dry 
 air in such proportions that the action of the mixture upon 
 the rays of heat was the same as that of the dried breath. 
 The percentage of the former being known, immediately 
 gave that of the latter. The same breath analyzed chemi- 
 cally by Dr. Frankland, and physically by Mr. Barrett, gave 
 the following results : 
 
 Percentage of Carbonic Acid in the Human Breath. 
 
 Chemical analysis. Physical analysis. 
 
 4.66 4.56 
 
 6.33 5.22 
 
 It is thus proved that in the quantity of ethereal motion 
 which it is competent to take up, we have a practical meas- 
 ure of the carbonic acid of the breath, and hence of the 
 combustion going on in the human lungs. 
 
 Still this question of period, though of the utmost im- 
 portance, is not competent to account for the whole of the 
 observed facts. The ether, as far as we know, accepts 
 vibrations of all periods with the same readiness. To it 
 the oscillations of an atom of oxygen are just as acceptable 
 as those of a molecule of olefiant gas ; that the vibrating 
 oxygen then stands so far below the olefiant gas in radiant 
 
RADIATION. 205 
 
 power must be referred not to period, but to some other 
 peculiarity of the elementary gas. The atomic group which 
 constitutes the molecule of olefiant gas, produces many 
 thousand times the disturbance caused by the oxygen, be- 
 cause the group is able to lay a vastly more powerful hold 
 upon the ether than the single atoms can, The cavities and 
 indentations of a molecule composed of spherical atoms 
 may be one cause of this augmented hold. Another, and 
 probably very potent one may be, that the ether itself, con- 
 densed and entangled among the constituent atoms of a 
 compound, virtually increases the magnitude of the group, 
 and hence augments the disturbance. But whatever may 
 be the fate of these attempts to visualize the physics of the 
 process, it will still remain true that, to account for the 
 phenomena of radiation and absorption we must take into 
 consideration the shape, size, and complexity of the mole- 
 cules by which the ether is disturbed. 
 
 16. Summary and Conclusion. 
 
 Let us now cast a momentary glance over the ground 
 that we have left behind. The general nature of light and 
 heat was first briefly described : the compounding of matter 
 from elementary atoms and the influence of the act of com- 
 bination on radiation and absorption were considered and 
 experimentally illustrated. Through the transparent ele- 
 mentary gases radiant heat was found to pass as through a 
 vacuum, while many of the compound gases presented 
 almost impassable obstacles to the calorific waves. This 
 deportment of the simple gases directed our attention to 
 other elementary bodies, the examination of which led to 
 the discovery that the element iodine, dissolved in bisul- 
 phide of carbon, possesses the power of detaching, with 
 extraordinary sharpness, the light of the spectrum from its 
 heat, intercepting all luminous rays up to the extreme red, 
 
206 FRAGMENTS OF SCIENCE. 
 
 and permitting the calorific rays beyond the red to pass 
 freely through it. This substance was then employed to 
 filter the beams of the electric light, and to form foci of . 
 invisible rays so intense as to produce almost all the effects 
 obtainable in an ordinary fire. Combustible bodies were 
 burnt and refractory ones were raised to a white heat by the 
 concentrated invisible rays. Thus, by exalting their re- 
 frangibility, the invisible rays of the electric light were 
 rendered visible, and all the colors of the solar spectrum 
 were extracted from utter darkness. The extreme richness 
 of the electric light in invisible rays of low refrangibility 
 was demonstrated, one-ninth only of its radiation consisting 
 of luminous rays. The deadness of the optic nerve to those 
 invisible rays was proved, and experiments were then 
 added, to show that the bright and the dark rays of a 
 solid body raised gradually to intense incandescence, are 
 strengthened together ; intense dark heat being an inva- 
 riable accompaniment of intense white heat. A sun could 
 not be formed, or a meteorite rendered luminous, on any 
 other condition. The light-giving rays, constituting only a 
 small fraction of the total radiation, their unspeakable im- 
 portance to us is due to the fact that their periods are 
 attuned to the special requirements of the eye. 
 
 Among the vapors of volatile liquids vast differences 
 were also found to exist, as regards their powers of absorp- 
 tion. We followed various molecules from a state of liquid 
 to a state of gas, and found in both states of aggregation, 
 the power of the individual molecules equally asserted. 
 The position of a vapor as an absorber of radiant heat was 
 shown to be determined by that of the liquid from which it 
 is derived. Reversing our conceptions, and regarding the 
 molecules of gases and vapors not as the recipients, but as 
 the originators of wave-motion; not as absorbers but as 
 radiators ; it was proved that the powers of absorption and 
 radiation went hand in hand, the self-same chemical act 
 
RADIATION. 207 
 
 which rendered a body competent to intercept the waves 
 of ether, rendering it competent in the same degree to gen- 
 erate them. Perfumes were next subjected to examination, 
 and notwithstanding their extraordinary tenuity, they were 
 found vastly superior, in point of absorptive power, to the 
 body of the air in which they were diffused. We were led 
 thus slowly up to the examination of the most widely 
 diffused and most important of all vapors the aqueous 
 vapor of our atmosphere and we found in it a potent 
 absorber of the purely calorific rays. The power of this 
 substance to influence climate, and its general influence 
 on the temperature of the earth, were then briefly dwelt 
 upon. A cobweb spread above a blossom is sufficient to 
 protect it jrom nightly chill ; and thus the aqueous vapor 
 of our air, attenuated as it is, checks the drain of terrestrial 
 heat, and saves the surface of our planet from the refriger- 
 ation which would assuredly accrue, were no such sub- 
 stance interposed between it and the voids of space. We 
 considered the influence of vibrating period and molecular 
 form on absorption and radiation, and finally deduced, from 
 its action upon radiant heat, the exact amount of carbonic 
 acid expired by the human lungs. 
 
 Thus in brief outline were placed before you some of 
 the results of recent inquiries in the domain of Radiation, 
 and my aim throughout has been to raise in your minds 
 distinct physical images of the various processes involved 
 in our researches. It is thought by some that natural 
 science has a deadening influence on the imagination, and 
 a doubt might fairly be raised as to the value of any study 
 which would necessarily have this effect. But the experi- 
 ence of the last hour must, I think, have convinced you 
 that the study of natural science goes hand in hand with 
 the culture of the imagination. Throughout the greater 
 part of this discourse we have been sustained by this 
 faculty. We have been picturing atoms, and molecules, 
 
208 FRAGMENTS OF SCIENCE. 
 
 and vibrations, and waves, which eye has never seen nor 
 ear heard, and which can only be discerned by the exercise 
 of imagination. This, in fact, is the faculty which enables 
 us to transcend the boundaries of sense, and connect the 
 phenomena of our visible world with those of an invisible 
 one. Without imagination we never could have risen to 
 the conceptions which have occupied us here to-day ; and 
 in proportion to your power of exercising this faculty aright, 
 and of associating definite mental images with the terms 
 employed, will be the pleasure and the profit which you 
 will derive from this lecture. The outward facts of Nature 
 are insufficient to satisfy the mind. We cannot be content 
 with knowing that the light and heat of the sun illuminate 
 and warm the world. We are led irresistibly to inquire 
 what is light, and what is heat ? and this question leads us 
 at once out of the region of sense into that of imagination. 
 Thus pondering, and questioning, and striving to sup- 
 plement that which is felt and seen, but which is incom- 
 plete, by something unfelt and unseen which is necessary 
 to its completeness, men of genius have in part discerned, 
 not only the nature of light and heat, but also, through 
 them, the general relationship of natural phenomena. The 
 working power of Nature is the power of actual or poten- 
 tial motion, of which all its phenomena are but special 
 forms. This motion manifests itself in tangible and in in- 
 tangible matter, being incessantly transferred from the one 
 to the other, and incessantly transformed by the change. 
 It is as real in the waves of the ether as in the waves of 
 the sea ; the latter, derived as they are from winds, which 
 in their turn are derived from the sun, being nothing more 
 than the heaped-up motion of the former. It is the calo- 
 rific waves emitted by the sun which heat our air, produce 
 our winds, and hence agitate our ocean. And whether they 
 break in foam upon the shore, or rub silently against the 
 ocean's bed, or subside by the mutual friction of their own 
 
RADIATION. 209 
 
 parts, the sea-waves, which cannot subside without pro- 
 ducing heat, finally resolve themselves into waves of ether, 
 thus regenerating the motion from which their temporary 
 existence was derived. This connection is typical. Nature 
 is not an aggregate of independent parts, but an organic 
 whole. If you open a piano and sing into it, a certain 
 string will respond. Change the pitch of your voice ; the 
 first string ceases to vibrate, but another replies. Change 
 again the pitch ; the first two strings are silent, while an- 
 other resounds. Now, in altering the pitch you simply 
 change the form of the motion communicated by your vocal 
 chords to the air, one string responding to one form, and 
 another to another. And thus is sentient man acted on by 
 Nature, the optic, the auditory, and other nerves of the 
 human body being so many strings differently tuned and 
 responsive to different forms of the universal power. 
 
 NOTE. The statements regarding the action of aqueous vapor, made 
 in sections 12 and 13 of this Lecture, have been controverted by the late 
 Professor Magnus, of Berlin. I therefore wish the reader to hold in sus- 
 pension his judgment of these two sections until new light can be thrown 
 upon the subject. This will soon be done. 
 
IX. 
 ON RADIANT HEAT 
 
 IN RELATION TO THE COLOR AND CHEMICAL CONSTITUTION 
 OF BODIES. 
 
 A DISCOURSE. 
 
 DELIVEKED IN THE EOYAL INSTITUTION OF GKEAT BKITAIN. 
 
 January 19, 1866. 
 
" I took a number of little square pieces of broadcloth from a tailor's 
 pattern-card, of various colors. They were black, deep blue, lighter blue, 
 green, purple, red, yellow, white, and other colors, or shades of color. I 
 laid them all out upon the snow on a bright, sunshiny morning. In a few 
 hours (I cannot now be exact as to the time), the black, being warmed 
 most by the sun, was sunk so low as to be below the stroke of the sun's 
 rays ; the dark blue almost as low, the lighter blue not quite so much as 
 the dark, the other colors less as they were lighter. The white remained 
 on the surface of the snow, not having entered it at all. 
 
 " What signifies philosophy that does not apply to some use ? May 
 we not learn from hence that black clothes are not so fit to wear in a hot, 
 sunny climate or season as white ones ; because in such clothes the body 
 is more heated by the sun when we walk abroad, and are at the same time 
 heated by the exercise, which double heat is apt to bring on putrid, dan- 
 gerous fevers ? That soldiers and seamen, who must march and labor in 
 the sun, should, in the East or West Indies, have a uniform of white ? 
 That summer hats for men or women should be white, as repelling that 
 heat which gives headaches to so many, and to some the fatal stroke that 
 the French call coup de soleil ? That the ladies' summer hats, however, 
 should be lined with black, as not reverberating on their faces those rays 
 which are reflected upward from the earth or water ? That the putting 
 of a white cap of paper or linen within the crown of a black hat, as most 
 do, will not keep out the heat, though it would if placed without ? That 
 fruit walls being blacked may receive so much heat from the sun in the 
 daytime as to continue warm in some degree through the night, and 
 thereby preserve the fruit from frosts, or forward its growth with sun- 
 dry other particulars of greater or less importance that will occur from 
 time to time to attentive minds ? " 
 
 BENJAMIN FRANKLIN, 
 Letter to Miss Mary Stevenson. 
 
IX. 
 
 ON RADIANT HEAT IN RELATION TO THE COLOR 
 AND CHEMICAL CONSTITUTION OF BODIES. 
 
 ONE of the most important functions of physical sci- 
 ence, considered as a discipline of the mind, is to enable us by 
 means of the tangible processes of Nature to apprehend the 
 intangible. The tangible processes give direction to the 
 line of thought ; but this once given, the length of the line 
 is not limited by the boundaries of the senses. Indeed, the 
 domain of the senses in Nature is almost infinitely small in 
 comparison with the vast region accessible to thought 
 which lies beyond them. From a few observations of a 
 comet, when it comes within the range of his telescope, an 
 astronomer can calculate its path in regions which no tele- 
 scope can reach ; and in like manner, by means of data 
 furnished in the narrow world of the senses, we make our- 
 selves at home in other and wider worlds, which can be 
 traversed by the intellect alone. 
 
 From the earliest ages the questions, " What is light ? " 
 and " What is heat ? " have occurred to the minds of men ; 
 but these questions never would have been answered had 
 they not been preceded by the question, " What is sound ? " 
 Amid the grosser phenomena of acoustics the mind was 
 first disciplined, conceptions being thus obtained from 
 direct observation, which were afterward applied to phe- 
 nomena of a character far too subtle to be observed directly. 
 Sound we know to be due to vibratory motion. A vibrating 
 
214 FRAGMENTS OF SCIENCE. 
 
 tuning-fork, for example, moulds the air round it into un- 
 dulations or waves, which speed away on all sides with a 
 certain measured velocity, impinge upon the drum of the 
 ear, shake the auditory nerve, and awake in the brain the 
 sensation of sound. When sufficiently near a sounding 
 body, we can feel the vibrations of the air. A deaf man, 
 for example, plunging his hand into a bell when it is 
 sounded, feels through the common nerves of his body those 
 tremors which, when imparted to the nerves of healthy ears, 
 are translated into sound. There are various ways of ren- 
 dering those sonorous vibrations not only tangible but 
 visible; and it was not until numberless experiments of 
 this kind had been executed, that the scientific investigator 
 abandoned himself wholly, and without a shadow of uncer- 
 tainty, to the conviction that what is sound within us is, 
 outside of us, a motion of the air. 
 
 But once having established this fact once having 
 proved beyond all doubt that the sensation of sound is 
 produced by an agitation of the nerve of the ear, the 
 thought soon suggested itself that light might be due to 
 an agitation of the nerve of the eye. This was a great step 
 in advance of that ancient notion which regarded light as 
 something emitted by the eye, and not as any thing 
 imparted to it. But if light be produced by" an agitation 
 of the optic nerve or retina, what is it that produces the 
 agitation ? Newton, you know, supposed minute particles 
 to be shot through the humors of the eye against the 
 retina, which hangs like a target at the back of the eye. 
 The impact of these particles against the target, Newton 
 believed to be the cause of light. But Newton's notion 
 has not held its ground, being entirely driven from the 
 field by the more wonderful and far more philosophical 
 notion that light, like sound, is a product of wave-motion. 
 
 The domain in which this motion of light is carried on 
 lies entirely beyond the reach of our senses. The waves 
 
RADIANT HEAT AND ITS RELATIONS. 215 
 
 of light require a medium for their formation and propaga- 
 tion, but we cannot see, or feel, or taste, or smell this 
 medium. How, then, has its existence been established ? 
 By showing that by the assumption of this wonderful 
 intangible ether all the phenomena of optics are accounted 
 for with a fulness and clearness and conclusiveness which 
 leave no desire of the intellect unfulfilled. When the law 
 of gravitation first suggested itself to the mind of Newton, 
 what did he do ? He set himself to examine whether it 
 accounted for all the facts. He determined the courses of 
 the planets ; he calculated the rapidity of the moon's fall 
 toward the earth; he considered the precession of the 
 equinoxes, the ebb and flow of the tides, and found all 
 explained by the law of gravitation. He therefore regarded 
 this law as established, and the verdict of science sub- 
 sequently confirmed his conclusion. On similar, and, if 
 possible, on stronger grounds, we found our belief in the 
 existence of the universal ether. It explains facts far 
 more various and complicated than those on which New- 
 ton based his law. If a single phenomenon could be 
 pointed out which the ether is proved incompetent to 
 explain, we should have to give it up ; but no such phe- 
 nomenon has ever been pointed out. It is, therefore, at 
 least as certain that space is filled with a medium by 
 means of which suns and stars diffuse their radiant power, 
 as that it is traversed by that force which holds, not only 
 our planetary system, but the immeasurable heavens them- 
 selves, in its grasp. 
 
 There is no more wonderful instance than this of the 
 production of a line of thought from the world of the senses 
 into the region of pure imagination. I mean by imagination 
 here, not that play of fancy which can give to " airy nothing 
 a local habitation and a name," but that power which 
 enables the mind to conceive realities which lie beyond the 
 range of the senses to present to itself distinct physical 
 
216 FRAGMENTS OF SCIENCE. 
 
 images of processes which, though mighty in the aggregate 
 beyond all conception, are so minute individually as to 
 elude all observation. It is the waves of air excited by 
 this tuning-fork which render its vibrations audible. It is 
 the waves of ether sent forth from those lamps overhead 
 which render them luminous to us fout so minute are these 
 waves, that it would take from 30,000 to 60,000 of them 
 placed end to end to cover a single inch. Their number, 
 however, compensates for their minuteness. Trillions of 
 them have entered your eyes and hit the retina at the back 
 of the eye in the time consumed in the utterance of the 
 shortest sentence of this discourse. This is the steadfast 
 result of modern research ; but we never could have reached 
 it without previous discipline. ("We never could have 
 measured the waves of light, nor even imagined them to 
 exist, had we not previously exercised ourselves among the 
 waves of sound. Sound and light are now mutually help- 
 ful, the conceptions of each being expanded, strengthened, 
 and denned, by the conceptions of the other. 
 
 The ether which conveys the pulses of light and heat 
 not only fills the celestial spaces, bathing the sides of suns 
 and planets, but it also encircles the atoms of which these 
 suns and planets are composed. It is the motion of these 
 atoms, and not that of any sensible parts of bodies, that 
 the ether conveys ; it is this motion that constitutes the 
 objective cause of what in our sensations are light and 
 /neat. An atom, then, sending its pulses through the infi- 
 \y nite ether, resembles a tuning-fork sending its pulses 
 through the air. Let us look for a moment at this thrilling 
 ether, and briefly consider its relation to the bodies whose 
 vibrations it conveys. Different bodies, when heated to the 
 same temperature, possess very different powers of agitat- 
 ing the ether: some are good radiators, others are bad 
 radiators ; which means that some are so constituted as to 
 communicate their motion freely to the ether, producing 
 
RADIANT HEAT AND ITS RELATIONS. 217 
 
 therein powerful undulations; while others are unable 
 thus to communicate their motion, but glide through the 
 ether without materially disturbing its repose. Recent 
 experiments have proved that elementary bodies, except 
 under certain anomalous conditions, belong to the class of 
 bad radiators. An atom vibrating in the ether resembles 
 this naked tuning-fork vibrating in the air. The amount 
 of motion communicated to the air by these thin prongs is 
 too small to evoke at any distance the sensation of sound. 
 But if we permit the atoms to combine chemically and 
 form molecules, the result in many cases is an enormous 
 change in the power of radiation. The amount of ethereal 
 disturbance produced by the combined atoms of a body 
 may be many thousand times that produced by its constitu- 
 ent atoms when uncombined. The effect is roughly typi- 
 fied by this tuning-fork when connected with its resonant 
 case. The fork and its case now swing as a compound 
 system, and the vibrations which were before inaudible, are 
 now the source of a musical sound so powerful that it 
 might be plainly heard by thousands at once. The fork 
 and its case combined may be roughly regarded as a good 
 radiator of sound. 
 
 The pitch of a musical note depends upon the rapidity 
 of its vibrations, or, in other words, on the length of its 
 waves. Now, the pitch of a note answers to the color of 
 light. Taking a slice of white light from the beam of an 
 electric lamp, I cause that light to pass through an arrange- 
 ment of prisms. It is decomposed, and we have the effect 
 obtained by Newton, who first unrolled the solar beam into 
 the splendors of the solar spectrum. At one end of this 
 spectrum we have red light, at the other violet, and be- 
 tween those extremes lie the other prismatic colors. As 
 we advance along the spectrum from the red to the violet, 
 the pitch of the light if I may use the expression height- 
 ens, the sensation of violet being produced by a more rapid 
 10 
 
218 FRAGMENTS OF SCIENCE. 
 
 succession of impulses than that which produces the im- 
 pression of red. The vibrations of the violet are about 
 twice as rapid as those of the red ; in other words, the 
 range of the visible spectrum is about an octave. 
 
 There is no solution of continuity in this spectrum ; 
 one color changes into another by insensible gradations. 
 It is as if an infinite number of tuning-forks, of gradually 
 augmenting pitch, were vibrating at the same time. But 
 turning to another spectrum that, namely, obtained from 
 the incandescent vapor of silver you observe that it con- 
 sists of two narrow and intensely luminous green bands. 
 Here it is as if two forks only, of slightly different pitch, 
 were vibrating. The length of the waves which produce 
 this first band is such that 47,460 of them, placed end to 
 end, would fill an inch. The waves which produce the 
 second band are a little shorter ; it would take of these 
 47,920 to fill an inch. In the case of the first band, the 
 number of impulses imparted in one second to every 
 eye which now sees it, is 577 millions of millions ; while 
 the number of impulses imparted in the same time by the 
 second band is 600 millions of millions. I now cast upon 
 - the screen before you the beautiful stream of green light 
 from which these bands were derived. This luminous 
 stream is the incandescent vapor of silver. The rates of 
 vibration of the atoms of that vapor are as rigidly fixed as 
 those of two tuning-forks; and to whatever height the 
 temperature of the vapor may be raised, the rapidity of its 
 vibrations, and consequently its color, which wholly de- 
 pends upon that rapidity, remains unchanged. 
 
 The vapor of water, as well as the vapor of silver, has 
 its definite periods of vibration, and these are such as to 
 disqualify the vapor, when acting freely as such, from 
 being raised to a white heat. The oxyhydrogen flame, for 
 example, consists of hot aqueous vapor. It is scarcely 
 visible in the air of this room, and it would be still less 
 
RADIANT HEAT AND ITS RELATIONS. 219 
 
 visible if we could burn the gas in a clean atmosphere. 
 But the atmosphere, even at the summit of Mont Blanc, 
 is dirty ; in London it is more than dirty ; and the burning 
 dirt gives to this flame the greater portion of its present 
 light. But the heat of the flame is enormous. Cast-iron 
 fuses at a temperature of 2,000 Fahr. A piece of platinum 
 is heated to vivid redness at a distance of two inches be- 
 yond the visible termination of the flame. The vapor 
 which produces incandescence is here absolutely dark. In 
 the flame itself the platinum is raised to dazzling white- 
 ness, and is finally pierced by the flame. "When this flame 
 impinges on a piece of lime, we have the dazzling Drum- 
 mond light. But the light is here due to the fact that 
 when it impinges upon the solid body, the vibrations ex- 
 cited in that body by the flame are of periods different from 
 its own. 
 
 Thus far we have fixed our attention on atoms and 
 molecules in a state of vibration, and surrounded by a 
 medium which accepts their vibrations, and transmits them 
 through space. But suppose the waves generated by one 
 system of molecules to impinge upon another system, how 
 will the waves be affected ? Will they be stopped, or will 
 they be permitted to pass ? Will they transfer their mo- 
 tion to the molecules on which they impinge, or will they 
 glide round the molecules, through the intermolecular 
 spaces, and thus escape ? 
 
 The answer to this question depends upon a condition 
 which may be beautifully exemplified by an experiment on 
 sound. These two tuning-forks are tuned absolutely alike. 
 They vibrate with the same rapidity, and mounted thus 
 upon their resonant stands, you hear them loudly sounding 
 the same musical note. I stop one of the forks, and throw 
 the other into strong vibration. I now bring that other 
 near the silent fork, but not into contact with it. Allow- 
 ing them to continue in this position for four or five seconds, 
 
220 FRAGMENTS OF SCIENCE. 
 
 I stop the vibrating fork ; but the sound has not ceased. 
 The second fork has taken up the vibrations of its neigh- 
 bor, and is now sounding in its turn. I dismount one of 
 the forks, and permit the other to remain upon its stand. 
 I throw the dismounted fork into strong vibration, but you 
 cannot hear it sound. Detached from its stand the amount 
 of motion which it can communicate to the air is too small 
 to make itself sensible to the ear at any distance. I now 
 bring the dismounted fork close to the mounted one, but 
 not into actual contact with it. Out of the silence rises a 
 mellow sound. Whence comes it ? From the vibrations 
 which have been transferred from the dismounted fork to 
 the mounted one. 
 
 That motion should thus transfer itself through the air 
 it is necessary that the two forks should be in perfect unison. 
 If I place on one of the forks a morsel of wax not larger 
 than a pea, it is rendered thereby powerless to affect, or to 
 be affected by, the other. It is easy to understand this 
 experiment. The pulses of the one fork can affect the other, 
 because they are perfectly timed. A single pulse causes 
 the prong of the silent fork to vibrate through an infinitesi- 
 mal space. But just as it has completed this small vibra- 
 tion, another pulse is ready to strike it. Thus, the small 
 impulses add themselves together. In the five seconds 
 during which the forks were held near each other, the vi- 
 brating fork sent 1,280 waves against its neighbor, and 
 those 1,280 shocks, all delivered at the proper moment, all, 
 as I have said, perfectly timed, have given such strength 
 to the vibrations of the mounted fork as to render them 
 audible to you all. 
 
 Let me give you one other curious illustration ' of the 
 influence of synchronism on musical vibrations. Here are 
 three small gas-flames inserted in three glass tubes of dif- 
 ferent lengths. Each of these flames can be caused to emit 
 a musical note, the pitch of which is determined by the 
 
RADIANT HEAT AND ITS RELATIONS. 221 
 
 length of the tube surrounding the flame. The shorter the 
 tube the higher is the pitch. The flames are now silent 
 within their respective tubes, but each of them can be 
 caused to respond to a proper note sounded anywhere in 
 this room. Here is an instrument called a siren, by which 
 a powerful musical note can be produced. Beginning with 
 a note of low pitch, and ascending gradually to a higher 
 one, I finally reach the note of the flame in the longest tube. 
 The moment it is reached, the flame bursts into song. But 
 the other flames are still silent within their tubes. I urge 
 the instrument on to higher notes ; the second flame has 
 now started, and the third alone remains. But a still higher 
 note starts it also. Thus, as the sound of the siren rises 
 gradually in pitch, it awakens every flame in passing, by 
 striking it with a series of waves whose periods of recur- 
 rence are similar to its own. 
 
 Now the wave-motion from the siren is in part taken up 
 by the flame which synchronizes with the waves ; and had 
 these waves to impinge upon a multitude of flames, instead 
 of upon one flame only, the transference might be so great 
 as to absorb the whole of the original wave-motion. (JLet 
 us apply these facts to radiant heat. This blue flame is 
 the flame of carbonic oxide ; this transparent gas is carbonic- 
 acid gas. In the blue flame we have carbonic acid intensely 
 heated ; or, in other words, in a state of intense vibration. 
 It thus resembles the sounding tuning-fork, while this cold 
 carbonic acid resembles the silent one. What is the con- 
 sequence ? Through the synchronism of the hot and cold 
 gas transmission of motion through the gas is prevented ; 
 it is all transferred. The cold gas is intensely opaque to 
 the radiation from this particular flame, though highly 
 transparent to heat of every other kind. We are here 
 manifestly dealing with that great principle which lies at 
 the basis of spectrum analysis, and which has enabled 
 scientific men to determine the substances of which the sun, 
 
222^ FRAGMENTS OF SCIENCE. 
 
 the stars, and even the nebulae, are composed : the principle, 
 namely, that a body which is competent to emit any ray, 
 whether of heat or light, is competent in the same degree 
 to absorb that ray. The absorption depends on the syn- 
 chronism which exists between the vibrations of the atoms 
 from which the rays, or more correctly the waves, issue, and 
 those of the atoms against which they impinge. 
 
 To its incompetence to emit white light, aqueous vapor 
 adds incompetence to absorb white light. It cannot, for 
 example, absorb the luminous rays of the sun, though it 
 can absorb the non-luminous rays of the earth. This in- 
 competence of aqueous vapor to absorb luminous rays is 
 shared by water and ice in fact, by all really transparent 
 substances. Their transparency is due to their inability to 
 absorb luminous rays. The molecules of such substances 
 are in dissonance with the luminous waves, and hence such 
 waves pass through transparent substances without dis- 
 turbing the molecular rest. A purely luminous beam, how- 
 ever intense may be its heat, is sensibly incompetent to 
 melt the smallest particle of ice. We can, for example, 
 converge a powerful luminous beam upon a surface covered 
 with hoar-frost without melting a single spicula of the ice- 
 crystals. How then, it may be asked, are the snows of the 
 Alps swept away by the sunshine of summer ? I answer 
 they are not swept away by sunshine at all, but by solar 
 rays which have no sunshine whatever in them. The lumi- 
 nous rays of the sun fall upon the snow-fields and are flashed 
 in echoes from crystal to crystal, but they find next to no 
 lodgment within the crystals. They are hardly at all ab- 
 sorbed, and hence they cannot produce fusion. But a body 
 of powerful dark rays is emitted by the sun, and it is these 
 rays that cause the glaciers to shrink and the snows to dis- 
 appear ; it is they that fill the banks of the Arve and Ar- 
 veyron, and liberate from their frozen captivity the Rhone 
 and the Rhine. 
 
RADIANT HEAT AND ITS RELATIONS. 223 
 
 Placing a concave silvered mirror behind the electric 
 light I converge its rays to a focus of dazzling brilliancy. 
 I place in the path of the rays, between the light and the 
 focus, a vessel of water, and now introduce at the focus a 
 piece of ice. The ice is not melted by the concentrated 
 beam which has passed through the water, though matches 
 are ignited at the focus and wood is set on fire. The pow- 
 erful heat, then, of this luminous beam is incompetent to 
 melt the ice. I withdraw the cell of water ; the ice imme- 
 diately liquefies, and you see the water trickling from it 
 in drops. I reintroduce the cell of water; the fusion is 
 arrested and the drops cease to fall. The transparent water 
 of the cell exerts no sensible absorption on the luminous 
 rays, still it withdraws something from the beam, which, 
 when permitted to act, is competent to melt the ice. This 
 something is the dark radiation of the electric light. Again, 
 I place a slab of pure ice in front of the electric lamp ; send 
 a luminous beam first through our cell of water and then 
 through the ice. By means of a lens an image of the slab 
 is cast upon a white screen. The beam, sifted by the water, 
 has no power upon the ice. But observe what occurs when 
 the water is removed ; we have here a star and there a star, 
 each star resembling a flower of six petals, and growing 
 visibly larger before our eyes. As the leaves enlarge their 
 edges become serrated, but there is no deviation from the 
 six-rayed type. We have here, in fact, the crystallization 
 of the ice inverted by the invisible rays of the electric beam. 
 They take the molecules down in this wonderful way, and 
 reveal to us the exquisite atomic structure of the substance 
 with which Nature every winter roofs our ponds and lakes. 
 
 Numberless effects, apparently anomalous, might be ad- 
 duced in illustration of the action of these lightless rays. 
 Here, for example, are two powders, both white, and undis- 
 tinguishable from each other by the eye. The luminous 
 rays of the lamp are unabsorbed by both powders from 
 
224 FRAGMENTS OF SCIENCE. 
 
 those rays they acquire no heat ; still one of the substances, 
 sugar, is heated so highly by the concentrated beam of the 
 electric lamp that it first smokes violently and then inflames, 
 while the other substance, salt, is barely warmed at the 
 focus. Here, again, are two perfectly transparent liquids 
 placed in a test-tube at the focus ; one of them boils in a 
 couple of seconds, while the other in a similar position is 
 hardly warmed. The boiling-point of the first liquid is 78 
 C., which is speedily reached; that of the second liquid is 
 only 48 C., which is never reached at all. These anoma- 
 lies are entirely due to the unseen element which mingles 
 with the luminous rays of the electric beam, and, indeed, 
 constitutes 90 per cent, of its calorific power. 
 
 I have here a substance by which these dark rays may 
 be detached from the total emission of the electric lamp. 
 This ray-filter is a black liquid that is to say, black as 
 pitch to the luminous, but bright as a diamond to the non- 
 luminous radiation. It mercilessly cuts off the former, but 
 allows the latter free transmission. I bring these invisible 
 rays to a focus at a distance of several feet frcm the electric 
 lamp ; the dark rays form there an invisible image of the 
 source from which they issue. By proper means this in- 
 visible image may be transformed into a visible one of 'daz- 
 zling brightness. I could, moreover, show you, if .time per- 
 mitted, how out of those perfectly dark rays we might ex- 
 tract, by a process of transmutation, all the colors of the 
 solar spectrum. I could also prove to you that those rays, 
 powerful as they are, and sufficient to fuse many metals, 
 may be permitted to enter the eye and to break upon the 
 retina without producing the least luminous impression. 
 
 The dark rays are now collected before you ; you see 
 nothing at their place of convergence ; with a proper ther- 
 mometer it could be proved that even the air at the focus 
 is just as cold as the surrounding air. And mark the con- 
 clusion to which this leads'. It proves the ether at the focus 
 
RADIANT HEAT AND ITS RELATIONS. 225 
 
 to be practically detached from the air that the most vio- 
 lent ethereal motion may there exist without the least 
 aerial motion. But though you see it not, there is suffi- 
 cient heat at that focus to set London on fire. The heat 
 there at the present, moment is competent to raise iron to a 
 temperature at which it throws off brilliant scintillations. 
 It can heat platinum to whiteness and almost fuse that re- 
 fractory metal. It actually can fuse gold, silver, copper, 
 and aluminium. The moment, moreover, that wood is 
 placed at the focus it bursts into a blaze. 
 
 It has been already affirmed that whether as regards ra- 
 diation or absorption the elementary atoms possess but little 
 power. This might be illustrated by a long array of facts ; 
 and one of the most singular of these is furnished by the 
 deportment of that extremely combustible substance, phos- 
 phorus, when placed at this dark focus. It is impossible to 
 ignite there a fragment of amorphous phosphorus. But or- 
 dinary phosphorus is a far quicker combustible, and its de- 
 portment to radiant heat is still more impressive. It may 
 be exposed to the intense radiation of an ordinary fire with- 
 out bursting into flame. It may also be exposed for twenty 
 or thirty seconds at an obscure focus of sufficient power to 
 raise platinum to a white heat, without ignition. Notwith- 
 standing the energy of the ethereal waves here concentrated, 
 notwithstanding the extremely inflammable character of 
 the elementary body exposed to their action, the atoms of 
 that body refuse to partake of the motion of the waves, and 
 consequently cannot be powerfully affected by their heat. 
 
 The knowledge which we now possess will enable us to 
 analyze with profit a practical question. White dresses are 
 worn in summer because they are found to be cooler than 
 dark ones. The celebrated Benjamin Franklin made the fol- 
 lowing experiment: He placed bits of cloth of various colors 
 upon snow, exposed them to direct sunshine, and found that 
 they sank to different depths in the snow. The black cloth 
 
226 FRAGMENTS OF SCIENCE. 
 
 sank the deepest, the white did not sink at all. Franklin 
 inferred from his experiment that black bodies are the best 
 absorbers, and white ones the worst absorbers, of radiant 
 heat. Let us test the generality of this conclusion. I have 
 here two cards, one of which is coated with a very dark 
 powder, and the other with a perfectly white one. I place 
 the powdered surfaces before the fire, and leave them there 
 until they have acquired as high a temperature as they can 
 attain in this position. Which of the cards is most highly* 
 heated ? It requires no thermometer to answer this ques- 
 tion ? Simply pressing the back of the card, on w r hich the 
 white powder is strewn, against my cheek or forehead, I 
 find it intolerably hot. Placipg the dark card in the same 
 position I find it cool. The white powder has absorbed far 
 more heat than the dark one. This simple result abolishes 
 a hundred conclusions which have been hastily drawn from 
 the experiment of Franklin. Again, here are suspended 
 two delicate mercurial thermometers at the same distance 
 from a gas-flame. The bulb of one of them is covered by a 
 dark substance, the bulb of the other by a white one. Both 
 bulbs have received the radiation from the flame, but the 
 white bulb has absorbed most, and its mercury stands much 
 higher than that of the other thermometer. I might vary 
 this experiment in a hundred ways, and show you that from 
 the darkness of a body you can draw no certain conclusion 
 regarding its power of absorption. 
 
 The reason of this simply is, that color gives us intelli- 
 gence of only one portion, and that the smallest one, of 
 the rays impinging on the colored body. Were the rays 
 all luminous we might with certainty infer from the color 
 of a body its power of absorption ; but the great mass of 
 the radiation from our fire, our gas-flame, and even from 
 the sun itself, consists of invisible calorific rays, regarding 
 which color teaches us nothing. A body may be highly trans- 
 parent to one class of rays, and highly opaque to the other 
 
RADIANT HEAT AND ITS RELATIONS. 227 ' 
 
 class. Thus the white powder, which has shown itself so 
 powerful an absorber, has been specially selected on account 
 of its extreme perviousness to the visible rays, and its ex- 
 treme imperviousness to the invisible ones ; while the dark 
 powder was chosen on account of its extreme transparency 
 to the invisible, and its extreme opacity to the visible rays. 
 In the case of the radiation from our fire, about 98 per cent, 
 of the whole emission consists of invisible rays ; the body, 
 therefore, which was most opaque to these triumphed as 
 an absorber, though that body was a white one. ^ 
 
 I would here invite you to consider the manner in 
 which we obtain from natural facts what may be called 
 their intellectual value. Throughout the processes of Na- 
 ture there is interdependence and harmony, and the main 
 value of our science, considered as a mental discipline, con- / 
 sists in the tracing of this interdependence and the demon- 
 stration of this harmony. The outward and visible phe- 
 nomena are with us the counters of the intellect ; and our 
 science would not be worthy of its name and fame if it 
 halted at facts, however practically useful, and neglected 
 the laws which accompany and rule phenomena. Let us 
 endeavor, then, to extract from the experiment of Franklin 
 its full intellectual value, calling to our aid the knowledge 
 which our predecessors have already stored. Let us im- 
 agine two pieces of cloth of the same texture, the one 
 black and the other white, placed upon sunned snow. Fix- 
 ing our attention on the white piece, let us inquire whether 
 there is any reason to expect that it will sink into the 
 snow at all. There is knowledge at hand which enables 
 us to reply at once in the negative. There is, on the con- 
 trary, reason to expect that after a sufficient exposure the 
 bit of cloth will be found on an eminence instead of in a 
 hollow ; that instead of a depression, we shall have a rela- 
 tive elevation of the bit of cloth. For, as regards the lu- 
 minous rays of the sun, the cloth and the snow are alike 
 
228 FRAGMENTS OF SCIENCE. 
 
 powerless; the one cannot be warmed, nor the other 
 melted, by such rays. The cloth is white and the snow is 
 white, because then: confusedly mingled particles and fibres 
 are incompetent to absorb luminous rays. Whether, then, 
 the cloth will sink or not depends entirely upon the dark 
 rays of the sun. Now the substance which absorbs the 
 dark rays of the sun with the greatest avidity is ice or 
 snow, which is merely ice in powder. A less amount of 
 heat will be lodged in the cloth than in the surrounding 
 snow. The cloth must, therefore, act as a shield to the 
 snow on which it rests ; and in consequence of the more 
 rapid fusion of the exposed snow, the cloth must in due 
 time be left behind, perched upon an eminence like a gla- 
 cier-table. 
 
 But though the snow transcends the cloth both as a 
 radiator and absorber, it does not much transcend it. 
 Cloth is very powerful in both these respects. Let us 
 now turn our attention to the piece of black cloth,' the 
 texture and fabric of which I assume to be the r same as 
 that of the white. For our object being to compare the 
 effects of color, we must, in order to study this effect in its 
 purity, preserve all other conditions constant. Let us then 
 suppose the black cloth to be obtained from the dyeing 
 of the white. The cloth itself, without reference to the 
 dye, is nearly as good an absorber of heat as the snow 
 around it. But to the absorption of the dark solar rays 
 by the undyed cloth is now added the absorption of the 
 whole of the luminous rays, and this great additional in- 
 flux of heat is far more than sufficient to turn the balance 
 in favor of the black cloth. The sum of its actions on the 
 dark and luminous rays exceeds the action of the snow 
 on the dark rays alone. Hence the cloth will sink in the 
 snow, and this is the philosophy of Franklin's experi- 
 ment. 
 
 Throughout this discourse the main stress has been laid 
 
RADIANT HEAT AND ITS RELATIONS. 229 
 
 on chemical constitution, as influencing most powerfully 
 the phenomena of radiation and absorption. With regard 
 to gases, vapors, and to the liquids from which these va- 
 pors are derived, it has been proved by the most varied 
 and conclusive experiments that the acts of radiation and 
 absorption are molecular that they depend upon chemical 
 and not upon mechanical condition. In attempting to ex- 
 tend this principle to solids I was met by a multitude of 
 facts obtained by celebrated experimenters, which seemed 
 flatly to forbid such extension. Melloni, for example, found 
 the same radiant and absorbent power for chalk and lamp- 
 black. MM. Masson and Courtepee performed a most 
 elaborate series of experiments on chemical precipitates of 
 various kinds, and found that they one and all manifested 
 the same power of radiation. They concluded from their 
 researches, that where bodies are reduced to an extremely 
 fine state of division the influence of this state is so power- 
 ful as entirely to mask and override whatever influence 
 may be due to chemical constitution. 
 
 But it appears to me that through the whole of these 
 researches a serious oversight has run, the mere mention 
 of which will show you what caution is essential in the 
 operations of experimental philosophy. Let me state 
 wherein I suppose this oversight to consist. I have here 
 a metal cube with two of its sides brightly polished. I 
 fill the cube with boiling water and determine the quan- 
 tity of heat emitted by the two bright surfaces. One of 
 them far transcends the other as a radiator of heat. Both 
 surfaces appear to be metallic ; what, then, is the cause of 
 the observed difference in their radiative power ? Simply 
 this: I have coated, one of the surfaces with transparent 
 gum, through which, of course, is seen the metallic lustre 
 behind. Now this varnish, though so perfectly transparent 
 to luminous rays, is as opaque as pitch or lamp-black to 
 non-luminous ones. It is a powerful emitter of dark rays ; 
 
230 FRAGMENTS OF SCIENCE. 
 
 it is also a powerful absorber. While, therefore, at the 
 present moment it is copiously pouring forth radiant heat 
 itself, it does not allow a single ray from the metal behind 
 to pass through it. The varnish then, and not the metal, is 
 the real radiator. 
 
 Now Melloni, and Masson, and Courtepee, experimented 
 thus: they mixed their powders and precipitates with 
 gum-water, and laid them by means of a brush upon the 
 surfaces of a cube like this. True they saw their red pow- 
 ders red, their white ones white, and their black ones black, 
 but they saw these colors through the coat of varnish 
 which encircled every particle of their powders. When, 
 therefore, it was concluded that color had no influence on 
 radiation, no chance had been given to it of asserting its 
 influence ; when it was found that all chemical precipitates 
 radiated alike, it was the radiation from a varnish common 
 to them all which showed the observed constancy. Hun- 
 dreds, perhaps thousands, of experiments on radiant heat 
 have been performed in this way by various inquirers, but 
 I fear the work will have to be done over again. I am not, 
 indeed, acquainted with an instance in which an oversight 
 of so trivial a character has been committed in succession 
 by so many able men, and vitiated so large an amount of 
 otherwise excellent work. 
 
 Basing our reasonings, then, on demonstrated facts, we 
 arrive at the extremely probable conclusion that the envel- 
 ope of the particles, and not the particles themselves, was 
 the real radiator in the experiments just referred to. To 
 reason thus, and deduce their more or less probable conse- 
 quences from experimental facts, is an incessant exercise 
 of the student of physical science. But having thus fol- 
 lowed for a time the light of reason alone through a series 
 of phenomena, and emerged from them with a purely intel- 
 lectual conclusion, our duty is to bring that conclusion to 
 an experimental test. In this way we fortify our science, 
 
KADIANT HEAT AND ITS RELATIONS. 231 
 
 sparing no pains, shirking no toil to secure sound materials 
 for the edifice which it is our privilege to raise. 
 
 For the purpose of testing our conclusion regarding the 
 influence of the gum I take two powders of the same physi- 
 cal appearance ; one of them is a compound of mercury and 
 the other a compound of lead. On two surfaces of this 
 cube are spread these bright-red powders without varnish 
 of any kind. Filling the tube with boiling water, and de- 
 termining the radiation from the two surfaces, one of them 
 is found to emit thirty-nine rays, while the other emits 
 seventy-four. This, surely, is a great difference. Here, 
 however, is a second cube, having two of its surfaces coated 
 with the same powders, the only difference being that now 
 the powders are laid on by means of a transparent gum. 
 Both surfaces are now absolutely alike in radiative power. 
 Both of them emit somewhat more than was emitted by 
 either of the unvarnished powders, simply because the gum 
 employed is a better radiator than either of them. Exclud- 
 ing all varnish, and comparing white with white, I find 
 vast differences ; comparing black with black, I find them 
 also different; and when black and white are compared, in 
 some cases the black radiates far more than the white, 
 while in other cases the white radiates far more than the 
 black. Determining, moreover, the absorptive power of 
 those powders, it is found to go hand-in-hand with their 
 radiative power. The good radiator is a good absorber, and 
 the bad radiator is a bad absorber. From all this it is evi- 
 dent that as regards the radiation and absorption of non- 
 luminous heat, color teaches us nothing ; and that even as 
 regards the radiation of the sun, consisting as it does main- 
 ly of non-luminous rays, conclusions as to the influence of 
 color may be altogether delusive. This is the strict scien- 
 tific upshot of our researches. But it is not the less true 
 that in the case of wearing apparel and this for reasons 
 which I have given in analyzing the experiment of Frank- 
 
232 FRAGMENTS OF SCIENCE. 
 
 lin black dresses are more potent than white ones as ab- 
 sorbers of solar heat. 
 
 Thus, in brief outline, I have brought before you a few 
 of the results of recent inquiry. If you ask me what is the 
 use of them, I can hardly answer you, unless you define the 
 term use. If you meant to ask me whether those dark 
 rays which clear away the Alpine snows will ever be ap- 
 plied to the roasting of turkeys or the driving of steam- 
 engines, while affirming their power to do both, I would 
 frankly confess that they are not at present capable of 
 competing profitably with coal in these particulars. Still 
 they may have great uses unknown to me ; and when our 
 coal-fields are exhausted, it is possible that a more ethereal 
 race than ourselves may cook their victuals and perform 
 their work in this transcendental way. But is it necessary 
 that the student of science should have his labors tested by 
 their possible practical applications ? What is the prac- 
 tical value of Homer's Iliad? You smile, and possibly 
 think that Homer's Iliad is good as a means of culture. 
 There's the rub. The people who demand of science prac- 
 tical uses, forget, or do not know, that it also is great as a 
 means of culture ; that the knowledge of this wonderful 
 universe is a thing profitable in itself, and requiring no 
 practical application to justify its pursuit. But while the 
 student of Nature distinctly refuses to have his labors 
 judged by their practical issues, unless the term practical 
 be made to include mental as well as material good, he 
 knows full well that the greatest practical triumphs have 
 been episodes in the search after pure natural truth. The 
 electric telegraph is the standing wonder of this age, and 
 the men whose scientific knowledge and mechanical skill 
 have made the telegraph what it is are deserving of all 
 honor. In fact, they have their reward, both in reputation 
 and in those more substantial benefits which the direct ser- 
 vice of the public always carries in its train. But who, I 
 
RADIANT HEAT AND ITS RELATIONS. 233 
 
 would ask, put the soul into this telegraphic body ? Who 
 snatched from heaven the fire that flashes along the line ? 
 This, I am bound to say, was done by two men, the one a 
 dweller in Italy, 1 the other a dweller in England, and, there- 
 fore, not a thousand miles distant from the spot where I 
 now stand, 2 who never in their inquiries consciously set a 
 practical object before them whose only stimulus was 
 the fascination which draws the climber to a never-trodden 
 peak, and would have made Csesar quit his victories to 
 seek the sources of the Nile. That the knowledge brought 
 us by those prophets, priests, and kings of science, is what 
 the world calls useful knowledge, the triumphant applica- 
 tion of their discoveries proves. But science has another 
 function to fulfil, in the storing and the training of the hu- 
 man mind ; and I would base my appeal to you on the 
 poor specimen which has been brought before you this 
 evening, whether any system of education at the present 
 day can be deemed even approximately complete in which 
 the knowledge of Nature is neglected or ignored. 
 
 1 Volta. 2 Faraday. 
 
X. 
 
 ON CHEMICAL RAYS AND THE STRUCT- 
 URE AND LIGHT OF THE SKY. 
 
 A DISCOURSE. 
 
 DELIVERED IN THE ROYAL INSTITUTION OF GREAT BEITAIN. 
 On Friday, January 15, 1869. 
 
" This is a very mysterious and a very beautiful phenomenon when 
 observed by the aid of a polariscope, consisting of a tourmaline plate, 
 with a slice of Iceland crystal or nitre, cut at right angles to the optic 
 axis, and applied on the side of the tourmaline farthest from the eye. In 
 a cloudless day, if the sky be explored in all parts by looking through 
 this compound plate, the polarized rings will be seen developed with more 
 or less intensity in every region but that nearest the sun and that most 
 distant from it the maximum of polarization taking place on a zone of 
 the sky 90 from the sun, or in a great circle, having the sun for one of 
 its poles, so that the cause of polarization is evidently a reflection of the 
 sun's light on something. The question is, on what ? Were the angle of 
 maximum polarization 76 we should look to water or ice for the reflect- 
 ing body. But though we were once of this opinion (art. Light, Encycl. 
 Metropol. 1143), careful observation has satisfied us that 90, or there- 
 abouts, is the correct angle, and that therefore, whatever be the body on 
 which the light has been reflected, if polarized by a single reflection, the 
 c polarizing angle ' must be 45, and the index of refraction, which is the 
 tangent of that angle, unity ; in other words, the reflection would require 
 to be made in air upon air ! The only imaginable way in which this could 
 happen would be at the plane of contact of two portions of air differently 
 heated, such as might be supposed to occur at almost every point of the 
 atmosphere in a bright sunny day ; but against this there seems to be an 
 insuperable objection. The polarization is most regular and complete, as 
 we have lately been able to satisfy ourselves under the most favorable 
 possible atmospheric conditions, after sunset, in the bright twilight of a 
 summer night, with the sun some degrees below the horizon, and long 
 after all the tremor and turmoil of the air, due to irregular heating, must 
 have completely subsided. On the other hand, if effected by several suc- 
 cessive reflections, what is to secure a large majority of them being in 
 one plane (in which case only their polarizing effect would accumulate) ; 
 and of those which become ultimately effective, what, is there to deter- 
 mine an ultimate deviation of 90 as that of the maximum ? The more 
 the subject is considered, the more it will be found beset with difficulties ; 
 and its explanation, when arrived at, will probably be found to carry 
 with it that of the blue color of the sky itself." 
 
 SIR JOHN HERSCHEL. 
 
X. 
 
 ON CHEMICAL BAYS AND THE. STRUCTURE AND 
 LIGHT OF THE SKY. 
 
 THE first physical investigation of any importance in 
 which, jointly with my friend Professor Knoblauch, I took 
 part, bore the title : " The Magneto-optic Properties of 
 Crystals, and the Relation of Magnetism and Diamagnetism 
 to Molecular Arrangement." 1 This investigation compelled 
 me to reflect upon the structure of crystals, on their optical 
 properties in relation to that structure, and more particu- 
 larly on the striking phenomena exhibited by many of 
 them in the field of a sufficiently powerful magnet. These 
 were evidently due to the manner in which the molecules 
 of the crystals were built together by the force of- crystal- 
 lization ; and it was natural, if not necessary for me, to em- 
 ploy such strength of imagination as I possessed in obtain- 
 ing a mental picture of this molecular architecture. The 
 inquiry gave a tinge and bias to my subsequent scientific 
 thought, rendering, as it did, the conceptions and pursuits 
 of molecular physics pleasant to me. Its influence is to be 
 traced in most of my scientific work. The first lecture, for 
 example, which I ever delivered in this theatre, was " On 
 the Influence of Material Aggregation on the Manifestations 
 of Force ; " by " material aggregation " being meant the 
 way in which, by Nature or by Art, the molecules of mat- 
 
 1 Philosophical Magazine, July, 1850. 
 
238 FKAGMENTS OF SCIENCE. 
 
 ter are arranged together. In 1853 I also published a 
 paper " On Molecular Influences," in which common heat 
 was made the explorer of organic structure. In the " Ba- 
 kerian Lecture," given before the Royal Society in 1855, 
 the same idea and phraseology crop out. The Bakerian 
 Lecture for 1864 bears the title " Contributions to Molec- 
 ular Physics." And all through the investigations which 
 have occupied me during the last ten years, my wish and 
 aim have been to make radiant heat an instrument by 
 which to lay hold of the ultimate particles of matter. 
 
 The labors now to be considered lie in the same direc- 
 tion. In the researches just referred to, tubes of glass and 
 brass were employed, called, for the sake of distinction, 
 " experimental tubes," in which radiant heat was acted 
 upon by the gases and vapors subjected to examination. 
 Two or three months ago, with a view of seeing what oc- 
 curred within these tubes on the entrance of the gases or 
 vapors, it was found necessary to intensely illuminate their 
 interiors. The source of illumination chosen was the elec- 
 tric light, the beam of which, converged by a suitable lens, 
 was sent along the axis of the tube. The dirt and filth in 
 which we habitually live were strikingly revealed by this 
 method 'of illumination. For, wash the tube as we might 
 with water, alcohol, acid, or alkali, until its appearance in 
 ordinary daylight was that of absolute purity, the delusive 
 character of this appearance was in most cases revealed by 
 the electric beam. In fact, in air so charged with sus- 
 pended matter as that which supplies our lungs in London, 
 it is not possible to be more than approximately clean. 
 
 Vapors of various kinds were sent into a glass experi- 
 mental tube, a yard in length, and about three inches in 
 diameter. As a general rule, the vapors were perfectly 
 transparent ; the tube, when they were present, appearing 
 as empty as when they were absent. In two or three cases, 
 however, a faint cloudiness showed itself within the tube. 
 
CHEMICAL RAYS. 239 
 
 This caused me a momentary anxiety, for I did not know 
 how far, in describing my previous experiments, actions 
 might have been ascribed to pure cloudless vapor, which 
 were really due to those newly-observed nebulas. Inter- 
 mittent discomfort, however, is the normal feeling of the 
 investigator ; for it drives him to closer scrutiny, to greater 
 accuracy, and often, as a consequence, to new discovery. 
 It was soon found that the nebulae revealed by the beam 
 were also generated by the beam, and the observation 
 opened a new door into that region inaccessible to sense, 
 which embraces so much of the intellectual life of the phys- 
 ical investigator. 
 
 "What are those vapors of which we have been speak- 
 ing ? They are aggregates of molecules^ or small masses of 
 matter, and every molecule is itself an aggregate of smaller 
 parts called atoms. A molecule of aqueous vapor, for ex- 
 ample, consists of two atoms of hydrogen and one of oxy- 
 gen. A molecule of ammonia consists of three atoms of 
 hydrogen, and one of nitrogen, and so of other substances. 
 Thus the molecules, themselves inconceivably small, are 
 made up of distinct parts still smaller. When, therefore, 
 a compound vapor is spoken of, the corresponding mental 
 image is an aggregate of molecules separated from each 
 other, though still exceedingly near, each of these being 
 composed of a group of atoms still nearer to each other. 
 So much for the matter which enters into our conception of 
 a vapor. 1 To this must now be added the idea of motion. 
 The molecules have motions of their own as wholes y their 
 constituent atoms have also motions of their own, which 
 are executed independently of those of the molecules ; just 
 
 1 Newton seemed to consider that the molecules might be rendered 
 visible by microscopes ; but of the atoms he appears to have entertained 
 a different opinion. He finely remarks : "It seems impossible to see 
 the more secret and noble works of Nature within the corpuscles, by 
 reason of their transparency." (Herschel, On Light, Art. 1145.) 
 
240 FKAGMENTS OF SCIENCE. 
 
 as the various movements of the earth's surface are exe- 
 cuted independently of the orbital revolution of our planet. 
 
 The vapor molecules are kept asunder by forces which, 
 virtually or actually, are forces of repulsion. Between 
 these elastic forces and the atmospheric pressure under 
 which the vapor exists, equilibrium is established as soon 
 as the proper distances between the molecules have been 
 assumed. If, after this, the molecules be urged nearer to 
 each 'other by a momentary force, they recoil as soon as 
 the force is expended. If they be separated more widely 
 apart, when the separating force ceases to act they again 
 approach each other. The case is different as regards the 
 constituent atoms. 
 
 And here let it be remarked, that we are now upon the 
 very outmost verge of molecular physics ; and that I am 
 attempting to familiarize your minds with conceptions 
 which have not yet obtained universal currency even among 
 chemists ; which many chemists, moreover, might deem un- 
 tenable. But, tenable or untenable, it is of the highest sci- 
 entific importance to discuss them. Let us, then, look men- 
 tally at our atoms grouped together to form a molecule. 
 Every atom is held apart from its neighbors by a force of 
 repulsion ; why, then, do not the mutually repellant mem- 
 bers of this group part company ? The molecules separate 
 from each other when the external pressure is lessened or 
 removed, but the atoms do not. The reason of this stabil- 
 ity is that two forces, the one attractive and the other re- 
 pulsive, are in operation between every two atoms ; and the 
 position of every atom its distance from its fellows is 
 determined by the equilibration of these two forces. If the 
 atoms come too near, repulsion predominates and drives 
 them apart; if too distant, attraction predominates and 
 draws them together. The point at which attraction and 
 repulsion are equal to each other is the atom's position of 
 equilibrium. If not absolutely cold and there is no such 
 
CHEMICAL RAYS. 241 
 
 thing as absolute coldness in our corner of Nature the 
 atoms are always in a state of vibration, their vibrations 
 being executed to and fro across their positions of equilib- 
 rium. 
 
 Into a vapor thus constituted, we have now to pour a 
 beam of light ; which most of you know to be a train of 
 minute waves, excited in, and propagated through, an al- 
 most infinitely attenuated and elastic medium, which fills 
 all space, and which we name the ether. It is hardly neces- 
 sary to remind you that these waves of light are not all of 
 the same size ; that some of them are much longer and 
 higher than others ; that the short waves and the long ones 
 move with the same rapidity through space, just as short 
 and long waves of sound travel with the same rapidity 
 through air, and that, therefore, the shorter waves must fol- 
 low each other in quicker succession than the longer ones ; 
 that the different rapidities with which the waves of light 
 impinge upon the retina, or optic nerve, give rise in con- 
 sciousness to differences of color / that there are, moreover, 
 numberless waves emitted by the sun and other luminous 
 bodies which reach the retina, but which are incompetent 
 to excite the sensation of light ; for, if the lengths of the 
 waves exceed a certain limit, or if they fall short of a cer- 
 tain other limit, they cannot generate vision. And it is to 
 be particularly borne in mind, that the capacity to excite 
 vision does not depend so much on the strength of the 
 waves as on their periods of recurrence. I have, as many 
 of you know, permitted waves to enter my own eye, which, 
 if their energy were that of light, would have instantly and 
 utterly ruined the optic nerve, but which failed to produce 
 any impression whatever upon consciousness, because their 
 periods were not those competent to excite the retina. 
 
 The elements of all the conceptions with which we shall 
 have subsequently to deal are now in your possession. And 
 you will observe that, though we are speaking of things 
 11 
 
242 FRAGMENTS OF SCIENCE. 
 
 which lie entirely beyond the range of the senses, the con- 
 ceptions are as truly mechanical as they would be if w^e 
 were dealing with ordinary masses of matter, and with 
 w r aves of sensible magnitude. No really scientific mind at 
 the present day will be disposed to draw a substantial dis- 
 tinction between chemical and mechanical phenomena. 
 They differ from each other as regards the magnitude of 
 the masses involved ; but in this sense the phenomena of 
 astronomy differ, also, from those of ordinary mechanics. 
 The main bent of the natural philosophy of a future age 
 will probably be to chasten into order, by subjecting it to 
 mechanical laws, the existing chaos of chemical phe- 
 nomena. 
 
 Whether we see rightly or wrongly whether our in- 
 tellection be real or imaginary it is of the utmost im- 
 portance in science to aim at perfect clearness in the de- 
 scription of all that comes, or seems to come, within the 
 range of the intellect. For, if we are right, clearness of 
 utterance forwards the cause of right ; while, if we are 
 wrong, it insures the speedy correction of error. In this 
 spirit, and with the determination at all events to speak 
 plainly, let us deal with our conceptions of ether-waves and 
 molecules. Supposing a wave, or a train of waves, to im- 
 pinge upon a molecule so as to urge all its parts with the 
 same motion, the molecule would move bodily as a whole, 
 but because they are animated by a common motion there 
 would be no tendency of its constituent atoms to separate 
 from each other. Differential motions among the atoms 
 themselves would be necessary to effect a separation, and 
 if such motions be not introduced by the shock of the 
 waves, there is no mechanical ground for the decomposition 
 of the molecule. 
 
 Thus the conception of the decomposition of compound 
 molecules by the waves of ether comes to us recommended 
 by a priori probability. But a closer examination of the 
 
CHEMICAL RAYS. 243 
 
 question compels us to supplement, if not materially to 
 qualify, this conception. It is a most remarkable fact that 
 the waves which have thus far been found most effectual in 
 shaking- asunder the atoms of compound molecules are 
 those of least mechanical power. Billows, to use a strong 
 comparison, are incompetent to produce effects which are 
 readily produced by ripples. It is, for example, the violet 
 and ultra-violet rays of the sun that are most effectual in 
 producing these chemical decompositions ; and, compared 
 with the red and ultra-red solar rays, the energy of these 
 " chemical rays " is infinitesimal. This energy would 
 probably in some cases have to be multiplied by millions 
 to bring it up to that of the ultra-red rays : and still the 
 latter are powerless where the smaller waves are potent. 
 We here observe a remarkable similarity between the be- 
 havior of chemical molecules and that of the human retina. 
 The energy transmitted to the eye from a candle-flame half 
 a mile distant is more than sufficient to inform conscious- 
 ness ; while waves of a different period, possessing twenty 
 thousand million times this energy, have been suffered to 
 impinge upon my own retina, with an absolute unconscious- 
 ness of any effect whatever mechanical, physiological, 
 chemical, or thermal. 
 
 If, then, the power of these smaller waves to unlock the 
 bonds of chemical union be not a result of their strength, it 
 must be, as in the case of vision, a result of their periods 
 of recurrence. But how are we to figure this action ? The 
 shock of a single wave produces no more than an infini- 
 tesimal effect upon an atom or a molecule. To produce a 
 larger effect, the motion must accumulate, and for wave- 
 impulses to accumulate, they must arrive in periods iden- 
 tical with the periods of vibration of the atoms on which 
 they impinge. In this case each successive wave finds the 
 atom in a position which enables that wave to add its shock 
 to the sum of the shocks of its predecessors. The effect is 
 
244 FRAGMENTS OF SCIENCE. 
 
 mechanically the same as that due to the timed impulses 
 of a boy upon a swing. The single tick of a clock has no 
 appreciable effect upon the unvibrating and equally long 
 pendulum of a distant clock ; but a succession of ticks, each 
 of which adds, at the proper moment, its infinitesimal push 
 to the sum of the pushes preceding it, will, as a matter of 
 fact, set the second clock going. So likewise a single puff 
 of air against the prong of a heavy tuning-fork produces 
 no sensible motion, and, consequently, no audible sound ; 
 but a succession of puffs, which follow each other in periods 
 identical with the tuning-fork's period of vibration, will 
 render the fork sonorous. I think the chemical action of 
 light is to be regarded in this way. Fact and reason point 
 to the conclusion that it is the heaping up of motion on the 
 atoms, in consequence of their synchronism with the shorter 
 waves, that causes them to part company. This I take to 
 be the mechanical cause of these decompositions which are 
 effected by the waves of ether. 
 
 And now let us return to that faint cloudiness already 
 mentioned, from which, as from a germ, these considerations 
 and speculations have sprung. It has been long known that 
 light effected the decomposition of a certain number of 
 bodies. The transparent iodide of ethyl, or of methyl, for 
 example, becomes brown and opaque on exposure to light, 
 through the discharge of its iodine. The art of photography 
 is founded on the chemical actions of light ; so that it is 
 well known that the effects for which the foregoing theoretic 
 considerations would have prepared us, are not only proba- 
 ble, but actual. 
 
 But the method employed in the experiments in which 
 the cloudiness above referred to was observed, and which 
 consists simply in offering the vapors of volatile substances 
 to the action of light, enables us not only to give such ex- 
 periments a beautfiul form, but also to give a great exten- 
 sion to the operations of light, or rather of radiant force, as 
 
CHEMICAL RAYS. 
 
 245 
 
 a chemical agent. It also enables us to illustrate in our 
 laboratories actions which have been hitherto performed 
 only in the laboratory of Nature. A few of these actions 
 of a representative character will now be brought before 
 you; and advantage will be taken of the fact that, in a 
 great number of cases, one or more of the substances into 
 which the waves of light break up , 
 compound molecules are compar- 
 atively involatile. The products 
 of decomposition require a greater 
 heat than is required by the va- 
 pors from which they are derived 
 to keep them in the gaseous form ; 
 and hence, if the space in which 
 these new bodies are liberated be 
 of the "proper temperature, they 
 will not remain in the vaporous 
 condition, but will precipitate 
 themselves as liquid particles, thus 
 forming visible clouds upon the 
 beam, to the action of which they 
 owe their existence. 
 
 The little flask, F, in the an- 
 nexed figure, is stopped by a cork, 
 pierced in two places. Through 
 one orifice passes a narrow glass 
 tube, #, which terminates imme- 
 diately under the cork; through 
 the other orifice passes a similar 
 tube, #, descending to the bottom of the little flask, which 
 is filled to a height of about an inch with a transparent 
 liquid. The name of this liquid is nitrite of amyl, in every 
 molecule of which we have 5 atoms of carbon, 11 of hydro- 
 gen, 1 of nitrogen, and 2 of oxygen. Upon this group the 
 waves of our electric light will be immediately let loose. 
 
246 FRAGMENTS OF SCIENCE. 
 
 The large horizontal tube that you see before you is called 
 an " experimental tube ; " it is connected with our small 
 flask ; between them, however, a stop-cock intervenes, by 
 means of which the passage between the flask and the ex- 
 perimental tube can be opened or closed at pleasure. The 
 other tube, passing through the cork of the flask and de- 
 scending into the liquid, is connected with a U-shaped ves- 
 sel, filled with fragments of clean glass, covered with sul- 
 phuric acid. In front of the U-shaped vessel is a narrow 
 tube stuffed with cotton-wool. At one end of the experi- 
 mental tube is our electric lamp ; and here, finally, is an air- 
 pump, by means of which the tube has been exhausted. 
 We are now ready for experiment. 
 
 Opening the cock cautiously, the air of the room passes, 
 in the first place, through the cotton-wool, which holds 
 back the numberless organic germs and inorgani6 dust- 
 particles floating in the atmosphere. The air, thus cleansed, 
 passes into the U-shaped vessel, where it is dried by the 
 sulphuric acid. It then descends through the narrow tube 
 to the bottom of the little flask, and escapes there through 
 a small orifice into the liquid. Through this it bubbles, 
 loading itself to some extent with the nitrite-of-amyl vapor, 
 and then the air and vapor enter the experimental tube 
 together. 
 
 The closest scrutiny would now fail to discover any 
 thing within this tube ; it is, to all appearance, absolutely 
 empty. The air and the vapor are both invisible. We 
 will permit the electric beam to play upon this mixture. 
 The lens of the lamp is so situated as to render the beam 
 slightly convergent, the focus being formed in the vapor 
 at about the middle of the tube. You will notice that the 
 tube remains dark for a moment after the turning on of the 
 beam ; but the chemical action will be so rapid that atten- 
 tion is requisite to mark this interval of darkness. I ignite 
 the lamp ; the tube for a moment seems empty ; but sud- 
 
CHEMICAL RAYS. 247 
 
 denly the beam darts through a luminous white cloud, which 
 has banished the preceding darkness. It has, in fact, shaken 
 asunder the molecules of the nitrite of amyl, and brought 
 down upon itself a shower of liquid particles which causes 
 it to flash forth in your presence like a solid luminous 
 spear. It is worth while to mark how this experiment 
 illustrates the fact that, however intense a luminous beam 
 may be, it remains invisible unless it has something to 
 shine upon. Space, though traversed by the rays from all 
 suns and all stars, is itself unseen. Not even the ether 
 which fills space, and whose motions are the light of the 
 universe, is itself visible. 
 
 You notice that the end of the experimental tube most 
 distant from the lamp is free from cloud. Now the nitrite- 
 of-amyl vapor is there also, but it is unaffected by the 
 powerful beam passing through it. Let us make the trans- 
 mitted beam more concentrated by receiving it on a con- 
 cave silver mirror, and causing it to return by reflection into 
 the tube. It is still powerless. Though a cone of light of 
 extraordinary intensity now traverses the vapor, no pre- 
 cipitation occurs, no trace of cloud is formed. Why ? Be- 
 cause the very small portion of the beam competent to de- 
 compose the vapor is quite exhausted by its work in the 
 frontal portions of the tube. The great body of the light 
 which remains, after this sifting out of the few effectual 
 rays, has no power over the molecules of nitrite of amyl. 
 We have here, strikingly illustrated, what has been already 
 stated regarding the influence of period, as contrasted with 
 that of strength. For the portion of the beam which is here 
 ineffectual has probably more than a million times the ab- 
 solute energy of the effectual portion. It is energy specially 
 related to the atoms that we here need, which specially re- 
 lated energy being possessed by the feeble waves, invests 
 them with their extraordinary power. When the experi- 
 mental tube is reversed so as to bring the undecomposed 
 
248 FRAGMENTS OF SCIENCE. 
 
 vapors under the action of the unsifted beam, you have in- 
 stantly this fine luminous cloud precipitated. 
 
 The light of the sun also effects the decomposition of 
 the uitrite-of-amyl vapor. A small room in the Royal 
 Institution, into which the sun shone, was partially dark- 
 ened, the light being permitted to enter through an open 
 portion of the window-shutter. In the track of the beam 
 was placed a large plano-convex lens, which formed a fine 
 convergent cone in the dust of the room behind it. The 
 experimental tube was filled in the laboratory, covered 
 with a black cloth, and carried into the partially-dark- 
 ened room. On thrusting one end of the tube into the 
 cone of rays behind the lens, precipitation within the cone 
 was copious and immediate. The vapor at the distant end 
 of the tube was shielded by that in front ; but on revers- 
 ing the tube, a second and similar splendid cone was pre- 
 cipitated. 
 
 Now let us pause for a moment and glance at the 
 ground over which we have passed. We have defined a 
 vapor as an aggregate of. molecules mutually repellent, 
 but hindered from indefinitely retreating from each other 
 by an external pressure. We have defined a molecule as 
 an aggregate of atoms maintained in positions of equi- 
 librium by the equalized action of two opposing forces, 
 and always oscillating to and fro across those positions. 
 We have defined a beam of light as a train of innumerable 
 waves, and have illustrated then* chemical action. We 
 have learned that it is not the magnitude or power of the 
 waves, so much as their periods of recurrence, that renders 
 them effectual as chemical agents. We have also seen 
 how the luminous beam is sifted by the vapor which it 
 decomposes, and deprived of those rays which are com- 
 petent to effect the decomposition. The effects, moreover, 
 obtained with the electric beam are also produced by the 
 beams of the sun. 
 
CHEMICAL RAYS. 249 
 
 And here I would ask you to make familiar to your 
 minds the idea that no chemical action can be produced by 
 a ray that does not involve the destruction of the ray. But 
 the term " ray " is unsatisfactory to us at present, when 
 our desire is to abolish all vagueness, and to affix a definite 
 physical significance to each of our terms. Abandoning 
 the term ray as loose and indefinite, we have to fix our 
 thoughts upon the waves of light ; and to render clear to 
 our minds that those waves which produce chemical action 
 do so by delivering up their own motion to the molecules 
 which they decompose. We have here forestalled to some 
 extent a question of great importance in molecular physics, 
 which, however, is worthy of being fixed more definitely in 
 your mind ; it is this : When the waves of ether are in- 
 tercepted by a compound vapor, is the motion of the waves 
 transferred to the molecules of the vapor, or to the atoms 
 of the molecules ? We have thus far leaned to the con- 
 clusion that the motion is communicated to the atoms ; for 
 if not to these individually, why should they be shaken 
 asunder ? The question, however, is capable of, and is 
 worthy of, another test, the bearing and significance of 
 which you will immediately appreciate. 
 
 As already explained, the molecules are held in their 
 positions of equilibrium by their mutual repulsion on the 
 one side, and by an external pressure on the other. Their 
 rate of vibration, if they vibrate at all, must depend upon 
 the elastic force which they mutually exert. If this force 
 be changed, the rate of vibration must change along with 
 it; and after the change the molecules could no longer 
 absorb the waves which they absorbed prior to the change. 
 Now, the elastic force between molecule and molecule is 
 utterly altered when a vapor passes to the liquid state. 
 Hence, if the liquid absorbs waves of the same period as its 
 vapor, it is a proof that the absorption is not effected by 
 the molecules. Let us be perfectly clear on this important 
 
250 FRAGMENTS OF SCIENCE. 
 
 point. Those waves are absorbed whose vibrations syn- 
 chronize with those of the molecules or atoms on which 
 they impinge ; a principle which is sometimes expressed by 
 saying that bodies radiate and absorb the same rays. This 
 great law, as you know, is the foundation of spectrum- 
 analysis ; it enabled Kirchhoff to explain the lines of Frauen- 
 hofer, and to determine the chemical composition of the 
 atmosphere of the sun. If, then, after such a change as 
 that involved in the passage of a vapor to the liquid state, 
 the same waves are absorbed as were absorbed prior to the 
 passage, it is a proof that the molecules, which must have 
 utterly changed their periods, cannot be the seat of the ab- 
 sorption ; and we are driven to conclude that it is to the 
 atoms, whose rates of vibration are unchanged by the 
 change of aggregation, that the wave-motion is transferred. 
 If experiment should prove this identity of action on the 
 part of a vapor and its liquid, it would establish in a new 
 and striking manner the conclusion to which we have pre- 
 viously leaned. 
 
 We will now resort to the experimental test. In front 
 of this experimental tube, which contains a quantity of the 
 nitrite-of-amyl vapor, is placed a glass cell a quarter of an 
 inch in thickness, filled with the liquid nitrite of amyl. I 
 send the electric beam first through the liquid and then 
 through its vapor. The luminous power of this beam is 
 very great, but it can make no impression upon the vapor. 
 The liquid has robbed it completely of its effective waves. 
 When the liquid is removed chemical action immediately 
 commences, and in a moment we have the apparently 
 empty tube filled with this bright cloud, precipitated by 
 one portion of the beam, and illuminated by another. Thus 
 we uncover to some extent the secrets of this world of 
 molecules and atoms. 
 
 Instead of employing air as the vehicle by which the 
 vapor is carried into the experimental tube, we may em- 
 
CHEMICAL RAYS. 251 
 
 ploy oxygen, hydrogen, or nitrogen. With hydrogen curi- 
 ous effects are observed, due to the sinking of the clouds 
 through the extremely light gas in which they float. They 
 illustrate, but do not prove, the untenable notion of those 
 who say that the clouds of our own atmosphere could not 
 float if the cloud-particles were not little bladders instead 
 of full spheres. Before you is a tube filled with the nitrite- 
 of-amyl vapor, which has been carried into the tube by 
 hydrogen gas. On sending the beam through the tube a 
 delicate bluish-white cloud is precipitated. A few strokes 
 of the pump clear the tube of this cloud, but leave a resi- 
 due of vapor behind. Again, turning on the beam we have 
 a second cloud, more delicate than the first. This may be 
 done half a dozen times in succession. A residue of vapor 
 will still linger in the tube sufficient to yield a cloud of ex- 
 quisite delicacy, both as regards color and texture. 
 
 Besides the nitrite of amyl, a great number of other 
 substances might be employed, which, like the nitrite, have 
 been hitherto not known to be chemically susceptible to 
 light. This is, in fact, a representative case. One point 
 in addition I wish to illustrate, chiefly because the effect is 
 the same in kind as one of great importance in nature. Our 
 atmosphere contains carbonic-acid gas, which furnishes food 
 to the vegetable world. But this food, as many of you 
 know, could not be consumed by plants and vegetables 
 without the intervention of the sun's rays. As far as we 
 know, however, these rays are powerless upon the free car- 
 bonic acid of our atmosphere ; the sun can only decompose 
 the gas when it is absorbed by the leaves of plants. In the 
 leaves the carbonic acid is in close proximity with sub- 
 stances ready to take advantage of the loosening of the 
 molecules by the waves of light. Incipient disunion being 
 introduced by the solar rays, the carbon of the gas is seized 
 upon by the leaf and appropriated, while the oxygen is dis- 
 charged into the atmosphere. 
 
252 FRAGMENTS OF SCIENCE. 
 
 The experimental tube now before you contains a 
 quantity of a different vapor from that which we have 
 hitherto employed. The liquid from which this vapor is 
 derived is called the nitrite of butyl. On sending the elec- 
 tric beam through the vapor, which has been carried in by 
 air, the chemical action is insensible. I add to the vapor a 
 quantity of air which has been permitted to bubble through 
 hydrochloric acid. When the beam is now turned on, so 
 rapid is the action and so dense the clouds precipitated, 
 that you could hardly, by an effort of attention, observe the 
 dark interval which preceded the precipitation of the cloud. 
 This enormous augmentation of the action is due to the 
 presence of the hydrochloric acid. Like the chlorophyl in 
 the leaves of plants, it takes advantage of the loosening of 
 the molecules of nitrite of butyl by the waves of the electric 
 light. 
 
 In these experiments we have employed a luminous 
 beam for two different purposes. A small portion of it has 
 been devoted to the decomposition of our vapors, while the 
 great body of the light has served to render luminous the 
 clouds resulting from the decomposition. It is possible to 
 impart to these clouds any required degree of tenuity, for 
 it is in our power to limit at pleasure the amount of vapor 
 in our experimental tube. When the quantity is duly 
 limited, the precipitated particles are at first inconceivably 
 small, defying the highest microscopic power to bring them 
 within the range of vision. Probably their, diameters might 
 then be expressed in millionths of an inch. They grow 
 gradually, and as they augment in size they scatter a con- 
 tinually increasing quantity of wave-motion, until finally 
 the cloud which they form becomes so luminous as to fill 
 this theatre with light. During the growth of the particles 
 the most splendid iridescences are often exhibited. Such 
 I have sometimes seen with delight and wonder in the 
 atmosphere of the Alps, but never any thing so gorgeous 
 
STRUCTURE AND LIGHT OF THE SKY. 253 
 
 as those which our laboratory experiments reveal. It is 
 not, however, with the iridescences, however beautiful they 
 may be, that we have now to occupy our thoughts, but 
 with other effects which bear upon the two great standing 
 enigmas of meteorology the color of the sky and the polar- 
 ization of its light. 
 
 It is possible, as stated, by duly regulating the quantity 
 of vapor, to make our precipitated particles grow from an 
 infinitesimal and altogether ultra-microscopic size to masses 
 of sensible magnitude ; and by means of these particles, in 
 a certain stage of their growth, we can produce a blue 
 which shall rival, if it does not transcend, that of the deepest 
 and purest Italian sky. Let this point be in the first place 
 established. Associated with our experimental tube is a 
 barometer, the mercurial column of which now indicates 
 that the tube is exhausted. Into the tube is introduced a 
 quantity of the mixed air and nitrite-of-butyl vapor sufficient 
 to depress the mercurial column one-twentieth of an inch ; 
 that is to say, the air and vapor together exert a pressure 
 of one six-hundredth of an atmosphere. I now add a quan- 
 tity of air and hydrochloric acid sufficient to depress the 
 mercury half an inch farther, and into this compound and 
 highly-attenuated atmosphere I discharge the beam of the 
 electric light. The effect is slow ; but gradually within the 
 tube arises this splendid azure, which strengthens for a time, 
 reaches a maximum of depth and purity, and then, as the 
 particles grow larger, passes into whitish blue. This. ex- 
 periment is representative, and it illustrates a general 
 principle. Various other colorless substances of the most 
 diverse properties, optical and chemical, might be employed 
 for this experiment. The incipient cloud in every case 
 would exhibit this superb blue ; thus proving to demonstra- 
 tion that particles of infinitesimal size, without any color of 
 their own, and irrespective of those optical properties ex- 
 hibited by the substance in a massive state, are competent 
 to produce the color of the sky. 
 
254 FRAGMENTS OF SCIENCE. 
 
 But there is another subject connected with our firma- 
 ment, of a more subtle and recondite character than even 
 its color. I mean that " mysterious and beautiful phenom- 
 enon," ' the polarization of the light of the sky. The po- 
 larity of a magnet consists in its two-endedness, both ends, 
 or poles, acting in opposite ways. Polar forces, as most of 
 you know, are those in which the duality of attraction and 
 repulsion is manifested. And a kind of two-sidedness 
 noticed by Huyghens, commented on by Newton, and dis- 
 covered by a French philosopher, named Malus, in a beam 
 of light which had been reflected from one of the windows 
 of the Luxembourg Palace in Paris receives the name of 
 polarization. "We must now, however, attach a distinct- 
 ness to the idea of a polarized beam, which its discoverers 
 were not able to attach to it. For in their day men's 
 thoughts were not sufficiently ripe, nor optical theory suffi- 
 ciently advanced, to seize upon or express the physical 
 meaning of polarization. When a gun is fired, the explo- 
 sion is propagated as a wave through the air. The shells 
 of air, if I may use the term, surrounding the centre of con- 
 cussion, are successively thrown into motion, each shell 
 yielding up its motion to that in advance of it, and return- 
 ing to its position of equilibrium. Thus, while the wave 
 travels through long distances, each individual particle of 
 air concerned in its transmission performs merely a small 
 excursion to and fro. 2 In the case of sound, the vibrations 
 of the air-particles are executed in the direction in which 
 the sound travels. They are, therefore, called longitudinal 
 vibrations. In the case of light, on the contrary, the vibra- 
 tions are transversal; that is to say, the individual particles 
 of ether move to and fro across the direction in which the 
 light is propagated. In this respect waves of light resem- 
 ble ordinary water-waves, more than waves of sound. In 
 
 1 Herschel's Meteorology, Art. 233. 
 
 2 Lectures on Sound, p. 3. (Longmans.) 
 
STRUCTURE AND LIGHT OF THE SKY. 255 
 
 the case of an ordinary beam of light, the vibrations of the 
 ether-particles are executed in every direction perpendicular 
 to it ; but let the beam impinge obliquely upon a plane- 
 glass surface, as in the case of Malus, the portion reflected 
 will no longer have its particles vibrating in all directions 
 round it. By the act of reflection, if it occur at the proper 
 angle, the vibrations are all confined to a single plane, and 
 light thus circumstanced is called plane polarized light. 
 
 A beam of light passing through ordinary glass executes 
 its vibrations within the substance exactly as it would do 
 in air, or in ether-filled space. Not so when it passes 
 through many transparent crystals. For these have also 
 their two-sidedness, the arrangement of their molecules 
 being such as to tolerate vibrations only in certain definite 
 directions. There is the well-known crystal tourmaline, 
 which shows a marked hostility to all vibrations executed 
 at right angles to the axis of the crystal. It speedily ex- 
 tinguishes such vibrations, while those executed parallel to 
 the axis are freely propagated. The consequence is, that a 
 beam of light, after it has passed through any thickness of 
 this crystal, emerges from it polarized. So also as regards 
 the beautiful crystal known as Iceland spar, or as double- 
 refracting spar. In one direction, but in one only, it acts 
 like a piece of glass ; in all other directions it splits the 
 beam of light passing through it into two distinct halves, 
 both of which are perfectly polarized, their vibrations being 
 executed in two planes, at right angles to each other. 
 
 It is possible by a suitable contrivance to get rid of one 
 of the two polarized beams into which Iceland spar divides 
 an ordinary beam of light. This was done so ingeniously 
 and effectively by a man named Nicol, that the Iceland spar, 
 cut in his fashion, is now universally known as Nicol's prism. 
 Such a prism can polarize a beam of light, and if the beam, 
 before it impinges on the prism, be already polarized, in 
 one position of the prism it is stopped, while in another 
 
256 FRAGMENTS OF SCIENCE. 
 
 position it is transmitted. Our way is now, to some extent, 
 cleared toward an examination of the light of the sky. 
 Looking at various points of the blue firmament through a 
 Nicol's prism, and turning the prism round its axis, we im- 
 mediately notice variations of brightness. In certain posi- 
 tions of the prism, and from certain points of the firmament, 
 the light appears to be freely transmitted ; while it is only 
 necessary to turn the prism round its axis through an angle 
 of 90 to materially diminish the intensity of the light. On 
 close scrutiny it is found that the difference produced by 
 the rotation of the prism is greatest when the sky is re- 
 garded in a direction at right angles to that of the solar 
 rays through the air. 
 
 Let me describe a few actual observations made some 
 days ago on Primrose Hill. The sun was near setting, and 
 a few scattered neutral-tint clouds, which failed to catch 
 the dying light, were floating in the air. When these were 
 looked at across the track of the solar beams, it was pos- 
 sible, by turning the Nicol round, to see them either as 
 white clouds on a dark ground, or as dark clouds on a bright 
 ground. 1 In certain positions of the prisms the sky-light 
 was in great part quenched, and then the clouds, projected 
 against the darkness of space, appeared white. Turning 
 the Nicol 90 round its axis, the brightness of the sky was 
 restored, the clouds becoming dark through contrast with 
 this brightness. Experiments of this kind prove that the 
 blue light sent to us by the firmament is polarized, and that 
 the direction of most perfect polarization is perpendicular 
 to the solar rays. Were the heavenly azure like the light 
 scattered from a thick cloud, the turning of the prism would 
 have no effect upon it ; it would be transmitted equally dur- 
 ing the entire rotation of the prism. The light of the sky is 
 in great part quenched, because it is in great part polarized. 
 
 1 1 was not aware when these words were written that this observation 
 was made by the indefatigable Brewster. 
 
STRUCTURE AND LIGHT OF THE SKY. 257 
 
 When a luminous beam impinges at the proper angle 
 on a plane-glass surface it is polarized by reflection. It is 
 polarized, in part, by all oblique reflections ; but at one 
 particular angle, the reflected light is perfectly polarized. 
 An exceedingly beautiful and simple law, discovered by 
 Sir David Brewster, enables us readily to find the polarizing 
 angle of any substance whose refractive index is known. 
 This law was discovered experimentally by Brewster ; but 
 the "Wave Theory of light renders a complete reason for 
 the law. A geometrical image of it is thus given : When 
 a beam of light impinges obliquely upon a plate of glass it 
 is in part reflected and in part refracted. At one particular 
 incidence the reflected and the refracted portions of the 
 beam are at right angles to each other. The angle of inci- 
 dence is then the polarizing angle. It varies with the re- 
 fractive index of the substance ; being for water 52^, for 
 glass 57, and for diamond 68. 
 
 It has been already stated that, in order to obtain the 
 most perfect polarization of the firmamental light, the sky 
 must be regarded in a direction at right angles to the solar 
 beams. This is sometimes expressed by saying that the 
 place of maximum polarization is at an angular distance of 
 90 from the sun. This angle, enclosed as it is between the 
 direct and reflected rays, comprises both the angles of inci- 
 dence and reflection, supposing the polarization to be due 
 to a single reflection. Hence the angle of incidence is half 
 of 90, or 45. This is the atmospheric polarizing angle, 
 and the question is, what known substance possesses an 
 index of refraction to correspond with this polarizing angle ? 
 " If," says Sir John Herschel, " we knew this substance, we 
 might be tempted to conclude that particles of it, scattered 
 in the atmosphere, produce the polarization of the sky. 
 Were the angle of maximum polarization 76 (instead of 
 90), we should look to water or ice, as the reflecting body, 
 however inconceivable the existence in a cloudless atmos- 
 
258 FRAGMENTS OF SCIENCE. 
 
 phere and a hot summer day, of unevaporated particles of 
 water." But a polarizing angle of 45 corresponds to a 
 refractive index of 1 ; this means that there is no refraction 
 at all, in which case we ought to have no reflection. Brew- 
 ster, therefore, and others came to the conclusion that the 
 reflection was from the particles of air themselves. Dr. 
 Rubenson, of Upsala, made the angle enclosed between the 
 direct and reflected beams 90 2' ; " the half of which," says 
 Mr. Buchan, in his excellent little "Handy Book of Me- 
 teorology," " is so near the polarizing angle of air as to leave 
 no doubt that the light of the sky, as first stated by Brew- 
 ster, is polarized by reflection from the particles of air." 
 
 If you doubt the wisdom, acknowledge, at all events, 
 the faith in your capacity which has caused me to bring 
 so entangled a subject before you. I would fain believe, 
 however, that even the intellect which draws its culture 
 from a totally different source, may have its interest excited 
 in subjects like the present, dark and difficult though they 
 seem. I do not expect that you will grasp all the details 
 of this discussion ; but everybody present will, I think, see 
 the extremely important part hitherto played by the law 
 of Brewster in speculations as to the color and polarization 
 of the sky. Let me now endeavor to demonstrate in your 
 presence, firstly, and in confirmation of our former experi- 
 ments, that sky-blue may be produced by exceedingly mi- 
 nute particles of any kind of matter ; secondly, that polari- 
 zation identical with that of the sky is produced by such 
 particles ; and thirdly, that matter in this fine state of di- 
 vision, where its particles are small in comparison with the 
 height and span of a wave of light, releases itself completely 
 from the law of Brewster ; the direction of maximum polari- 
 zation being absolutely independent of the polarizing angle 
 as hitherto defined. 
 
 Into this experimental tube, in the manner already de- 
 scribed, I introduce a vapor which is decomposable by the 
 
STRUCTURE- AND LIGHT OF THE SKY. 259 
 
 waves of light. The mixed air and vapor are sufficient to 
 depress the mercurial column one inch. I add to this mix- 
 ture air, which has been permitted to bubble through dilute 
 hydrochloric acid, until the column is depressed thirty inches : 
 in other words, until the tube is full. And now I permit 
 the electric beam to play upon the mixture. For some 
 time nothing is seen. The chemical action is doubtless 
 progressing, and condensation is going on ; but the con- 
 densing molecules have not yet coalesced to particles suffi- 
 ciently large to reflect sensibly the waves of light. As 
 before stated and the statement rests upon an experimental 
 basis the particles here generated are at first so small that 
 their diameters would probably have to be expressed in 
 millionths of an inch ; while to form each of these particles 
 whole crowds of molecules are probably aggregated. Helped 
 by such considerations the intellectual vision plunges more 
 profoundly into atomic Nature, and shows us, among other 
 things, how far we are from the realization of Newton's 
 hope that the molecules might one day be seen by micro- 
 scopes. While I am speaking, you observe this delicate blue 
 color, forming and strengthening within the experimental 
 tube. No sky-blue could exceed it in richness and purity ; 
 but the particles which produce this color lie wholly beyond 
 our microscopic range. A uniform color is here developed, 
 which has as little breach of continuity which yields as 
 little evidence of the particles concerned in its production, 
 as that yielded by a body whose color is due to true mo- 
 lecular absorption. This blue is at first as deep and dark 
 as the sky seen from the highest Alpine peaks, and for the 
 same reason. But it grows gradually brighter, still main- 
 taining its blueness, until at length a whitish tinge mingles 
 with the pure azure ; announcing that the particles are now 
 no longer of that infinitesimal size which mainly scatters 
 the shortest waves. 1 
 
 1 Possibly a photographic impression might be taken long before the 
 blue becomes visible, for the ultra-blue rays are first reflected. 
 
260 FRAGMENTS OF SCIENCE. 
 
 The liquid here employed is the iodide of allyl, but I 
 might choose any one of a dozen substances here before 
 me to produce the effect. You have seen what may be 
 done with the nitrite of butyl. With nitrite of amyl, bisul- 
 phide of carbon, benzol, benzoic ether, etc., the same blue 
 color may be produced. In all cases, where matter slowly 
 passes from the molecular to the massive state the transi- 
 tion is marked by the production of the blue. More than 
 this : you have seen me looking at the blue color (I hardly 
 like to call it a blue " cloud," its texture and properties are 
 so different from ordinary clouds) through this bit of spar. 
 This is a Nicol's prism, and it is to be wished that one of 
 them could be placed in the hands of each of you. Now, 
 this blue that I have been regarding turns out to be, if the 
 expression be allowed, a bit of more perfect sky than the 
 sky itself. On looking across the illuminating beam as we 
 look across the solar rays in the atmosphere, we obtain not 
 only partial polarization, but perfect polarization. In one 
 position of the Nicol the blue light passes freely to the eye ; 
 in the other it is absolutely cut off, the experimental tube 
 being reduced to optical emptiness. It is well to place a 
 black surface behind the experimental tube, so as to prevent 
 foreign light from troubling the eye. In one position of 
 the prism this black surface is seen without softening or 
 qualification; for the particles within the tube are them- 
 selves invisible, and the light which they scatter is quenched. 
 If the light of the sky were polarized with the same per- 
 fection, on looking properly toward it through a Nicol we 
 should meet, not the mild radiance of the firmament, but 
 the unillumined blackness of space. 
 
 The construction of a Nicol's prism is such that it 
 allows the passage of vibrations which are executed in a 
 certain determinate direction, and these only. All vibra- 
 tions executed at right angles to this direction are com- 
 pletely stopped : while components only of those executed 
 
STRUCTURE AND LIGHT OF THE SKY. 261 
 
 obliquely to it are transmitted. It is easy, therefore, to see 
 that, from the position in which the prism must be held to 
 transmit or to quench the light of our incipient cloud, we 
 can infer the direction of the vibrations of that light. You 
 will be able to picture those vibrations without difficulty. 
 Suppose a line drawn from any point of the " cloud " per- 
 pendicular to the illuminating beam. The particles of ether 
 along that line, which carry the light from the cloud to the 
 eye, vibrate in a direction perpendicular both to the line 
 and to the beam. And if any number of lines be drawn 
 in the same way from the cloud, like the spokes of a wheel, 
 the particles of ether along all of them oscillate in the same 
 manner. Wherefore, if a plane surface be imagined cutting 
 the incipient cloud at rigbt angles to its length, the vibra- 
 tions discharged laterally are all parallel to this surface. 
 This is the plane of vibration of the polarized light. 
 
 Our incipient blue cloud is a virtual Nicol's prism, and, 
 between it and the real prism, we can produce all the 
 effects obtainable between the polarizer and analyzer of a 
 polariscope. When, for example, a thin plate of selenite, 
 which is crystallized sulphate of lime, is placed between 
 the Nicol and the incipient cloud, we obtain the splendid 
 chromatic phenomena of polarized light. The color of the 
 gypsum-plate, as many of you know, depends upon its 
 thickness. If this be uniform, the color is uniform. If, on 
 the contrary, the plate be wedge-shaped, thickening grad- 
 ually and uniformly from edge to back, we have brilliant 
 bands of color produced parallel to the edge of the wedge. 
 Perhaps the best form of plate for experiments of this 
 character is that now in my hand, which was prepared for 
 me some years ago by a man of genius in his way, the late 
 Mr. Darker, of Lambeth. It consists of a plate of selenite 
 thin at the centre, and gradually thickening toward the 
 circumference. Placing this film between the Nicol and 
 the cloud, we obtain, instead of a series of parallel bands, a 
 
262 FRAGMENTS OF SCIENCE. 
 
 system of splendidly-colored rings. Precisely the same 
 phenomena are observed when we look at the blue firma- 
 ment in a direction perpendicular to the solar rays. 
 
 We have thus far illuminated our artificial sky with 
 ordinary light. We will now examine the effects produced 
 when the light which illuminates the particles is itself 
 polarized. In front of the electric lamp, and between it 
 and the experimental tube, is placed this fine Nicol's prism, 
 which is sufficiently large to embrace and to polarize the 
 entire beam. The plane of vibration of the light now 
 emergent from the prism, and falling upon the cloud, is 
 vertical ; and we find that this formless aggregate of infini- 
 tesimal particles, without definite structure, is absolutely 
 incompetent to scatter the light upward or downward, 
 while it freely discharges the light horizontally, right and 
 left. I turn the polarizing Nicol so as to render the plane 
 of vibration horizontal ; the cloud now freely scatters the 
 light vertically upward and downward, but it is absolutely 
 incompetent to shed a ray horizontally to the right or left. 
 
 Suppose the atmosphere of our planet to be surrounded 
 by an envelope impervious to light, with an aperture on 
 the sunward side, through which a solar beam could enter. 
 Surrounded on all sides by air not directly illuminated, the 
 track of the sunlight would resemble that of the electric 
 beam in a dark space filled with our incipient cloud. The 
 course of the sunbeam would be blue, and it would dis- 
 charge laterally, in all directions round it, light in precisely 
 the same polarized condition as that discharged from the 
 incipient cloud. In fact, the azure revealed by the sunbeam 
 would be the azure of such a cloud. And if, instead of 
 permitting the ordinary light of the sun to enter the aper- 
 ture, a NicoPs prism were placed there, which should 
 polarize the sunlight on its entrance into our atmosphere, 
 the particles producing the color of the sky would act 
 precisely like those of our incipient cloud. In two directions 
 
STRUCTURE AND LIGHT OF THE SKY. 263 
 
 we should have the solar light reflected ; in two others un- 
 reflected. In fact, out of such a solitary beam, traversing 
 the unilluminated air, we should be able to extract every 
 effect shown by our incipient cloud. In the production of 
 such clouds we virtually carry bits of the sky into our 
 laboratories, and obtain with them all the effects obtainable 
 in the open firmament of heaven. 
 
 The real sky is, as I have said, less perfect than our 
 artificial one may be made. For, mingled with the infini- 
 tesimal particles which constitute the true matter of the 
 sky, there are others too coarse to scatter perfectly po- 
 larized light at right angles to the solar beams. Hence, 
 when the brilliancy of the sky is diminished to the utter- 
 most, there is still a residue of light; the extinction is 
 partial, and not total, as in the case of our incipient cloud. 
 Let us consider this matter. The perfect polarization can 
 only be produced by excessively minute particles ; imagine 
 them growing gradually larger as they actually do in our 
 experiments. The extinction by the Nicol is perfect as 
 long as the polarization is complete. But what would you 
 expect? Manifestly, that after a time the polarization 
 would cease to be perfect. But here again the relation of 
 the size of the particles to the size of the waves must come 
 into play. In relation to the blue waves the particles are 
 larger than in relation to the red ; the blue waves, there- 
 fore, will be the first liberated from a condition dependent 
 on the smallness of the particles. They will first escape 
 from the trammels of polarization ; and on their liberation 
 they exhibit an azure far purer and more brilliant than that 
 produced by the first precipitation of the particles. Could 
 we overarch ourselves with a sky of this color for a single 
 day, it would make us discontented with our present lack- 
 lustre firmament ever afterward. It will be observed that 
 in all these cases reason and experiment go hand in hand, 
 the one predicting, the other verifying; every such verift- 
 
264 FRAGMENTS OF SCIENCE. 
 
 cation lending its weight of proof to the undulatory theory 
 on which the predictions are founded. 
 
 The selenite ring-system, already referred to, is a most 
 delicate reagent for the detection of polarized light. When 
 we look normally r , or perpendicularly, at an incipient cloud, 
 the colors of the rings are most vividly developed, a dim- 
 inution of the color being immediately apparent when 
 the incipient cloud is regarded obliquely. But let us con- 
 tinue to look through the Nicol and selenite normally at 
 the cloud : the particles augment in size, the cloud becomes 
 coarser and whiter, the strength of the selenite colors be- 
 coming gradually feebler. At length the cloud ceases to 
 discharge polarized light along the normal, and then the 
 selenite colors entirely disappear. If, now, the cloud be re- 
 garded obliquely the colors are restored, very vividly, if not 
 with their first vividness and clearness. Thus the cloud 
 that has ceased to discharge polarized light at right angles 
 to the illuminating beam, pours out such light copiously in 
 oblique directions. The direction of maximum polariza- 
 tion changes with the texture of the cloud. 
 
 But this is not all ; and to understand, even partially, 
 what remains, a word must be said regarding the appear- 
 ance of the colors of our plate of selenite. If, as before 
 stated, the plate be of uniform thickness, its hue in polar- 
 ized light is uniform. Suppose, then, that by arranging 
 the Nicol the color of the plate is raised to its maximum 
 brilliancy, and suppose the color produced to be green" on 
 turning the Nicol round its axis the green becomes fainter. 
 When the angle of rotation amounts to 45 the color dis- 
 appears ; we then pass what may be called a neutral posi- 
 tion, where the selenite behaves, not as a crystal, but as a 
 bit of glass. Continuing the rotation, a color reappears, 
 but it is no longer green but red. This attains its maxi- 
 mum at a distance of 45 from the neutral position, or, in 
 other words, at a distance of 90 from the position which 
 
STRUCTURE AND LIGHT OF THE SKY. 265 
 
 showed the green at its maximum. At a further distance 
 of 45 from the position of maximum red, the color disap- 
 pears a second time. We have there a second neutral 
 point, beyond which the green comes again into view, at- 
 taining its maximum brilliancy at the end of a rotation of 
 180. By the rotation of the Nicol, therefore, through an 
 angle of 90, we produce a color complementary to that 
 with which we started. 
 
 As may be inferred from this result, the selenite ring- 
 system changes its character when the Nicol is turned. 
 It is possible to have the centre of the circle dark, the 
 surrounding rings being vividly colored. The turning of 
 the Nicol through an angle of 90 renders the centre bright, 
 while every point occupied by a certain color in the first 
 instance is occupied by the complement of that color in the 
 second. By curious internal actions, not here to be de- 
 scribed, the cloud in our experimental tube sometimes 
 divides itself into sections of different textures. Some sec- 
 tions are coarser than others, while it often happens that 
 some are iridescent to the naked eye, and others not. 
 Looking normally at such a cloud through the selenite and 
 Nicol, it often happens that in passing from section to sec- 
 tion the whole character of the ring-system is changed. 
 You start with a section producing a dark centre and a 
 corresponding system of rings ; you pass through a neutral 
 point to another section and find there the centre bright, 
 and each of the first rings displaced by one of the comple- 
 mentary color. Sometimes as many as four such rever- 
 sions occur in the cloud of an experimental tube a yard 
 long. Now, the changes here indicated mean that in passing 
 from section to section of the cloud the plane of vibration 
 of the polarized light turns suddenly through an angle of 
 90 ; this change being entirely due to the different texture 
 of the two parts of the cloud. 
 
 You will now be able to understand, as far as it is cat 
 12 
 
266 FRAGMENTS OF SCIENCE. 
 
 pable of being understood, a very beautiful effect which, 
 under favorable circumstances, might be observed in our 
 atmosphere. This experimental tube contains an inch of 
 the iodide-of-allyl vapor, the remaining 29 inches necessary 
 to fill the tube being air, which has bubbled through aque- 
 ous hydrochloric acid. Besides, therefore, the vapor of 
 iodide of allyl, we have those of water and of acid within 
 the tube. The light has been acting on the mixture for 
 some time, a beautiful incipient blue cloud being formed. As 
 before stated, the " incipient cloud " is wholly different in 
 texture and optical properties from an ordinary cloud ; but 
 it is possible to precipitate in the midst of the azure the 
 aqueous vapor so as to cause it to form in the tube a cloud 
 similar to the clouds of our atmosphere. An exhausted 
 vessel of about one-third of the capacity of the experi- 
 mental tube is connected with it, the passage uniting both 
 being closed by a stop-cock. On opening this cock the 
 mixed air and vapor rush from the experimental tube into 
 the empty vessel ; and, in consequence of the chilling due 
 to rarefaction, the vapor in the experimental tube is pre- 
 cipitated as a true cloud. What is the result ? Instantly 
 the centre of the system of colored rings becomes bright, 
 and the whole series of colors corresponding to definite ra- 
 dial distances, complementary: While you continue to 
 look at the cloud, it gradually melts away as an atmos- 
 pheric cloud might do in the azure of heaven. And there 
 is our azure also remaining behind. The coarser cloud 
 seems drawn aside like a veil, the blue reappears, the first 
 ring-system, with its dark centre and correspondingly col- 
 ored circles, being restored. 
 
 Thus patiently you have accompanied me over a piece 
 of exceedingly difficult ground ; and I think, as a prudent 
 guide, we ought to halt . upon the eminence we have now 
 attained. We might go higher, but the bowlders begin 
 here to be very rough. At a future day we shall, I doubt 
 
STRUCTURE AND LIGHT OF THE SKY. 267 
 
 not, be able to overcome this difficulty, and to reach to- 
 gether a greater elevation. 
 
 THE SKY OF THE ALPS. 
 
 THE vision of an object always implies a differential 
 action on the retina of the observer. The object is dis- 
 tinguished from surrounding space by its excess or defect 
 of light in relation to that space. By altering the illumi- 
 nation, either of the object itself or of its environment, we 
 alter the appearance of the object. Take the case of clouds 
 floating in the atmosphere with patches of blue between 
 them. Any thing that changes the illumination of either 
 alters the appearance of both, that appearance depending, 
 as stated, upon differential action. Now the light of the 
 sky being polarized, may, as the reader of the foregoing 
 pages knows, be in great part quenched by a Nicol's 
 prism, while the light of a cloud, being unpolarized, cannot 
 be thus extinguished. Hence the possibility of very re- 
 markable variations, not only in the aspect of the firma- 
 ment, which is really changed, but also in the aspect of the 
 clouds which have that firmament as a background. It is 
 possible, for example, to choose clouds of such a depth of 
 shade that when the Nicol quenches the light behind them, 
 they shall vanish, being undistinguishable from the residual 
 dull tint which outlives the extinction of the brilliance of 
 the sky. A cloud less deeply shaded, but still deep enough, 
 when viewed with the naked eye, to appear dark on a 
 bright ground, is suddenly changed to a white cloud on a 
 dark ground by the quenching of the sky behind it. When 
 a reddish cloud at sunset chances to float in the region of 
 maximum polarization, the quenching of the sky behind it 
 causes it to flash with a brighter crimson. Last Easter eve 
 the Dartmoor sky, which had just been cleansed by a snow- 
 
268 FRAGMENTS OF SCIENCE. 
 
 storm, wore a very wild appearance. Round the horizon it 
 was of steely brilliancy, while reddish cumuli and cirri 
 floated southward. When the sky was quenched behind 
 them these floating masses seemed like dull embers sud- 
 denly blown upon ; they brightened like a fire. In the 
 Alps we have the most magnificent examples of crimson 
 clouds and snows, so that the effects just referred to may 
 be here studied under the best possible conditions. On 
 August 23, 1869, the evening Alpenglow was very fine, 
 though it did not reach its maximum depth and splendor. 
 Toward sunset I walked up the slopes to obtain a better 
 view of the Weisshorn. The side of the peak seen from the 
 Bel Alp, being turned from the sun, was tinted mauve ; 
 but I wished to see one of the rose-colored buttresses of the 
 mountain. Such was visible from a point a few hundred 
 feet above the hotel. The Matterhorn also, though for the 
 most part in shade, had a crimson projection, while a deep 
 ruddy red lingered along its western shoulder. Four dis- 
 tinct peaks and buttresses of the Dom, in addition to its 
 dominant head all covered with pure snow were red- 
 dened by the light of sunset. The shoulder of the Alphu- 
 bel was similarly colored, while the great mass of the Flet- 
 schorn was all a-glow, and so was the snowy spine of the 
 Monte Leone. 
 
 Looking at the Weisshorn through the Nicol, the glow 
 of its protuberance was strong or weak according to the 
 position of the prism. The summit also underwent a 
 change. In one position of the prism it exhibited a pale 
 white against a dark background ; in the rectangular posi- 
 tion, it was a dark mauve against a light background. The 
 red of the Matterhorn changed in a similar manner ; but 
 the whole mountain also passed through striking changes 
 of definition. The air at the time was filled with a silvery 
 haze, in which the Matterhorn almost disappeared. This 
 could be wholly quenched by the Nicol, and then the 
 
THE SKY OF THE ALPS. 269 
 
 mountain sprang forth with astonishing solidity and detach- 
 ment from the surrounding air. The changes of the Dom 
 were still more wonderful. A vast amount of light could 
 be removed from the sky behind it, for it occupied the po- 
 sition of maximum polarization. By a little practice with 
 the Nicol it was easy to render the extinction of the light, 
 or its restoration, almost instantaneous. When the sky was 
 quenched, the four minor peaks and buttresses, and the 
 summit of the Dom, together with the shoulder of the Al- 
 phubel, glowed as if set suddenly on fire. This was imme- 
 diately dimmed by turning the Nicol through an angle of 
 90. It was not the stoppage of the light of the sky behind 
 the mountains alone which produced this startling effect ; 
 the air between them and me was highly opalescent, and 
 the quenching of this intermediate glare augmented re- 
 markably the distinctness of the mountains. 
 
 On the morning of August 24th similar effects were fine- 
 ly shown. At 10 A. M. all three mountains, the Dom, the 
 Matterhorn, and the Weisshorn, were powerfully affected 
 by the Nicol. But in this instance also the line drawn to 
 the Dom being accurately perpendicular to the direction of 
 the solar shadows, and consequently very nearly perpen- 
 dicular to the solar beams, the effects on this mountain were 
 most striking. The gray summit of the Matterhorn at the 
 same time could scarcely be distinguished from the opales- 
 cent haze around it ; but when the Nicol quenched the 
 haze, the summit became instantly isolated, and stood out 
 in bold definition. It is to be remembered that in the pro- 
 duction of these effects the only things changed are the 
 sky behind and the luminous haze in front of the moun- 
 tains ; that these are changed because the light emitted 
 from the sky and from the haze is plane polarized light, 1 
 and that the light from the snows and from the mountains 
 being sensibly unpolarized, is not directly affected by the 
 1 Defined at page 255. 
 
270 FRAGMENTS OF SCIENCE. 
 
 Nicol. It will also be understood that it is not the interposi- 
 tion of the haze as an opaque body that renders the moun- 
 tains indistinct, but that it is the light of the haze which 
 dims and bewilders the eye, and thus weakens the defini- 
 tion of objects seen through it. 
 
 The results have a direct bearing upon what artists call 
 " aerial perspective." As we look from the summit of the 
 Aletschhorn, or from a lower elevation, at the serried crowd 
 of peaks, especially if the mountains be darkly colored 
 covered with pines, for example every peak and ridge is 
 separated from the mountains behind it by a thin blue haze 
 which renders the relations of the mountains as to distance 
 unmistakable. When this haze is regarded through the 
 Nicol perpendicular to the sun's rays, it is in many cases 
 wholly quenched, because the light which it emits in this 
 direction is wholly polarized. When this happens, aerial 
 perspective is abolished, and mountains very differently dis- 
 tant appear to rise in the same vertical plane. Close to the 
 Bel Alp, for instance, is the gorge of the Massa, and beyond 
 the gorge is a high ridge darkened by pines. This ridge 
 may be projected upon the dark slopes at the opposite side 
 of the Rhone valley, and between both we have the blue 
 haze referred to, throwing the distant mountains far away. 
 But at certain hours of the day this haze may be quenched, 
 and then the Massa ridge and the mountains beyond the 
 Rhone seem almost equally distant from the eye. The one 
 appears, as it were, a vertical continuation of the other. 
 The haze varies with the temperature and humidity of the 
 atmosphere. At certain times and places it is almost as 
 blue as the sky itself ; but to see its color, the attention 
 must be withdrawn from the mountains and from the trees 
 which cover them. In point of fact, the haze is a piece of 
 more or less perfect sky ; it is produced in the same man- 
 ner, and is subject to the same laws, as the firmament it- 
 self. We live in the sky, not under it. 
 
THE SKY OF THE ALPS. 271 
 
 These points were further elucidated by the deportment 
 of the selenite plate, with which the readers of the fore- 
 going discourse are already acquainted. On some of the 
 sunny days of August the haze in the valley of the Rhone, 
 as looked at from the Bel Alp, was very remarkable. Tow- 
 ward evening the sky above the mountains opposite to my 
 place of observation yielded a series of the most splendidly- 
 colored iris-rings ; but on lowering the selenite until it had 
 the darkness of the pines at the opposite side of the Rhone 
 valley, instead of the darkness of space as a background, 
 the colors were not much diminished in brilliancy. I should 
 estimate the distance across the valley, as the crow flies, to 
 the opposite mountains, at nine miles ; so that a body of 
 air nine miles thick can, under favorable circumstances, 
 produce chromatic effects of polarization almost as vivid as 
 those produced by the sky itself. 
 
 Again : the light of a landscape, as of most other things, 
 consists of two parts: the one part comes purely from 
 superficial reflection, and this light is always of the same 
 color as that which falls upon the landscape ; the other 
 part comes to us from a certain depth within the objects 
 which compose the landscape, and it is this portion of the 
 total light which gives these objects their distinctive 
 colors. The white light of the sun enters all substances to 
 a certain depth, and is partially ejected by internal reflec- 
 tion ; each distinct substance absorbing and reflecting the 
 light in accordance with the laws of its own molecular con- 
 stitution. Thus the solar light is sifted by the landscape, 
 which appears in such colors and variations of color as, 
 after the sifting process, reach the observer's eye. Thus 
 the bright green of grass, or the darker color proper to the 
 pine, never comes to us alone, but is always mingled with 
 an amount of really foreign light derived from superficial 
 reflection. A certain hard brilliancy is conferred upon the 
 woods and meadows by this superficially-reflected light. 
 
272 FRAGMENTS OF SCIENCE. 
 
 Under certain circumstances, it may be quenched by a 
 Nicol's prism, and we then obtain the true color of the grass 
 and foliage. Trees and meadows thus regarded exhibit a 
 richness and softness of tint which they never show as long 
 as the superficial light is permitted to mingle with the true 
 interior emission. The needles of the pines show this effect 
 very well, large-leaved trees still better ; while a glimmer- 
 ing field of maize exhibits the most extraordinary variations 
 when looked at through the rotating Nicol. 
 
 Thoughts and questions like those here referred to took 
 me, in August, 1869, to the top of the Aletschhorn. The 
 effects described in the foregoing paragraphs were for the 
 most part reproduced in the summit of the mountain. I 
 scanned the whole of the sky with my Nicol. Both alone 
 and in conjunction with the selenite it pronounced the per- 
 pendicular to the solar beams to be the direction of maxi- 
 mum polarization. But at no portion of the firmament was 
 the polarization complete. The artificial sky produced in 
 the experiments recorded in the preceding discourse could, 
 in this respect, be rendered more perfect than the natural 
 one ; while the gorgeous " residual blue " which makes its 
 appearance when the polarization of the artificial sky ceases 
 to be perfect, was strongly contrasted with the lack-lustre 
 hue which, in the case of the firmament, outlived the ex- 
 tinction of the brilliance. With certain substances, how- 
 ever, artificially treated, this dull residue may also be ob- 
 tained. 
 
 All along the arc from the Matterhorn to Mont Blanc 
 the light of the sky immediately above the mountains was 
 powerfully acted upon by the Nicol. In some cases the 
 variations of intensity were astonishing. I have already 
 said that a little practice enables the observer to shift the 
 Nicol from one position to another so rapidly as to render 
 the alternate extinction and restoration of the light imme- 
 diate. When this was done along the arc to which I have 
 
THE SKY OF THE ALPS. 273 
 
 referred, the alternations of light and darkness resembled 
 the play of sheet-lightning behind the mountains. My 
 notes state that there was an element of awe connected 
 with the suddenness with which the mighty masses, ranged 
 along the line referred to, changed their aspect and defi- 
 nition under the operation of the prism. 
 
 JLibrary* 
 
 California 
 
XI. 
 DUST AND DISEASE. 
 
 A DISCOURSE. 
 
 DELIVEEED IN THE ROYAL INSTITUTION OP GEEAT BRITAIN. 
 
 January 21, 1870. With Additions. 
 
" Tout rniasme contagieux ales proprictcs, 1 s de reproduire son ana- 
 logue dans une maladie qu'il a occasionnee ; 2 de se repandre et dc 
 s'entendre a 1'infini, en vertu de ce developpement secondaire, c'est-a- 
 dire, aussi longtemps qu'il existe unematiere propre a recevoir le miasme, 
 et en a produire un nouveau. Ces deux proprietcs lui sont communes 
 avec les germes des animaux et des plantes. 
 
 HlLDEBRAND. 
 

 XL 
 
 ON DUST AND DISEASE. 
 
 Experiments on Dusty Air. 
 
 SOLAR light in passing through a dark room reveals its 
 track by illuminating the dust floating in the air. " The 
 sun," says Daniel Culverwell, " discovers atonies, though 
 they be invisible by candle-light, and makes them dance 
 naked in his beams." 1 
 
 In my researches on the decomposition of vapors by 
 light, I was compelled to remove these " atomes " and this 
 dust. It was essential that the space containing the vapors 
 should embrace no visible thing ; that no substance capable 
 of scattering the light in the slightest sensible degree 
 should, at the outset of an experiment, be found in the " ex- 
 perimental tube " traversed by the luminous beam. 
 
 For a long time I was troubled by the appearance there 
 of floating dust, which though invisible in diffuse daylight 
 was at once revealed by a powerfully-condensed beam. Two 
 tubes were placed in succession in the path of the air : the 
 one containing fragments of glass wetted with concentrated 
 sulphuric acid ; the other, fragments of marble wetted with 
 a strong solution of caustic potash. To my astonishment 
 the dust passed through both. The air of the Royal Insti- 
 tution sent through these tubes at a rate sufficiently slow 
 
 1 On a day of transient shadows there is something almost magical in 
 the rise and dissolution of the luminous beams among the scaffolding 
 poles of the Royal Albert Hall. 
 
278 FRAGMENTS OF SCIENCE. 
 
 to dry it, and to remove its carbonic acid, carried into the 
 experimental tube a considerable amount of mechanically 
 suspended matter, which was illuminated when the beam 
 passed through the tube. The effect was substantially the 
 same when the air was permitted to bubble through the 
 liquid acid and through the solution of potash. 
 
 Thus, on October 5, 1868, successive charges of air were 
 admitted through the potash and sulphuric acid into the ex- 
 hausted experimental tube. Prior to the admission of the 
 air the tube was optically empty ; it contained nothing 
 competent to scatter the light. After the air had entered 
 the tube, the conical track of the electric beam was in all 
 cases clearly revealed. This, indeed, was a daily observa- 
 tion at the time to which I now refer. 
 
 I tried to intercept this floating matter in various ways ; 
 and on the day just mentioned, prior to sending the air 
 through the drying apparatus, I carefully permitted it to 
 pass over the tip of a spirit-lamp flame. The floating mat- 
 ter no longer appeared, having been burnt up by the flame. 
 It was, therefore, of organic origin. I was by no means 
 prepared for this result ; for I had thought that the dust of 
 our air was, in great part, inorganic and non-combustible. 
 
 I had constructed a small gas-furnace, now much em- 
 ployed by chemists, containing a platinum tube, which 
 could be heated to vivid redness. 1 The tube contained a 
 roll of platinum gauze, which, while it permitted the air to 
 pass through it, insured the practical contact of the dust 
 with the incandescent metal. The air of the laboratory 
 was permitted to enter the experimental tube, sometimes 
 through the cold, and sometimes through the heated, tube 
 of platinum. The rapidity of admission was also varied. 
 In the first column of the following table the quantity of 
 air operated on is expressed by the number of inches which 
 the mercury gauge of the air-pump sank when the air en- 
 
 1 Pasteur was, I believe, the first to employ such a tube. 
 
DUST AND DISEASE. 279 
 
 tered. In the second column the condition of the platinum 
 tube is mentioned, and in the third the state of the air 
 which had entered the experimental tube. 
 
 Quantity of Air. State of Platinum Tube. State of Experimental Tube. 
 
 15 inches Cold Full of particles. 
 
 15 " Red hot Optically empty. 
 
 The phrase " optically empty " shows that when the con- 
 ditions of perfect combustion were present, the floating 
 matter totally disappeared. It was wholly burnt up, leav- 
 ing no sensible residue. The experiment was repeated 
 many times, with the same invariable result. 
 
 The whole of the visible particles floating in the air of 
 London rooms being thus proved to be of organic origin, 1 
 I sought to burn them up at the focus of a concave re- 
 flector. One of the powerfully convergent mirrors em- 
 ployed in my experiments on combustion by dark rays was 
 here made use of, but I failed in the attempt. Doubtless 
 the floating particles are in part transparent to radiant heat, 
 and are so far incombustible by such heat. Their rapid 
 motion through the focus also aids their escape. They do 
 not linger there sufficiently long to be consumed. A flame 
 it was evident would burn them up, but I at first thought 
 the presence of the flame would mask its own action among 
 the particles. 
 
 1 According to an analysis kindly furnished to me by Dr. Percy, the dust 
 collected from the walk of the British Museum contains fully 50 per cent, 
 of inorganic matter. I have every confidence in the results of this dis- 
 tinguished chemist ; they show that the floating dust of our rooms is, as it 
 were, winnowed from the heavier matter. As bearing directly upon this 
 point, I may quote the following passage from Pasteur : " Mais ici se 
 presente une remarque : la poussiere que 1'on trouve a la surface de tous 
 les corps est soumise constamment a des courants d'air, qui doivent sou- 
 lever ses particules les plus legeres, au nombre desquelles se trouvent, sans 
 doute, de preference les corpuscules organises, oaufs ou spores, moins 
 lourds generalement que les particules minerales." 
 
280 FRAGMENTS OF SCIENCE. 
 
 In a cylindrical beam, which strongly illuminated the 
 dust of the laboratory, was placed an ignited spirit-lamp. 
 Mingling with the flame, and round its rim, were seen 
 curious wreaths of darkness resembling an intensely-black 
 smoke. On lowering the flame below the beam the same 
 dark masses stormed upward. They were at times blacker 
 than the blackest smoke that I have ever seen issuing from 
 the funnel of a steamer ; and their resemblance to smoke 
 was so perfect as to lead the most practised observer to 
 conclude that the apparently-pure flame of the alcohol-lamp 
 required but a beam of sufficient intensity to reveal its 
 clouds of liberated carbon. 
 
 But is the blackness smoke ? This question presented 
 itself in a moment. A red-hot poker was placed under- 
 neath the beam, and from it the black wreaths also as- 
 cended. A large hydrogen-flame was next employed, and 
 it produced those whirling masses of darkness far more 
 copiously than either the spirit-flame or poker. Smoke was 
 therefore out of the question. 
 
 What, then, was the blackness ? It was simply that of 
 stellar space ; that is to say, blackness resulting from the 
 absence from the track of the beam of all matter competent 
 to scatter its light. When the flame was placed below the 
 beam, the floating matter was destroyed in situ and the 
 air, freed from this matter, rose into the beam, jostled aside 
 the illuminated particles, and substituted for their light the 
 darkness due to its own perfect transparency. Nothing 
 could more forcibly illustrate the invisibility of the agent 
 which renders all things visible. The beam crossed, un- 
 seen, the black chasm formed by the transparent air, while 
 at both sides of the gap the thick-strewn particles shone 
 out like a luminous solid under the powerful illumina- 
 tion. 
 
 But here a rather perplexing difficulty meets us. It is 
 not necessary to burn the particles to produce a stream of 
 
DUST AND DISEASE. 281 
 
 darkness. Without actual combustion, currents may be 
 generated which shall exclude the floating matter, and 
 therefore appear dark amid the surrounding brightness. I 
 noticed this effect first on placing a red-hot copper ball be- 
 low the beam, and permitting it to remain there until its 
 temperature had fallen below that of boiling water. The 
 dark currents, though much enfeebled, were still produced. 
 They may also be produced by a flask filled with hot 
 water. 
 
 To study this effect a platinum wire was stretched 
 across the beam, the two ends of the wire being connected 
 with the two poles of a voltaic battery. To regulate the 
 strength of the current a rheostat was placed in the circuit. 
 Beginning with a feeble current the temperature of the 
 wire was gradually augmented ; but, before it reached the 
 heat of ignition, a flat stream of air rose from it, which 
 when looked at edgeways appeared darker and sharper than 
 one of the blackest lines of Fraunhofer in the solar spec- 
 trum. Bight and left of this dark vertical band the float- 
 ing matter rose" upward, bounding definitely the non-lumi- 
 nous stream of air. What is the explanation? Simply 
 this : The hot wire rarefied the air in contact with it, but 
 it did not equally lighten the floating matter. The con- 
 vection current of pure air therefore passed upward among 
 the, inert particles, dragging them after it right and left, but 
 forming between them an impassable black partition. This 
 elementary experiment enables us to render an account of 
 the dark currents produced by bodies at a temperature be- 
 low that of combustion. 
 
 When the wire is white hot, it sends up a band of in- 
 tense darkness. This, I say, is due to the destruction of 
 the floating matter. But even when its temperature does 
 not exceed that of boiling water, the wire produces a dark 
 ascending current. This, I say, is due to the distribution 
 of the floating matter. Imagine the wire clasped by the 
 
282 FKAGMENTS OF SCIENCE. 
 
 mote-filled air. My idea is that it heats the air and lightens 
 it, without in the same degree lightening the floating mat- 
 ter. The tendency, therefore, is to start a current of clean 
 air through the mote-filled air. Figure the motion of the 
 air all round the wire. Looking at its transverse section 
 we should see the air at the bottom of the wire bending 
 round it right and left in two branch-currents, ascend- 
 ing its sides and turning to fill the^ partial vacuum created 
 above the wire. Now as each new supply of air, filled with 
 its motes, comes in contact with the hot wire, the clean 
 air, as just stated, is first started through the inert motes. 
 They are dragged after it, but there is a fringe of cleansed 
 air in advance of the motes. The two purified fringes of 
 the two branch-currents unite above the wire, and, keep- 
 ing the motes that once belonged to them right and left, 
 they form by their union the dark band observed in the 
 experiment. This process is incessant. Always the mo- 
 ment the mote-filled air touches the wire this distribution 
 is effected, a permanent dark band being thus produced. 
 Could the air and the particles under the wire pass through 
 its mass we should have a vertical current of particles, but 
 no dark band. For here, though the motes would be left 
 behind at starting, they would hotly follow the ascending 
 current and thus abolish the darkness. 
 
 It has been said that when the platinum wire is intensely 
 heated, the floating matter is not only distributed, but de- 
 stroyed. Let this be proved. I stretched a wire about 4 
 inches long through the air of an ordinary glass shade, rest- 
 ing on its stand. Its lower rim rested on cotton-wool, which 
 also surrounded the rim. The wire was raised to a white 
 heat by an electric current. The air expanded, and some 
 of it was forced through the cotton-wool, while, when the 
 current was interrupted and the air within the shade cooled, 
 the expelled air in its return did not carry motes along with 
 it. At the beginning of this experiment the shade was 
 
DUST AND DISEASE. 283 
 
 charged with floating matter ; at the end of half an hour it 
 was optically empty. 
 
 A second experiment was thus arranged : on the wooden 
 base of a cubical glass shade, measuring 11|- inches a side, 
 upright supports were fixed, and from one support to the 
 other 38 inches of platinum wire were stretched in four 
 parallel lines. The ends of the platinum wire were soldered 
 to two stout copper wires, which passed through the base 
 of the shade and could be connected with a battery. As 
 in the last experiment, the shade rested upon cotton-wool. 
 A beam sent through the shade revealed the suspended 
 matter. The platinum wire was then raised to whiteness. 
 In iive minutes there was a sensible diminution of the mat- 
 ter, and in ten minutes it was totally consumed. This 
 proves that when the platinum wire is sufficiently heated, 
 the floating matter, instead of being distributed, is destroyed. 
 
 But is not the matter really of a character which permits 
 of its destruction by the moderately-heated platinum wire ? 
 Here is the reply : 
 
 1. A platinum tube, with its plug of platinum gauze, 
 was connected with an experimental tube, through which a 
 powerful beam could be sent from an electric lamp placed 
 at its end. The platinum tube was heated till it glowed 
 feebly but distinctly in the dark. The experimental tube 
 was exhausted, and then filled with air which had passed 
 through the red-hot tube. A considerable amount of float- 
 ing matter which had escaped combustion was revealed by 
 the electric beam. 
 
 2. The tube was raised to brighter redness and the air 
 permitted to pass slowly through it. Though diminished 
 in quantity, a certain amount of floating matter passed into 
 the exhausted experimental tube. 
 
 3. The platinum tube was rendered still hotter ; a 
 barely perceptible trace of the floating matter now passed 
 through it. 
 
284 FRAGMENTS OF SCIENCE. 
 
 4. The experiment was repeated, with the difference 
 that the air was sent more slowly through the red-hot tube. 
 The floating matter was totally destroyed. 
 
 5. The platinum tube was now lowered until it bordered 
 upon a visible red heat. The air sent through it still more 
 slowly than in the last experiment carried with it a cloud 
 of floating matter. 
 
 If, then, the suspended matter is destroyed by a bright- 
 red heat, much more is it destroyed by a flame, whose tem- 
 perature is vastly higher than any here employed. So that 
 the blackness introduced into a luminous beam where a 
 flame is placed beneath it is due, as stated, to the destruc- 
 tion of the suspended matter. At a dull-red heat, how- 
 ever, and still more when only on the verge of redness, the 
 platinum tube permitted the motes to pass freely. In the 
 latter case the temperature was 800 or 900 Fahrenheit. 
 This was unable to destroy the suspended matter ; much 
 less, therefore, would a platinum wire heated to 212 be 
 competent to do so. Such a wire can only distribute the 
 matter, not destroy it. 
 
 The floating dust is revealed by intense local illumina- 
 tion. It is seen by contrast with the adjacent illuminated 
 space ; the brighter the illumination the more sensible is 
 the difference. Now, the beam employed in the foregoing 
 experiments is not of the same brightness throughout its 
 entire transverse section. Pass a white switch, or an ivory 
 paper-cutter, rapidly across the beam, the impression of its 
 section will linger on the retina. The section seems to float 
 for a moment in the air as a luminous circle, with a rim much 
 brighter than its central portion. The core of the beam is 
 thus seen to be enclosed by an intensely-luminous sheath. 
 An effect complementary to this is observed when the beam 
 is intersected by the dark band from the platinum wire. 
 The brighter the illumination the greater must be the rela- 
 tive darkness consequent on the withdrawal of the light. 
 
DUST AND DISEASE. 285 
 
 Hence the cross-section of the sheath surrounds the dark 
 band as a darker ring. 
 
 Oxygen, hydrogen, nitrogen, carbonic acid, so prepared 
 as to exclude all floating particles, produce the darkness 
 when poured or blown into the beam. Coal-gas does the 
 same. An ordinary glass shade placed in the air with its 
 mouth downward permits the track of the beam to be seen 
 crossing it. Let coal-gas or hydrogen enter the shade by a 
 tube reaching to its top, the gas gradually fills the shade 
 from the top downward. As soon as it occupies the space 
 crossed by the beam, the luminous track is instantly abol- 
 ished. Lifting the shade so as to bring the common bound- 
 ary of gas and air above the beam, the track flashes forth. 
 After the shade is full, if it be inverted, the gas passes up- 
 ward like a black smoke among the illuminated particles. 
 
 The air of our London rooms is loaded with this organic 
 dust, nor is the country air free from its presence. How r ever 
 ordinary daylight may permit it to disguise itself, a suffi- 
 ciently powerful beam causes dust suspended in air to ap- 
 pear almost as a semi-solid. Nobody could, in the first 
 instance, without repugnance, place the mouth at the illu- 
 minated focus of the electric beam and inhale the thickly- 
 massed dust revealed there. Nor is the repugnance abol- 
 ished by the reflection that, although we do not see the 
 floating particles, we are taking them into our lungs every 
 hour and minute of our lives. 
 
 The Germ- Theory of Contagious Disease. 
 
 There is no respite to this contact with the floating mat- 
 ter of the air ; and the wonder is, not that we should suffer 
 occasionally from its presence, but that so small a portion 
 of it, and even that but rarely diffused over large areas, 
 should appear to be deadly to man. And what is this por- 
 tion ? It was some time ago the current belief that epidemic 
 
286 FRAGMENTS OF SCIENCE. 
 
 diseases generally were propagated by a kind of malaria, 
 which consisted of organic matter in a state of motor-decay; 
 that when such matter was taken into the body through 
 the lungs, skin, or stomach, it had the power of spreading 
 there the destroying process which had attacked itself. 
 Such a power was visibly exerted in the case of yeast. A 
 little leaven was seen to leaven the whole lump, a mere 
 speck of matter in this supposed state of decomposition be- 
 ing apparently competent to propagate indefinitely its own 
 decay. Why should not a bit of rotten malaria work in a 
 similar manner within the human frame ? In 1836 a very 
 wonderful reply was given to this question. In that year 
 Cagniard de la Tour discovered the yeast-plant, a living or- 
 ganism, which, when placed in a proper medium, feeds, grows, 
 and reproduces itself, and in this way carries on the process 
 which we name fermentation. By this striking discovery 
 fermentation was connected with organic growth. 
 
 Schwann, of Berlin, discovered the yeast-plant inde- 
 pendently about the same time ; and in February, 1837, he 
 also announced the important result that, when a decoction 
 of meat is effectually screened from ordinary air, and sup- 
 plied solely with calcined air, putrefaction never sets in. 
 Putrefaction, therefore, he affirmed to be caused by some- 
 thing derived from the air, which something could be de- 
 stroyed by a sufficiently high temperature. The results of 
 Swann were confirmed by the independent experiments of 
 Helmholtz, Ure, and Pasteur, while other methods, pursued 
 by Schultze and by Schroeder and Dusch, led to the same 
 result. But as regards fermentation, the minds of chemists, 
 influenced probably by the great authority of Gay-Lussac, 
 fell back upon the old notion of matter in a state of decay. 
 It was not the living yeast-plant, but the dead or dying 
 parts of it, which, assailed by oxygen, produced the fer- 
 mentation. This notion was finally exploded by Pasteur. 
 He proved that the so-called " ferments " are not such ; 
 
DUST AND DISEASE. 287 
 
 that the true ferments are organized beings, which find in 
 the reputed ferments their necessary food. 
 
 Side by side with these researches and discoveries, and 
 fortified by them and others, has run the germ-theory of 
 epidemic disease. The notion was expressed by Kircher 
 and favored by Linnaeus, that epidemic diseases are due 
 to germs which float in the atmosphere, enter the body, and 
 produce disturbance by the development within the body 
 of parasitic life. While it was still struggling against 
 great odds, this theory found an expounder and a defender 
 in the President of this institution. At a time when most 
 of his medical brethren considered it a wild dream, Sir 
 Henry Holland contended that some form of the germ- 
 theory was probably true. The strength of this theory 
 consists in the perfect parallelism of the phenomena of con- 
 tagious disease with those of life. As a planted acorn 
 gives birth to an oak competent to produce a whole crop 
 of acorns, each gifted with the power of reproducing its 
 parent-tree ; and as thus from a single seedling a whole 
 forest may spring ; so, it is contended, these epidemic dis- 
 eases literally plant their seeds, grow, and shake abroad 
 new germs, which, meeting in the human body their proper 
 food and temperature, finally take possession of whole 
 populations. There is nothing to my knowledge in pure 
 chemistry which resembles the power of self-multiplication 
 possessed by the matter which produces epidemic disease. 
 If you sow wheat you do not get barley ; if you sow small- 
 pox you do not get scarlet fever, but small-pox indefinitely 
 multiplied, and nothing else. The matter of each con- 
 tagious disease reproduces itself as rigidly as if it were (as 
 Miss Nightingale puts it) dog or cat. 
 
 "arasitic Diseases of SilJc-ivorms. Pasteups ResearcJies. 
 
 It is admitted on all hands that some diseases are the 
 product of parasitic growth. Both in man and lower crea- 
 
288 FRAGMENTS OF SCIENCE. 
 
 tures, the existence of such diseases has been demonstrated. 
 I am enabled to lay before you an account of an epidemic 
 of this kind, thoroughly investigated and successfully com- 
 batted by M. Pasteur. For fifteen years a plague had 
 raged among the silk-worms of France. They had sickened 
 and died in multitudes, while those that succeeded in spin- 
 ning their cocoons furnished only a fraction of the normal 
 quantity of silk. In 1853 the silk culture of France pro- 
 duced a revenue of one hundred and thirty millions of 
 francs. During the twenty previous years the revenue had 
 doubled itself, and no doubt was entertained as to its future 
 augmentation. The weight of the cocoons produced in 
 1853 was twenty-six millions of kilogrammes ; in 1865 it 
 had fallen to four millions, the fall entailing in the single 
 year last mentioned a loss of one hundred millions of francs. 
 The country chiefly smitten by this calamity happened 
 to be that of the celebrated chemist, Dumas, now perpetual 
 secretary of the French Academy of Sciences. He turned 
 to his friend, colleague, and pupil, Pasteur, and besought 
 him with an earnestness which the circumstances rendered 
 almost personal, to undertake the investigation of the 
 malady. Pasteur at this time had never seen a silk-worm, 
 and he urged his inexperience in reply to his friend. But 
 Dumas knew too well the qualities needed for such an in- 
 quiry to accept Pasteur's reason for declining it. " Je 
 mets," said he, " un prix extreme a' voir votre attention 
 fixee sur la question qui interesse mon pauvre pays; la 
 misere surpasse tout ce que vous pouvez imaginer." 
 Pamphlets about the plague had been showered upon the 
 public, the monotony of waste-paper being broken at rare 
 intervals by a more or less useful publication. " The 
 Pharmacopoeia of the Silk-worm," wrote M. Cornalia in 
 1860, "is now as complicated as that of man. Gases, 
 liquids, and solids have been laid under contribution. 
 From chlorine to sulphurous acid, from nitric acid to rum, 
 
DUST AND DISEASE. 289 
 
 from sugar to sulphate of quinine all has been invoked in 
 behalf of this unhappy insect." The helpless cultivators, 
 moreover, welcomed with ready trustfulness every new 
 remedy, if only pressed upon them with sufficient hardi- 
 hood. It seemed impossible to diminish their blind confi- 
 dence in their blind guides. In 1863 the French Minister 
 of Agriculture himself signed an agreement to pay 500,000 
 francs for the use of a remedy which its promoter declared 
 to be infallible. It was tried in twelve different depart- 
 ments of France, and found perfectly useless. In no single 
 instance was it successful. It was under these circum- 
 .stances that M. Pasteur, yielding to the entreaties of his 
 friend, betook himself to Alais in the beginning of June, 
 1865. As regards silk husbandry, this was the most im- 
 portant department in France, and it was also that which 
 had been most sorely smitten by the epidemic. 
 
 The silk-worm had been previously attacked by mus- 
 cardine, a disease proved by Bassi to be caused by a vege- 
 table parasite. Though not hereditary, this malady was 
 propagated annually by the parasitic spores, which, wafted 
 by winds, often sowed the disease in places far removed 
 from the centre of infection. Muscardine is now said to 
 be very rare ; but for the last fifteen or twenty years a 
 deadlier malady has taken its place. A frequent outward 
 sign of this new disease are the black spots which cover 
 the silk-worms, hence the name pebrine, first applied to 
 the plague by M. de Quatrefages, and adopted by Pasteur. 
 Pebrine declares itself in the stunted and unequal growth 
 of the worms, in the languor of their movements, in their 
 fastidiousness as regards food, and in their premature 
 death. The track of discovery as regards the epidemic 
 is this: In 1849 Guerin M<neville noticed in the blood 
 of silk-worms vibratory corpuscles which he supposed to 
 be endowed with independent life. Filippi proved him 
 wrong, and showed that the motion of the corpuscles was 
 13 
 
290 FRAGMENTS OF SCIENCE. 
 
 the well-known Brownian motion. But Filippi himself 
 committed the error of supposing the corpuscles ta be 
 normal to the life of the insect. They are really the cause 
 of its mortality the form and substance of its disease. 
 This was well described by Cornalia ; while Lebert and 
 Frey subsequently found the corpuscles not only in the 
 blood, but in all the tissues of the insect. Osimo, in 1857, 
 discovered them in the eggs, and on this observation Vitta- 
 diani founded, in 1859, a practical method of distinguishing 
 healthy from diseased eggs. The test often proved falla- 
 cious, and it was never extensively applied. 
 
 These corpuscles take possession of the intestinal canal, 
 and spread thence throughout the body of the worm. 
 They fill the silk cavities, the stricken insect often going 
 through the motions of spinning without any material to 
 answer to the act. Its organs, instead of being filled with 
 the clear viscous liquid of the silk, are packed to disten- 
 tion by the corpuscles. On this feature of the plague Pas- 
 teur fixed his entire attention. The cycle of the silk- 
 worm's life is briefly this: From the fertile egg comes 
 the little worm, which grows, and casts its skin. This pro- 
 cess of moulting is repeated two or three times at subse- 
 qjaent intervals during the life of the insect. After the last 
 moulting the worm climbs the brambles placed to receive 
 it, and spins among them its cocoon. It passes thus into 
 a chrysalis ; the chrysalis becomes a moth, and the moth 
 when liberated lays the eggs which form the starting-point 
 of a new cycle. Now Pasteur proved that the plague-cor- 
 puscles might be incipient in the egg, and escape detec- 
 tion ; they might also be germinal in the worm, and still 
 baffle the microscope. But as the worm grow r s, the cor- 
 puscles grow also, becoming larger and more defined. In 
 the aged chrysalis they are more pronounced than in the 
 worm ; while in the moth, if either the egg or the worm 
 from which it comes should have been at all stricken^ the 
 
DUST AND DISEASE. 291 
 
 corpuscles infallibly appear, offering no difficulty of detec- 
 tion. This was the first great point made out in 1865 by 
 Pasteur. The Italian naturalists, as aforesaid, recom- 
 mended the examination of the eggs before risking their 
 incubation. Pasteur showed that both eggs and worms 
 might be smitten and still pass muster, the culture of such 
 eggs or such worms being sure to entail disaster. He made 
 the moth his starting-point in seeking to regenerate the race. 
 Pasteur made his first communication on this subject to 
 the Academy of Sciences in September, 1865. It raised 
 a cloud of criticism. Here, forsooth, was a chemist rashly 
 quitting his proper metier and presuming to lay down the / 
 law for the physician and biologist on a subject which was 
 eminently theirs. " On trouva etrange que je fusse si peu 
 au courant de la question ; on m'opposa des travaux qui 
 avaient paru depuis longtemps en Italie, dont les r<sultats 
 montraient Finutilit^ de mes efforts, et I'impossibilit6 d'ar- 
 river a un resultat pratique dans la direction que je m'etais 
 engage. Que mon ignorance fut grande au sujet des re- 
 cherches sans nombre qui avaient paru depuis quinze an- 
 nees." Pasteur heard the buzz, but he continued his work. 
 In choosing the eggs intended for incubation, the cultiva- 
 tors selected those produced in the successful "educa- 
 tions " of the year. But they could not understand the 
 frequent and often disastrous failures of their selected 
 eggs ; for they did not know, and nobody prior to Pasteur 
 was competent to tell them, that the finest cocoons may 
 envelope doomed corpusculous moths. It was not, how- 
 ever, easy to make the cultivators accept new guidance. 
 To strike their imagination, and if possible determine their 
 practice, Pasteur hit upon the expedient of prophecy. In 
 1866 he inspected at St. Hippolyte-du-Fort fourteen differ- 
 ent parcels of eggs intended for incubation. Having ex- 
 amined a sufficient number of the moths which produced 
 these eggs, he wrote out the prediction of what would oc- 
 
292 FRAGMENTS OF SCIENCE. 
 
 cur in 1867, and placed the prophecy as a sealed letter in 
 the hands of the Mayor of St. Hippolyte. 
 
 In 1867 the cultivators communicated to the mayor 
 their results. The letter of Pasteur was then opened and 
 read, and it was found that in twelve out of fourteen cases 
 there was absolute conformity between his prediction and 
 the observed facts. Many of the groups had perished to- 
 tally ; the others had perished almost totally ; and this 
 was the prediction of Pasteur. In two out of the fourteen 
 cases, instead of the prophesied destruction, half an aver- 
 age crop was obtained. Now, the parcels of eggs here re- 
 ferred to were considered healthy by their owners. They 
 had been hatched and tended in the firm hope that the la- 
 bor expended on them would prove remunerative. The appli- 
 cation of the moth-test for a few minutes in 1866 would 
 have saved the labor and averted the disappointment. Two 
 additional parcels of eggs were at the same time submitted 
 to Pasteur. He pronounced them healthy ; and his words 
 were verified by the production of an excellent crop. 
 Other cases of prophecy still more remarkable, because 
 more circumstantial, are recorded in Pasteur's work. 
 
 Pasteur subjected the development of the corpuscles to 
 a searching investigation. "With admirable skill and com- 
 pleteness he examined the various modes by which the 
 plague is propagated. He obtained perfectly healthy 
 worms from moths perfectly free from corpuscles, and se- 
 lecting from them 10, 20, 30, 50, as the case might be, he 
 introduced into the worms the corpusculous matter. It was 
 first permitted to accompany the food. Let us take a sin- 
 gle example out of many. Rubbing up a small corpuscu- 
 lous worm in water, he smeared the mixture over the mul- 
 berry-leaves. Assuring himself that the leaves had been 
 eaten, he watched the consequences from day to day. Side 
 by side with the infected worms he reared their fellows, 
 keeping them as much as possible out of the way of infec- 
 
DUST AND DISEASE. 293 
 
 tion. These constituted his " lot temoign," his standard of 
 comparison. On the 16th of April, 1868, he thus infected 
 thirty worms. Up to the 23d they remained quite well. 
 On the 25th they seemed well, but on that day corpuscles 
 were found in the intestines of two of them. They first 
 form in the tunic of the intestine. On the 27th, or eleven 
 days after the infected repast, two fresh worms were exam- 
 ined, and not only was the intestinal canal found in each 
 case invaded, but the silk-organ itself was found charged 
 with corpuscles. On the 28th, the twenty-six remaining 
 worms were covered by the black spots of pebrine. On the 
 30th, the difference of size between the infected and non- 
 infected worms was very striking, the sick worms being not 
 more than two-thirds of the size of the healthy ones. On 
 the 2d of May, a worm which had just finished its fourth 
 moulting was examined. Its whole body was so filled with 
 corpuscles as to excite astonishment that it could live. 
 The disease advanced, the worms died and were examined, 
 and on the llth of May only six out of the thirty remained. 
 They were the strongest of the lot, but, on being searched, 
 they also were found charged with corpuscles. Not one of 
 the thirty worms had escaped ; a single corpusculous meal 
 had poisoned them all. The standard lot, on the contrary, 
 spun their fine cocoons, and two only of their moths were 
 found to contain any trace of corpuscles, which had, doubt- 
 less, been introduced during the rearing of the worms. 
 
 As his acquaintance with the subject increased, Pas- 
 teur's desire for precision augmented, and he finally gives 
 the growing number of corpuscles seen in the field of his 
 microscope from day to day. After a contagious repast, 
 the number of worms containing the parasite gradually 
 augmented until finally it became cent, per cent. The 
 number of corpuscles would at the same time rise from to 
 1, to 10, to 100, and sometimes even to 1,000 or 1,500 for a 
 single field of his microscope. He then varied the mode 
 
294 FRAGMENTS OF SCIENCE. 
 
 of infection. He inoculated healthy worms with the cor- 
 pusculous matter, and watched the consequent growth of 
 the disease. He showed how the worms inoculate each 
 other by the infliction of visible wounds with their claws. 
 In various cases he washed the claws, and found corpuscles 
 in the water. He demonstrated the spread of infection by 
 the simple association of healthy and diseased worms. The 
 diseased worms sullied the leaves by their dejections, they 
 also used their claws, and spread infection -in both ways. 
 It was no hypothetical infected medium that killed the 
 worms, but a definitely-organized and isolated thing. He 
 examined the question of contagion at a distance, and de- 
 monstrated its existence. In fact, as might be expected 
 from Pasteur's antecedents, the investigation was exhaus- 
 tive, the skill and beauty of his manipulation finding fitting 
 correlatives in the strength and clearness of his thought. 
 
 The following quotation from Pasteur's work clearly 
 shows the relation in which his researches stand to this 
 great question : 
 
 " Place," he says, " the most skilful educator, even the most expert 
 rnicroscopist, in presence of large educations which present the symp- 
 toms described in our experiments ; his judgment will necessarily be er- 
 roneous if he confines himself to the knowledge which preceded my re- 
 searches. The worms will not present to him the slightest spot of 
 pcbrine ; the microscope will not reveal the existence of corpuscles ; the 
 mortality of the worms will be null or insignificant ; and the cocoons 
 leave nothing to be desired. Our observer would, therefore, conclude 
 without hesitation that the eggs produced will be good for incubation. 
 The truth is, on the contrary, that all the worms of these fine crops have 
 been poisoned ; that, from the beginning, they carried in them the germ 
 of the malady ; ready to multiply itself beyond measure in the chrysa- 
 lides and the moths, thence to pass into the eggs and smite with sterih'ty 
 the next generation. And what is the first cause of the evil concealed 
 under so deceitful an exterior ? In our experiments we can, so to speak, 
 touch it with our fingers. It is entirely the effect of a single corpuscu- 
 lous repast ; an effect more or less prompt according to the epoch of life 
 of the worm that has eaten the poisoned food." 
 
DUST AND DISEASE. 295 
 
 Pasteur describes in detail his method of securing 
 healthy eggs, which is nothing less than a mode of restor- 
 ing to France her ancient prosperity in silk husbandry. 
 And the justification of his work is to be found in the re- 
 ports which reached him of the application, and the unpar- 
 alleled success of his method, at the time he was putting 
 his researches together for final publication. In France 
 and Italy his method has been pursued with the most sur- 
 prising results. It was an up-hill fight which led to this 
 triumph, but opposition stimulated Pasteur, and thus, with- 
 out meaning it, did good service. " Ever," he says, " since 
 the commencement of these researches, I have been ex* 
 posed to the most obstinate and unjust contradictions ; but 
 I have made it a duty to leave no trace of these contests in 
 this book." And, in reference to parasitic diseases, he uses 
 the following weighty words : " II est au pouvoir de 1'homme 
 de faire disparaitre de la surface du globe les maladies par- 
 asitaires, si, comme c'est ma conviction, la doctrine des gene"- 
 rations spontanees est une chimere." 
 
 Pasteur dwells upon the ease with which an island like 
 Corsica might be absolutely isolated from the silk-worm 
 epidemic. And, with regard to other epidemics, Mr. Simon 
 describes the extraordinary exemption of the Scilly Isles for 
 the ten years extending from 1851 to 1860. Of the 627 
 registration districts of England, one only had an entire es- 
 cape from diseases which, in whole or in part, were preva- 
 lent in all the others : " In all the ten years it had not a 
 single death by measles, nor a single death by small-pox, 
 nor a single death by scarlet fever. And why ? Not be- 
 cause of its general sanitary merits, for it had an average 
 amount of other evidence of unhealthiness. Doubtless, the 
 reason of its escape was that it was insular. It was the 
 district of the Scilly Isles to which it was most improb- 
 able that any febrile contagion should come from without. 
 And its escape is an approximative proof that, at least for 
 
296 FRAGMENTS OF SCIENCE. 
 
 those ten years, no contagium of measles, nor any contagium 
 of scarlet fever, nor any contagium of small-pox, had arisen 
 spontaneously within its limits." It may be added that 
 there were only seven districts in England in which no 
 death from diphtheria occurred, and that, of those seven S 
 districts, the district of the Scilly Isles was one. s 
 
 A second parasitic disease of silk-worms, called in France 
 laflacherie, coexistent with pebrine but quite distinct from 
 it, has also been investigated. Enough, however, has been 
 said to send such of you as are interested in these questions 
 to the original volumes for further information. To one 
 important practical point M. Pasteur, in a letter written to 
 me, directs attention : 
 
 " Permettez-moi de terminer ces quelques lignes que je dois dieter, 
 vaincu que je suis par la maladie, en vous faisant observer que vous 
 rendriez service aux Colonies de la Grande-Bretagne en repandant la 
 connaissance de ce livre, etdes principes que j'etablistou chant la maladie 
 des vers a soie. Beaucoup de ces colonies pourraient cultiver le murier 
 avec succes, et en jetant les yeux sur mon ouvrage vous vous convaincrez 
 aisement qu'il est facile aujourd'hui, non-seulement d'eloigner la maladie 
 regnante, mais en outre de donner aux recoltes de la soie une prosperite 
 qu'elles n'ont jamais eue." 
 
 Origin and Propagation of Contagious Matter. 
 
 Prior to Pasteur, the most diverse and contradictory 
 opinions were entertained as to the contagious character of 
 pebrine ; some stoutly affirmed it, others as stoutly denied 
 it. But on one point all were agreed. " They believed in 
 the existence of a deleterious medium, rendered epidemic 
 by some occult and mysterious influence, to which was at- 
 tributed the cause of the disease." Those acquainted with 
 medical literature will not fail to observe an instructive 
 analogy here. We have on the one side accomplished 
 writers ascribing epidemic diseases to " deleterious media," 
 which arise spontaneously in crowded hospitals and over 
 
DUST AND DISEASE. 297 
 
 ill-smelling drains. According to them the matter of epi- 
 demic disease is formed de novo in a putrescent atmosphere. 
 On the other side we have writers, clear, vigorous, with 
 well-defined ideas and methods of research, contending that 
 the matter which produces epidemic disease comes always 
 from a parent-stock. It behaves as germinal matter, and 
 they do not hesitate to regard it as such. They no more 
 believe in the spontaneous generation of such diseases than 
 they do in the spontaneous generation of mice. Pasteur, 
 for example, found that pebrine had been known for an in- 
 definite time as a disease among silk-worms. The develop- 
 ment of it which he combated was merely the expansion of 
 an already existing power, the bursting into open confla- 
 gration of a previously smouldering fire. There is nothing 
 surprising in this ; for though epidemic disease requires a 
 special contagium to produce it, surrounding conditions 
 must have a potent influence on its development. Common 
 seeds may be duly sown, but the conditions of temperature 
 and moisture may be such as to restrict or altogether pre- 
 vent the subsequent growth. Looked at, therefore, from 
 the point of view of the germ-theory, the exceptional energy 
 which epidemic disease from time to time exhibits is not 
 out of harmony with the method of Nature. You some- 
 times hear diphtheria spoken of as if it were a new disease 
 of the last twenty years ; but Mr. Simon tells me that from 
 about three centuries ago, when tremendous epidemics of it 
 began to rage in Spain (where it was named Garrotitto), 
 and soon afterward in Italy, the disease has been well 
 known to all successive generations of doctors ; and that, 
 for instance, in or about 1758, Dr. Starr, of Liskeard, in a 
 communication to the Royal Society, particularly described 
 the disease, with all the characters which have recently 
 again become familiar, but under the name of morbus stran- 
 gulatorius, as then severely epidemic in Cornwall ; a fact 
 the more interesting as diphtheria, in its more modern re- 
 
298 FRAGMENTS OF SCIENCE. 
 
 appearance, again showed predilection for that remote 
 county. Many also believe that the black death of five 
 centuries ago.has disappeared as mysteriously as it came, 
 but Mr. Simon finds that it is believed to be prevalent at 
 this hour in some of the northwestern parts of India. 
 
 Let me here state an item of my own experience. When 
 I was at the Bel Alp last year the clergyman appointed to 
 that station received letters informing him of the breaking 
 out of scarlet fever among his children. He lived, if I re- 
 member rightly, on the healthful eminence of Dartmoor, 
 and it was difficult to imagine how scarlet fever could have 
 been wafted to the place. A drain ran close to his house, 
 and on it his suspicions were manifestly fixed. Some of 
 our medical writers would fortify him in this notion, while 
 those of another school would deny to a drain, however 
 foul, the power of producing a specific disease. After close 
 inquiry, he recollected that a hobby-horse had been used 
 both by his boy and another that a short time previously 
 had passed through scarlet fever. Drains and cesspools 
 are by no means in such evil odor as they used to be. A 
 fetid Thames and a low death-rate occur from time to time 
 together in London. For, if the special matter or germs of 
 epidemic disorder be not present, a corrupt atmosphere, 
 however obnoxious otherwise, will not produce the disorder. 
 Corrupted air may promote an epidemic, but cannot origi- 
 nate it. On the other hand, through the transport of the 
 special germ or virus, disease may develop itself in regions 
 where the drainage is good and the atmosphere pure. 
 
 If you see a new thistle growing in your field you feel 
 sure that its seed has been wafted thither. Just as sure 
 does it seem that the contagious matter of scarlatina, or 
 any other contagious fever, has been transplanted to the 
 place where it newly appears. With a clearness and con- 
 clusiveness not to be surpassed Dr. William Budd has 
 traced such diseases from place to place; showing how 
 
DUST AND DISEASE. 299 
 
 they plant themselves at distinct foci among populations 
 subjected to the same atmospheric influences, just as grains 
 of corn might be carried in the pocket and sown. Hilde- 
 brand, to whose remarkable work, Du Typhus Contagieux, 
 Dr. de Mussy has directed my attention, gives the following 
 striking case, both of the durability and the transport of 
 the virus of scarlatina : " Un habit noir que j'avais en visi- 
 tant une malade attaque*e de scarlatine, et que je portai de 
 Vienne de Podolie, sans 1' avoir mis depuis plus d'un an et 
 demi, me communiqua, des que je fus arrive, cette maladie 
 contagieuse, que je repandis ensuite dans cette province, otl 
 elle etait jusqu'alors presque inconnue." Some years ago 
 Dr. de Mussy himself was summoned to a country-house in 
 Surrey to see a young lady who was suffering from a dropsy, 
 evidently the consequence of scarlatina. The original dis- 
 ease being of a very mild character had been quite over- 
 looked, but circumstances were recorded which could leave 
 no doubt upon the mind as to the nature and cause of the 
 complaint. But then the question arose, how did the young 
 lady catch the scarlatina ? She had come there on a visit 
 two months previously, and it was only after she had been 
 a month in the house that she was taken ill. The house- 
 keeper at once cleared up the mystery. The young lady 
 on her arrival had expressed a particular wish to occupy a 
 nice room in an isolated tower, and in that room six months 
 previously a visitor had been confined with an attack of 
 scarlatina. The room had been swept and whitewashed, 
 but the carpets had been permitted to remain. 
 
 Thousands of cases could probably be cited in which 
 the disease has shown itself in this mysterious way, but 
 where a strict examination has revealed its true parentage 
 and extraction. Is it then philosophical to take refuge in 
 the fortuitous concourse of atoms as a cause of specific dis- 
 ease, merely because in special cases the parentage may be 
 indistinct? Those best acquainted with atomic Nature, 
 
300 FRAGMENTS OF SCIENCE. 
 
 and who are most ready to admit, as regards even higher 
 things than this, the potentialities of matter, will be the 
 Jast to accept these rash hypotheses. 
 
 The Germ-Theory applied to Surgery. 
 
 Not only medical but surgical science is now seeking 
 light and guidance from this germ-theory. Upon it the 
 antiseptic system of Professor Lister, of Edinburgh, is 
 founded ; and if the facts be correctly given, the results 
 are extraordinary. As already stated, the germ-theory of 
 putrefaction was started by Schwann, but the illustrations 
 of this theory adduced by Professor Lister are of such, pub- 
 lic moment as not only to justify, but to render imperative, 
 their introduction here : 
 
 Schwann's observations, says Professor Lister, did not receive the 
 attention which they appear to me to have deserved. The fermentation 
 of sugar was generally allowed to be occasioned by the torula cerevisice ; 
 but it was not admitted that putrefaction was due to an analogous 
 agency. And yet the two cases present a very striking parallel. In 
 each a stable chemical compound, sugar in the one case, albumen in the 
 other, undergoes extraordinary chemical changes under the influence of 
 an excessively minute quantity of a substance which, regarded chemi- 
 cally, we should suppose inert. As an example of this in the case of 
 putrefaction, let us take a circumstance often witnessed in the treatment 
 of large chronic abscesses. In order to guard against the access of at- 
 mospheric air, we used to draw off the matter by means of a canula and 
 trocar, such as you see here, consisting of a silver tube with a sharp- 
 pointed steel rod fitted into it, and projecting beyond it. The instru- 
 ment, dipped in oil, was thrust into the cavity of the abscess, the trocar 
 was withdrawn, and the pus flowed out through the canula, care being 
 taken by gentle pressure over the part to prevent the possibility of 
 regurgitation. The canula was then drawn out with due precaution 
 against the reflux of air. This method was frequently successful as to 
 its immediate object, the patient being relieved from the mass of the ac- 
 cumulated fluid, and experiencing no inconvenience from the operation. 
 But the pus was pretty certain to reaccumulate in course of time, and it 
 became necessary again and again to repeat the process. And unhappily 
 
DUST AND DISEASE. 301 
 
 there was no absolute security of immunity from bad consequences. 
 However carefully the procedure was conducted, it sometimes happened, 
 even though the puncture seemed healing by first intention, that feverish 
 symptoms declared themselves in the course of the first or second day, 
 and, on inspecting the seat of the abscess, the skin was, perhaps, seen to 
 be red, implying the presence of some cause of irritation, while a rapid 
 reaccumulation of the fluid was found to have occurred. Under these 
 circumstances, it became necessary to open the abscess by free incision, 
 when a quantity, large in proportion to the size of the abscess, say, for 
 example, a quart, of pus escaped, fetid from putrefaction. Now, how 
 had this change been brought about? "Without the germ-theory, I 
 venture to say, no rational explanation of it could have been given. It 
 must have been caused by the introduction of something from without. 
 Inflammation of the punctured wound, even supposing it to have oc- 
 curred, would not explain the phenomenon. For mere inflammation, 
 whether acute or chronic, though it occasions the formation of pus, does 
 not induce putrefaction. The pus originally evacuated was perfectly 
 sweet, and we know of nothing to account for the alteration in its quality 
 but the influence of something derived from the external world. And 
 what could that something be ? The dipping of the instrument in oil, 
 and the subsequent precautions, prevented the entrance of oxygen. Or 
 even if you allowed that a few atoms of the gas did enter, it would be 
 an extraordinary assumption to make that these could in so short a time 
 effect such changes in so large a mass of albuminous material. Besides, 
 the pyogenic membrane is abundantly supplied with capillary vessels, 
 through which arterial blood, rich in oxygen, is perpetually flowing; 
 and there can be little doubt that the pus, before it was evacuated at 
 all, was liable to any action which the element might be disposed to 
 exert upon it. 
 
 On the oxygen-theory, then, the occurrence of putrefaction under 
 these circumstances is quite inexplicable. But if you admit the germ- 
 theory, the difficulty vanishes at once. The canula and trocar having 
 been lying exposed to the air, dust will have been deposited upon them, 
 and will be present in the angle between the trocar and the silver tube, 
 and in that protected situation will fail to be wiped off when the instru- 
 ment is thrust through the tissues. Thus when the trocar is withdrawn, 
 some portions of this dust will naturally remain upon the margin of the 
 canula, which is left projecting into the abscess, and nothing is more 
 likely than that some particles may fail to be washed off by the stream 
 of out-flowing pus, but may be dislodged when the tube is taken out, 
 and left behind in the cavity. The germ-theory tells us that these par- 
 
302 FRAGMENTS OF SCIENCE. 
 
 tides of dust will be pretty sure to contain the germs of putrefactive 
 organisms, and if one such is left in the albuminous liquid, it will 
 rapidly develop at the high temperature of the body, and account for 
 all the phenomena. 
 
 But striking as is the parallel between putrefaction in this instance 
 and the vinous fermentation, as regards the greatness of the effort pro- 
 duced, compared with the minuteness and the inertness, chemically 
 speaking, of the cause, you will naturally desire further evidence of the 
 similarity of the two processes. You can see with the microscope the 
 torula of fermenting must or beer. Is there, you may ask, any organism 
 to be detected in the putrefying pus ? Yes, gentlemen, there is. If any 
 drop of the putrid matter is examined with a good glass, it is found to 
 be teeming with myriads of minute jointed bodies, called vibrios, which 
 indubitably proclaim their vitality by the energy of their movements. It 
 is not an affair of probability, but a fact, that the entire mass of that 
 quart of pus has become peopled with living organisms as the result of 
 the introduction of the canula and trocar ; for the matter first let out 
 was as free from vibrios as it was from putrefaction. If this be so, 
 the greatness of the chemical changes that have taken place in the pus 
 ceases to be surprising. We know that it is one of the chief peculiari- 
 ties of living structures that they possess extraordinary powers of effect- 
 ing chemical changes in materials in their vicinity, out of all proportion 
 to their energy as mere chemical compounds. And we can hardly doubt 
 that the animalcules which have been developed in the albuminous liquid, 
 and have grown at its expense, must have altered its constitution, just 
 as we ourselves alter that of the materials on "which we feed. 1 
 
 Secured from the danger of putrefaction, it is amazing 
 how, under the hands of a really able surgeon, the human 
 flesh and bones may be cut, torn, and crunched with im- 
 punity. The accounts of the operations of our eminent 
 surgeons read like romance. On this, however, I must not 
 dwell further than to recommend to your attention a case 
 described in the British Medical Journal for the 14th of 
 January last. In the operations of Professor Lister care 
 is taken that every portion of tissue laid bare by the knife 
 shall be defended from germs ; that if they fall upon the 
 wound they shall be killed as they fall. With this in view 
 1 Introductory Lecture before the University of Edinburgh. 
 
DUST AND DISEASE. 303 
 
 he showers upon his exposed surfaces the spray of diluted 
 carbolic acid, which is particularly deadly to the germs, 
 and he surrounds the wound in the most careful manner 
 with antiseptic bandages. To those accustomed to strict 
 experiment it is manifest that we have a strict experimenter 
 here a man with a perfectly distinct object in view, which 
 he pursues with never-tiring patience and unwavering faith. 
 And the result, in his hospital practice, as described by him- 
 self, has been, that even in the midst of abominations too 
 shocking to be mentioned here, and in the neighborhood 
 of wards where death was rampant from pyaemia, erysipe- 
 las, and hospital gangrene, he was able to keep his patients 
 absolutely free from these terrible scourges. Let me here 
 recommend to your attention Professor Lister's "Intro- 
 ductory Lecture before the University of Edinburgh," which 
 I have already quoted ; his paper on "The Effect of the 
 Antiseptic System of Treatment on the Salubrity of a Sur- 
 gical Hospital ; " and the article in the British Medical 
 Journal, to which I have just referred. 
 
 If, instead of using carbolic-acid spray, he could sur- 
 round his wounds with properly-filtered air, the result 
 would, he contends, be the same. In a room where the 
 germs not only float but cling to clothes and walls, this 
 would be difficult if not impossible. But surgery is ac- 
 quainted with a class of wounds in which the blood is freely 
 mixed with air that has passed through the lungs, and it is a 
 most remarkable fact that such air does not produce putre- 
 faction. Professor Lister, as far as I know, was the first to 
 give a philosophical interpretation of this fact, which he 
 decribes and comments upon thus : 
 
 I have explained to my own mind the remarkable fact that in simple 
 fracture of the ribs, if the lung be punctured by a fragment, the blood 
 effused into the pleural cavity, though freely mixed with air, undergoes 
 no decomposition. The air is sometimes pumped into the pleural cavity 
 in such abundance that, making its way through the wound in the pleura 
 
304 FRAGMENTS OF SCIENCE. 
 
 cos tails, it inflates the cellular tissue of the whole body. Yet this occa- 
 sions no alarm to the surgeon (although if the blood in the pleura were to 
 putrefy, it would infallibly occasion dangerous suppurative pleurisy). 
 Why air introduced into the pleural cavity through a wounded lung 
 should have such wholly different effects from that entering directly 
 through a wound in the chest was to me a complete mystery until I heard 
 of the germ-theory of putrefaction, when it at once occurred to me that 
 it was only natural, that air should be filtered of germs by the air-pas- 
 sages, one of whose offices is to arrest inhaled particles of dust, and pre- 
 vent them from entering the air-cells. 
 
 I shall have occasion to refer to this remarkable hy- 
 pothesis further on. 
 
 The advocates of the germ-theory, both of putrefaction 
 and epidemic disease, hold that both arise, not from the air, 
 but from something contained in the air. They hold, more- 
 over, that " something " to be not a vapor nor a foreign 
 gas, nor indeed a molecule of any kind, but a particle? 
 The term " particulate " has been used in the Reports of 
 the Medical Department of the Privy Council to describe 
 this supposed constitution of contagious matter ; a,nd Dr. 
 Sanderson's experiments render it in the highest degree 
 probable, if they do not actually demonstrate, that the virus 
 of small-pox is " particulate." Definite knowledge upon 
 this point is of exceeding importance, because in the treat- 
 ment of particles methods are available which it would be 
 futile to apply to molecules. 
 
 Application of iMminous Beams to researches of this 
 nature. 
 
 My own interference with this great question, while 
 sanctioned by eminent names, has been also an object of 
 
 1 As regards size, there is probably no sharp line of division between 
 molecules and particles ; the one gradually shades into the other. But 
 the distinction that I would draw is this : the atom or the molecule, if 
 free, is always part of a gas, the particle is never so. A particle is a bit 
 of liquid or solid matter formed by the aggregation of atoms or molecules. 
 
DUST AND DISEASE. 305 
 
 varied and ingenious attack. On this point I will only say 
 that when feeling escapes from behind the intellect, where 
 it is a useful urging force, and places itself in front of the 
 intellect, it is liable to produce glamour and all manner 
 of delusions. Thus my censors, for the most part, have lev- 
 elled their remarks against positions which I never assumed, 
 and against claims which I never made. The simple his- 
 tory of the matter is this : During the autumn of 1868 I 
 was much occupied with the observations referred to at the 
 beginning of this discourse. For fifteen years I had ha- 
 bitually employed the electric light, making use of the 
 floating dust to reveal the paths of luminous beams ; but 
 until 1868, when I was driven to it, I did not intentionally 
 reverse the process and employ a luminous beam to reveal 
 and examine the dust. In a paper presented to the Royal 
 Society in December, 1869, 1 thus described the observa- 
 tions which induced me to give more special attention to 
 the question of spontaneous generation and the germ- 
 theory of epidemic disease. 
 
 The Floating Matter of the Air. 
 
 Prior to the discovery of the foregoing action (the chemical action of 
 light upon vapors), and also during the experiments just referred to, the 
 nature of my work compelled me to aim at obtaining experimental tubes 
 absolutely clean upon the surface, and absolutely empty within. Neither 
 condition is, however, easily attained. 
 
 For however well the tubes might be washed and polished, and how- 
 ever bright and pure they might appear in ordinary daylight, the electric 
 beam infallibly revealed signs and tokens of dirt. The air was always 
 present, and it was sure to deposit some impurity. All chemical pro- 
 cesses, not conducted in a vacuum, are open to this disturbance. When 
 the experimental tube was exhausted it exhibited no trace of floating 
 matter, but on admitting the air through the U-tubes containing caustic 
 potash and sulphuric acid, a dust-cone more or less distinct was always re- 
 vealed by the powerfully-condensed electric beam. 
 
 The floating motes resembled minute particles of liquid which had 
 
306 FRAGMENTS OF SCIENCE. 
 
 been carried mechanically into the experimental tube. Precautions were 
 therefore taken to prevent any such transfer. They produced little or no 
 mitigation. I did not imagine at the time that the dust of the external 
 air could find such free pa'ssage through the caustic potash and the sul- 
 phuric-acid tubes. But the motes really came from without. They also 
 passed with freedom through a variety of ethers and alcohols. In fact, 
 it requires long-continued action on the part of an acid first to wet the 
 motes and afterward to destroy them. By carefully passing the air 
 through the flame of a spirit-lamp or through a platinum tube heated to 
 bright redness, the floating matter was sensibly destroyed. It was there- 
 fore combustible, in other words, organic matter. I tried to intercept it 
 by a large respirator of cotton-wool. Close pressure was necessary to 
 render the wool effective. A plug of the wool rammed pretty tightly into 
 the tube through which the air passed was finally found competent to 
 hold back the motes. They appeared from time to time afterward and 
 gave me much trouble ; but they were invariably traced in the end to 
 some defect in the purifying apparatus to some crack or flaw in the 
 sealing-wax employed to render the tubes air-tight. Thus through proper 
 caie, but not without a great deal of searching out of disturbances, the 
 experimental tube, even when filled vdth pure air or vapor, contains 
 nothing competent to scatter the light. The space within it has the as- 
 pect of an absolute vacuum. 
 
 An experimental tube in this condition I call optically empty. 
 
 The simple apparatus employed in these experiments will be at once 
 understood by reference to the figure on page 307. S S' is the glass ex- 
 perimental tube which has varied in length from 1 to 5 feet, and which may 
 be from 2 to 3 inches in diameter. From the end S the pipe p p' passes to 
 an air-pump. Connected with the other end S' we have the flask F, con- 
 taining the liquid whose vapor is to be examined ; then follows a U-tube, 
 T, filled with fragments of clean glass wetted with sulphuric acid ; then 
 a second U-tube, T', containing fragments of marble wetted with caustic 
 potash ; and finally a narrow straight tube, 1 ', containing a tolerably 
 tightly-fitting plug of cotton-wool. To save the air-pump gauge from the 
 attack of such vapors as act on mercury, as also to facilitate observa- 
 tion, a separate barometer tube was employed. 
 
 Through the cork which stops the flask F two glass tubes, a and 5, pass 
 air-tight. The tube a ends immediately under the cork ; the tube b, on 
 the contrary, descends to the bottom of the flask and dips into the liquid. 
 The end of the tube b is drawn out so as to render very small the orifice 
 through which the air escapes into the liquid". 
 
 The experimental tube S S' being exhausted, a cock at the end S' is 
 
308 FRAGMENTS OF SCIENCE. 
 
 carefully turned on. The air passes slowly through the cotton-wool, the 
 caustic potash, and the sulphuric acid in succession. Thus purified it 
 enters the flask F and bubbles through the liquid. Charged with vapor 
 it finally passes into the experimental tube, where it is submitted to ex- 
 amination. The electric lamp L placed at the end of the experimental 
 tube furnished the necessary beam. 
 
 "Wanting the cotton-wool the floating matter of the air 
 ran the gantlet of this system. The fact thus forced upon 
 my attention had a bearing too obvious to be overlooked. 
 It rendered at once evident to the senses why air filtered 
 through cotton- wool is incompetent to generate animalcular 
 life. The air is rendered by this treatment optically pure ; 
 in other words, freed from all floating matter, germs in- 
 cluded. But the observations also revealed the great 
 liability to error in experiments of this nature. They 
 showed that without an amount of care which was hardly 
 to be expected in all cases, error would be inevitable. It 
 was especially manifest that the chemical method of 
 Schultze might lead to the 'most erroneous consequences ; 
 that neither acids nor alkalies had the power of rapid 
 destruction which they had been supposed to possess. In 
 short, the employment of the luminous beam rendered 
 evident the cause of success in experiments rigidly con- 
 ducted like those of Pasteur ; while it made equally evident 
 the certainty of failure in experiments less severely and less 
 skilfully carried out. 
 
 Dr. Bennetts Experiments. 
 
 Take, for example, the well-conceived experiments of 
 Dr. Hughes Bennett, described before the Royal Society 
 of Surgeons in Edinburgh, on January 17, 1868. 1 Into 
 flasks containing decoctions of liquorice-root, hay, or tea, 
 Dr. Bennett, by an ingenious method, forced air. The air 
 was driven through two U-tubes, the one containing a so- 
 
 1 British Medical Journal, 13, pt. ii. 1868. 
 
DUST AND DISEASE. 309 
 
 lution of caustic potash, the other sulphuric acid. " All the 
 bent tubes," says Dr. Bennett, " were filled with fragments 
 of pumice-stone to break up the air, so as to prevent the 
 possibility of any germs passing through in the centre of 
 bubbles." The air also passed through a Liebig's bulb 
 containing sulphuric acid, and also through a bulb contain- 
 ing gun-cotton. 
 
 It was only natural for Dr. Bennett to believe that his 
 bent tubes entirely cut off the germs. Previous to the 
 observations just referred to I also believed in their compe- 
 tence to do this. But these observations destroy any such 
 notion. The gun-cotton, moreover, will fail to arrest the 
 whole of the floating matter unless it is tightly packed, 
 and there is no indication in Dr. Bennett's memoir that it 
 was so packed. On the whole, I should infer from the 
 mere inspection of the apparatus the very results which 
 Dr. Bennett has described a retardation of the develop- 
 ment of life, a total absence of it in some cases, and its 
 presence in others. 
 
 In his first series of experiments eight flasks were fed 
 with his sifted air, and five with common air. In ten or 
 twelve days all the five had fungi in them; while it re- 
 quired from four to nine months to develop fungi in the 
 others. In one case, moreover, even after this interval, no 
 fungi appeared. In a second series of experiments there 
 was a similar exception. In a third series of experiments 
 he abandoned the cork stoppers used in the first and second 
 series, and employed glass stoppers. Flasks containing 
 decoctions of tea, beef, and hay, were filled with common 
 air, and other flasks with sifted air. In every one of the 
 former fungi appeared, and in not one of the latter. These 
 experiments simply ruin the doctrine that Dr. Bennett 
 finally espouses. 
 
 In all these cases the prepared air was forced into the 
 infusion when it was boiling hot. Dr. Bennett made a 
 
310 FRAGMENTS OF SCIENCE. 
 
 fourth series of experiments, in which,- previous to forcing 
 in the air, he permitted the flasks to cool. Into four bottles 
 thus treated he forced prepared air, and after a time found 
 fungi in all of them. What is his conclusion ? Not that 
 the boiling hot liquid employed in his first experiments had 
 destroyed such germs as had run the gantlet of his ap- 
 paratus ; but that air which, previous to being sealed up, 
 had been exposed to a temperature of 212 is too rare to 
 support life. This conclusion is so remarkable that it ought 
 to be stated in Dr. Bennett's own words : " It may be 
 easily conceived that air subjected to a boiling temperature 
 is so expanded as scarcely to merit the name of air, and 
 that it is more or less unfit for the purpose of sustaining 
 animal or vegetable life," 
 
 Now numerical data are attainable here, and, as a 
 matter of fact, I live and flourish for a considerable portion 
 of each year in air of less density than that which Dr. 
 Bennett describes as scarcely meriting the name of air; 
 the Swiss men, women, children, flocks, herds, tadpoles, 
 grasshoppers, flowers, and grasses, do the same, while the 
 chamois rears its kids in air rarer still. 
 
 In a fifth series of experiments sixteen bottles were 
 filled with infusions. Into four of them, while cold, or- 
 dinary unheated and unsifted air was pumped. In these 
 four bottles fungi were developed. Into four other bottles, 
 containing a boiling infusion, ordinary air was also pumped 
 - no fungi were here developed. Into four other bottles 
 containing an effusion which had been boiled and permitted 
 to cool, sifted air was pumped no fungi were developed. 
 Finally, into four bottles containing a boiling infusion, 
 sifted air was pumped no fungi were developed. Only, 
 therefore, in the four cases where the infusions were cold 
 infusions, and the air ordinary air, did fungi appear. 
 
 Dr. Bennett does not draw from these experiments the 
 conclusion to which they so obviously point. On them, on 
 
DUST AND DISEASE. 311 
 
 the contrary, their author founds a defence of the doctrine 
 of spontaneous generation, and a general theory of sponta- 
 neous development. So strongly was he impressed with 
 the idea that the germs could not possibly pass through his 
 potash and sulphuric-acid tubes, that the appearance of 
 fungi, even in a small minority of cases, where the air had 
 been sent through these tubes, was to him conclusive evi- 
 dence of the spontaneous origin of such fungi. And he 
 accounts for the absence of life in many of his experi- 
 ments by resorting to an hypothesis which will not bear 
 a moment's consideration. But now that we know that 
 organic particles may pass unscathed through alkalies and 
 acids, the experiments of Dr. Bennett are precisely what 
 ought, under the circumstances, to be expected. Indeed, 
 their harmony, with the conditions now revealed, is a proof 
 of the honesty and accuracy with which they were executed. 
 On another point also the luminous beam will cast a 
 light. Pasteur opened flasks upon the Mer de Glace, and, 
 being careful not to come between the wind and his flasks, 
 found the air incompetent, in the great majority of cases, 
 to generate life. M. Pouchet repeated Pasteur's experiment 
 in the Pyrenees, adding the precaution of holding the flasks, 
 when they were opened, above his head. The luminous 
 beam at once shows us the effect of this additional precau- 
 tion. Let smoking brown paper be placed at the open 
 mouth of a glass shade so that the smoke shall ascend and 
 fill the shade. A beam sent through the shade forms a 
 bright track through the smoke. When the closed fist is 
 placed underneath the shade, a vertical wind of surprising 
 violence, considering the small elevation of temperature, 
 rises from the hand, displacing by comparatively dark air 
 the illuminated smoke. Such a wind infallibly rose from 
 M. Pouchet's body as he held his flasks above his head, and 
 thus the precaution of Pasteur of not coming between the 
 wind and the flask was annulled. 
 
312 FRAGMENTS OF SCIENCE. 
 
 Again, in order to utterly destroy all germs M. Pouchet 
 produced water from the combustion of hydrogen in air ; 
 but even in this water he found organisms. Had he seen, 
 however, as you have, the manner in which the air is clouded 
 with floating matter, would he have concluded that the 
 deportment of water which had been permitted to trickle 
 through such air could have the least influence in deciding 
 this great question ? I think not. Here is a quantity of 
 water produced and collected by allowing a hydrogen-flame 
 to play upon the polished bottom of a silver basin, in which 
 ice had been placed. This water is clear in the common 
 light ; but in the condensed electric beam it is seen to be 
 laden with particles, so thick-strewn and minute, as to pro- 
 duce a continuous cone of light. In passing through the 
 air the water loaded itself with this matter, and doubtless 
 became charged with incipient life. 
 
 Let me now draw your attention to another experiment 
 of Pasteur. He prepared twenty-one flasks, each contain- 
 ing a decoction of yeast, filtered and clear. He boiled the 
 decoction, so as to destroy whatever germs it might contain, 
 and while the space above the liquid was filled with pure 
 steam he sealed his flasks with a blow-pipe. He opened ten 
 of them in the deep, damp caves of the Paris Observatory, 
 and eleven of them in the court-yard of the establishment. 
 Of the former, one only showed signs of life subsequently. 
 In nine out of the ten flasks no organisms of any kind 
 were developed. In all the others organisms speedily ap- 
 peared. 
 
 Now here is an experiment conducted in Paris ; let us 
 see whether we cannot throw light upon it in London. I 
 place this large flask in the beam, and you see the luminous 
 track crossing it from side to side. The flask is filled with 
 the air of this room, charged with its germs and its dust, 
 and hence capable of illumination. But here is another 
 similar flask, which cuts a clear gap out of the beam. It is 
 
DUST AND DISEASE. 313 
 
 filled with unfiltered air, and still no trace of the beam is 
 visible. Why ? By pure accident I stumbled on this flask 
 in our apparatus-room, where it had remained quiet for 
 some time. Here are three other flasks which have also 
 been kept quiet for a couple of days ; they are all optically 
 empty. The still air of the flasks has deposited its dust, 
 germs and all, and is itself practically free from suspended 
 matter. Hence, manifestly, the result of Pasteur. 
 
 I have had a chamber erected, the lower half of which 
 is of wood, its upper half being enclosed by four glazed 
 window-frames. The chamber tapers to a truncated cone 
 at the top* It measures in plan 3 ft. by 2 ft. 6 in., and its 
 height is 5 ft. 10 in. On the 6th of February this chamber 
 was closed, every crevice that could admit dust, or cause 
 displacement of the air, being carefully pasted over with 
 paper. The electric beam at first revealed the dust within 
 the chamber as it did in the air of the laboratory. The 
 chamber was examined almost daily ; a perceptible diminu- 
 tion of the floating matter being noticed as time advanced. 
 At the end of a week the chamber was optically empty, 
 exhibiting no trace of matter competent to scatter the light. 
 But where the beam entered, and where it quitted the 
 chamber, the white circles stamped upon the interior sur- 
 faces of the glass showed what had become of the dust. It 
 clung to those surfaces, and from them instead of from the 
 air, the light was scattered. If the electric beam were sent 
 through the air of the Paris caves, the cause of its impotence 
 as generator of life would, I venture to predict, be revealed. 
 
 These experiments illustrate the application of a lumi- 
 nous beam to researches of this kind. They prove that the 
 germs which produce infusorial and fungoid life share the 
 fate of the ordinary visible dust with which they are inter- 
 mixed ; that such germs attach themselves to the sides of 
 vessels, and fall gradually to the bottom of spaces filled 
 with perfectly still air. But I will now turn to a far more 
 14 
 
314 FRAGMENTS OF SCIENCE. 
 
 interesting application of the luminous beam than any 
 hitherto described. My reference to Professor Lister's 
 interpretation of the fact that air which has passed through 
 the lungs cannot produce putrefaction is fresh in your mem- 
 ories. He there assumed that the air was rendered innocu- 
 ous by the filtering action of the lungs. Can this filtering 
 process be taken out of the region of assumption and placed 
 in that of demonstration ? It can. 
 
 Here is the concentrated beam with which we operated 
 at the commencement of this discourse. Its track through 
 the dust is luminous, and you have seen the blackness in- 
 troduced when the dust is burnt, or otherwise removed. I 
 fill my lungs with ordinary air and breathe through a glass 
 tube across the beam. The condensation of the aqueous 
 vapor of the breath is here shown by the formation of a 
 luminous white cloud of delicate texture. It is necessary 
 to abolish this cloud, and this may be done by drying the 
 breath previous to its entering the beam ; or, still more 
 simply, by warming the glass-tube. When this is done the 
 luminous track of the beam is for a time uninterrupted, 
 because the dust returning from the lungs makes good, in 
 great part, the particles displaced. After some time, how- 
 ever, an obscure disk appears in the beam, the darkness of 
 which increases, until finally, toward the end of the expira- 
 tion, the beam is, as it were, pierced by an intensely black 
 hole, in which no particles whatever can be discerned. The 
 deeper air of the lungs is absolutely free from suspended 
 matter. It is therefore in the precise condition required by 
 Professor Lister's explanation. This experiment may* be 
 repeated any number of times with the same result. I think 
 it must be regarded as a crowning piece of evidence both 
 of the correctness of Professor Lister's views and of the 
 impotence, as regards vital development, of optically pure 
 air, 
 
DUST AND DISEASE. 315 
 
 Cotton-wool ^Respirator. 
 
 I now empty my lungs as perfectly as possible, and 
 placing a handful of cotton-wool against my mouth and 
 nostrils, inhale through it. There is no difficulty in thus 
 filling the lungs with air. On expiring this air through a 
 glass tube, its freedom from floating matter is at once 
 manifest. From the very beginning of the act of expira- 
 tion the beam is pierced by a black aperture. The first 
 puff from the lungs abolishes the illuminated dust, and puts 
 a patch of darkness in its place ; and the darkness con- 
 tinues throughout the entire course of the expiration. 
 When the tube is placed below the beam and moved to 
 and fro, the same smoke-like appearance as that obtained 
 with a flame is observed. In short, the cotton-wool, when 
 used in sufficient quantity, and with due care, completely 
 intercepts the floating matter on its way to the lungs. 1 
 
 The application of these experiments is obvious. If a 
 physician wishes to hold back from the lungs of his patient, 
 or from his own, the germs or virus by which contagious 
 disease is propagated, he will employ a cotton-wool res- 
 pirator. If perfectly filtered, attendants may breathe the 
 air unharmed. In all probability the protection of the lungs 
 and mouth will be the protection of the entire system. For 
 it is exceedingly probable that the germs which lodge in 
 the air-passages, or find their way with the saliva into the 
 stomach with its absorbent system, are those which sow in 
 
 1 Since the first publication of these results, Professor Lister has 
 availed himself of the filtering power of cotton-wool in the treatment of 
 wounds. He first destroys the germs adhering to the wool, and by a 
 proper lotion kills those that may be scattered on the flesh. The cleansed 
 wool placed upon the wound permits of a free diffusion of the air, but 
 entirely intercepts the germs, and thus keeps the blood perfectly sweet. 
 It is here essential that no matter from the wound should reach the out- 
 side air, for such matter would open a highway to the organisms. 
 
316 FKAGMENTS OF SCIENCE. 
 
 the body epidemic disease. If this be so, then disease can 
 be warded off by carefully-prepared filters of cotton-wool. 
 I should be most willing to test their efficacy in my own 
 person. But apart from all doubtful applications, it is per- 
 fectly certain that various noxious trades in England may 
 be rendered harmless by the use of such filters. I have 
 had conclusive evidence of this from people engaged in 
 such trades. A form of respirator devised by Mr. Garrick, 
 a hotel proprietor in Glasgow, in which inhalation and ex- 
 halation occur through two different valves, the one per- 
 mitting the air to enter through the cotton-wool, and the 
 other permitting the exit of the air direct into the atmos- 
 phere, is well adapted for this purpose. But other forms 
 might readily be devised. 
 
 Fireman's Respirator. 
 
 Smoke is often the fireman's greatest obstacle in his 
 efforts to save life ; I thought, therefore, of inventing a res- 
 pirator for the use of firemen. Schroeder was the first to 
 use cotton-wool as a filter. To catch the atmospheric 
 germs, M. Pouchet employed a film of adhesive glycerine 
 spread upon glass ; while Dr. Stenhouse turned charcoal to 
 important account in respirators. By a combination of all 
 three a respirator of peculiar efficacy is obtained. For the 
 smoke of dried leaves the cotton-wool alone was found an 
 adequate protection ; but for the far more pungent smoke 
 of resinous deal it was found totally inadequate. At the 
 suggestion of a friend I moistened the wool with glycerine, 
 and found it a great improvement. It was the notion of 
 Pouchet in another form. Still about five minutes in dense 
 smoke was all that could be endured. I then associated 
 fragments of charcoal with the moistened cotton ; the effect 
 was excellent. 1 Armed with a respirator of this kind, one 
 1 Mr. Ladd, of Beak Street, makes these respirators. 
 
DUST AND DISEASE. 317 
 
 can breathe without annoyance in a space so crammed 
 with smoke, that a single inhalation without the respirator 
 would be intolerable. I wrote to the chief officer of the 
 Metropolitan Fire Brigade, asking him whether such a res- 
 pirator would be useful. He replied to me that it would, 
 but added that he was aware of every invention of the kind 
 in all the countries of Europe, and that none had been 
 found of any use. At my invitation he was kind enough 
 to come to the Royal Institution with two firemen and an 
 assistant. The three latter, wearing such respirators, went 
 in succession into the smoke-filled space, and on returning, 
 stated that they had not experienced the slightest discom- 
 fort, that they could have remained there all day long. 
 Captain Shaw himself repeated the experiment with the 
 same result. I am confident that sooner or later this res- 
 pirator will be employed, to the great benefit of a class of 
 men' whose actions in critical circumstances I have often 
 had occasion to admire. 
 
 Application of Luminous Seams to Water. 
 
 The method of examination here pursued is also appli- 
 cable to water. It is in some sense complementary to that 
 of the microscope, and may I think materially aid inquiries 
 conducted with that instrument. In microscopic examina- 
 tion attention is directed to a small portion of the liquid, 
 and the aim is to detect the individual suspended particles. 
 By the present method a large portion of the liquid is il- 
 luminated, its general condition being revealed, through 
 the light scattered by suspended particles. Care is taken 
 to defend the eye from the access of all other light, and, 
 thus defended, it becomes an organ of inconceivable deli- 
 cacy. Indeed, an amount of impurity so infinitesimal as to 
 be scarcely expressible in numbers, and the individual par- 
 ticles of which are so small as wholly to elude the micro- 
 
318 FRAGMENTS OF SCIENCE. 
 
 scope, may, when examined by the method alluded to, pro- 
 duce not only sensible but striking effects upon the eye. 
 
 I take, for instance, this bottle of water intended to 
 quench your lecturer's thirst. In the track of the beam it 
 simply reveals itself as dirty water. So you see that we 
 are invaded with dirt not only in the air we breathe, but 
 in the water we drink. And this water is no worse than 
 the other London waters. Thanks to the kindness of Pro- 
 fessor Frankland, I have been furnished with specimens of 
 the water of eight London companies. They are all laden 
 with impurities mechanically suspended. But you will ask 
 whether filtering will not remove the suspended matter ? 
 The grosser matter, undoubtedly, but not the more finely- 
 divided matter. Here is water which has been passed four 
 times through a filter of bibulous paper, but it is still laden 
 with fine matter. Here, also, is a bottle kindly sent me by 
 Mr. Lipscomb, and passed once through his charcoal filter. 
 But the track of the beam through it is more luminous 
 than through air, because the quantity of matter suspended 
 in the w r ater is greater than that suspended in air. Here 
 is another specimen courteously sent to me by the Silicated 
 Carbon Company. All the grosser matter has been re- 
 moved, but it is thick with fine matter. Nine-tenths of the 
 light scattered by these particles is perfectly polarized in a 
 direction at right angles to the beam, and this release of 
 the particles from the ordinary law of polarization is a 
 demonstration of their smallness. I should say by far the 
 greater number of the particles concerned in this scattering 
 are wholly beyond the range of the microscope, and no or- 
 dinary filter can intercept them. There is an aesthetic 
 pleasure in the drinking of a glass of cold sparkling water, 
 and I fear these experiments will destroy this pleasure if 
 you ever enjoyed it. And it is next to impossible by arti- 
 ficial means to produce pure water. Mr. Hartley, for ex- 
 ample, some time ago distilled water while it was sur- 
 
DUST AND DISEASE. 319 
 
 rounded by hydrogen, but the water was not free from 
 floating matter. It is so hard to be clean in the midst of 
 dirt. Here, however, is an approach to pure water. It is 
 from the Lake of Geneva, and the bottle was carefully 
 filled for me by my distinguished friend Soret. The track 
 of the beam through it is of a delicate sky blue ; there is 
 scarcely a trace of grosser matter. 
 
 The purest water that I have seen probably the purest 
 which has been seen hitherto has been obtained from the 
 fusion of selected specimens of ice. But extraordinary 
 precautions are required to obtain this degree of purity. 
 An apparatus was devised and constructed by my assistant 
 for this purpose. Through the plate of an air-pump passes 
 the shank of a large funnel, attached to which below the 
 plate is a glass bulb. In the funnel is placed a block of the 
 most transparent ice, and over the funnel is a glass receiver. 
 This is first exhausted and refilled several times with air, 
 which has been filtered by its passage through cotton-wool, 
 the ice being thus surrounded by pure moteless air. But 
 the ice has previously been in contact with mote-filled air'; 
 it is, therefore, necessary to let it wash its own face, and 
 wash the bulb which is to receive the water of liquefaction. 
 The ice is permitted to melt, the bulb is filled and emptied 
 several times, until finally the large block dwindles to a 
 small one. We may be sure that all impurity has been 
 thus removed from the surface of the ice. These two bulbs 
 contain water obtained in this way, the purity of which is 
 the maximum hitherto attained. Still I should hesitate to 
 call the water absolutely pure. When the concentrated 
 beam is sent through it the track of the beam is not invis- 
 ible, but of the most exquisitely delicate blue. This blue 
 is purer than that of the sky, so that the matter which pro- 
 duces it must be finer than that of the sky. It may be, 
 and indeed has been, contended that this blue is scattered 
 by the very molecules of the water, and not by matter sus- 
 
320 FRAGMENTS OF SCIENCE. 
 
 pended in it. But when we remember that this perfection 
 of blue is approached gradually through stages of less per- 
 fect blue; and when we consider that a blue in all re- 
 spects similar is demonstrably obtainable from particles 
 mechanically suspended, we should hesitate, I think, to 
 conclude that we have arrived here at the last stage of pu- 
 rification. The evidence, I think, points distinctly to the 
 conclusion that, could we push the process of purification 
 still further, even this last delicate trace of blue would dis- 
 appear. 
 
 Chalk - Water. Clark's /Softening Process. 
 
 But is it not possible to match the water of the Lake 
 of Geneva here in England ? Undoubtedly it is. We have 
 in England a kind of rock which constitutes at once an ex- 
 ceedingly clean recipient and a natural filter, and from which 
 we can obtain water extremely free from mechanical im- 
 purities. I refer to the chalk-formation, in which large 
 quantities of water are held in store. Our chalk-hills are 
 in most cases covered with thin layers of soil, and with 
 very scanty vegetation. Neither opposes much obstacle 
 the entry of the rain into the chalk, where any organic 
 impurity which the water may carry in is soon oxidized 
 and rendered harmless. Those who have scampered like 
 myself over the downs of Hants and Wilts will remember 
 the scarcity of water in these regions. In fact, the rain- 
 fall, instead of washing the surface and collecting in streams, 
 sinks into the fissured chalk and percolates through it, and 
 when this formation is suitably tapped we obtain water of 
 exceeding briskness and purity. Here is a large globe filled 
 with the water of a well near Tring. It is wonderfully free 
 from mechanical impurity ; indeed, it stands to reason that 
 water wholly withdrawn from surface contamination and 
 percolating through so clean a substance should be pure. 
 Sending a beam through this glass of water its purity is 
 
DUST AND DISEASE. 321 
 
 conspicuous ; you see the track of the beam, but it is not 
 the thick and muddy track revealed in London waters. It 
 has been a subject much debated whether the supply of 
 excellent water which the chalk holds in store could not be 
 rendered available for London. Many of the most eminent 
 engineers and chemists have ardently recommended this 
 source, and have sought to show that not only is its 
 purity unrivalled, but that its quantity is practically inex- 
 haustible. Data sufficient to test this are now, I believe, 
 in existence ; the number of wells sunk in the chalk is so 
 considerable and the quantity of water which they yield is 
 so well known. 
 
 But this water, so admirable as regards freedom from 
 mechanical impurity, labors under the disadvantage of 
 being very hard. It is rendered hard by the large quantity 
 of carbonate of lime which it holds in solution. The chalk- 
 water in the neighborhood of Watford holds in solution 
 about seventeen grains of carbonate of lime per gallon. 
 This, in the old terminology, used to be called seventeen 
 degrees of hardness. Now this hard water is bad for tea, 
 bad for washing ; it furs your boilers, because the lime 
 held in solution is precipitated by boiling. If the water be 
 used cold, its hardness must be neutralized at the expense 
 of soap before it will give a lather. These are serious ob- 
 jections to the use of chalk-water in London. But they 
 are now successfully met by the experimental demonstration 
 that such water can be softened inexpensively, and on a 
 grand scale. I had long known the method of softening 
 water called Clark's process, but not until recently, under 
 the guidance of Mr. Homersham, did I see proof of its 
 larger applications. The chalk-water is softened for the 
 supply of the city of Canterbury ; at the Chiltern Hills it 
 is softened for the supply of Tring and Aylesbury. Carter- 
 ham also enjoys the luxury. 
 
 I have visited all these places, and made myself ac- 
 
322 FRAGMENTS OF SCIENCE. 
 
 quainted with the works. At Canterbury there are three 
 reservoirs covered in and protected by a concrete roof and 
 layers of pebbles both from the summer's heat and the win- 
 ter's cold. Each reservoir contains 120,000 gallons of chalk- 
 water. Adjacent to these reservoirs are others containing 
 pure slacked lime the so-called " cream of lime." These 
 are filled with water, the lime and water being thoroughly 
 mixed by air forced in by an engine through apertures in 
 the bottom of the reservoir. The water thus well mixed 
 with the lime soon dissolves all of this substance that it is 
 capable of dissolving. The lime is then allowed to subside 
 to the bottom, leaving a perfectly clear lime-water behind. 
 The object is now to soften the chalk-water. Into the 
 empty reservoir is introduced a certain quantity of the clear 
 lime-water, and after this about nine times the quantity of the 
 chalk-water. The transparency immediately disappears 
 the mixture of the two clear liquids becomes thickly turbid. 
 The carbonate of lime is precipitated, and the precipitate is 
 permitted to subside ; it is crystalline and heavy, and there- 
 fore sinks rapidly. In about twelve hours you find a layer 
 of pure white carbonate of lime at the bottom of the reservoir, 
 with a water of extraordinary beauty and purity overhead. 
 A few days ago I pitched some halfpence into a reservoir 
 sixteen feet deep at the Chilton Hills. The sixteen feet 
 hardly perceptibly dimmed the coin. Had I cast a pin in, 
 it could, I am persuaded, have been seen at the bottom. By 
 this process of softening the water is reduced from about 
 seventeen degrees of hardness to three degrees of hardness. 
 It yields a lather immediately. Its temperature is constant 
 throughout the year. In the hottest summer it is cool, its 
 temperature being 20 above the freezing-point; and it 
 does not freeze in winter if conveyed in proper pipes. It 
 is not exposed to the contamination of either earth or air. 
 The reservoirs are covered; a leaf cannot blow into the 
 water, no surface contamination can reach it, it passes di- 
 
DUST AND DISEASE. 323 
 
 rect from the main into the house-tap ; no cisterns are em- 
 ployed, the supply is always fresh and pure. It is highly 
 charged with air. This is the kind of water which is sup- 
 plied to the fortunate people of Tring, Caterham, and Can- 
 terbury. 
 
 Let me, in conclusion, remind you that I do not con- 
 sider the floating matter revealed by the electric beam to be 
 all living matter. I believe that only in exceptional cases, 
 such as those cited in the excellent reports of Dr. Angus 
 Smith, does the quantity of living matter suspended in the 
 air of our streets and rooms amount to more than a small 
 fraction of the total dust. But I believe it to be perfectly 
 well established that, during epidemics, air and water are 
 charged with the specific " materies morbi " by which the 
 disease is spread ; that these two media are, in fact, the 
 chief vehicles of its dissemination. I believe there are the 
 strongest grounds for holding the contagious matter to be 
 " particulate," and further, that the particles are to all in- 
 tents and purposes germs / exhibiting as they do the fun- 
 damental characteristic of propagating their own kind 
 through countless generations, and over vast geographical 
 areas. Their life and reproduction run parallel to, and are 
 an incident of the life of man himself. I do not doubt the 
 ability of these particles to scatter light, nor that the means 
 by which the visible floating dust of our air is arrested, and 
 which demonstrably arrest -with it the germs of various 
 forms of fungoid and animalcular life, including those con- 
 cerned in the phenomena of putrefaction, will also be found 
 effectual in arresting contagium. 
 
 The following extract from a private letter written to 
 me by Dr. William Budd, is so important, and its reasoning 
 is so cleaT, that I asked and obtained the permission of its 
 exceedingly able writer to publish it : 
 
 " Another point of great practical importance is, as far 
 
324 FRAGMENTS OF SCIENCE. 
 
 as possible, to appraise the respective shares which air and 
 water take in the great act of distribution. That both 
 cholera and typhoid fever are sometimes disseminated by 
 drinking-water has been amply proved. I have myself re- 
 lated many instances of the fact, and have in my notes the 
 record of many others still more striking. But that water 
 is the sole or even the chief vehicle of cholera and typhoid 
 is a notion which, if I may trust my own experience, facts 
 do not warrant. 
 
 " Limiting myself, for the moment, to the case of typhoid, 
 I am in a position to state that all the worst and most wide- 
 spread outbreaks of that fever which I have ever witnessed, 
 have occurred among communities supplied by drinking- 
 water wilich was absolutely blameless. Two illustrations 
 will suffice. 
 
 " I live in a town in which the divorce between sewage 
 and drinking-water has long been consummated. Bristol 
 is supplied with drinking-water which from its source in the 
 Mendips to the tap from which it is delivered under high 
 pressure to the consumer, flows through conduits out of all 
 reach of sewage contamination. 
 
 " And yet typhoid fever has not only not ceased to exist 
 in Bristol, but about eight or ten years ago (before the 
 appointment of a health-officer), there occurred in the Parish 
 of St. James one of the worst outbreaks of this fever which 
 I have ever seen in the city. . In the course of a circuit 
 which I took one morning with the late Dr. Pring (at that 
 time Poor Law Medical Officer) I saw within a compara- 
 tively small area more than eighty cases of the disease. 
 
 " Now, with the exception of a single household, all the 
 patients were drinking the Mendip water ; the very same 
 water which, as far as fever is concerned, more than 150,000 
 of their fellow-citizens outside, the infected area were drink- 
 ing with absolute impunity. 
 
 " Some four or five years ago I was sent for to advise 
 
DUST AND DISEASE. 325 
 
 what measures should be taken to stay an outbreak of 
 typhoid fever which had occurred in a large convent about 
 two miles from Bristol. The inmates were divided into 
 three distinct divisions ; the largest being a reformatory for 
 girls, who occupied the central block of the building. Into 
 this reformatory the fever was brought by a girl already 
 suffering from it, and who had contracted it in a sea-side 
 place more than twenty miles off. From this girl the dis- 
 ease spread until, at the date of my visit, more than fifty 
 girls were lying ill of it. From first to last the fever was 
 confined to the reformatory girls, and to persons in immedi- 
 ate attendance upon them. 
 
 " Now, the facts as to the drinking-water were these : 
 
 " 1. The water was proved by examination of the well, 
 and by chemical analysis, to be entirely free from sewage 
 contamination. 
 
 " 2. The inmates of another large division of the con- 
 vent, who remained entirely free from fever, drank the same 
 water as the girls among whom fever was raging like a 
 plague. 
 
 " 3. From the very time when disinfection was brought 
 to bear on the excreta the disease ceased to spread, although 
 the inmates of the infected division continued to drink the 
 same water as before. 
 
 " Lastly, nothing has since been done to the well the 
 water remains what it was but no fever has occurred in 
 the convent since. 
 
 " The evidence in both these cases is, as you see, of that 
 crucial, decisive order that admits of no reply. 
 
 " They show, at least, that typhoid fever may do its 
 worst where drinking-water takes absolutely no part in the 
 distribution of the poison. 
 
 " But if water be excluded, the air is the only other 
 possible vehicle by which a poison generated in the living 
 body can find its way back to other living bodies on a scale 
 
,126 FRAGMENTS OF SCIENCE. 
 
 sufficiently large to cause the resulting disease to assume 
 an epidemic form. 
 
 " I may remark further that the infection of the air in 
 these two cases was obviously not the work of chance, but 
 only represented the effect of agencies which are always in 
 operation where this fever prevails. 
 
 " The phenomenon is, in fact, merely the expression of a 
 general law. 
 
 " The germs cast off in the liquid excreta of contagious 
 diseases rise into the air by no power of their own, but in 
 virtue of the very same physical conditions which cause the 
 germs of the great tribe of Infusoria, which, as their name 
 bespeaks, breed in liquids, to rise in swarms into the same 
 medium. 
 
 " If there were time or need, I could show, by evidence 
 quite as decisive, that all these statements apply equally to 
 cholera also. 
 
 " I do not know that there is any thing in these data to 
 suggest additional matter for your essay, but I have thought 
 it worth while to bring them under your notice, harmoniz- 
 ing as they do with your own investigations which show 
 by such striking phenomena that air and water are equally 
 objects of distrust. 
 
 " As to the germ-theory itself, that is a matter on which 
 I have long since made up my mind. From the day when 
 I first began to think upon these subjects, I have never had 
 a doubt that the specific cause of contagious fevers must be 
 living organisms. 
 
 " It is impossible, in fact, to make any statement bearing 
 upon the essence or distinctive characters of these fevers, 
 without using terms which are of all others the most distinc- 
 tive of life. Take up the writings of the most violent oppo- 
 nent of the germ-theory, and, ten to one, you will find them 
 full of such terms as ' propagation,' ' self-propagation,' ' re- 
 production,' 'self-multiplication,' and so on. Try as he 
 
DUST AND DISEASE. 327 
 
 may if he has any tiling to say of these diseases which is 
 characteristic of them he cannot evade the use of these 
 terms or the exact equivalents of them. While perfectly 
 applicable to life and to living things, these terms express 
 qualities which are not only inapplicable to common chemi- 
 cal agents, but, as far as I can see, actually inconceivable 
 of them." 
 
XII. 
 LIFE AND LETTERS OF FARADAY. 
 
 BY DR. HENRY BENCE JONES. 
 
 [The Academy for May and June, 18TO.] 
 
Fame is the spur that the clear spirit doth raise 
 (That last infirmity of noble minds) 
 To scorn delights and live laborious days ; 
 But the fair guerdon when we hope to find, 
 And think to burst out into sudden blaze, 
 Comes the blind fury with the abhorred shears, 
 And slits the thin-spun life. But not the praise 
 Phoebus replied, and touched my trembling ears ; 
 Fame is no plant that grows on mortal soil, 
 Nor in the glistering foil 
 Set off to the world, nor in broad rumor lies, 
 But lives and spreads aloft by those pure eyes 
 And perfect witness of all-judging Jove." 
 
 SflLTOX. 
 
XII. 
 LIFE AND LETTERS OF FARADAY. 
 
 UNDERTAKEN and executed in a reverent and loving 
 spirit, the work of Dr. Bence Jones makes Faraday the 
 virtual writer of his own life. Everybody now knows the 
 story of the philosopher's birth; that his father was a 
 smith ; that he was born at Newington Butts in 1791 ; 
 that he slid along the London pavements, a bright-eyed 
 errand-boy, with a load of brown curls upon his head and a 
 packet of newspapers under his arm ; that the lad's master 
 was a bookseller and bookbinder a kindly man, who 
 became attached to the little fellow and in due time made 
 him his apprentice without fee ; that during his appren- 
 ticeship he found his appetite for knowledge provoked and 
 strengthened by the books he stitched and covered. Thus 
 he grew in wisdom and stature to his year of legal man- 
 hood, when he appears in the volumes before us as a writer 
 of letters, which reveal his occupation, acquirements, and 
 tone of mind. His correspondent was Mr. Abbott, a mem- 
 ber of the Society of Friends, who, with a forecast of his 
 friend's greatness, preserved his letters and produced them 
 at the proper time. 
 
 In later years Faraday always carried in his pocket a 
 blank card on which he jotted down in pencil his thoughts 
 and memoranda. He made his notes in the laboratory, 
 in the theatre, and in the streets. This distrust of his 
 memory reveals itself in his first letter to Abbott. To a 
 
332 FKAGMENTS OF SCIENCE. 
 
 proposition that no new inquiry should be started between 
 them before the old one had been exhaustively discussed, 
 Faraday objects. " Your notion," he says, " I can hardly 
 allow, for the following reason : ideas and thoughts spring 
 up in my mind which are irrevocably lost for want of 
 noting at the time." Gentle as he seemed, he wished to 
 have his own way, and he had it throughout his life. 
 Differences of opinion sometimes arose between the two 
 friends, and then they resolutely faced each other. "I 
 accept your offer to fight it out with joy, and shall in the 
 battle of experience cause not pain, but, I hope, pleasure." 
 Faraday notes his own impetuosity, and incessantly checks 
 it. There is at times something mechanical in his self- 
 restraint. In another nature it would have hardened into 
 mere " correctness " of conduct ; but his overflowing affec- 
 tions prevented this in his case. The habit became a second 
 nature to him at last, and lent serenity to his later years. 
 
 In October, 1812, he was engaged by a Mr. De la 
 Roche as a iourneyman bookbinder ; but the situation did 
 not suit him. His master appears to have been an austere 
 and passionate man, and Faraday was to the last degree 
 sensitive. All his life he continued so. He suffered at 
 times from dejection ; and a certain grimness, too, pervaded 
 his moods. " At present," he writes to Abbott, " I am as 
 serious as you can be* and would not scruple to speak a 
 truth to any human being, whatever repugnance it might 
 give rise to. Being in this state of mind, I should have 
 refrained from writing to you, did I not conceive from the 
 general tenor of your letters that your mind is, at proper 
 times, occupied upon serious subjects to the exclusion of 
 those that are frivolous." Plainly he had fallen into that 
 stern Puritan mood which not only crucifies the flesh, affec- 
 tions, and lusts of him who harbors it, but is often a cause 
 of disturbed digestion to his friends. 
 
 About three months after his engagement with De la 
 
FARADAY. 333 
 
 Roche, Faraday quitted him and bookbinding together. 
 He had heard Davy, copied his lectures, and written to him 
 entreating to be released from trade, which he hated, and 
 enabled to pursue science. Davy recognized the merit of 
 his correspondent, kept his eye upon him, and when oc- 
 casion offered, drove to his door and sent in a letter offer- 
 ing him the post of assistant in the laboratory of the Royal 
 Institution. He was engaged upon March 1, 1812, and on 
 the 8th we find him extracting the sugar from beet-root. 
 He joined the City Philosophical Society which had been 
 founded by Mr. Tatum in 1808. "The discipline was very 
 sturdy, the remarks very plain, and the results most valu- 
 able." Faraday derived great profit from this little asso- 
 ciation. In the laboratory he had a discipline sturdier still. 
 Both Davy and himself were at this time cut and bruised 
 by explosions of chloride of nitrogen. One explosion was 
 so rapid " as to blow my hand open, tear away a part of 
 one nail, and make my fingers so sore that I cannot use 
 them easily." In another experiment " the tube and re- 
 ceiver were blown to pieces ; I got a cut on the head, and 
 Sir Humphry a bruise on his hand." And again, speaking 
 of the same substance, he says : " When put in the pump 
 and exhausted, it stood for a moment, and then exploded 
 with a fearful noise. Both Sir H. and I had masks on, but 
 I escaped this time the best. Sir H. had his face cut in 
 two places about the chin, and a violent blow on the fore- 
 head struck through a considerable thickness of silk and 
 leather." It was this same substance that blew out the 
 eye of Dulong. 
 
 Over and over again, even at this early date, we can 
 discern the quality which, compounded with his rare intel- 
 lectual power, made him a great experimental philosopher. 
 This was his desire to see facts, and not to rest contented 
 with the descriptions of them. He frequently pits the eye 
 against the ear, and affirms the enormous superiority of the 
 
834 FRAGMENTS OF SCIENCE. 
 
 organ of vision. Late in life I have heard him say that he 
 could never have fully understand an experiment until he 
 had seen it. But he did not confine himself to experiment. 
 He aspired to be a teacher, and reflected and wrote upon 
 the method of scientific exposition. " A lecturer," he ob- 
 serves, " should appear easy and collected, undaunted and 
 unconcerned : " still " his whole behavior should evince 
 respect for his audience." These recommendations were 
 afterward in great part embodied by himself. I doubt 
 his unconcern, but his fearlessness was often manifested. 
 It used to rise within him as a wave, which carried both 
 him and his audience along with it. On rare occasions 
 also, when he felt himself and his subject hopelessly unin- 
 telligible, he suddenly evoked a certain recklessness of 
 thought, and without halting to extricate his bewildered 
 followers, he would dash alone through the jungle into 
 which he had unwittingly led them ; thus saving them from 
 ennui by the exhibition of a vigor which, for the time being, 
 they could neither share nor comprehend. 
 
 In October, 1813, he quitted England with Sir Hum- 
 phry and Lady Davy. During his absence he kept a 
 journal, from which copious and interesting extracts have 
 been made by Dr. Bence Jones. Davy was considerate, 
 preferring at times to be his own servant rather than 
 impose on Faraday duties which he disliked. But Lady 
 Davy was the reverse. She treated him as an underling ; 
 he chafed under the treatment, and was often on the point 
 of returning home. They halted at Geneva. De la Rive 
 the elder had known Davy in 1799, and by his writings in 
 the " Bibliotheque Britannique," had been the first to make 
 the English chemist's labors known abroad. He welcomed 
 Davy to his country residence in 1814. Both were sports- 
 men, and they often went out shooting together. On these 
 occasions Faraday charged Davy's gun, while De la Rive 
 charged his own. Once the Genevese philosopher found 
 
FARADAY. 335 
 
 himself by the side of Faraday, and in his frank and genial 
 way entered into conversation with the young man. It was 
 evident that a person possessing such a charm of manner 
 and such high intelligence could be no mere servant. On 
 inquiry De la Rive was somewhat shocked to find that the 
 soi-disant domestique was really preparateur in the labora- 
 tory of the Royal Institution : and he immediately proposed 
 that Faraday thenceforth should join the masters instead 
 of the servants at their meals. To this Davy, probably out 
 of weak deference to his wife, objected ; but an arrangement 
 was come to that Faraday thenceforward should have his 
 food in his own room. Rumor states that a dinner in 
 honor of Faraday was given by De la Rive. This is a 
 delusion ; there was no such banquet ; but Faraday never 
 forgot the kindness of the friend who saw his merit when 
 he was a mere gar$on de laboratoire* 
 
 He returned in 1815 to the Royal Institution. Here he 
 helped Davy for years; he worked also for himself, and 
 lectured frequently at the City Philosophical Society. He 
 took lessons in elocution, happily without damage to his 
 natural force, earnestness, and grace of delivery. He was 
 never pledged to theory, and he changed in opinion as 
 knowledge advanced. With him life was growth. In those 
 early lectures we hear him say, " In knowledge, that man 
 only is to be contemned and despised who is not in a state 
 of transition." And again, " Nothing is more difficult and 
 requires more caution than philosophical deduction, nor is 
 there any thing more adverse to its accuracy than fixity of 
 opinion." Not that he was wafted about by every wind of 
 
 1 While confined last autumn at Geneva by 'the effects of a fall in the 
 Alps, my friends, with a kindness I can never forget, did all that friend- 
 ship could suggest to render my captivity pleasant to me. M. de la Rive 
 then wrote out for me the full account, of which the foregoing is a con- 
 densed abstract. It was at the desire of Dr. Bence Jones that I asked 
 him to do so. The rumor of a banquet at Geneva illustrates the ten 
 dency to substitute for the youth of 1814 the Faraday of later years. 
 
336 FRAGMENTS OF SCIENCE. 
 
 doctrine ; but that he united flexibility with his strength. 
 In striking contrast with this intellectual expansiveness is 
 his fixity in religion, but this is a subject which cannot be 
 discussed here. 
 
 Of all the letters published in these volumes none 
 possess a greater charm than those of Faraday to his wife. 
 Here, as Dr. Bence Jones truly remarks, " he laid open all 
 his mind and the whole of his character, and what can be 
 made known can scarcely fail to charm every one by its 
 loveliness, its truthfulness, arid its earnestness." Abbott 
 and he sometimes swerved into word-play about love ; but 
 up to 1820, or thereabouts, the passion was potential 
 merely. Faraday's journal, indeed, contains entries which 
 show that he took pleasure in the assertion of his contempt 
 for love ; but these very entries became links in his destiny. 
 It was through them that he became acquainted with one 
 who inspired him with a feeling which only ended with his 
 life. His biographer has given us the means of tracing the 
 varying moods which preceded his acceptance. They reveal 
 more than the common alternations of light and gloom ; at 
 one moment he wishes that his flesh might melt and he 
 become nothing ; at another he is intoxicated with hope. 
 The impetuosity of his character was then unchastened by 
 the discipline to which it was subjected in after-years. The 
 very strength of his passion proved for a time a bar to its 
 advance, suggesting as it did to the conscientious mind of 
 Miss Barnard doubts of her capability to return it with 
 adequate force. But they met again and again, and at 
 each successive meeting he found his heaven clearer, until 
 at length he was able to say, " Not a moment's alloy of this 
 evening's happiness occurred. Every thing was delightful 
 to the last moment of my stay with my companion, because 
 she was so." The turbulence of doubt subsided, and a calm 
 and elevating confidence took its place. " What can I call 
 nyself," he writes to her in a subsequent letter, " to convey 
 
FARADAY. 337 
 
 most perfectly my affection and love for you ? Can I or 
 can truth say more than that for this world I am yours ? " 
 Assuredly he made his profession good, and no fairer light 
 falls upon his character than that which reveals his relations 
 to his wife. Never, I believe, existed a manlier, purer, 
 steadier love. Like a burning diamond it continued to shed 
 for six-and-forty years its white and smokeless glow. 
 
 Faraday was married on June 12, 1821 ; and up to this 
 date Davy appears throughout as his friend. Soon after- 
 ward, however, disunion occurred between them, which, 
 while it lasted, must have given Faraday intense pain. It 
 is impossible to doubt the honesty of conviction with which 
 this subject has been treated by Dr. Bence Jones, and there 
 may be facts known to him, but not appearing in these 
 volumes, which justify his opinion that Davy in those days 
 had become jealous of Faraday. This, which is the preva- 
 lent belief, is also reproduced in an excellent article in 
 the March number of Eraser's Magazine. But the best 
 analysis I can make of the data fails to present Davy in 
 this light to me. The facts, as I regard them, are briefly 
 these : 
 
 In 1820, Oersted, of Copenhagen, made the celebrated 
 discovery which connects electricity with magnetism, and 
 immediately afterward the acute mind of Wollaston per- 
 ceived that a wire carrying a current ought to rotate 
 round it own axis under the influence of a magnetic pole. 
 In 1821 he tried, but failed, to realize this result in the 
 laboratory of the Royal Institution. Faraday was not pres- 
 ent at the moment, but he came in immediately afterward, 
 and heard the conversation of Wollaston and Davy about 
 the experiment. He had also heard a rumor of a wager 
 that Dr. Wollaston would eventually succeed. 
 
 This was in April. In the autumn of the same year 
 Faraday wrote a history of electro-magnetism, and repeated 
 for himself the experiments which he described. It was 
 15 
 
338 FRAGMENTS OF SCIENCE. 
 
 while thus instructing himself that he succeeded in causing 
 a wire carrying an electric current to rotate round a mag- 
 netic pole. This was not the result sought by Wollaston, 
 but it was closely related to it. 
 
 The strong tendency of Faraday's mind to look upon 
 the reciprocal actions of natural forces gave birth to his 
 greatest discoveries ; and we, who know this, should be 
 justified in concluding that, even had Wollaston not pre- 
 ceded him, the result would have been the same. But in 
 judging Davy we ought to transport ourselves to his time, 
 and carefully exclude from our thoughts and feelings that 
 noble subsequent life which would render simply impossible 
 the ascription to Faraday of anything unfair. It would be 
 unjust to Davy to put our knowledge in the place of his, 
 or to credit him with data which he could not have pos- 
 sessed. Rumor and fact had connected the name of Wol- 
 laston with these supposed interactions between magnets 
 and currents. When, therefore, Faraday in October pub- 
 lished his successful experiment without any allusion to 
 Wollaston, general, though really ungrounded, criticism 
 followed. I say ungrounded because, firstly, Faraday's ex- 
 periment was not that of Wollaston, and secondly, Faraday, 
 before he published it, had actually called upon Wollaston, 
 and not finding him at home did not feel himself authorized 
 to mention his name. 
 
 In December Faraday published a second paper on the 
 same subject, from which, through a misapprehension, the 
 name of Wollaston was also omitted. Warburton and 
 others thereupon affirmed that Wollaston's ideas had been 
 appropriated without acknowledgment, and it is plain that 
 Wollaston himself, though cautious in his utterance, was 
 also hurt* Censure grew till it became intolerable. "I 
 hear," writes Faraday to. his friend Stodart, " every day 
 more and more of these sounds, which, though only whis- 
 pers, to me, arc, I suspect, spoken aloud among scientific 
 
FARADAY. 339 
 
 men." He might have written explanations and defences, 
 but he went straighter to the point. He wished to see the 
 principals face to face to plead his cause before them per- 
 sonally. There is a certain vehemence in his desire to do 
 this. He saw Wollaston, he saw Davy, he saw Warbur- 
 ton ; and I am inclined to think that it was the irresistible 
 candor and truth of character which these viva voce de- 
 fences revealed, as much as the defences themselves, that 
 disarmed resentment at the time. 
 
 As regards Davy, another cause of dissension arose in 
 1823. In the spring of that year Faraday analyzed the 
 hydrate of chlorine, a substance once believed to be the 
 element chlorine, but .proved by Davy to be a compound of 
 that element and water. The analysis was looked over b}' 
 Davy, who then and there suggested to Faraday to heat 
 the hydrate in a closed glass tube. This was done, the 
 substance was decomposed, and one of the products of de- 
 composition was proved by Faraday to be chlorine liquefied 
 by its own pressure. On the day of its discovery he com- 
 municated this result to Dr. Paris. Davy, on being in- 
 formed of it, instantly liquefied another gas in the same 
 way. Having struck thus into Faraday's inquiry, ought he 
 not to have left the matter in Faraday's hands ? I think 
 he ought. But, considering his relation to both Faraday 
 and the hydrate of chlorine, Davy, I submit, may be excused 
 for thinking differently. A father is not always wise enough 
 to see that his son has ceased to be a boy, and estrange- 
 ment on this account is not rare ; nor was Davy wise enough 
 to discern that Faraday had passed the mere assistant 
 stage and become a discoverer. It is now hard to avoid 
 magnifying this error. But had Faraday died or ceased to 
 work at this time, or had his subsequent life been devoted 
 to money-getting instead of to research, would anybody 
 now dream of ascribing jealousy to Davy? Assuredly 
 not. Why should he be jealous ? His reputation at this 
 
340 FRAGMENTS OF SCIENCE. 
 
 time was almost without a parallel : his glory was without 
 a cloud. He had added to his other discoveries that of 
 Faraday, and after having been his teacher for seven years, 
 his language to him was this : " It gives me great pleasure 
 to hear that you are comfortable at the Royal Institution, 
 and I trust that you will not only do something good and 
 honorable for yourself, but also for science." This is not 
 the language of jealousy, potential or actual. But the 
 chlorine business introduced irritation and anger, to which, 
 and not to any ignobler motive, Davy's opposition to the 
 election of Faraday to the Royal Society is, I am per- 
 suaded, to be ascribed. 
 
 These matters are touched upon with perfect candor 
 and becoming consideration in the volumes of Dr. Bence 
 Jones, but in " society " they are not always so handled. 
 Here a name of noble intellectual associations is surrounded 
 by injurious rumors which I would willingly scatter for- 
 ever. The pupil's magnitude and the splendor of his posi- 
 tion are too great and absolute to need as a foil the humilia- 
 tion of his master. Brothers in intellect, Davy and Fara- 
 day, however, could never have become brothers in feeling ; 
 their characters were too unlike. Davy loved the pomp 
 and circumstance of fame, Faraday the inner consciousness 
 that he had fairly won renown. They were both proud 
 men. But with Davy pride projected itself into the outer 
 world, while with Faraday it became a steadying and dig- 
 nifying inward force. In one great particular they agreed. 
 Each of them could have turned his science to immense 
 commercial profit, but neither of them did so. The noble 
 excitement of research, and the delight of discovery, con- 
 stituted their reward. I commend them to the reverence 
 which great gifts greatly exercised ought to inspire. They 
 were both ours, and through the coming centuries England 
 will be able to point with just pride to the possession of 
 such men. 
 
FARADAY. 341 
 
 The first volume of the " Life and Letters " reveals to 
 us the youth who was to be father to the man. Skilful, 
 aspiring, resolute, he grew steadily in knowledge and in 
 power. Consciously or unconsciously, the relation of action 
 to reaction was ever present to Faraday's mind. It had 
 been fostered by his discovery of magnetic rotations, and 
 it planted in him more daring ideas of a similar kind. Mag- 
 netism he knew could be evoked by electricity, and he 
 thought that electricity, in its turn, ought to be capable of 
 evolution by magnetism. On August 29, 1831, his experi- 
 ments on this subject began. He had been fortified by 
 previous trials, which, though failures, had begotten in- 
 stincjts directing him toward the truth. He, like every 
 strong worker, might at times miss the outward object, but 
 he always gained the inner light education and expansion. 
 Of this Faraday's life was a constant illustration. By No- 
 vember he had discovered and colligated a multitude of 
 the most wonderful and unexpected phenomena. He had 
 generated currents by currents ; currents by magnets, per- 
 manent and transitory; and he afterward generated cur- 
 rents by the earth itself. Arago's " Magnetism of Rota- 
 tion," which had for years offered itself as a challenge to 
 the best scientific intellects of Europe, now fell into his 
 hands. It proved to be a beautiful but still special illustra- 
 tion of the great principle of magneto-electric induction. 
 Nothing equal to this, in the way of pure experimental in- 
 quiry, had previously been achieved. 
 
 Electricities from various sources were next examined, 
 and their differences and resemblances revealed. He thus 
 assured himself of their substantial identity. He then took 
 up conduction, and gave many striking illustrations of the 
 influence of fusion on conducting power. Renouncing pro- 
 fessional work, from which at this time he might have de- 
 rived an income of many thousands a year, he poured his 
 whole momentum into his researches. He was long en- 
 
342 FRAGMENTS OF SCIENCE. 
 
 tangled in electro-chemistry. The light .of law was for a 
 time obscured by the thick umbrage of novel facts ; but he 
 finally emerged from his researches with the great principle 
 of definite electro-chemical decomposition in his hands. If 
 his discovery of magneto-electricity may be ranked with 
 that of the pile by Volta, this new discovery may almost 
 stand beside that of definite combining proportions in 
 chemistry. He passed on to static electricity its conduc- 
 tion, induction, and mode of propagation. He discovered 
 and illustrated the principle of inductive capacity; and, 
 turning to theory, he asked himself how electrical attrac- 
 tions and repulsions are transmitted. Are they, like gravity, 
 actions at a distance, or do they require a medium ? If the 
 former, then, like gravity, they will act in straight lines ; if 
 the latter, then, like sound or light, they may turn a corner. 
 Faraday held, and his views are gaining ground, that his 
 experiments proved the fact of curvilinear propagation, and 
 hence the operation of a medium. Others denied this ; but 
 none can deny the profound and philosophic character of 
 his leading thought. 1 The first volume of the researches 
 contains all the papers here referred to. 
 
 Faraday had heard it stated that henceforth physical 
 discovery would be made solely by the aid of mathematics ; 
 that we had our data, and needed only to work deductively. 
 Statements of a similar character crop out from time to 
 time in our day. They arise from an imperfect acquaintance 
 with the nature, present condition, and prospective vastness 
 of the field of physical inquiry. The tendency of natural 
 science doubtless is to bring all physical phenomena under 
 the dominion of mechanical laws ; to give them, in other 
 words, mathematical expression. But our approach to this 
 result is asymptotic ; and for ages to come possibly for 
 
 1 In a very remarkable paper published in PoggendorfFs Annalen for 
 1857, Werner Siemens develops and accepts Faraday's theory of molecu- 
 lar induction. 
 
FARADAY. 343 
 
 all the ages of the human race Nature will find room for 
 both the philosophical experimenter and the mathematician. 
 Faraday entered his protest against the foregoing statement 
 by labelling his investigations " Experimental Researches 
 in Electricity." They were completed in 1854, and three 
 volumes of them have been published. For the sake of 
 reference he numbered every paragraph, the last number 
 being 3,362. In 1859 he collected and published a fourth 
 volume of papers under the title, " Experimental Researches 
 in Chemistry and Physics." Thus the apostle of experi- 
 ment magnified his office. 
 
 The second volume of the Researches embraces memoirs 
 on the Electricity of the Gymnotus ; on the Source of Power 
 in the Voltaic Pile ; on the Electricity evolved by the Friction 
 of Water and Steam, in which the phenomena and principles 
 of Sir William Armstrong's Hydro-electric machine are 
 described and developed ; a paper on Magnetic Rotations, 
 and Faraday's letters in relation to the controversy it 
 aroused. The contribution of the most permanent value 
 here is that on the Source of Power in the Voltaic Pile. By 
 it the Contact Theory, pure and simple, was totally over- 
 thrown, and the necessity of chemical action to the main- 
 tenance of the current demonstrated. 
 
 The third volume of the Researches opens with a me- 
 moir entitled, " The Magnetization of Light, and the Illu- 
 mination of Magnetic Lines of Force." It is difficult even 
 now to affix a definite meaning to this title ; but the dis- 
 covery of the rotation of the plane of polarization which it 
 announced seems pregnant with great results. The writ- 
 ings of William Thomson on the theoretic aspects of the 
 discovery ; the excellent electro-dynamic measurements of 
 Wilhelm Weber, which are models of experimental com- 
 pleteness and skill ; Weber's labors in conjunction with his 
 lamented friend Kohlrausch above all, the researches of 
 Clerk Maxwell on the Electro-magnetic Theory of Light 
 
344 FRAGMENTS OF SCIENCE. 
 
 point to that wonderful and mysterious medium which is 
 the vehicle of light and radiant heat as the probable basis 
 also of magnetic and electric phenomena. The hope of 
 such a combination was first raised by the discovery here 
 referred to. 1 Faraday himself seemed to cling with partic- 
 ular affection to this discovery. He felt that there was 
 more in it than he was able to unfold. He predicted that 
 it would grow in meaning with the growth of science. This 
 it has done ; this it is doing now. Its right interpretation 
 will probably mark an epoch in scientific history. 
 
 Rapidly following it is the discovery of Diamagnetism, 
 or the Repulsion of Matter by a magnet. Brugmans had 
 shown that bismuth repelled a magnetic needle. Here he 
 stopped.. Le Bailliff proved that antimony did the same. 
 Here he stopped. Seebeck, Becquerel, and others, also 
 touched the discovery. These fragmentary gleams excited 
 a momentary curiosity, and were almost forgotten, when 
 Faraday, independently, alighted on the same facts ; and, 
 instead of stopping, made them the inlets to a new and 
 vast region of research. The value of a discovery is to be 
 measured by the intellectual action it calls forth ; and it 
 was Faraday's good fortune to strike such lodes of scientific 
 truth as give some of the best intellects of the age occupa- 
 tion. 
 
 The salient quality of Faraday's scientific character re- 
 veals itself from beginning to end of these volumes : a union 
 
 1 A letter addressed to me by Professor Weber, on the 18th of last 
 March, contains the following reference to the connection here mentioned : 
 " Die Hoffnung einer solchen Combination ist durch Faraday's Entdeckung 
 dor Drehung der Polarisationsebene durch magnetische Directionskraft 
 zuerst, und sodann durch die Uebereinstimmung derjenigen Geschwindig- 
 keit, welche das Verhaltniss der electro-dynamischen Einheit zur electro- 
 statischen ausdriickt, mit der Geschwindigkeit des Lichts angeregt 
 worden ; und mir scheint von alien Versuchen, welche zur Verwirklichung 
 dieser Hoffnung gemacht worden sind, das von Efernn Maxwell gemachte 
 am erfolgreichsten." 
 
FARADAY. 345 
 
 of ardor and patience the one prompting the attack, the 
 other holding him on to it till defeat was final or victory 
 assured. Certainty in one sense or the other was necessary 
 to his peace of mind. The right method of investigation 
 is, perhaps, incommunicable ; it depends on the individual 
 rather than on the system, and the mark is missed when 
 Faraday's researches are pointed to as merely illustrative 
 of the power of the inductive philosophy. The brain may 
 be filled with that philosophy, but without the energy and 
 insight which this man possessed, and which with him were 
 personal and distinctive, we should never rise to the level 
 of his achievements. His power is that of individual genius, 
 rather than of philosophic method ; the energy of a strong 
 soul expressing itself after its own fashion, and acknowl- 
 edging no mediator between it and Nature. 
 
 The second volume of the " Life and Letters," like the 
 first, is an historic treasury as regards Faraday's work and 
 character, and his scientific and social relations. It contains 
 letters from Humboldt, Herschel, Hachette, De la Rive, Du- 
 mas, Liebig, Melloni, Becquerel, Oersted, PlUcker, Du Bois- 
 Reymond, Lord Melbourne, Prince Louis Napoleon, and many 
 other distinguished men. I notice with particular pleasure 
 a letter from Sir John Herschel in reply to a sealed packet 
 addressed to him by Faraday, but which he had permission 
 to open if he pleased. The packet referred to one of the 
 many unfulfilled hopes which spring up in the mind of fer- 
 tile investigators : 
 
 " Go on and prosper, * from strength to strength,' like a 
 victor marching with assured step to further conquests ; and 
 be certain that no voice will join more heartily in the paeans 
 that already begin to rise, and will speedily swell into a 
 shout of triumph, astounding even to yourself, than that of 
 J. F. W. Herschel." 
 
 As an encourager of the scientific worker, this fine spirit 
 is still active. 
 
346 FRAGMENTS OF SCIENCE. 
 
 Faraday's behavior to Melloni in 1835 merits a word of 
 notice. The young man was a political exile in Paris. He 
 had newly-fashioned and applied the thermo-electric pile, 
 and had obtained with it results of the greatest importance. 
 But they were not appreciated. With the sickness of dis- 
 appointed hope, Melloni waited for the report of the Com- 
 missioners appointed by the Academy of Sciences to exam- 
 ine his labors. At length he published his researches in 
 the " Annales de Chimie." They thus fell into the hands 
 of Faraday, who, discerning at once their extraordinary 
 merit, obtained for their author the Rumford Medal of the 
 Royal Society. A sum of money always accompanies this 
 medal, and the pecuniary help was at this time even more 
 essential than the mark of honor to the young refugee. 
 Melloni' s gratitude was boundless : 
 
 " Et vous, monsieur," he writes to Faraday, " qui appar- 
 tenez a une societe" a laquelle je n'avais rien oflfert, vous qui 
 me connaissiez & peine le nom ; vous n'avez pas demande 
 si j'avais des ennemis faibles ou puissants, ni calcule quel 
 en 6tait le nombre ; mais vous avez parle pour Popprime 
 Stranger, pour celui qui n'avait pas le moindre droit a tant 
 de bienveillance, et vos paroles ont e'te' accueillies favorable- 
 ment par des collegues consciencieux ! Je reconnais bien 
 la des hommes dignes de leur noble mission, les ve"ritables 
 repre"sentants de la science d'un pays libre et ge"n6reux." 
 
 Within the prescribed limits of this article it would be 
 impossible to give even the slenderest summary of Fara- 
 day's correspondence, or to carve from it more than the 
 merest fragments of his character. His letters, written to 
 Lord Melbourne and others in 1836, regarding his pension, 
 illustrate his uncompromising independence. The Prime 
 Minister had offended him, but assuredly the apology de- 
 manded and given was complete. I think it certain that, 
 notwithstanding the very full account of this transaction 
 given by Dr. Bence Jones, motives and influences were at 
 
FARADAY. 347 
 
 work which even now are not entirely revealed. The min- 
 ister was bitterly attacked, but he bore the censure of the 
 press with great dignity. Faraday, while he disavowed 
 having either directly or indirectly furnished the matter of 
 those attacks, did not publicly exonerate his lordship. The 
 Hon. Caroline Fox had proved herself Faraday's ardent 
 friend, and it was she who had healed the breach between 
 the philosopher and the minister. She manifestly thought 
 that Faraday ought to have come forward in Lord Mel- 
 bourne's defence, and there is a flavor of resentment in one 
 of her letters to him on the subject. No doubt Faraday had 
 good grounds for his reticence, but they are to me unknown. 
 In 1841 his health broke down utterly, and he went to 
 Switzerland with his wife and brother-iri-law. His bodily 
 vigor soon revived, and he accomplished feats of walking 
 respectable even for a trained mountaineer. The published 
 extracts from his Swiss journal contain many beautiful and 
 touching allusions. Amid references to the tints of the 
 Jungfrau, the blue rifts of the glaciers, and the noble Niesen, 
 towering over the Lake of Thun, we come upon the charm- 
 ing little scrap which I have elsewhere quoted : " Clout-nail 
 making goes on here rather considerably, and is a very neat 
 and pretty operation to observe. I love a smith's shop, and 
 any thing relating to smithery. My father was a smith." 
 This is from his journal ; but he is unconsciously speaking 
 to somebody perhaps to the world. 
 
 His descriptions of the Staub-bach, Giessbach, and of 
 the scenic effects of sky and mountain, are all fine and sym- 
 pathetic. But amid it all,, and in reference to it all, he tells 
 his sister that " true enjoyment is from within, not from 
 without." In those days Agassiz was living under a slab 
 of gneiss on the glacier of the Aar. Faraday met Forbes 
 at the Grimsel, and arranged with him an excursion to the 
 " H6tel des Neuchatelois ; " but indisposition put the pro- 
 ject out. 
 
348 FRAGMENTS OF SCIENCE. 
 
 From the Fort of Ham, in 1843, Faraday received a let- 
 ter addressed to him by Prince Louis Napoleon Bonaparte. 
 He read this letter to me many years ago, and the desire, 
 shown in various ways by the French Emperor, to turn 
 modern science to account, has often reminded me of it 
 since. At the age of thirty-five the prisoner of Ham speaks 
 of " rendering his captivity less sad by studying the great 
 discoveries " which science owes to Faraday ; and he asks 
 a question which reveals his cast of thought at the time : 
 "What is the most simple combination to give to a voltaic 
 battery, in order to produce a spark capable of setting fire 
 to powder under water or under ground ? n Should the 
 necessity arise, the French Emperor will not lack at the 
 outset the best appliances of modern science ; while we, I 
 fear, shall have to learn the magnitude of the resources we 
 are now neglecting amid the pangs of actual war. 1 
 
 One turns with renewed pleasure to Faraday's letters 
 to his wife, published in the second volume. Here surely 
 the loving essence of the man appears more distinctly than 
 anywhere else. From the house of Dr. Percy, in Birming- 
 ham, he writes thus : 
 
 "Here even here the moment I leave the table I 
 wish I were with you IN QUIET. Oh, what happiness is 
 ours ! My runs into the world in this way only serve to 
 make me esteem that happiness the more." 
 
 And again * 
 
 " We have been to a grand conversazione in the town- 
 hall, and I have now returned to my room to talk with you, 
 as the pleasarstest and happiest thing that I can do. Noth- 
 ing rests me so much as communion with you. I feel it 
 even now as I write, and catch myself saying the words 
 aloud as I write them." 
 
 1 The " science " has since been applied with astonishing effect by 
 those who had studied it far more thoroughly than the Emperor oT the 
 French, 
 
FARADAY, 349 
 
 Take this, moreover, as indicative of his love for Na- 
 ture : 
 
 " After writing, I walk out in the evening hand-in-hand 
 with my dear wife to enjoy the sunset ; for to me who love 
 scenery, of all that I have seen or can see there is none 
 surpasses that of heaven. A glorious sunset brings with it 
 a thousand thoughts that delight me." 
 
 Of the numberless lights thrown upon him by the " Life 
 and Letters," some fall upon his religion. In a letter to a 
 lady he describes himself as belonging a " a very small and 
 despised sect of Christians, known, if known at all, as San- 
 demanians, and our hope is founded on the faith that is in 
 Christ." He adds : " I do not think it at all necessary to 
 tie the study of the natural sciences and religion together, 
 and in my intercourse with my fellow-creatures, that which 
 is religious, and that which is philosophical, have ever been 
 two distinct things." He saw clearly the danger of quit- 
 ting his moorings, and his science became the safeguard 
 of his particular faith. For his investigations so filled his 
 mind as to leave no room for skeptical questionings, thus 
 shielding from the assaults of philosophy the creed of his 
 youth. His religion was constitutional and hereditary. It 
 was implied in the eddies of his blood and in the tremors 
 of his brain ; and however its outward and visible form 
 might have changed, Faraday would still have possessed 
 its elemental constituents awe, reverence, truth, and love. 
 
 It is worth inquiring how so profoundly religious a mind, 
 and so great a teacher, would be likely to regard our pres- 
 ent discussions on the subject of education. Faraday 
 would be a " secularist " were he now alive. He had no 
 sympathy with those who contemn knowledge unless it be 
 accompanied by dogma. A lecture delivered before the 
 City Philosophical Society in 1818, when he was twenty- 
 six years of age, expresses the views regarding education 
 which he entertained to the end of his life. " First, then," 
 
350 FRAGMENTS OF SCIENCE. 
 
 he says, "all theological considerations are banished from 
 the society, and of course from my remarks ; and whatever 
 I may say has no reference to a future state, or to the means 
 which are to be adopted in this world in anticipation of it. 
 Next, I have no intention of substituting any thing for re- 
 ligion, but I wish to take that part of human nature whicli 
 is independent of it. Morality, philosophy, commerce, the 
 various institutions and habits of society, are independent 
 of religion, and may exist either with or without it. They 
 are always the same, and can dwell alike in the breasts of 
 those who from opinion are entirely opposed in the set of 
 principles they include in the term religion, or in those who 
 have none. 
 
 " To discriminate more closely, if possible, I will ob- 
 serve that we have no right to judge religious opinions, but 
 the human nature of this evening is that part of man which 
 we have a right to judge ; and I think it will be found, on 
 examination, that this humanity as it may perhaps be 
 called will accord with what I have before described as 
 being in our own hands so improvable and perfectible." 
 
 Among my old papers I find the following remarks on 
 one of my earliest dinners with Faraday : " At two o'clock 
 he came down for me. He, his niece, and myself, formed 
 the party. ' I never give dinners,' he said. ' I don't know 
 how to give dinners, and I never dine out. But I should 
 not like my friends to attribute this to a wrong cause. I 
 act thus for the sake of securing time for work, and not 
 through religious motives, as some imagine.' He said 
 grace. I am almost ashamed to call his prayer a ' saying ' 
 of grace. In the language of Scripture, it might be de- 
 scribed as the petition of a son, into whose heart God had 
 sent the Spirit of His Son, and who with absolute trust 
 asked a blessing from his father. We dined on roast beef, 
 Yorkshire pudding, and potatoes ; drank sherry, talked of 
 research and its requirements, and of his habit of keeping 
 
FARADAY. 351 
 
 himself free from the distractions of society. He was bright 
 and joyful boylike, in fact, though he is now sixty-two. 
 His work excites admiration, but contact with him warms 
 and elevates the heart. Here, surely, is a strong man. I 
 love strength, but let me not forget the example of its 
 union with modesty, tenderness, and sweetness, in the char- 
 acter of Faraday." 
 
 Faraday's progress in discovery, and the salient points 
 of his character, are well brought out by the wise choice of 
 letters and extracts published in these volumes. I will not 
 call the labors of the biographer final. So great a char- 
 acter will challenge reconstruction. In the coming time 
 some sympathetic spirit, with the requisite strength, knowl- 
 edge, and solvent power, will, I doubt not, render these 
 materials plastic, give them more perfect organic form, and 
 send through them, with less of interruption, the currents 
 of Faraday's life. " He was too good a man," writes his 
 present biographer, " for me to estimate rightly, and too 
 great a philosopher for me to understand thoroughly." 
 That may be, but the reverent affection to which we owe 
 the discovery, selection, and arrangement of the materials 
 here placed before us, is probably a surer guide than mere 
 literary skill. The task of the artist who may wish in 
 future times to reproduce the real though unobtrusive 
 grandeur, the purity, beauty, and childlike simplicity of 
 him whom we have lost, will find his chief treasury already 
 provided for him by Dr. Bence Jones's labor of love. 
 
 Library 
 
XIII. 
 
 AN 
 
 ELEMENTARY LECTURE ON MAGNETISM. 
 
 ADDKESS TO THE TEACHERS OF PEIMAET SCHOOLS AT THE SOUTH 
 KENSINGTON MUSEUM. 
 
 April 30, 1861. 
 
" Next in order I will proceed to discuss by what law of Nature it 
 comes to pass that iron can be attracted by that stone which the Greeks 
 call the Magnet from the name of its native place, because it has its origin 
 within the bounds of the country of the Magnesians. This stone is more 
 wondered at because it often produces a chain of [iron] rings hanging 
 down from it. Thus you may see five and more suspended in succession 
 and tossing about in the light airs, one always hanging from the other 
 and attached to its lower side, and each in turn one from the other ex- 
 periencing the binding power of the stone : with such a continued cur- 
 rent its force flies through all. 
 
 " In things of this kind, many things must be established before you 
 can assign the true law of the thing in question, and it must be ap- 
 proached by a very circuitous road ; wherefore all the more I call for an 
 attentive ear and mind." LUCRETIUS, DC Rerum Natura, Lib. VI., 
 Munro's Translation, p. 317. 
 
 This lecture is a plain statement of the elementary facts of magnet- 
 ism, of one magnetic theory, and of the methods to be pursued in master- 
 ing both. It has already circulated among the teachers mentioned on its 
 title-page, and I had some doubts as to the propriety of its insertion 
 here. But, on reading it, it seemed so likely to be helpful, that my 
 scruples disappeared. J. T. 
 
MAGNETIC LINES OF FORCE. 
 
 From a Photograph by Professor MAYER, Lehigh University, United States. 
 
XIII. 
 
 A LECTURE ON MAGNETISM. 
 
 WE have no reason to believe that the sheep or the dog, 
 or, indeed, any of the lower animals, feel an interest in the 
 laws by which natural phenomena are regulated. A herd 
 may be terrified by a thunder-storm ; birds may go to roost, 
 and cattle return to their stalls during a solar eclipse ; but 
 neither birds nor cattle, as far as we know, ever think of 
 inquiring into the causes of these things. It is otherwise 
 with man. The presence of natural objects, the occurrence 
 of natural events, the varied appearances of the universe in 
 which he dwells, penetrate beyond his organs of sense, and 
 appeal to an inner power of which the senses are the mere 
 instruments and excitants. No fact is to him either final 
 or original. He cannot limit himself to the contemplation 
 of it alone, but endeavors to ascertain its position in a 
 series to which the constitution of his mind assures him it 
 must belong. He regards all that he witnesses in the 
 present as the efflux and sequence of something that has 
 gone before, and as the source of a system of events which 
 is to follow. The notion of spontaneity, by which in his 
 ruder .state he accounted for natural events, is abandoned; 
 the idea that Nature is an aggregate of independent parts 
 also disappears, as the connection and mutual dependence 
 of physical powers become more and more manifest : until 
 he is finally led, and that chiefly by the science of which 
 I happen this evening to be the exponent, to regard 
 
358 FRAGMENTS OF SCIENCE. 
 
 Nature as an organic whole, as a body each of whose 
 members sympathizes with the rest, changing, it is true, 
 from ages to ages, but without one real break of continuity, 
 or a single interruption of the fixed relations of cause and 
 effect. 
 
 The system of things which we call Nature is, however, 
 too vast and various to be studied first-hand by any single 
 mind. As knowledge extends there is always a tendency 
 to subdivide the field of investigation, its various parts be- 
 ing taken up by different individuals, and thus receiving a 
 greater amount of attention than could possibly be bestowed 
 on them if each investigator aimed at the mastery of the 
 whole. East, west, north, and south, the human mind 
 pushes its conquests ; but the centripetal form in which 
 knowledge, as a whole, advances, spreading ever wider on 
 all sides, is due in reality to the exertions of individuals, 
 each of whom directs his efforts, more or less, along a single 
 line. Accepting, in many respects, his culture from his 
 fellow-men, taking it from spoken words and from written 
 books, in some one direction, the student of Nature must 
 actually touch his work. He may otherwise be a dis- 
 tributor of knowledge, but not a creator, and fails to attain 
 that vitality of thought and correctness of judgment which 
 direct and habitual contact with natural truth can alone 
 impart. 
 
 One large department of the system of Nature which 
 forms the chief subject of my own studies, and to which it 
 is my duty to call your attention this evening, is that of 
 physics, or natural philosophy. This term is large enough 
 to cover the study of Nature generally, but it is usually 
 restricted to a department which, perhaps, lies closer to 
 our perceptions than any other. It deals with the phe- 
 nomena and laws of light and heat with the phenomena 
 and laws of magnetism and electricity with those of 
 sound with the pressures and motions of liquids and 
 
A LECTURE ON MAGNETISM. 359 
 
 gases, whether in a state of translation or of undulation. 
 The science of mechanics is a portion of natural philosophy, 
 though at present so large as to need the exclusive atten- 
 tion of him who would cultivate it profoundly. Astronomy 
 is the application of physics to the motions of the heavenly 
 bodies, the vastness of the field causing it, however, to be 
 regarded as a department in itself. In chemistry physical 
 agents play important parts. By heat and light we cause 
 bodies to combine, and by heat and light we decompose 
 them. Electricity tears asunder the locked atoms of com- 
 pounds, through their power of separating carbonic acid into 
 its constituents ; the solar beams build up the whole vege- 
 table world, and .by it the animal, while the touch of the 
 self-same beams causes hydrogen and chlorine to unite with 
 sudden explosion and form by their combination a powerful 
 acid. Thus physics and chemistry intermingle, physical 
 agents being employed by the chemist as a means to an 
 end ; while in physics proper the laws and phenomena of 
 the agents themselves, both qualitative and quantitative, 
 are the primary objects of attention. 
 
 My duty here to-night is to spend an hour in telling how 
 this subject is to be studied, and how a knowledge of it is 
 to be imparted to others. When first invited to do this, I 
 hesitated before accepting the responsibility. It would be 
 easy to entertain you with an account of what natural phi- 
 losophy has accomplished. I might point to those applica- 
 tions of science regarding which we hear so much in the 
 newspapers, and which we often find mistaken for science 
 itself. I might, of course, ring changes on the steam- 
 engine and the telegraph, the electrotype and the photo- 
 graph, the medical applications of physics, and the million 
 other inlets by which scientific thought filters into prac- 
 tical life. That would be easy compared with the task 
 of informing you how you are to make the study of physics 
 the instrument of your own culture, how you are to pos- 
 
360 FRAGMENTS OF SCIENCE. 
 
 sess its facts and make them living" seeds which shall 
 take root and grow in the mind, and not lie like dead 
 lumber in the store-house of memory. This is a task much 
 heavier than the mere cataloguing of scientific achieve- 
 ments ; and it is one which, feeling my own want of time 
 and power to execute it aright, I might well hesitate to 
 accept. 
 
 But let me sink excuses, and attack the work to the 
 best of my ability. First and foremost, then, I would ad- 
 vise you to get a knowledge of facts from actual observa- 
 tion. Facts looked at directly are vital ; when they pass 
 into words half the sap is taken out of them. You wish, 
 for example, to get a knowledge of magnetism ; well, pro- 
 vide yourself with a good, book on the subject, if you can, 
 but do not be content with what the book tells you ; do 
 not be satisfied with its descriptive woodcuts ; see the 
 actual thing yourself. Half of our book-writers describe 
 experiments which they never made, and their descrip- 
 tions often lack both force and truth ; but no matter how 
 clever or conscientious they may be, their written words 
 cannot supply the place of actual observation. Every fact 
 has numerous radiations, which are shorn off by the man 
 who describes it. Go, then, to a philosophical instrument- 
 maker, and give, according to your means, for a straight 
 bar-magnet, say, half a crown, or, if you can afford it, five 
 shillings for a pair of them ; or get a smith to cut a length 
 of ten inches from a bar of steel an inch wide and half an 
 inch thick ; file its ends decently, harden it, and get some- 
 body like myself to magnetize it. Two bar-magnets are 
 better than one. Procure some darning-needles such as 
 these. Provide yourself also with a little unspun silk; 
 which will give you a suspending fibre void of torsion ; 
 make a little loop of paper or of wire, thus, and attach your 
 fibre to it. Do it neatly. In the loop place your darning- 
 needle, and bring the two ends or poles, as they are called, 
 
A LECTURE ON MAGNETISM. 361 
 
 of your magnet successively up to either end of the needle. 
 Both the poles, you find, attract both ends of the needle. 
 Replace the needle by a bit of annealed iron wire, the same 
 effects ensue. Suspend successively little rods of lead, 
 copper, silver, or brass, of wood, glass, ivory, or whalebone ; 
 the magnet produces no sensible effect upon any of these 
 substances. You thence infer a special property in the 
 case of steel and iron. Multiply your experiments, how- 
 ever, and you will find that some other substances besides 
 iron are acted upon by your magnet. A rod of the metal 
 nickel, or of the metal cobalt, from which the blue color 
 used by painters is derived, exhibits powers similar to those 
 observed with the iron and steel. 
 
 In studying the character of the force you may, how- 
 ever, confine yourself to iron and steel, which are always 
 at hand. Make your experiments with the darning-needle 
 over and over again ; operate on both ends of the needle ; 
 try both ends of the magnet. Do not think the work stu- 
 pid ; you are conversing with Nature, and must acquire a 
 certain grace and mastery over her language ; and these 
 practice can alone impart. Let every movement be made 
 with care, and avoid slovenliness from the outset. In every 
 one of your experiments endeavor to feel the responsibility 
 of a moral agent. Experiment, as I have said, is the lan- 
 guage by which we address Nature, and through which she 
 sends her replies ; in the use of this language a lack of 
 straightforwardness is as possible and as prejudicial as in 
 the spoken language of the tongue. If you wish to become 
 acquainted with the truth of Nature, you must from the first 
 resolve to deal with her sincerely. 
 
 Now remove your needle from its loop, and draw it from 
 end to end along one of the ends of the magnet ; resuspend 
 it, and repeat your former experiment. You find the result 
 different. You now find that each extremity of the magnet 
 attracts one end of the needle and repels the other. The 
 16 
 
362 FRAGMENTS OF SCIENCE. 
 
 simple attraction observed in the first instance is now re- 
 placed by a dual force. Repeat the experiment till you 
 have thoroughly observed the ends which attract and those 
 which repel each other. 
 
 Withdraw the magnet entirely from the vicinity of your 
 needle, and leave the latter freely suspended by its fibre. 
 Shelter it as well as you can from currents of air, and if 
 you have iron buttons on your coat or a steel penknife in 
 your pocket, beware of their action. If you work at night, 
 beware of iron candlesticks, or of brass ones with iron rods 
 inside. Freed from such disturbances, the needle takes up 
 a certain determinate position. It sets its length nearly 
 north and south. Draw it aside from this position and let 
 it go. After several oscillations it will again come to it. 
 If you have obtained your magnet from a philosophical-in- 
 strument maker, you will see a mark on one of its ends. 
 Supposing, then, that you drew your needle along the end 
 thus marked, and that the eye-end of your needle was the 
 last to quit the magnet, you will find that the eye turns to 
 the south, the point of the needle turning toward the north. 
 Make sure of this, and do not take this statement on my 
 authority. 
 
 Now take a second darning-needle like the first, and 
 magnetize it in precisely the same manner: freely sus- 
 pended it also will turn its point to the north and its eye 
 to the south. Your next step is to examine the action of 
 the two needles which you have thus magnetized upon each 
 other. 
 
 Take one of them in your hand, and leave the other sus- 
 pended ; bring the eye-end of the former near the eye-end 
 of the latter ; Vthe suspended needle retreats : it is repelled. 
 Make the sam0 experiment with the two points, you obtain 
 the same result, the suspended needle is repelled. Now 
 cause the dissimilar ends to act on each other you have 
 attraction point attracts eye and eye attracts point. Prove 
 
A LECTURE ON MAGNETISM. 363 
 
 the reciprocity of this action by removing the suspended 
 needle, and putting the other in its place. You obtain the 
 same result. The attraction, then, is mutual, and the re- 
 pulsion is mutual, and you have thus demonstrated in the 
 clearest manner the fundamental law of magnetism, that 
 like poles repel, and that unlike poles attract each other. 
 You may say that this is all easily understood without do- 
 ing ; but do it, and your knowledge will not be confined to 
 what I have uttered here. 
 
 I have said that one end of your magnet has a mark 
 upon it ; lay several silk fibres together, so as to get suffi- 
 cient strength, or employ a thin silk ribbon, and form a loop 
 large enough to hold your magnet. Suspend it ; it turns 
 its marked end toward the north. This marked end is that 
 which in England is called the north pole. If a common 
 smith has made your magnet, it will be convenient to deter- 
 mine its north pole yourself, and to mark it with a file. 
 You vary your experiments by causing your magnetized 
 darning-needle to attract and repel your large magnet ; it 
 is quite competent to do so. In magnetizing the needle, I 
 have supposed the eye-end to be the last to quit the marked 
 end of the magnet ; that end of the needle is a south pole. 
 The end which last quits the magnet is always opposed in 
 polarity to the end of the magnet with which it has been in 
 contact. Brought near each other they mutually attract, 
 and thus demonstrate that they are unlike poles. 
 
 You may perhaps learn all this in a single hour ; but 
 spend several at it, if necessary; and remember, under- 
 standing it is not sufficient: you must obtain a manual 
 aptitude in addressing Nature. If you speak to your fellow- 
 man, you are not entitled to use jargon. Bad experiments 
 are jargon addressed to Nature, and just as much to be dep- 
 recated. A manual dexterity in illustrating the interaction 
 of magnetic poles is of the utmost importance at this stage 
 of your progress, and you must not neglect attaining this 
 
364 FRAGMENTS OF SCIENCE. 
 
 power over your implements. As you proceed, moreover, 
 you will be tempted to do more than I can possibly suggest. 
 Thoughts will occur to you which you will endeavor to fol- 
 low out ; questions will arise which you will try to answer. 
 The same experiment may be twenty things to twenty 
 people. Having witnessed the action of pole on pole 
 through the air, you will perhaps try whether the magnetic 
 power is not to be screened off. You use plates of glass, 
 \vood, slate, pasteboard, or gutta-percha, but find them all 
 pervious to this wondrous force. One magnetic pole acts 
 upon another through these bodies as if they were not 
 present. And should you become a patentee for the regu- 
 lation of ships' compasses, you will not fall, as some pro- 
 jectors have done, into the error of screening off the mag- 
 netism of the ship by the interposition of such substances. 
 
 If you wish to teach a class you must contrive that the 
 effects which you have thus far witnessed for yourself shall 
 be witnessed by twenty or thirty pupils. And here your 
 private ingenuity must come into play. You will attach 
 bits of paper to your needles, so as to render their move- 
 ments visible at a distance, denoting the north and south 
 poles by different colors, say green and red. You may also 
 improve upon your darning-needle. Take a strip of sheet- 
 steel, the rib of a lady's stays will answer, heat it to vivid 
 redness and plunge it into cold water. It is thereby hard- 
 ened, rendered, in fact, almost as brittle as glass. Six 
 inches of this, magnetized in the manner of the darning- 
 needle, will be better able to carry your paper indexes. 
 Having secured such a strip, you proceed thus : 
 
 Magnetize a small sewing-needle and determine its 
 poles ; or, break half an inch or an inch off your magnetized 
 darning-needle, and suspend it by a fine silk fibre. The 
 sewing-needle or the fragment of the darning-needle is now 
 to be used as a test-needle to examine the distribution of 
 the magnetism in your strip of steel. Hold the strip up- 
 
A LECTURE ON MAGNETISM. 365 
 
 right in your left hand, and cause the test-needle to ap- 
 proach the lower end of your strip; one end is attracted, 
 the other is repelled. Raise your needle along the strip ; 
 its oscillations, which at first were quick, become slower ; 
 opposite the middle of the strip they cease entirely ; neither 
 end of the needle is attracted ; above the middle the test- 
 needle turns suddenly round, its other end being now at- 
 tracted. Go through the experiment thoroughly ; you thus 
 learn that the entire lower half of the strip attracts one end 
 of the needle, while the entire upper half attracts the oppo- 
 site end. Supposing the north end of your little needle to 
 be that attracted below, you infer that the entire lower half 
 of your magnetized strip exhibits south magnetism, while 
 the entire upper half exhibits north magnetism. So far, 
 then, you have determined the distribution of magnetism in 
 your strip of steel. 
 
 You look at this fact, you think of it ; in its suggestive- 
 ness the value of the experiment chiefly consists. The 
 thought arises, " What will occur if I break my strip of 
 steel across in the middle ? Shall I obtain two magnets, 
 each possessing a single pole ? " Try the experiment ; 
 break your strip of steel, and test each half as you tested 
 the whole. The mere presentation of its two ends in suc- 
 cession to your test-needle suffices to show you that you 
 have not a magnet with a single pole, that each half pos- 
 sesses two poles with a neutral point between them. And 
 if you again break the half into two other halves, you will 
 find that each quarter of the original strip exhibits precisely 
 the same magnetic distribution as the strip itself. You 
 may continue the breaking process ; no matter how small 
 your fragment may be, it still possesses two opposite poles 
 and a neutral point between them. Well, your hand ceases 
 to break where breaking becomes a mechanical impossi- 
 bility ; but does the mind stop there ? No : you follow 
 the breaking process in idea when you can no longer realize 
 
366 FRAGMENTS OF SCIENCE. 
 
 it in fact ; your thoughts wander amid the very atoms of 
 your steel, and you conclude that each atom is a magnet, 
 and that the force exerted by the strip of steel is the mere 
 summation or resultant of the forces of its ultimate par- 
 ticles. 
 
 Here, then, is an exhibition of power which we can call 
 forth or cause to disappear at pleasure. "We magnetize our 
 strip of steel by drawing it along the pole of a magnet ; 
 we can demagnetize it, or reverse its magnetism, by prop- 
 erly drawing it along the same pole in the opposite direc- 
 tion. What, then, is the real nature of this wondrous 
 change ? What is it that takes place among the atoms of 
 the steel when the substance is magnetized ? The question 
 leads us beyond the region of sense, and into that of imagi- 
 nation. This faculty, indeed, is the divining-rod of 'the man 
 of science. Not, however, an imagination which catches 
 its creations from the air, but one informed and inspired by 
 facts, capable of seizing firmly on a physical image as a 
 principle, of discerning its consequences, and of devising 
 means whereby these forecasts of thought may be brought 
 to an experimental test. If such a principle be adequate to 
 account for all the phenomena, if from an assumed cause 
 the observed facts necessarily follow, we call the assump- 
 tion a theory, and, once possessing it, we can not only re- 
 vive at pleasure facts already known, but we can predict 
 others which we have never seen. Thus, then, in the prose- 
 cution of physical science, our powers of observation, mem- 
 ory, imagination, and inference, are all drawn upon. We 
 observe facts and store them up ; imagination broods upon 
 these memories, and by the aid of reason tries to discern 
 their interdependence. The theoretic principle flashes, or 
 slowly dawns upon the mind, and then the deductive fac- 
 ulty interposes to carry out the principle to its logical con- 
 sequences. A perfect theory gives dominion over natural 
 facts; and even an assumption which can only partially 
 
A LECTURE ON MAGNETISM. 367 
 
 stand the test of a comparison with facts, may be of emi- 
 nent use in enabling us to connect and classify groups of 
 phenomena. The theory of magnetic fluids is of this latter 
 character, and with it we must now make ourselves familiar. 
 
 With the view of stamping the thing more firmly on 
 your minds, I will make use of a strong and vivid image. 
 In optics, red and green are called complementary colors ; 
 their mixture produces white. Now I ask you to imagine 
 each of these colors to possess a self-repulsive power ; that 
 red repels red, and that green repels green ; but that red 
 attracts green and green attracts red, the attraction of the 
 dissimilar colors being equal to the repulsion of the similar 
 ones. Imagine the two colors mixed so as to produce 
 white, and suppose two strips of wood painted with this 
 white ; what will be their action upon each other ? Sus- 
 pend one of them freely as we suspended our darning- 
 needle, and bring the other near it ; what will occur ? The 
 red component of the strip you hold in your hand will re- 
 pel the red component of your suspended strip, but then it 
 will attract the green ; and the forces being equal they neu- 
 tralize each other.. In fact, the least reflection shows you 
 that the strips will be as indifferent to each other as two 
 unmagnetized darning-needles would be under the same 
 circumstances. 
 
 But suppose, instead of mixing the colors, we painted 
 one half of each strip from centre to end red, and the other 
 half green, it is perfectly manifest that the two strips would 
 now behave toward each other exactly as our two magnet- 
 ized darning-needles the red end would repel the red and 
 attract the green, the green would repel the green and at- 
 tract the red ; so that, assuming two colors thus related to 
 each other, we could by their mixture produce the neutral- 
 ity of an unmagnetized body, while by their separation we 
 could produce the duality of action of magnetized bodies. 
 
 But you have already anticipated a defect in my con- 
 
368 FRAGMENTS OF SCIENCE. 
 
 ception ; for if we break one of our strips of wood in the 
 middle we have one half entirely red and the other entirely 
 green, and with these it would be impossible to imitate the 
 action of our broken magnet. How, then, must we modify 
 our conception? We must evidently suppose each atom 
 of wood painted green on one face and red on the opposite 
 one. If this were done the resultant action of all the atoms 
 would exactly resemble the action of a magnet. Here, also, 
 if the two opposite colors of each atom could be caused 
 to mix so as to produce white, we should have, as before, 
 perfect neutrality. 
 
 Substitute in your minds for these two self-repellant 
 and mutually attractive colors two invisible self-repellant 
 and mutually attractive fluids, which in ordinary steel are 
 mixed to form a neutral compound, but which the act of 
 magnetization separates from each other, placing the oppo- 
 site fluids on the opposite faces of each atom, and you have 
 a perfectly distinct conception of the celebrated theory of 
 magnetic fluids. The strength of the magnetism excited is 
 supposed to be proportional to the quantity of neutral fluid 
 decomposed. According to this theory nothing is actually 
 transferred from the exciting magnet to the excited steel. 
 The act of magnetization consists in the forcible separation 
 of two powers which existed in the steel before it was mag- 
 netized, but which then neutralized each other by their coa- 
 lescence. And if you test your magnet after it has excited 
 a hundred pieces of steel, you will find that it has lost no 
 force no more, indeed, than I should lose had my words such 
 a magnetic influence on your minds, as to excite in them a 
 strong resolve to study natural philosophy. I should, in 
 fact, be the gainer by my own utterance and by the reac- 
 tion of your strength ; and so also the magnet is the gainer 
 by the reaction of the body which it magnetizes. 
 
 Look now to your excited piece of steel ; figure each 
 atom to your minds with its opposed fluids spread over its 
 
A LECTURE ON MAGNETISM. 369 
 
 opposite faces. How can this state of tilings be perma- 
 nent ? The fluids, by hypothesis, attract each other ; 
 what, then, keeps them apart ? Why do they not instantly 
 rush together across the equator of the atom, and thus neu- 
 tralize each other ? To meet this question, philosophers 
 have been obliged to infer the existence of a special force 
 which holds the fluids asunder. They call it coercive 
 force and it is found that those kinds of steel which offer 
 most resistance to being magnetized, which require the 
 greatest amount of coercion to tear their fluids asunder, 
 are the very ones which offer the greatest resistance to the 
 reunion of the fluids after they have been once separated. 
 Such kinds of steel are most suited to the formation of per- 
 manent magnets. It is manifest, indeed, that without 
 coercive force a permanent magnet -would not be at all pos- 
 sible. 
 
 You have not forgotten that, previous to magnetizing your 
 darning-needle, both its ends were attracted by your mag- 
 net 5 and that both ends of your bit of iron wire were acted 
 upon in the same way. Probably also long before this you 
 will have dipped the end of your magnet among iron filings, 
 and observed how they cling to it, or into a nail-box, and 
 found how it drags the nails after it. I know very well 
 that if you are not the slaves of routine, you will have by 
 this time done many things that I have not told you to do, 
 and thus multiplied your experience beyond what I have 
 indicated. You are almost sure to have caused a bit of 
 iron to hang from the end of your magnet, and you have 
 probably succeeded in causing a second piece to attach 
 itself to the first, a third to the second ; until finally the 
 force has become too feeble to bear the weight of more. 
 If you have operated with nails, you may have observed 
 that the points and edges hold together with the greatest 
 tenacity ; and that a bit of iron clings more firmly to the 
 corner of your magnet than to one of its flat surfaces. In 
 
370 FRAGMENTS OF SCIENCE. 
 
 short, you will, in all likelihood, have enriched your expe- 
 rience in many ways without any special direction from 
 me. 
 
 Well, the magnet attracts the nail, and that nail attracts 
 a second one. This proves that the nail in contact with 
 the magnet has had the magnetic quality developed in it 
 by that contact. If it be withdrawn from the magnet, its 
 power to attract its fellow-nail ceases. Contact, however, 
 is not necessary. A sheet of glass or paper, or a space 
 of air, may exist between the magnet and the nail ; the 
 latter is still magnetized, though not so forcibly as when 
 in actual contact. The nail then presented to the magnet 
 is itself a temporary magnet. That end which is turned 
 toward the magnetic pole has the opposite magnetism of 
 the pole which excites it ; the end most remote from the 
 pole has the same magnetism as the pole itself, and be- 
 tween the two poles the nail, like the magnet, possesses a 
 magnetic equator. 
 
 Conversant as you now are with the theory of magnetic 
 fluids, you have already, I doubt not, anticipated me in 
 imagining the exact condition of the iron under the in- 
 fluence of the magnet. You picture the iron as possessing 
 the neutral fluid in abundance, you picture the magnetic 
 pole, when brought near, decomposing the fluid ; repelling 
 the fluid of a like kind with itself, and attracting the unlike 
 fluid ; thus exciting in the parts of the iron nearest to itself 
 the opposite polarity. But the iron is incapable of becoming 
 a permanent magnet. It only shows its virtue as long as 
 the magnet acts upon it. "What, then, does the iron lack 
 which the steel possesses ? It lacks coercive force. Its 
 fluids are separated with ease, but, once the separating 
 cause is removed, they flow together again and neutrality 
 is restored. Your imagination must be quite nimble in 
 picturing these changes. You must be able to see the 
 fluids dividing and reuniting according as the magnet is 
 
A LECTURE ON MAGNETISM. 371 
 
 brought near or withdrawn. Fixing a definite pole in your 
 imagination, you must picture the precise arrangement of 
 the two fluids with reference to this pole. And you must 
 not only be well drilled in the use of this mental imagery 
 yourself, but you must be able to arouse the same pictures 
 in the minds of your pupils, and satisfy yourself that they 
 possess this power of placing actually before themselves 
 magnets and iron in various positions, and describing the 
 exact magnetic state of the iron in each particular case. 
 The mere facts of magnetism will have their interest im- 
 mensely augmented by an acquaintance with those hidden 
 principles whereon the facts depend. Still, w r hile you use 
 this theory of magnetic fluids to track out the phenomena 
 and link them together, be sure to tell your pupils that it 
 is to be regarded as a symbol merely a symbol, more- 
 over, which is incompetent to cover all the facts, 1 but which 
 does good practical service while we are waiting for the 
 actual truth. 
 
 This state of excitement into which the soft iron is 
 thrown by the influence of the magnet, is sometimes called 
 " magnetization by influence." More commonly, however, 
 the magnetism is said to be " induced " in the soft iron, 
 and hence this way of magnetizing is called "magnetic 
 induction." Now, there is nothing theoretically perfect in 
 Nature : there is no iron so soft as not to possess a certain 
 amount of coercive force, and no steel so hard as not to be 
 capable, in some degree, of magnetic induction. The 
 quality of steel is in some measure possessed by iron, and 
 the quality of iron is shared in some degree by steel. It is 
 in virtue of this latter fact that the unmagnetized darning- 
 
 1 This theory breaks down when applied to diamagnetic bodies, which 
 are repelled by magnets. Like soft iron, such bodies are thrown into a 
 state of temporary excitement in virtue of which they are repelled, but 
 any attempt to explain such a repulsion by the decomposition of a fluid 
 will demonstrate its own futility. 
 
372 FRAGMENTS OF SCIENCE. 
 
 needle was attracted in your first experiment ; and from 
 this you may at once deduce the consequence that, after the 
 steel has been magnetized, the repulsive action of a magnet 
 must be always less than its attractive action. For the 
 repulsion is opposed by the inductive action of the magnet 
 on the steel, while the attraction is assisted by the same 
 inductive action. Make this clear to your minds, and 
 verify it by your experiments. In some cases you can 
 actually make the attraction due to the temporary mag- 
 netism overbalance the repulsion due to the permanent 
 magnetism, and thus cause two poles of the same kind 
 apparently to attract each other. "When, however, good 
 hard magnets act on each other from a sufficient distance, 
 the inductive action practically vanishes, and the repulsion 
 of like poles is sensibly equal to the attraction of unlike 
 ones. 
 
 I dwell thus long on elementary principles, because 
 they are of the first importance, and it is the temptation 
 of this age of unhealthy cramming to neglect them. Now 
 follow me a little further. In examining the distribution 
 of magnetism in your strip of steel, you raised the needle 
 slowly from bottom to top, and found what we called a 
 neutral point at the centre. Now does the magnet really 
 exert no influence on the pole presented to its centre ? 
 Let us see. 
 
 FIG. 1. 
 
 I -IN 
 
 Let S N, Fig. 1, be your magnet, and let n represent a 
 particle of north magnetism placed exactly opposite the 
 
A LECTUEE ON MAGNETISM. 373 
 
 middle of the magnet. Of course this is an imaginary case, 
 as you can never in reality thus detach your north mag- 
 netism from its neighbor. What is the action of the two 
 poles of the magnet on n ? Your reply will of course be 
 that the pole S attracts n while the pole N repels it. 
 Let the magnitude and direction of the attraction be ex- 
 pressed by the line n m, and the magnitude and direction 
 of the repulsion by the line n o. Now the particle n being 
 equally distant from S and N, the line n o, expressing the 
 repulsion, will be equal to m n, which expresses the attrac- 
 tion, and the particle n y acted upon by two such forces, 
 must evidently move in the direction p n, exactly midway 
 between m n and n o. Hence you see that, although there 
 is no tendency of the particle n to move toward the mag- 
 netic equator, there is a tendency on its part to -move 
 parallel to the magnet. If instead of a particle of north 
 magnetism we placed a particle of south magnetism op- 
 posite to the magnetic equator, it would evidently be urged 
 along the line n q ; and if instead of two separate particles 
 of magnetism we place a little magnetic needle, containing 
 both north and south magnetism, opposite the magnetic 
 equator, its south pole being urged along n q, and its north 
 along n p, the little needle will be compelled to set itself 
 parallel to the magnet S N. Make the experiment, and 
 satisfy yourselves that this is the case. 
 
 Substitute for your magnetic needle a bit of iron wire, 
 devoid of permanent magnetism, and it will set itself ex- 
 actly as the needle does. Acted upon by the magnet, the 
 wire, as you know, becomes a magnet and behaves as such ; 
 it will, of course, turn its north pole toward p 9 and south 
 pole toward , just like the needle. 
 
 But supposing you shift the position of your particle of 
 north magnetism, and bring it nearer to one end of your 
 magnet than to the other, the forces acting on the particle 
 are no longer equal ; the nearest pole of the magnet will 
 
374 FRAGMENTS OF SCIENCE. 
 
 act more powerfully on the particle than the more distant 
 one. Let S N, Fig. 2, be the magnet and n the particle 
 of north magnetism in its new position. Well, it is 
 repelled by N, and attracted by S. Let the repulsion be 
 represented in magnitude and direction by the line n o, 
 and* the attraction by the shorter line n m. The resultant 
 of these two forces will be found by completing the par- 
 allelogram m n o p, and drawing its diagonal n p. Along 
 n p, then, a particle of north magnetism would be urged 
 by the simultaneous action of S and N. Substituting a 
 
 particle of south magnetism for n, the same reasoning 
 would lead to the conclusion that the particle would be 
 urged along n q, and if we place at n a short magnetic 
 needle, its north pole will be urged along n p, its south 
 pole along n q, and the ouly position possible to the 
 needle, thus acted on, is along the line p q, which, as you 
 see, is no longer parallel to the magnet. Verify this by 
 actual experiment. 
 
 In this way we might go round the entire magnet, and 
 considering its two poles as two centres from which the 
 force emanates, we could, in accordance with ordinary me- 
 chanical principles, assign a definite direction to the mag- 
 netic needle at every particular place. And substituting, 
 as before, a bit of iron wire for the magnetic needle, the 
 positions of both will be the same. 
 
A LECTURE ON MAGNETISM. 375 
 
 Now, I think, without further preface, you will be able 
 to comprehend for yourselves, and explain to others, one 
 of the. most interesting effects in the whole domain of 
 magnetism. Iron filings you know are particles of iron, 
 irregular in shape, being longer in some directions than in 
 others. For the present experiment, moreover, instead of 
 the iron filings, very small scraps of thin iron wire might be 
 employed. I place a sheet of paper over the magnet ; it is 
 all the better if the paper be stretched on a wooden frame, 
 as this enables us to keep it quite level. I scatter the 
 filings, or the scraps of wire, from a sieve upon the paper, 
 and tap the latter gently, so as to liberate the particles for 
 a moment from its friction. The magnet acts on the filings 
 through the paper, and see how it arranges them ! They 
 embrace the magnet in a series of beautiful curves, which 
 are technically called magnetic curves, or lines of magnetic 
 force. Does the meaning of these lines yet flash upon 
 you ? Set your magnetic needle or your suspended bit of 
 wire at any point of one of the curves, and you will find 
 the direction of the needle or of the wire to be exactly that 
 of the particle of iron, or of the magnetic curve at the 
 point. Go round and round the magnet ; the direction of 
 your needle always coincides with the direction of the curve 
 on which it is placed. These, then, are the lines along 
 which a particle of south magnetism, if you could detach it, 
 would move to the north pole, and a bit of north magnetism 
 to the south pole ; they are the lines along which the de- 
 composition of the neutral fluid takes place, and in the case 
 of the magnetic needle, one of its poles being urged in one 
 direction, and the other pole in the opposite direction, the 
 needle must necessarily set itself as a tangent to the curve. 
 I will not seek to simplify this subject further. If there 
 be any thing obscure or confused or incomplete in my 
 statement, you ought now, by patient thought, to be able 
 to clear, away the obscurity, to reduce the confusion to 
 
376 FRAGMENTS ,OF SCIENCE. 
 
 order, and to supply what is needed to render the explana- 
 tion complete. Do not quit the subject until you thor- 
 oughly understand it ; and if you are able to look with 
 your mind's eye at the play of forces around a magnet, and 
 see distinctly the operation of those forces in the produc- 
 tion of the magnetic curves, the time which we have spent 
 together has not been spent in vain. 
 
 In this thorough manner we must master our materials, 
 reason upon them, and, by determined study, attain to clear- 
 ness of conception. Facts thus dealt with exercise an 
 expansive force upon the boundaries of thought; they 
 widen the mind to generalization. We soon recognize a 
 brotherhood between the larger phenomena of Nature and 
 the minute effects which we have observed in our private 
 chambers. Why, we inquire, does the magnetic needle set 
 north and south ? Evidently it is compelled to do so by 
 the earth; the great globe which we inherit is itself a 
 magnet. Let us learn a little more about it. By means of 
 a bit of wax or otherwise attach your silk fibre to your 
 magnetic needle by a single point at its middle, the needle 
 will thus be uninterfered with by the paper loop, and will 
 enjoy to some extent a power of dipping its point or its 
 eye below the horizon. Lay your magnet on a table, and 
 hold the needle over the equator of the magnet. The 
 needle sets horizontal. Move it toward the north end of 
 the magnet; the south end of the needle dips, the dip 
 augmenting as you approach the north pole, over which 
 the needle, if free to move, will set itself exactly vertical. 
 Move it back to the centre, it resumes its horizontally ; 
 pass it on toward the south pole, its north end now dips, 
 and directly over the south pole the needle becomes ver- 
 tical, its north end being now turned downward. Thus 
 we learn that on the one side of the magnetic equator the 
 north end of the needle dips ; on the other side the south 
 end dips, the dip varying from nothing to ninety degrees. If 
 
A LECTURE ON MAGNETISM. 377 
 
 we go to the equatorial regions of the earth with a suit- 
 ably-suspended needle, we shall find there the position of 
 the needle horizontal. If we sail north, one end of the 
 needle dips ; if we sail south, the opposite end dips ; and 
 over the north or south terrestrial magnetic pole the needle 
 sets vertical. The south magnetic pole has not yet been 
 found, but Sir James Ross discovered the north magnetic 
 pole on the 1st of June, 1831. In this manner we estab- 
 lish a complete parallelism between the action of the earth 
 and that of an ordinary magnet. 
 
 The terrestrial magnetic poles do not coincide with the 
 geographical ones ; nor does the earth's magnetic equator 
 quite coincide with the geographical equator. The direc- 
 tion of the magnetic needle in London, which is called the 
 magnetic meridian, encloses an angle of 24 degrees with 
 the true astronomical meridian, this angle being called the 
 Declination of the needle for London. The north pole of 
 the needle now lies to the west of the true meridian ; the 
 declination is westerly. In the year 1660, however, the 
 declination was nothing, while before that time it was 
 easterly. All this proves that the earth's magnetic con- 
 stituents are gradually changing their distribution. This 
 change is very slow ; it is technically called the secular 
 change, and the observation of it has not yet extended over 
 a sufficient period of -time to enable us to guess, even ap- 
 proximately, at its laws. 
 
 Having thus discovered, to some extent, the secret of 
 the earth's power, we can turn it to account. I hold in my 
 hand a poker formed of good soft iron ; it is now in the 
 line of dip, a tangent, in fact, to the earth's line of magnetic 
 force. The earth, acting as a magnet, is at this moment 
 constraining the two fluids of the poker to separate, making 
 the lower end of the poker a north pole, and the upper 
 end a south pole. Mark the experiment : I hold the knob 
 uppermost, and it attracts the north end of a magnetic 
 
378 FRAGMENTS OF SCIENCE. 
 
 needle. I now reverse the poker, bringing its knob under- 
 most ; the knob is now a north pole and attracts the south 
 end of a magnetic needle. Get such a poker and carefully 
 repeat this experiment ; satisfy yourselves that the fluids 
 shift their position according to the manner in which the 
 poker is presented to the earth. It has already been stated 
 that the softest iron possesses a certain amount of coercive 
 force. The earth, at this moment, finds in this force an an- 
 tagonist which opposes the full decomposition of the neu- 
 tral fluid. The component fluids may be figured as meet- 
 ing an amount of friction, or possessing an amount of ad- 
 hesion, which prevents them from gliding over the atoms 
 of the poker. Can we assist the earth in this case ? If we 
 wish to remove the residue of a powder from the interior 
 surface of a glass to which the powder clings, we invert 
 the glass, tap it, loosen the hold of the powder, and thus 
 enable the force of gravity to pull it down. So also by 
 tapping the end of the poker we loosen the adhesion of 
 the fluid to the atoms and enable the earth to pull them 
 apart. But, what is the consequence? The portion of 
 fluid which has been thus forcibly dragged over the atoms 
 refuses to return when the poker has been removed 
 from the line of dip; the iron, as you see, has become 
 a permanent magnet. By reversing its position and 
 tapping it again we reverse its magnetism. A thoughtful 
 and competent teacher will well know how to place these 
 remarkable facts before his pupils in a manner which will 
 excite their interest; he will know, and if not, will try to 
 learn, how, by the use of sensible images, more or less gross, 
 to give those he teaches definite conceptions, purifying 
 these conceptions more and more as the minds of his pupils 
 become more capable of abstraction. He will cause his 
 logic to run like a line of light through these images, and 
 by thus acting he will cause his boys to march at his 
 side with a profit and a joy, which the mere exhibition of 
 
A LECTURE ON MAGNETISM. 379 
 
 facts without principles, or the appeal to the bodily senses 
 and the power of memory alone, could never inspire. 
 
 As an expansion of the note at page 371, the following extract may 
 find a place here : 
 
 " It is well known that a voltaic current exerts an attractive force 
 upon a second current, flowing in the same direction ; and that when the 
 directions are opposed to each other the force exerted is a repulsive one. 
 By coiling wires into spirals, Ampere was enabled to make them produce 
 all the phenomena of attraction and repulsion exhibited by magnets, and 
 from this it was but a step to his celebrated theory of molecular cur- 
 rents. He supposed the molecules of a magnetic body to be surrounded 
 by such currents, which, however, in the natural state of the body mu- 
 tually neutralized each other, on account of their confused ' grouping. 
 The act of magnetization he supposed to consist in setting these molecu- 
 lar currents parallel to each other ; and, starting from this principle, he 
 reduced all the phenomena of magnetism to the mutual action of electric 
 currents. 
 
 " If we reflect upon the experiments recorded in the foregoing pages 
 from first to last, we can hardly fail to be convinced that diamagnetic 
 bodies operated on by magnetic forces possess a polarity " the same in 
 kind as, but the reverse in direction of that acquired by magnetic bodies." 
 But, if this be the case, how are we to conceive the physical mechanism 
 of this polarity ? According to Coulomb's and Poisson's theory, the act 
 of magnetization consists in the decomposition of a neutral magnetic 
 fluid ; the north pole of a magnet, for example, possesses an attraction 
 for the south fluid of a piece of soft iron submitted to its influence, draws 
 the said fluid toward it, and with it the material particles with which the 
 fluid is associated. To account for diamagnetic phenomena this theory 
 seems to fail altogether ; according to it, indeed, the oft-used phrase, " a 
 north pole exciting a north pole, and a south pole a south pole," involves 
 a contradiction. For if the north fluid be supposed to be attracted tow- 
 ard the influencing north pole, it is absurd to suppose that its presence 
 there could produce repulsion. The theory of Ampere is equally at a loss 
 to explain diamagnetic action ; for if we suppose the particles of bismuth 
 surrounded by molecular currents, then, according to all that is known 
 of electro-dynamic laws, these currents would set themselves parallel to, 
 and in the same direction as those of the magnet, and hence attraction, 
 and not repulsion, would be the result. The fact, however, of this not 
 
380 FRAGMENTS OF SCIENCE. 
 
 being the case proves that these molecular currents are not the mechan- 
 ism by which diamagnetic induction is effected. The consciousness of 
 this, I doubt not, drove M. Weber to the assumption that the phenomena 
 of diamagnetism are produced by molecular currents, not directed, but 
 actually excited in the bismuth by the magnet. Such induced currents 
 would, according to known laws, have a direction opposed to those of the 
 inducing magnet, and hence would produce the phenomena of repulsion. 
 To carry out the assumption here made, M. Weber is obliged to suppose 
 that the molecules of diamagnetic bodies are surrounded by channels, in 
 which the induced molecular currents, once excited, continue to flow 
 without resistance." DiG.magndis.rn and Magnc-cry&tallic Action, pp. 136, 
 137. 
 
XIV. 
 SHORTER ARTICLES. 
 
 SLATES. DEATH BY LIGHTNING. SCIENCE AND SPIRITS.- 
 VITALITY. ADDITIONAL REMARKS ON MIRACLES. 
 
SLATES. 
 
 (Part of a Lecture delivered in the Royal Institution of Great Britain, 
 June 6, 1856.) 
 
 WHEN the student of physical science has to investigate 
 the character of any natural force, his first care must be to 
 purify it from the mixture of other forces, and thus study 
 its simple action. If, for example, he wishes to know how 
 a mass of liquid would shape itself, if at liberty to follow 
 the bent of its own molecular forces, he must see that these 
 forces have free and undisturbed exercise. We might, per- 
 haps, refer him to the dew-drop for a solution of the ques- 
 tion ; but here we have to do, not only with the action of 
 the molecules of the liquid upon each other, but also with 
 the action of gravity upon the mass, which pulls the drop 
 downward and elongates it. If he would examine the 
 problem in its purity, he must do as Plateau has done, de- 
 tach the liquid mass from the action of gravity ; he would 
 then find the shape to be a perfect sphere. Natural pro- 
 cesses come to us in a mixed manner, and to the unin- 
 structed mind are a mass of unintelligible confusion. Sup- 
 pose half a dozen of the best musical performers to be placed 
 in the same room, each playing his own instrument to per- 
 fection, but no two playing the same tune ; though each in- 
 dividual instrument might be a source of perfect music, still 
 the mixture of all would produce mere noise. Thus it is 
 with the processes of Nature. Here mechanical and mo- 
 lecular laws intermingle and create apparent confusion. 
 Their mixture constitutes what may be called the noise of 
 
384 FRAGMENTS OF SCIENCE. 
 
 natural laws, and it is the vocation of the man of science to 
 resolve this noise into its components, and thus to detect 
 the " music " in which the foundations of Nature are laid. 
 
 The necessity of this detachment of one force from all 
 other forces is nowhere more strikingly exhibited than in the 
 phenomena of crystallization. Here, for example, is a so- 
 lution of common sulphate of soda or Glauber salt. Look- 
 ing into it mentally, we see the molecules of that liquid, 
 like disciplined squadrons under a governing eye, arranging 
 themselves into battalions, gathering round distinct centres, 
 and forming themselves into solid masses, which after a time 
 assume the visible shape of the crystal now held in my hand. 
 I may, like an ignorant meddler wishing to hasten matters, 
 introduce confusion into this order. This may be done by 
 plunging a glass rod into the vessel ; the consequent action 
 is not the pure expression of the crystalline forces ; the 
 molecules rush together with the confusion of an unorgan- 
 ized mob, and not -with the steady accuracy of a disciplined 
 host. In this mass of bismuth also we have an example of 
 confused crystallization ; but in the crucible behind me a 
 slower process is going on : here there is an architect at 
 work " who makes no chips, no din," and who is now build- 
 ing the particles into crystals, similar in shape and structure 
 to those beautiful masses which we see upon the table. By 
 permitting alum to crystallize in this slow way, we obtain 
 these perfect octahedrons ; by allowing carbonate of lime 
 to crystalize, Nature produces these beautiful rhomboids ; 
 when silica crystallizes, we have formed these hexagonal 
 prisms capped at the ends by pyramids ; by allowing salt- 
 petre to crystallize we have these prismatic masses, and 
 when carbon crystallizes, we have the diamond. If we wish 
 to obtain a perfect crystal, we must allow the molecular 
 forces free play : if the crystallizing mass be permitted to 
 rest upon a surface it will be flattened, and to prevent this 
 a small crystal must be so suspended as to be surrounded 
 
SLATES. 385 
 
 on all sides by the liquid, or, if it rest upon the surface, it 
 must be turned daily so as to present all its faces in succes- 
 sion to the working builder. 
 
 In building up crystals these little atomic bricks often 
 arrange themselves into layers which are perfectly parallel 
 to each other, and which can be separated by mechanical 
 means ; this is called the cleavage of the crystal. The crys- 
 tal of sugar I hold in my hand thus far escaped the solvent 
 and abrading forces which sooner or later determine the 
 fate of sugar-candy. I readily discover that it cleaves with 
 peculiar facility in one direction. Again, I lay my knife 
 upon this piece of rock-salt, and with a blow cleave it in 
 one direction. Laying the knife at right angles to its 
 former position, the crystal cleaves again; and finally, 
 placing the knife at right angles to the two former positions, 
 we find a third cleavage. Rock-salt cleaves in three di- 
 rections, and the resulting solid is this perfect cube, which 
 may be broken up into any number of smaller cubes. Ice- 
 land spar also cleaves in three directions, not at right angles, 
 but oblique to each other, the resulting solid being a rhom- 
 boid. In each of these cases the mass cleaves with equal 
 facility in all three directions. For the sake of complete- 
 ness I may say that many crystals cleave with unequal fa- 
 cility in different directions: heavy spar presents an ex- 
 ample of this kind of cleavage. 
 
 Turn we now to the consideration of some other phe- 
 nomena to which the term cleavage may be applied. Beech, 
 deal, and other woods, cleave with facility along the fibre, 
 and this cleavage is most perfect when the edge of the axe 
 is laid across the rings which mark the growth of the tree. 
 If you look at this bundle of hay severed from a rick, you 
 will see a sort of cleavage in it also ; the stalks lie in paral- 
 lel planes, and only a small force is required to separate 
 them laterally. But we cannot regard the cleavage of the 
 tree as the same in character as that of the hay-rick. In 
 17 
 
386 FRAGMENTS OF SCIENCE. 
 
 the one case it is the molecules arranging themselves ac- 
 cording to organic laws which produce a cleavable struct- 
 ure, in the other case the easy separation in one direction 
 is due to the mechanical arrangement of the coarse sensible 
 masses of the stalks of hay. 
 
 This sand-stone rock was once a powder, more or less 
 coarse, held in mechanical suspension by water. The pow- 
 der was composed of two distinct parts, fine grains of sand 
 and small plates of mica. Imagine a wide strand covered by 
 a tide, or an estuary with water which holds such powder in 
 suspension : how will it sink ? The rounded grains of sand 
 will reach the bottom first, because they encounter least re- 
 sistance, the mica afterward, and when the tide recedes we 
 have the little plates shining like spangles upon the surface 
 of the sand. Each successive tide brings its charge of 
 mixed powder, deposits its duplex layer day after day, and 
 finally masses of immense thickness are piled up, which by 
 preserving the alternations of sand and mica tell the tale 
 of their formation. Take the sand and mica, and mix them 
 together in water, and allow them to subside ; they will ar- 
 range themselves in the manner indicated, and by repeating 
 the process you can actually build up a mass which shall be 
 the exact counterpart of that presented by Nature. Now this 
 structure cleaves with readiness along the planes in which 
 the particles of mica are strewn. Specimens of such a rock 
 sent to me from Halifax, and other masses from the quarries 
 of Over Darwen in Lancashire, are here before you. With 
 a hammer and chisel I can cleave them into flags ; indeed, 
 these flags are employed for roofing purposes in the districts 
 from which the specimens have come, and receive the name 
 of " slate-stone." But you will discern without a word 
 from me, that this cleavage is not a crystalline cleavage 
 any more than that of a hay-rick. It is molar, not molec- 
 ular. 
 
 This, so far as I am aware of, has never been imagined, 
 
SLATES. 387 
 
 and it has been agreed among geologists not to call such 
 splitting as this cleavage at all, but to restrict the term to 
 a phenomenon of a totally different character. 
 
 Those who have visited the slate-quarries of Cumberland 
 and North Wales will have witnessed the phenomenon to 
 which I refer. We have long drawn our supply of roofing- 
 slates from such quarries ; school-boys ciphered on these 
 slates, they were used for tombstones in church-yards, and 
 for billiard-tables in the metropolis ; but not until a com- 
 paratively late period did men begin to inquire how their 
 wonderful structure was produced. What is the agency 
 which enables us to split Honister Crag, or the cliffs of 
 Snowdon, into laminae from crown to base ? This question 
 is at the present moment one of the great difficulties of 
 geologists, and occupies their attention perhaps more than 
 any other. You may wonder at this. Looking into the 
 quarry of Penrhyn, you may be disposed to offer the ex- 
 planation I heard given two years ago. " These planes of 
 cleavage," said a friend who stood beside me on the quarry's 
 edge, " are the planes of stratification which have been 
 lifted by some convulsion into an almost vertical position." 
 But this was a mistake, and indeed here lies the grand diffi- 
 culty of the problem. The planes of cleavage stand in most 
 cases at a high angle to the bedding. Thanks to Sir Roder- 
 ick Murchison, I am able to place the proof of this before 
 you. Here is a specimen of slate in which both the planes 
 of cleavage and of bedding are distinctly marked, one of 
 them making a large angle with the other. This is com- 
 mon. The cleavage of slates, then, is not a question of 
 stratification ; what, then, is its cause ? 
 
 In an able and elaborate essay published in 1835, Pro- 
 fessor Sedgwick proposed the theory that cleavage is due 
 to the action of crystalline or polar forces subsequent to the 
 consolidation of the rock. " We may affirm," he says, " that 
 no retreat of the parts, no contraction of dimensions in pass- 
 
388 FRAGMENTS OF SCIENCE. 
 
 ing to a solid state, can explain such .phenomena. They 
 appear to me only resolvable on the supposition that crystal- 
 line or polar forces acted upon the whole mass simultane- 
 ously in one direction and with adequate force." And again, 
 in another place : " Crystalline forces have rearranged whole 
 mountain-masses, producing a beautiful crystalline cleavage, 
 passing alike through all the strata." 1 The utterance of 
 such a man struck deep, as it ought to do, into the minds 
 of geologists, arid at the present day there are few who do 
 not entertain this view either in whole or in part. 2 The 
 boldness of the theory, indeed, has, in some cases, caused 
 speculation to run riot, and we have books published on the 
 action cf polar forces and geologic magnetism, which rather 
 astonish those who know something about the subject. Ac- 
 cording to this theory, whole districts of North Wales and 
 Cumberland, mountains included, are neither more nor less 
 than the parts of a gigantic crystal. These masses of slate 
 were originally fine mud, composed of the broken and 
 abraded particles of older rocks. They contain silica, alu- 
 mina, potash, soda, and mica, mixed mechanically together. 
 In the course of ages the mixture became consolidated, and 
 the theory before us assumes that a process of crystalliza- 
 tion afterward rearranged the particles and developed in it 
 a single plane of cleavage. Though a bold, and I think in- 
 admissible, stretch of analogies, this hypothesis has done 
 
 1 Transactions of the Geological Society, ser. ii. vol. iii. p. 477. 
 
 2 In a letter to Sir Charles Lyell, dated from the Cape of Good Hope 
 February 20, 1836, Sir John Herschel writes as follows : " If rocks have 
 been so heated as to allow of a commencement of crystallization, that is 
 to say, if they have been heated to a point at which the particles can be- 
 gin to move among themselves, or at least on their own axes, some gen- 
 eral law must then determine the position in which these particles will 
 rest on cooling. Probably that position will have some relation to the 
 direction in which the heat escapes. Now, when all or a majority of par- 
 ticles of the same nature have a general tendency to one position, that 
 must of course determine a cleavage plane." 
 
SLATES. 389 
 
 good service. Right or wrong, a thoughtfully - uttered 
 theory has a dynamic pow.er which operates against intel- 
 lectual stagnation; and even by provoking opposition is 
 eventually of service to the cause of truth. It would, how- 
 ever, have been remarkable if, among the ranks of geolo- 
 gists themselves, men were not found to seek an explana- 
 tion of slate-cleavage involving a less hardy assumption. 
 
 The first step in an inquiry of this kind is to seek facts. 
 This has been done, and the labors of Daniel Sharpe (the 
 late President of the Geological Society, who, to the loss 
 of science and the sorrow of all who knew him, has so sud- 
 denly been taken away from us), Mr. Henry Clifton Sorby, 
 and others, have furnished us with a body of facts associated 
 with slaty cleavage, and having a most important bearing 
 upon the question. 
 
 Fossil shells are found in these slate-rocks. I have here 
 several specimens of such shells in the actual rock, and oc- 
 cupying various positions in regard to the cleavage planes. 
 They are squeezed, distorted, and crushed ; in all cases the 
 distortion leads to the inference that the rock which con- 
 tains these shells has been subjected to enormous pressure 
 in a direction at right angles to the planes of cleavage. 
 The shells are all flattened and spread out in these planes. 
 Compare this fossil trilobite of normal proportions with 
 these others which have suffered distortion. Some have 
 lain across, some along, and some oblique to the cleavage of 
 the slate in which they are found ; but in all cases the dis- 
 tortion is such as required for its production a compressing 
 force acting at right angles to the planes of cleavage. As 
 the trilobites lay in the mud, the jaws of a gigantic vice ap- 
 pear to have closed upon them and squeezed them into the 
 shapes you see. 
 
 We sometimes find a thin layer of coarse, gritty mate- 
 rial, between two layers of finer rock, through which and 
 across the gritty layer pass the planes of lamination. The 
 
390 FRAGMENTS OF SCIENCE. 
 
 coarse layer is found bent by the pressure into sinuosities 
 like a contorted ribbon. Mr. Sorby has described a striking 
 case of this kind. This crumpling can be experimentally 
 imitated ; the amount of compression might, moreover, be 
 rough tly estimated by supposing the contorted bed to be 
 stretched out, its length measured and compared with the 
 shorter distance into which it has been squeezed. We find 
 in this way that the yielding of the mass has been consider- 
 able. 
 
 Let me now direct your attention to another proof of 
 pressure ; you see the varying colors which indicate the 
 bedding on this mass of slate. The dark portion is gritty, 
 being composed of comparatively coarse particles, which, 
 owing to their size, shape, and gravity, sink first and con- 
 stitute the bottom of each layer. Gradually, from bottom 
 to top the coarseness diminishes,, and near the upper sur- 
 face we have a layer of exceedingly fine mud. It is the 
 mud thus consolidated from which are derived the German 
 razor-stones, so much prized for the sharpening of surgical 
 instruments. When a bed is thin, the fine white mud is 
 permitted to rest upon a slab of the coarser slate in contact 
 with it : when the bed is thick it is cut into slices, which 
 are cemented to pieces of ordinary slate, and thus rendered 
 stronger. The mud thus deposited is, as might be ex- 
 pected, often rolled up into nodular masses, carried for- 
 ward, and deposited among coarser material by the rivers 
 from which the slate-mud has subsided. Here are such 
 nodules enclosed in sandstone. Everybody, moreover, who 
 has ciphered upon a school-slate must remember the whitish- 
 green spots which sometimes dotted the surface of the 
 slate, and over which the pencil usually slid as if the spots 
 were greasy. Now these spots are composed of the finer 
 mud, and they could not, on account of their fineness, bite 
 the pencil like the surrounding gritty portions of the slate. 
 Here is a beautiful example of these spots : you observe 
 
SLATES. 391 
 
 them on the cleavage surface in broad round patches. But 
 turn the slate edgeways and the section of each nodule is 
 seen to be a sharp oval, with its longer axis parallel to the 
 cleavage. This instructive fact has been adduced by Mr. 
 Sorby. I have made excursions to the quarries of Wales 
 and Cumberland, and to many of the slate-yards of London, 
 and found the fact general. Thus we elevate a common 
 experience of our boyhood into evidence of the highest 
 significance as regards a most important geological problem. 
 From the magnetic deportment of these slates, I was led to 
 infer that these spots contain a less amount of iron than the 
 surrounding dark slate. An analysis was made for me by 
 Mr. Hambly in the laboratory of Dr. Percy at the School of 
 Mines, with the following result : 
 
 ANALYSIS OF SLATE. 
 Dark Slate, two Analyses. 
 
 1. Percentage of iron 5.85 
 
 2. " " 6.13 
 
 Mean 5.99 
 
 WhitisJi-green Slate. 
 
 1. Percentage of iron 3.24 
 
 2. " " 3.12 
 
 According to these analyses, the quantity of iron in the 
 dark slate immediately adjacent to the greenish spot is 
 nearly double the quantity contained in the spot itself. 
 This is about the proportion which the magnetic experi- 
 ments suggested. 
 
 Let me now remind you that the facts brought before 
 you are typical each is the representative of a class. We 
 have seen shells crushed ; the unhappy trilobites squeezed, 
 beds contorted, nodules of greenish marl flattened ; and all 
 these sources of independent testimony point to one and 
 the same conclusion, namely, that slate-rocks have been 
 
392 FRAGMENTS OF SCIENCE. 
 
 subjected to enormous pressure in a direction at right angles 
 to the planes of cleavage. 
 
 In reference to Mr. Sorby's contorted bed, I have said 
 that by supposing it to be stretched out and its length 
 measured, it would give us an idea of the amount of yield- 
 ing of the mass above and below the bed. Such a measure- 
 ment, however, would not give the exact amount of yielding. 
 I hold in my hand a specimen of slate with its bedding 
 marked upon it ; the lower portions of each layer being 
 composed of a comparatively coarse gritty material some- 
 thing like what you may suppose the contorted bed to be 
 composed of. Now, in crossing these gritty portions, the 
 cleavage turns, as if tending to cross the bedding at an- 
 other angle. When the pressure began to act, the inter- 
 mediate bed, which is not entirely unyielding, suffered 
 longitudinal pressure ; as it bent, the pressure became grad- 
 ually more lateral, and the direction of its cleavage is ex- 
 actly such as you would infer from an action of this kind it 
 is neither quite across the bed nor yet in the same direction 
 as the cleavage of the slate above and below it, but inter- 
 mediate between both. Supposing the cleavage to be at 
 right angles to the pressure, this is the direction which it 
 ought to take across these more unyielding strata. 
 
 Thus we have established the concurrence of the phe- 
 nomena of cleavage and pressure that they accompany 
 each other ; but the question still remains, Is the pressure 
 sufficient to account for the cleavage ? A single geologist, 
 as far as I am aware, answers boldly in the affirmative. 
 This geologist is Sorby, who has attacked the question in 
 the true spirit of a physical investigator. Call to mind the 
 cleavage of the flags of Halifax and Over Darwen, which is 
 caused by the interposition of layers of mica between the 
 gritty strata. Mr. Sorby finds plates of mica to be also a 
 constituent of slate-rock. He asks himself, what will be 
 the effect of pressure upon a mass containing such plates 
 
SLATES. 393 
 
 confusedly mixed up in it ? It will be, he argues, and he 
 argues rightly, to place the plates with their flat surfaces 
 more or less perpendicular to the direction in which the 
 pressure is exerted. He takes scales of the oxide of iron, 
 mixes them with a fine powder, and on squeezing the mass 
 finds that the tendency of the scales is to set themselves at 
 right angles to the line of pressure. Along the planes of 
 weakness produced by the scales the mass cleaves. 
 
 By tests of a different character from those applied by 
 Mr. Sorby, it might be shown how true his conclusion is, 
 that the effect of pressure on elongated particles or plates 
 will be such as he describes it. But while the scales must 
 be regarded as a true cause, I should not ascribe to them a 
 large share in the production of the cleavage. I believe 
 that, even if the plates of mica were wholly absent, the 
 cleavage of slate-rocks would be much the same as it is at 
 present. 
 
 Here is a mass of pure white wax : it contains no mica 
 particles, no scales of iron, nor any thing analogous to them. 
 Here is the self-same substance submitted to pressure. I 
 would invite the attention of the eminent geologists now 
 before me to the structure of this wax. No slate ever ex- 
 hibited so clean a cleavage ; it splits into laminae of sur- 
 passing tenuity, and proves at a single stroke that pressure 
 is sufficient to produce cleavage, and that this cleavage is 
 independent of intermixed plates or scales. I have pur- 
 posely mixed this wax with elongated particles, and am 
 unable to say at the present moment that the cleavage is 
 sensibly affected by their presence if any thing, I should 
 say they rather impair its fineness and clearness than pro- 
 mote it. 
 
 The finer the slate is the more perfect will be the re- 
 semblance of its cleavage to that of the wax. Compare 
 the surface of the wax with the surface of this slate from 
 Borrodale in Cumberland. You have precisely the same 
 
394 FRAGMENTS OF SCIENCE. 
 
 features in both : you see flakes clinging to the surfaces of 
 each, which have been partially torn away in cleaving. 
 Let any close observer compare these two effects, he will, 
 I am persuaded, be led to the conclusion that they are the 
 product of a common cause. 1 
 
 But you will ask me how, according to my view, does 
 pressure produce this remarkable result. This may be 
 stated in a very few words : 
 
 There is no such thing in Nature as a body of perfectly 
 homogeneous structure. I breakr this clay which seems so 
 uniform, and find that the fracture presents to my eyes in- 
 numerable surfaces along which it has given way, and it 
 has yielded along those surfaces because in them the cohe- 
 sion of the mass is less than elsewhere. I break this mar- 
 ble, and even this wax, and observe the same result ; look 
 at the mud at the bottom of a dried pond ; look to some of 
 the ungra veiled walks in Kensington Gardens on drying af- 
 ter rain they are cracked and split, and, other circumstances 
 being equal, they crack and split where the cohesion is least. 
 Take then a mass of partially consolidated mud. Such a 
 mass is divided and subdivided by interior surfaces along 
 which the cohesion is comparatively small. Penetrate the 
 mass in idea, and you will see it composed of numberless 
 irregular polyhedra bounded by surfaces of weak cohesion. 
 Imagine such a mass subjected to pressure it yields and 
 spreads out in the direction of least resistance ; a the little 
 
 1 I have usually softened the wax by warming it, kneaded it with the 
 fingers, and pressed- it between thick plates of glass previously wetted. 
 At the ordinary summer temperature the pressed wax is soft, and tears 
 rather than cleaves ; on this account, I cool my compressed specimens in 
 a mixture of pounded ice and salt, and when thus cooled they split beau- 
 tifully. 
 
 2 It is scarcely necessary to say that, if the mass were squeezed equal- 
 ly in all directions, no laminated structure could be produced ; it must 
 have room to yield in a lateral direction. Mr. Warren De la Rue informs 
 me that he once wished to obtain white-lead in a fine granular state, and 
 
SLATES. 395 
 
 polyhedra become converted into laminae, separated from 
 each other by surfaces of weak cohesion, and the infallible 
 result will be a tendency to cleave at right angles to the 
 line of pressure. 
 
 Further, a mass of dried mud is full of cavities and fis- 
 sures. If you break dried pipe-clay you see them in great 
 numbers, and there are multitudes of them so small that you 
 cannot see them. A flattening of these cavities must take 
 place in squeezed mud, and this must to some extent facili- 
 tate the cleavage of the mass in the direction indicated. 
 
 Although the time at my disposal has not permitted me 
 duly to develop these thoughts, yet for the last twelve 
 months the subject has presented itself to me almost daily 
 under one aspect or another. I have never eaten a biscuit 
 during this period without remarking the cleavage devel- 
 oped by the rolling-pin. You have only to break a biscuit 
 across, and to look at the fracture, to see the laminated 
 structure. We have here the means of pushing the anal- 
 ogy further. I invite you to compare the structure of this 
 slate, which was subjected to a high temperature during 
 the conflagration of Mr. Scott Russell's premises, with that 
 of a biscuit. Air or vapor within the slate has caused it 
 to swell, and the mechanical structure it reveals is precisely 
 that of a biscuit. During these inquiries I have received 
 much instruction in the manufacture of puff-paste. Here 
 is some such paste baked under my own superintendence. 
 The cleavage of our hills is accidental cleavage, but this is 
 cleavage with intention. The volition of the pastry-cook 
 has entered into its formation. It has been his aim to pre- 
 serve a series of surfaces of structural weakness, along 
 which the dough divides into layers. Puff-paste in prepa- 
 
 to accomplish this he first compressed it. The mould was conical, and 
 permitted the lead to spread out a little laterally. The lamination was 
 as perfect as that of slate, and it quite defeated him in his effort to ob- 
 tain a granular powder. 
 
396 FRAGMENTS OF SCIENCE. 
 
 ration must not be handled too much ; it ought, moreover, 
 to be rolled on a cold slab, to prevent the 'butter from melt- 
 ing, and diffusing itself, thus rendering the paste more ho- 
 mogeneous and less liable to split. Puff-paste is, then, 
 ' simply an exaggerated case of slaty cleavage. 
 
 The principle which I have enunciated is so simple as 
 to be almost trivial ; nevertheless, it embraces not only the 
 cases mentioned, but, if time permitted, it might be shown 
 you that the principle has a much wider range of applica- 
 tion. When iron is taken from the puddling-furnace it is 
 more or less spongy, an aggregate in fact of small nod- 
 ules : it is at a welding heat, and at this temperature is 
 submitted to the process of rolling. Bright, smooth bars 
 are the result. But, notwithstanding the high heat, the 
 nodules do not perfectly blend together. The process of 
 rolling draws them into fibres. Here is a mass acted upon 
 by dilute sulphuric acid, which exhibits in a striking man- 
 ner this fibrous structure. The experiment was made by 
 my friend Dr. Percy, without any reference to the question 
 of cleavage. 
 
 Break a piece of ordinary iron, and you have a granular 
 fracture ; beat the iron, you elongate these granules, and 
 finally render the mass fibrous. Here are pieces of rails 
 along which the wheels of locomotives have slidden ; the 
 granules have yielded and become plates. They exfoliate 
 or come off in leaves ; all these effects belong, I believe, 
 to the great class of phenomena of which slaty cleavage 
 forms the most prominent example. 1 
 
 [I would now lay more stress on the lateral yielding, 
 referred to in the note at the bottom of page 394, accompa- 
 nied as it is by tangential sliding, than I was prepared to 
 do when this lecture was given. This sliding is, I think, 
 the principal cause of the planes of weakness both in 
 pressed wax and slate-rock. J. T. 1871.] 
 
 1 For some further observations on this subject by Mr. Sorby and 
 myself, see Philosophical Magazine for August, 1856. 
 
DEATH BY LIGHTNING. 
 
 PEOPLE in general imagine, when they think at all about 
 the matter, that an impression upon the nerves a blow, 
 for example, or the prick of a pin is felt at the moment it 
 is inflicted. But this is not the case. The seat of sensa- 
 tion is the brain, and to it the intelligence of any impression 
 made upon the nerves has to be transmitted before this 
 impression can become manifest in consciousness. The 
 transmission, moreover, requires time, and the consequence 
 is, that a wound inflicted on a portion of the body distant 
 from the brain is more tardily appreciated than one inflicted 
 adjacent to the brain. By an extremely ingenious experi- 
 mental arrangement, Helmholtz has determined the velocity 
 of this nervous transmission, and finds it to be about one 
 hundred feet a second, or less than one-tenth of the velocity 
 of sound in air. If, therefore, a whale fifty feet long were 
 wounded in the tail, it would not be conscious of the injury 
 till half a second after the wound had been inflicted. 1 But 
 this is not the only ingredient in the delay. There can 
 scarcely be a doubt that to every act of consciousness be- 
 longs a determinate molecular arrangement of the brain 
 that every thought or feeling has its physical correlative in 
 that organ; and nothing can be more certain than that 
 every physical change, whether molecular or mechanical, 
 requires time for its accomplishment. So that, besides the 
 
 1 A most admirable lecture on the velocity of nervous transmission 
 has been published by Dr. Du Bois-Raymond in the Proceedings of the 
 Royal Institution for 1866, vol. iv. p. 575. 
 
398 FKAGMENTS OF SCIENCE. 
 
 interval of transmission, a still further time is necessary for 
 the brain to put itself in order for its molecules to take 
 up the motions or positions necessary to the completion of 
 consciousness. Helmholtz considers that one-tenth of a 
 second is demanded for this purpose. Thus, in the case of 
 the whale above supposed, we have first half a second con- 
 sumed in the transmission of the intelligence through the 
 sensor nerves to the head, one-tenth of a second consumed 
 by the brain in completing the arrangements necessary to 
 consciousness, and, if the velocity of transmission through 
 the motor be the same as that through the sensor nerves, 
 half a second in sending a command to the tail to defend 
 itself. Thus one second and a tenth would elapse before 
 an impression made upon its caudal nerves could be re- 
 sponded to by a whale fifty feet long. 
 
 Now, it is quite conceivable that an injury might be 
 inflicted which would render the nerves unfit to be the con- 
 ductors of the motion which results in sensation ; and if 
 such a thing occurred, no matter how severe the injury 
 might be, we should not be conscious of it. Or it may 
 be that, long before the time required by the brain to 
 complete the arrangement necessary to consciousness, its 
 power of arrangement might be wholly suspended. In 
 such a case also, though the injury might be of a nature 
 to cause death, this would occur without feeling of any 
 kind. Death in this case would be simply the sudden 
 negation of life, without any intervention of consciousness 
 whatever. 
 
 Doubtless there are many kinds of death of this char- 
 acter. The passage of a musket-bullet through the brain 
 is a case in point ; and the placid aspect of a man thus killed 
 is in perfect accordance with the conclusion which might be 
 drawn a priori from the experiments of Helmholtz. Cases 
 of insensibility, moreover, are not uncommon which do not 
 result in death, and after which the persons affected have 
 
DEATH BY LIGHTNING. 399 
 
 been able to testify that no pain was felt prior to the loss 
 of consciousness. 
 
 The time required for a rifle-bullet to pass clean through 
 a man's head may be roughly estimated at a thousandth of 
 a second. Here, therefore, we should have no room for 
 sensation, and death would be painless. But there are 
 other actions which far transcend in rapidity that of the 
 rifle-bullet. A flash of lightning cleaves a cloud, appearing 
 and disappearing in less than a hundred-thousandth of a 
 second, and the velocity of electricity is such as would carry 
 it in a single second over a distance almost equal to that 
 which separates the earth and moon. It is well known 
 that a luminous impression once made upon the retina en- 
 dures for about one-sixth of a second, and that this is the 
 reason why we see a ribbon of light when a glowing coal 
 is caused to pass rapidly through the air. A body illumi- 
 nated by an instantaneous flash continues to be seen for the 
 sixth of a second after the flash has become extinct ; and 
 if the body thus illuminated be in motion, it appears at 
 rest at the place where the flash falls upon it. The color- 
 top is familiar to most of us. By this instrument a disk 
 with differently-colored sectors is caused to rotate rapidly ; 
 the colors blend together, and, if they are chosen in the 
 proper proportions, when the motion is sufficiently rapid 
 the disk appears white. Such a top, rotating in a dark room 
 and illuminated by an electric spark, appears motionless, 
 each distinct color being clearly seen. Professor Dove has 
 found that a flash of lightning produces the same effect. 
 During a thunder-storm he put a color-top in exceedingly 
 rapid motion, and found that every flash revealed the top 
 as a motionless object with its colors distinct. If illuminated 
 solely by a flash of lightning, the motion of all bodies on 
 the earth's surface would, as Dove has remarked, appear 
 suspended. A cannon-ball, for example, would have its 
 flight apparently arrested, and would seem to hang motion- 
 
400 FRAGMENTS OF SCIENCE. 
 
 less in space as long as the luminous impression which re- 
 vealed the ball remained upon the eye. 
 
 If, then, a rifle-bullet move with sufficient rapidity to 
 destroy life without the interposition of sensation, much 
 more is a flash of lightning competent to produce this effect. 
 Accordingly, we have well-authenticated cases of people 
 being struck senseless by lightning who, on recovery, had 
 no memory of pain. The following circumstantial case is 
 described by Hemmer : 
 
 On June 30, 1788, a soldier in the neighborhood of 
 Mannheim, being overtaken by rain, placed himself under 
 a tree, beneath which a woman had previously taken 
 shelter. He looked upward to see whether the branches 
 were thick enough to afford the required protection, and, 
 in doing so, was struck by lightning, and fell senseless to 
 the earth. The woman at his side experienced the shock 
 in her foot, but was not struck down. Some hours after- 
 ward the man revived, but remembered nothing about 
 what had occurred, save the fact of his looking up at the 
 branches. This was his last act of consciousness, and he 
 passed from the conscious to the unconscious condition 
 without pain. The visible marks of a lightning-stroke 
 are usually insignificant : the hair is sometimes burnt ; 
 slight wounds are observed ; while, in some instances, 
 a red streak marks the track of the discharge over the 
 skin. 
 
 Under ordinary circumstances, the discharge from a 
 small Leyden-jar is exceedingly unpleasant to myself. 
 Some time ago I happened to stand in the presence of a 
 numerous audience, with a battery of fifteen large Leyden- 
 jars charged beside me. Through some awkwardness on 
 my part, I touched a wire leading from the battery, and 
 the discharge went through my body. Life was absolutely 
 blotted out for a very sensible interval, without a trace of 
 pain. In a second or so consciousness returned ; I saw my- 
 
DEATH BY LIGHTNING. 401 
 
 self in the presence of the audience and apparatus, and, by 
 the help of these external appearances, immediately con- 
 cluded that I had received the battery discharge. The in- 
 tellectual consciousness of my position was restored with 
 exceeding rapidity, but not so the optical consciousness. 
 To prevent the audience from being alarmed, I observed 
 that it had often been my desire to receive accidentally 
 such a shock, and that my wish had at length been fulfilled. 
 But while making this remark, the appearance which my 
 body presented to myself was that of a number of separate 
 pieces. The arms, for example, were detached from the 
 trunk, and seemed suspended in the air. In fact, memory 
 and the power of reasoning appeared to be complete long 
 before the optic nerve was restored to healthy action. But 
 what I wish chiefly to dwell upon here is, the absolute pain- 
 lessness of the shock ; and there cannot be a doubt that, to 
 a person struck dead by lightning, the passage from life to 
 death occurs without consciousness being in the least de- 
 gree implicated. It is an abrupt stoppage of sensation, 
 unaccompanied by a pang. 
 July 8, 1865. 
 
SCIENCE AND SPIRITS. 
 
 THEIR refusal to investigate " spiritual phenomena " is 
 often urged as a reproach to scientific men. I here propose 
 to give a sketch of an attempt to apply to the " phenom- 
 ena " those methods of inquiry which are found available 
 in dealing with natural truth. 
 
 Some time ago, when the spirits were particularly 
 active in this country, a celebrated philosopher was invited, 
 or rather entreated, by one of his friends to meet and ques- 
 tion them. He had, however, already made their acquaint- 
 ance, and did not wish to renew it. I had not been so 
 privileged, and he therefore kindly arranged a transfer of 
 the invitation to me. The spirits themselves named the 
 time of meeting, and I was conducted to the place at the 
 day and hour appointed. 
 
 Absolute unbelief in the facts was by no means my con- 
 dition of mind. On the contrary, I thought it probable 
 that some physical principle, not evident to the spiritualists 
 themselves, might underlie their manifestations. Extraor- 
 dinary effects are produced by the accumulation of small 
 impulses. Galileo set a heavy pendulum in motion by the 
 well-timed puffs of his breath. Ellicot set one clock going 
 by the ticks of another, even when the two clocks were 
 separated by a wall. Preconceived notions can, moreover, 
 vitiate, to an extraordinary degree, the testimony of even 
 veracious persons. Hence my desire to witness those ex- 
 traordinary phenomena, the existence of which seemed 
 placed beyond a doubt by the known veracity of those who 
 had witnessed and described them. The meeting took 
 
SCIENCE AND SPIRITS. 403 
 
 place at a private residence in the neighborhood of Lon- 
 don. My host, his intelligent wife, and a gentleman who 
 may be called X., were in the house when I arrived. I 
 was informed that the " medium " had not yet made her 
 appearance ; that she was sensitive, and might resent sus- 
 picion. It was therefore requested that the tables and 
 chairs should be examined before her arrival, in order to be 
 assured that there was no trickery in the furniture. This 
 was done ; and I then first learned that my hospitable host 
 had arranged that the seance should be a dinner-party. 
 This was to me an unusual form of investigation ; but I 
 accepted it, as one of the accidents of the occasion. 
 
 The " medium " arrived a delicate-looking young lady, 
 who appeared to have suffered much from ill health. I 
 took her to dinner and sat close beside her. Facts were 
 absent for a considerable time, a series of very wonderful 
 narratives supplying their place. The duty of belief on 
 testimony was frequently insisted on. X. appeared to be 
 a chosen spiritual agent, and told us many surprising 
 things. He affirmed that, when he took a pen in his hand, 
 an influence ran from his shoulder downward, and impelled 
 him to write oracular sentences. I listened for a time, 
 offering no observation. " And now," continued X., " this 
 power has so risen as to reveal to me the thoughts of others. 
 Only this morning I told a friend what he was thinking of, 
 and what he intended to do during the day." Here, I 
 thought, is something that can be at once tested. I said 
 immediately to X. : " If you wish 'to win to your cause an 
 apostle, who will proclaim your principles to the world 
 without fear, tell me what I am now thinking of." X. 
 reddened, and did not tell me my thought. 
 
 Some time previously I had visited Baron Reichenbach, 
 in Vienna, and I now asked the young lady who sat beside 
 me, whether she could see any of the curious things which 
 he describes the light emitted by crystals, for example ? 
 
404 FRAGMENTS OF SCIENCE, 
 
 Here is the conversation which followed, as extracted from 
 my notes, written on the day following the seance : 
 
 Medium. " Oh, yes ; but I see light around all 
 bodies." 
 
 Z " Even in perfect darkness ? " 
 
 Medium. " Yes ; I see luminous atmospheres round all 
 people. The atmosphere which surrounds Mr. R. C. would 
 fill this room with light." 
 
 I. " You are aware of the effects ascribed by Baron 
 Reichenbach to magnets ? " 
 
 Medium. " Yes ; but a magnet makes me terribly ill." 
 
 Z " Am I to understand that, if this room were per- 
 fectly dark, you could tell whether it contained a magnet, 
 without being informed of the fact ? " 
 
 Medium. " I should know of its presence on entering 
 the room." 
 
 Z "How?" 
 
 Medium.'-'' I should be rendered instantly ill." 
 
 Z " How do you feel to-day ? " 
 
 Medium. " Particularly well ; I have not been so well 
 for months." 
 
 Z " Then, may I ask you whether there is, at the 
 present moment, a magnet in my possession ? " 
 
 The young lady looked at me, blushed, and stammered, 
 " No ; I am not en rapport with you." 
 
 I sat at her right hand, and a left-hand pocket, within 
 six inches of her person, contained a magnet. 
 
 Our host here deprecated discussion, as it " exhausted 
 the medium." The wonderful narratives were resumed ; 
 but I had narratives of my own quite as wonderful. These 
 spirits indeed, seemed clumsy creations, compared with 
 those with which my own researches had made me familiar. 
 I therefore began to match the wonders related to me by 
 other wonders. A lady present discoursed on spiritual 
 atmospheres, which she could see as beautiful colors when 
 
SCIENCE AND SPIRITS. 405 
 
 she closed her eyes. I professed myself able to see similar 
 colors, and more than that, to be able to see the interior of 
 my own eyes. The medium affirmed that she could see 
 actual waves of light coming from the sun. I retorted that 
 men of science could tell the exact number of waves 
 emitted in a second, and also their exact length. The 
 medium spoke of the performances of the spirits on musical 
 instruments. I said that such performance was gross, in 
 comparison with a kind of music which had been discovered 
 some time previously by a scientific man. Standing at a 
 distance of twenty feet from a jet of gas, he could command 
 the flame to emit a melodious note ; it would obey, and 
 continue its song for hours. So loud was the music emitted 
 by the gas-flame, that it might be 'heard by an assembly of 
 a thousand people. These were acknowledged to be as 
 great marvels as any of those of spiritdom. The spirits 
 were then consulted, and I was pronounced to be a first- 
 class medium. 
 
 During this conversation a low knocking was heard 
 from time to time under the table. These were the spirits' 
 knocks. I was informed that one knock, in answer to a 
 question, meant " No ; " that two knocks meant " Not 
 yet ; " and that three knocks meant " Yes." In answer to 
 the question whether I was a medium, the response was 
 three brisk and vigorous knocks. I noticed that the knocks 
 issued from a particular locality, and therefore requested 
 the spirits to be good enough to answer from another 
 corner of the table. They did not comply; but I was 
 assured that they would do it, and much more, by-and-by. 
 The knocks continuing, I turned a wine-glass upside down, 
 and placed my ear upon it, as upon a stethoscope. The 
 spirits seemed disconcerted by the act ; they lost their 
 playfulness, and did not quite recover it for a considerable 
 time. 
 
 Somewhat weary of the proceedings, I once threw my- 
 
406 FRAGMENTS OF SCIENCE. 
 
 self back against my chair, and gazed listlessly out of the 
 window. While thus engaged, the table was rudely pushed. 
 Attention was drawn to the wine, still oscillating in the 
 glasses, and I was asked whether that was not convincing. 
 I readily granted the fact of motion, and began to feel the 
 delicacy of my position. There were several pairs of arms 
 upon the table, and several pairs of legs under it ; but how 
 was I, without offence, to express the conviction which I 
 really entertained? To ward off the difficulty, I again 
 turned a wine-glass upside down and rested my ear upon it. 
 The rim of the glass was not level, and the hair on touch- 
 ing it caused it to vibrate and produce a peculiar buzzing 
 sound. A perfectly candid and warm-hearted old gentle- 
 man at the opposite side of the table, whom I may call A., 
 drew attention to the sound, and expressed his entire belief 
 that it was spiritual. I, however, informed him that it was 
 the moving hair acting on the glass. The explanation was 
 not well received, and X., in a tone of severe pleasantry, 
 demanded whether it was the hair that had moved the 
 table. The promptness of my negative probably satisfied 
 him that my notion was a very different one. 
 
 The superhuman power of the spirits was next dwelt 
 upon. The strength of man, it was stated, was unavailing 
 in opposition to theirs. No human power could prevent 
 the table from moving when they pulled it. During the 
 evening this pulling of the table occurred, or rather was 
 attempted, three times. Twice the table moved when my 
 attention was withdrawn from it ; on a third occasion, I 
 tried whether the act could be provoked by an assumed air 
 of inattention. Grasping the table firmly between my 
 knees, I threw myself back in the chair, and waited, with 
 eyes fixed on vacancy, for the pull. It came. For some 
 seconds it was pull spirit, hold muscle ; the muscle, how- 
 ever, prevailed, and the table remained at rest. Up to the 
 present moment, this interesting fact is known only to the 
 particular spirit in question and myself. 
 
SCIENCE AND SPIRITS. 407 
 
 A species of mental scene-painting, with which my own 
 pursuits had long rendered me familiar, was employed to 
 figure the changes and distribution of spiritual power. 
 The spirits were provided with atmospheres, which com- 
 bined with and interpenetrated each other, considerable 
 ingenuity being shown in demonstrating the necessity of 
 time in effecting the adjustment of the atmospheres. In 
 fact, just as in science, the senses, time, and space, con- 
 stituted the conditions of the phenomena. A rearrange- 
 ment of our positions was proposed and carried out ; and 
 soon afterward my attention was drawn to a scarcely 
 sensible vibration on the part of the table. Several persons 
 were leaning on the table at the time, and I asked permis- 
 sion to touch the medium's hand. " Oh, I know I tremble," 
 was her reply. Throwing one leg across the other, I ac- 
 cidentally nipped a muscle, and produced thereby an in- 
 voluntary vibration of the free leg. This vibration, I knew, 
 must be communicated to the floor, and thence to the 
 chairs of all present. I therefore intentionally promoted it. 
 My attention was promptly drawn to the motion ; and a 
 gentleman beside me, whose value as a witness I was par- 
 ticularly desirous to test, expressed his belief, that it was 
 out of the- compass of human power to produce so strange 
 a tremor. " I believe," he added earnestly, " that it is 
 entirely the spirits' work." " So do I," added, with heat, 
 the candid and warm-hearted old gentleman A. " Why, 
 sir," he continued, " I feel them at this moment shaking 
 my chair." I stopped the motion of the leg. " Now, sir," 
 A. exclaimed, " They are gone." I began again, and A. 
 once more ejaculated. I could, however, notice that there 
 were doubters present, who did not quite know what to 
 think of the manifestations. I saw their perplexity ; and, 
 as there was sufficient reason to believe that the disclosure 
 of the secret would simply provoke anger, I kept it to 
 myself. 
 
408 FRAGMENTS OF SCIENCE. 
 
 Again a period of conversation intervened, during which 
 the spirits became animated. The evening was confessedly 
 a dull one, but matters appeared to brighten toward its 
 close. The spirits were requested to spell the name by 
 which I am known in the heavenly world. Our host com- 
 menced repeating the alphabet, and when he reached the 
 letter " P " a knock was heard. He began again, and the 
 spirits knocked at the letter " O." I was puzzled, but 
 waited for the end. The next letter knocked down was 
 " E." I laughed, and remarked that the spirits were going 
 to make a poet of me. Admonished for my levity, I was 
 informed that the frame of mind proper for the occasion 
 ought to have been superinduced by a perusal of the Bible 
 immediately before the stance. The spelling, however, 
 went on, and sure enough I came out a poet. But matters 
 did not end here. Our host continued his repetition of the 
 alphabet, and the next letter of the name proved to be 
 " O." Here was manifestly an unfinished word ; and the 
 spirits were apparently in their most communicative mood. 
 The knocks came from under the table, but no person pres- 
 ent evinced the slightest desire to look under it. I asked 
 whether I might go underneath ; the permission was 
 granted ; so I crept under the table. Some tittered ; but 
 the candid old A. exclaimed, " He has a right to look into 
 the very dregs of it, to convince himself." Having pretty 
 well assured myself that no sound could be produced under 
 the table without its origin being revealed, I requested our 
 host to continue his questions. He did so, but in vain. He 
 adopted a tone of tender entreaty ; but the " dear spirits " 
 had become dumb dogs, and refused to be entreated. I 
 continued under that table for at least a quarter of an hour, 
 after which, with a feeling of despair as regards the pros- 
 pects of humanity never before experienced, I regained my 
 chair. Once there, the spirits resumed their loquacity, and 
 dubbed me " Poet of Science." 
 
SCIENCE AND SPIRITS. 409 
 
 This, then, is the result of an attempt made by a scien- 
 tific man to look into these spiritual phenomena. It is not 
 encouraging ; and for this reason : The present promoters 
 of spiritual phenomena divide themselves into two classes, 
 one of which needs no demonstration, while the other is 
 beyond the reach of proof. The victims like to believe, and 
 they do not like to be undeceived. Science is perfectly 
 powerless in the presence of this frame of mind. It is, 
 moreover, a state perfectly compatible with extreme intel- 
 lectual subtlety and a capacity for devising hypotheses 
 which only require the hardihood engendered by strong 
 conviction, or by callous mendacity, to render them impreg- 
 nable. The logical feebleness of science is not sufficiently 
 borne in mind. It keeps down the weed of superstition, 
 not by logic but by slowly rendering the mental soil unfit 
 for its cultivation. When science appeals to uniform ex- 
 perience, the spiritualist will retort, " How do you know 
 that a uniform experience will continue uniform ? You tell 
 me that the sun has risen for six thousand years : that is 
 no proof that it will rise to-morrow ; within the next twelve 
 hours it may be puffed out by the Almighty." Taking this 
 ground, a man may maintain the story of " Jack and the 
 Bean-stalk " in the face of all the science in the world. 
 You urge, in vain, that science has given us all the knowl- 
 edge of the universe which we now possess, while spiritual- 
 ism has added nothing to that knowledge. The drugged 
 soul is beyond the reach of reason. It is in vain that im- 
 postors are exposed, and the special demon cast out. He 
 has but slightly to change his shape, return to his house, 
 and find it " empty, swept, and garnished." * 
 
 December 10, 1864. 
 
 18 
 
VITALITY. 
 
 THE origin, growth, and energies of living things are 
 subjects which have always engaged the attention of think- 
 ing men. To account for them it was usual to assume a 
 special agent, to a great extent free from the limitations 
 observed among the powers of inorganic Nature. This 
 agent was called the vital force ; and, under its influence, 
 plants and animals were supposed to collect their materials 
 and to assume determinate forms. Within the last twenty 
 years, however, our ideas of vital processes have undergone 
 profound modifications ; and the interest, and even dis- 
 quietude, which the change has excited in some minds are 
 amply evidenced by the discussions and protests which are 
 now common regarding the phenomena of vitality. In 
 tracing out these phenomena through all their modifications 
 the most advanced philosophers of the present day declare 
 that they ultimately arrive at a single source of power, 
 from which all vital energy is derived ; and the disquieting 
 circumstance is that this source is not the direct fiat of a 
 supernatural agent, but a reservoir of what, if we do not 
 accept the creed of Zoroaster, must be regarded as inor- 
 ganic force. In short, it is considered as proved that all 
 the energy which we derive from plants and animals is 
 drawn from the sun. 
 
 A few years ago, when the sun was affirmed to be the 
 source of life, nine out of ten of those who are alarmed by 
 the form which this assertion has latterly assumed, would 
 have assented, in a general way, to its correctness,. Their 
 assent, however, was more poetical than scientific, and they 
 
VITALITY. 411 
 
 were by no means prepared to see a rigid mechanical sig- 
 nification attached to their words. This, however, is the 
 peculiarity of modern conclusions : that there is no creative f 
 energy whatever in the vegetable or animal organism, but 
 that all the power which we obtain from the muscles of 
 men and animals, as much as that which we develop by 
 the combustion of wood or coal, has been produced at the 
 sun's expense. The sun is so much colder that we may 
 have our fires ; he is also so much colder that we may have 
 our horse-racing and Alpine climbing. It is, for example, 
 certain that the sun has been chilled to an extent capable 
 of being accurately expressed in numbers, in order to fur- 
 nish the power which lifted this year a certain number of 
 tourists from the vale of Chamouni to the summit of Mont 
 Blanc. 
 
 To most minds, however, the energy of light and heat 
 presents itself as a thing totally distinct from ordinary me- 
 chanical energy. But either of them can be derived from 
 the other. By the friction of wood a savage can raise it 
 to the temperature of ignition ; by properly striking a piece 
 of iron a skilful blacksmith can cause it to glow, and thus, 
 by the rude agency of his hammer, he generates light and 
 heat. This action, if carried far enough, would produce the 
 light and heat of the sun. In fact the sun's light and heat 
 have actually been referred to the fall of meteoric matter 
 upon his surface ; and whether the sun is thus supported or 
 not, it is perfectly certain that he might be thus supported. 
 Whether, moreover, the whilom molten condition of our 
 planet was, as supposed by eminent men, due to the collision 
 of cosmic masses or not, it is perfectly certain that the mol- 
 ten condition might be thus brought about, pf, then, solar 
 light and heat can be produced by the impact of dead mat- 
 ter, and if from the light and heat thus produced we can 
 derive the energies which we have been accustomed to call 
 vital., it indubitably follows that vital energy may have a \ 
 proximately mechanical origin. 
 
412 FRAGMENTS OF SCIENCE. 
 
 In what sense, then, is the sun to be regarded as the 
 origin of the energy derivable from plants and animals ? 
 Let us try to give an intelligible answer to this question. 
 Water may be raised from the sea-level to a high elevation, 
 and then permitted to descend. In descending it may be 
 made to assume various forms to fall in cascades, to spurt 
 in fountains, to boil in eddies, or to flow tranquilly along a 
 uniform bed. It may, moreover, be caused to set complex 
 machinery in motion, to turn mill-stones, throw shuttles, 
 work saws and hammers, and drive piles. But every form 
 of power here indicated would be derived from the original 
 power expended in raising the water to the height from 
 which it fell. There is no energy generated by the ma- 
 chinery ; the work performed by the water in descending 
 is merely the parcelling out and distribution of the work 
 expended in raising it. In precisely this sense is all the 
 energy of plants and animals the parcelling out and distri- 
 bution of a power originally exerted by the sun. In the 
 case of the water, the source of the power consists in the 
 forcible separation of a quantity of the liquid from a low 
 level of the earth's surface and its elevation to a higher 
 position, the power thus expended being returned by the 
 water in its descent. In the case of vital phenomena, the 
 source of power consists in the forcible separation of the 
 atoms of compound substances by the sun. We name the 
 force which draws the water earthward " gravity," and that 
 which draws atoms together " chemical affinity ; " but these 
 different names must not mislead us regarding the qualita- 
 tive identity of the two forces. They are both attractions, 
 and, to the intellect, the falling of carbon atoms against 
 oxygen atoms is not more difficult of conception than the 
 falling of water to the earth. 
 
 The building up of the vegetable, then^ is effected by 
 the sun through the reduction of chemical compounds. The 
 phenomena of animal life are more or less complicated 
 
VITALITY. 413 
 
 reversals of these processes of reduction. We eat the vege- 
 table, and we breathe the oxygen of the air, and in our 
 bodies the oxygen which had been lifted from the carbon 
 and hydrogen by the action of the sun again falls toward 
 them, producing animal heat and developing animal forms. 
 Through the most complicated phenomena of vitality this 
 law runs : the vegetable is produced while a weight rises, 
 the animal is produced while a weight falls. But the ques- 
 tion is not exhausted here. The water employed in our 
 first illustration generates all the motion displayed in its 
 descent, but the form of the motion depends on the charac- 
 ter of the machinery interposed in the path of the water. 
 In a similar way the primary action of the sun's rays is 
 qualified by the atoms and molecules among which their 
 energy is distributed. Molecular forces determine the form 
 which the solar energy will assume. In the separation of 
 the carbon and oxygen this energy may be so conditioned 
 as to result in one case in the formation of a cabbage, and 
 in another case in the formation of an oak. So also as 
 regards the reunion of the carbon and the oxygen, the mo- 
 lecular machinery through which the combining energy 
 acts may, in one case, weave the texture of a frog, while 
 in another it may weave the texture of a man. 
 
 The matter of the animal body is that of inorganic Na- 
 ture. There is no substance in the animal tissues which is 
 not primarily derived from the rocks, the water, and the 
 air. Are the forces of organic matter, then, different in 
 kind from those of inorganic matter ? The philosophy of 
 the present day negatives the question. It is the com- 
 pounding in the organic world of forces belonging equally 
 to the inorganic that constitutes the mystery and the mir- 
 acle of vitality. Every portion of every animal body may 
 be reduced to purely inorganic matter. A perfect reversal 
 of this process of reduction would carry us from the inor- 
 ganic to the organic ; and such a reversal is at least con- 
 
414 . FRAGMENTS OF SCIENCE. 
 
 ceivable. The tendency, indeed, of modern science is to 
 break down the wall of partition between organic and inor- 
 ganic, and to reduce both to the operation of forces which 
 are the same in kind, but whose combinations differ in com- 
 plexity. 
 
 Consider now the question of personal identity, in rela- 
 tion to this of molecular form. Twenty-six years ago, 
 Mayer, of Heilbronn, with that power of genius which 
 breathes large meanings into scanty facts, pointed out that 
 the blood was " the oil of life," the combustion of which, 
 like that of coal in grosser cases, sustained muscular action. 
 The muscles are the machinery by which the dynamic power 
 of the blood is brought into play. Thus the blood is con- 
 sumed. But the whole body, though more slowly than the 
 blood, wastes also, so that after a certain number of years 
 it is entirely renewed. How is the sense of personal iden- 
 tity maintained across this flight of molecules ? To man as 
 we know him, matter is necessary to consciousness, but the 
 matter of any period may be all changed, while conscious- 
 ness exhibits no solution of continuity. Like changing 
 sentinels, the oxygen, hydrogen, and carbon that depart 
 seem to whisper their secret to their comrades that arrive, 
 and thus, while the Non-ego shifts the Ego remains intact. 
 Constancy of form in the grouping of the molecules, and 
 not constancy of the molecules themselves, is the correlative 
 of this constancy of perception. Life is a wave which in 
 no two consecutive moments of its existence is composed 
 of the same particles. 
 
 Supposing, then, the melocules of the human body 
 instead of replacing others, and thus renewing a preexist- 
 ing form, to be gathered first hand from Nature and put 
 together in the same relative positions as those which they 
 occupy in the body ; that they have the self-same forces and 
 distribution of forces, the self-same motions and distribution 
 of motions would this organized concourse of molecules 
 
VITALITY, 415 
 
 stand before us as a sentient thinking being ? There seerns 
 no valid reason to believe that it would not. Or, supposing 
 a planet carved from the sun, and set spinning round ail 
 axis, and revolving round the sun at a distance from him 
 equal to that of our earth, would one of the consequences 
 of its refrigeration be the development of organic forms ? 
 I lean to the affirmative. Structural forces are certainly 
 in the mass, whether or not those forces reach to the extent 
 of forming a plant or an animal. In an amorphous drop of 
 water lie latent all the marvels of crystalline force ; and 
 who will set limits to the possible play of molecules in a 
 cooling planet ? If these statements startle, it is because 
 matter has been denned and maligned by philosophers and 
 theologians who were equally unaware that it is, at bottom, 
 essentially mystical and transcendental. 
 
 Questions such as these derive their present interest in 
 great part from their audacity, which is sure, in due time, to 
 disappear. And the sooner the public dread is abolished 
 with reference to such questions the better for the cause of 
 truth. As regards knowledge, physical science is polar. 
 In one sense it knows, or is destined to know, every thing. 
 In another sense it knows nothing. Science knows much 
 of this intermediate phase of things that we call Nature, of 
 which it is the product ; but science knows nothing of the 
 origin or destiny of Nature. Who or what made the sun, 
 and gave his rays their alleged power ? Who or what made 
 and bestowed upon the ultimate particles of matter their 
 wondrous power of varied interaction ? Science does not 
 know : the mystery, though pushed back, remains unaltered. 
 To many of us who feel that there are more things in 
 heaven and earth than are dreamt of in the present phi' 
 losophy of science, but who have been also taught, by 
 baffled efibrts, how vain is the attempt to grapple with 
 the inscrutable, the ultimate frame of mind is that of 
 Goethe: 
 
416 FRAGMENTS OF SCIENCE. 
 
 "Who dares to name His name, 
 Or belief in Him proclaim, 
 Veiled in mystery as He is, the All-enfolder ? 
 Gleams across the mind His light, 
 Feels the lifted soul His might, 
 Dare it then deny His reign, the All-upholder? " 
 
 One or two interpolations excepted, the foregoing brief 
 article was written on an Alpine slope in the summer of 
 1863. Seven years afterward I was singularly interested 
 to learn, that nearly 300 years ago, in explaining the actions 
 and energies of the human body, Descartes employed 
 similar imagery and expressed similar views as far as the 
 knowledge of his time allowed. Professor Huxley, who 
 possesses a reading faculty which I can but envy, has pub- 
 lished in his " Lay Sermons " the following remarkable 
 extracts from the " Traite de 1'Homme : " 
 
 " In proportion as these spirits " (the animal spirits) " enter the cavi- 
 ties of the brain, they pass thence into the pores of its substance, and from 
 these pores into the nerves ; where, according as they enter, or even 
 only tend to enter, more or less, into one than into another, they have 
 the power of altering the figure of the muscles into which the nerves are 
 inserted, and by this means of causing all the limbs to move. Thus, aa 
 you may have seen in the grottoes and the fountains in royal gardens, 
 the force with which the water issues from its reservoir is sufficient to 
 move various machines, and even to make them play instruments, or 
 pronounce words, according to the different disposition of the pipes which 
 lead the water. 
 
 " And, in truth, the nerves of the machine which I am describing may 
 very well be compared to the pipes of these water-works ; its muscles 
 and its tendons to the other various engines and springs which seem to 
 move them ; its animal spirits to the water which impels them, of which 
 the heart is the fountain ; while the cavities of the brain are the central 
 office. Moreover, respiration and other such actions as are natural and 
 usual in the body, and which depend on the course of the spirits, are 
 like the movements of a clock, or of a mill, which may be kept up by the 
 ordinary flow of water. 
 
VITALITY. 417 
 
 " The external objects which, by their mere presence, act upon the 
 organs of the senses ; and which, by this means, determine the corporal 
 machine to move in many different ways, according as the parts of the 
 brain are arranged, are like the strangers who, entering into some of the 
 grottoes of those water- works, unconsciously cause the movements which 
 take place in their presence. For they cannot enter without treading upon 
 certain planks so arranged that, for example, if they approach a bathing 
 Diana, they cause her to hide among the reeds ; and if they attempt to 
 follow her, they see approaching a Neptune, who threatens them with his 
 trident ; or if they try some other way, they cause some monster who 
 vomits water into their faces, to dart out ; or like contrivances, according 
 to the fancy of the engineers who have made them. And lastly, when 
 the rational soul is lodged in this machine, it will have its principal seat 
 in the brain, and will take the place of the engineer, who ought to be 
 in that part of the works with which all the pipes are connected, when 
 he wishes to increase, or to slacken, or in some way to alter, their move- 
 ments." 
 
 " All the functions which I have attributed to this machine " (the 
 body), " as the digestion of food, the pulsation of the heart and of the 
 arteries ; the nutrition and the growth of the limbs ; respiration, wake- 
 fulness, and sleep ; the reception of light, sounds, odors, flavors, heat, and 
 such like qualities, in the organs of the external senses ; the impression 
 of the ideas of these in the organ of common-sense and in the imagina- 
 tion ; the retention, or the impression, of these ideas on the memory ; 
 the internal movements of the appetites and the passions ; and lastly, 
 the external movements of all the limbs, which follow so aptly, as well 
 the action of the objects which are presented to the senses, as the im- 
 pressions which meet in the memory, that they imitate as nearly as 
 possible those of a real man : I desire, I say, that you should consider 
 that these functions hi the machine naturally proceed from the mere 
 arrangement of its organs, neither more nor less than do the movements 
 of a clock, or other automaton, from that of its weights and its wheels ; 
 so that, so far as these are concerned, it is not necessary to conceive any 
 other vegetative or sensitive soul, nor any other principle of motion, or 
 of life, than the blood and the spirits agitated by the fire which burns 
 continually in the heart, and which is no wise essentially different from 
 all the fires which exist in inanimate bodies." 
 
ADDITIONAL REMARKS ON MIRACLES. 
 
 AMONG the scraps of manuscript written at the time 
 when Mr. Mozley's work occupied my attention I find the 
 following reflections : 
 
 With regard to the influence of modern science which 
 Mr. Mozley rates so low, one effect of it is certainly to en- 
 hance the magnitude of many of the recorded miracles, and 
 to increase proportionably the difficulties of belief. The 
 ancients knew but little of the vastness of the universe. 
 The Rev. Mr. Kirkman, for example, has shown what inad- 
 equate notions the Jews entertained regarding the " firma- 
 ment of heaven ; " and Professor Airy refers to the case of 
 a Greek philosopher who was persecuted for hazarding the 
 assertion, then deemed monstrous, that the sun might be as 
 large as the whole country of Greece. The concerns of a 
 universe, regarded from this point of view, were vastly 
 more commensurate with man and his concerns than those 
 of the universe which science now reveals to us ; and hence 
 that to suit man's purposes, or in compliance with his 
 prayers, changes should occur in the order of the universe, 
 was more easy of belief in the ancient world than it can 
 be now. In the very magnitude which it assigns to natural 
 phenomena, science has augmented the distance between 
 them and man, and increased the popular belief in their or- 
 derly progression. As a natural consequence, the demand 
 
REMAKES ON MIRACLES. 419 
 
 for evidence is more exacting than it used to be, whenever 
 it is affirmed that such order has been disturbed. 
 
 Let us take as an illustration the miracle by which the 
 victory of Joshua over the Amorites was rendered complete, 
 where the sun is reported to have stood still for " a whole 
 day" upon Gibeon, and the moon in the valley of Ajalon. 
 An Englishman of average education at the present day 
 would naturally demand a greater amount of evidence to 
 prove that this occurrence took place than would have sat- 
 isfied an Israelite in the age succeeding that of Joshua. 
 For, to the one, the miracle probably consisted of the stop- 
 page of a ball of fire less than a yard in diameter, while to 
 the other it would be the stoppage of an orb fourteen hun- 
 dred thousand times the earth in size. And, even accept- 
 ing the interpretation which instructed divines now put 
 upon this text, that Joshua dealt with what was apparent 
 merely, but that what really occurred was the suspension 
 of the earth's rotation, I think a greater reserve in accept- 
 ing the miracle, and a right to demand stronger evidence 
 in support of it, will be conceded to a modern man of sci- 
 ence than would have sufficed for an ancient Jew. 
 
 There is a scientific imagination as well as an historic 
 imagination, and when, by the exercise of the former, the 
 stoppage of the earth's rotation is clearly realized, the 
 event assumes proportions so vast in comparison with the 
 result to be obtained by it that belief reels under the reflec- 
 tion. The energy here involved is equal to that of six trill- 
 ions of horses working for the whole of the time employed 
 by Joshua in the destruction of his foes. The amount of 
 power thus expended would be sufficient to supply every 
 individual of an army a thousand times the strength of that 
 of Joshua, with a thousand times the fighting-power of 
 each of Joshua's soldiers, not for the few hours necessary to 
 the extinction of a handful of Amorites, but for millions 
 of years. All this wonder is silently passed over by the 
 
420 FRAGMENTS OF SCIENCE. 
 
 sacred historian, confessedly because he knew nothing 
 about it. Whether, therefore, we consider the miracle as 
 purely evidential, or as a practical means of vengeance, the 
 same lavish squandering of energy stares the scientific man 
 in the face. If evidential, the energy was wasted, because 
 the Israelites knew nothing of its amount ; if simply de- 
 structive, then the ratio of the quantity lost to that em- 
 ployed may be inferred from the foregoing figures. 
 
 To other miracles similar remarks apply. Transferring 
 thought from our little sand-grain of an earth to the im- 
 measurable heavens, where countless worlds, with their 
 freights of life, probably revolve unseen, the very suns which 
 warm them being barely seen by us across abysmal space ; 
 reflecting that beyond these sparks of solar fire suns innu- 
 merable may lie, whose light can never stir the optic nerve 
 at all ; and, bringing this conception face to face with the 
 idea that the Builder and Sustainer of it all should contract 
 himself to a burning bush, or behave in other familiar ways 
 ascribed to him it is easy to understand how astounding 
 the incongruity must appear to the scientific man. Did 
 this credulous prattle of the ancients about miracles stand 
 alone ; were it not locally associated with w r ords of imper- 
 ishable wisdom, and with examples of moral grandeur un- 
 matched elsewhere in the history of the human race, both 
 the miracles and their " evidences " would have long since 
 ceased to be the transmitted inheritance of intelligent men. 
 Under the pressure of the awe which this universe inspires, 
 well may we exclaim in David's spirit, if not in David's 
 words: "When I consider the heavens the work of thy 
 fingers, the moon, and the stars, which thou hast ordained ; 
 what is man that thou shouldst be mindful of him, or the 
 son of man that thou shouldst so regard him ? " 
 
 If you ask me who is to limit the outgoings of Al- 
 mighty power, my answer is, not I. If you should urge 
 that if the Builder and the Maker of this universe chose to 
 
REMARKS ON MIRACLES. 421 
 
 stop the rotation of the earth, or to take the form of a 
 burning bush, there is nothing to prevent him from doing 
 so, I am not prepared to contradict you. I neither agree 
 with you nor differ from you, for it is a subject of which I 
 know nothing. But I observe that in such questions re- 
 garding Almighty power, your inquiries relate, not to that 
 power as it is actually displayed in the universe, but to the 
 power of your own imagination. Your question is, not has 
 the Omnipotent done so and so ? or is it in the least likely 
 that the Omnipotent should do so and so ? but, is my imagi- 
 nation competent to picture a being able and willing to do 
 so and so ? I am not prepared to deny your competence. 
 To the human mind belongs the faculty of enlarging and 
 diminishing, of distorting and combining indefinitely the 
 objects revealed by the senses, or by its own consciousness. 
 It can imagine a mouse as large as an elephant, an elephant 
 as large as a mountain, and a mountain as high as the stars. 
 It can separate congruities and unite incongruities. We 
 see a fish and we see a woman ; we can drop one half of 
 each, and unite in idea the other two halves to a mermaid. 
 We see a horse and we see a man ; we are able to drop one 
 half of each, and unite the other two halves to a centaur. 
 Thus also the pictorial representations of the Deity, the 
 bodies and wings of cherubs and seraphs, the hoofs, horns, 
 and tail of the Evil One, the joys of the blessed, and the 
 torments of the damned, have been elaborated from mate- 
 rials furnished to the imagination by the senses. And it 
 behooves you and me to take care that our notions of the 
 Power which rules the universe are not mere fanciful or ig- 
 norant enlargements of human power. The capabilities of 
 what you call your reason are not denied. By the exercise 
 of the power here adverted to, and which may be called 
 the mytJiologic imagination, you can picture to yourself a 
 being able and willing to do any and every conceivable 
 thing. You are right in saying that in opposition to this 
 
422 FRAGMENTS OF SCIENCE. 
 
 power science is of no avail. Mr. Mozley would call it 
 " a weapon of air." The man of science, however, while 
 accepting the figure, would probably reverse its parts, 
 thinking that it is not science which is here the thing of 
 air, but the unsubstantial figment of the imagination to 
 which its solidity is opposed. 
 
 THE END. 
 
WOEKS OF HEEBEET SPENCEE, 
 
 PUBLISHED BY 
 
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 SYSTEM OF PHILOSOPHY 
 
 I. FIRST PRINCIPLES. 
 (New and Enlarged Edition.) 
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 PART II. THE INDUCTIONS OP BIOLOGY. 
 PART III. THE EVOLUTION OP LIFE. 
 
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 PART V. PHYSIOLOGICAL DEVELOPMENT. 
 PART VI. LAWS OF MULTIPLICATION. 
 
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 PART II. THE INDUCTIONS OP PSYCHOLOGY. 146 pages. Price, - $o!75 
 PART III. GENERAL SYNTHESIS. 100 pages. ) -o . 
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THE OBIGm OF CIVILIZATION; 
 
 OR, THE 
 
 PRIMITIVE CONDITION OF MAN. 
 By SIR JOHN LTTBBOCK, Bart., M, P., F. E. S, 
 
 3SO rages. Illustrated.. 
 
 This interesting work is the fruit of many years' research 
 by an accomplished naturalist, and one well trained in mod- 
 ern scientific methods, into the mental, moral, and social con- 
 dition of the lowest savage races. The want of a work of 
 this kind had long been felt, and, as scientific methods are 
 being more and more applied to questions of humanity, there 
 has been increasing need of a careful and authentic work de- 
 scribing the conditions of those tribes of men who are lowest 
 in the scale of development. 
 
 " This interesting work for it is intensely so in its aim, scope, and the 
 ability of its author treats of what the scientists denominate anthropology, 
 or the natural history of the human species ; the complete science of man, 
 body and soul, including sex, temperament, race, civilization, etc." Provi- 
 dence Press. 
 
 "A work which is most comprehensive in its aim, and most admirable in 
 its execution. The patience and judgment bestowed on the book are every, 
 where apparent ; the mere list of authorities quoted giving evidence of wide 
 and impartial reading. The work, indeed, is not only a valuable one on ac- 
 count of the opinions it expresses, but it is also most serviceable as a book 
 of reference. It offers an able and exhaustive table of a vast array of facts, 
 which no single student could well obtain for himself, and it has not been 
 made the vehicle for any special pleading on the part of the author." 
 London Athenceum. 
 
 " The book is no cursory and superficial review ; it goes to the very heart 
 of the subject, and embodies the results of all the later investigations. It ia 
 replete with curious and quaint information presented in a compact, luminous, 
 and entertaining form." Albany Evening Journal. 
 
 " The treatment of the subject is eminently practical, dealing more with 
 fact than theory, or perhaps it will be more just to say, dealing only with 
 theory amply sustained by fact." Detroit Free Press. 
 
 " This interesting and valuable volume illustrates, to some extent, the 
 way in which the modern scientific spirit manages to extract a considerable 
 treasure from the chaff and refuse neglected or thrown aside by former in 
 quirers." London Saturday Review. 
 
 D, APPLETON & CO., Publishers. 
 
D. Appleton & Company^ Publications. 
 
 LAY SERMONS, 
 ADD11ESSES, AND KEYIEWS, 
 
 BY THOMAS HENRY HUXLEY. 
 Cloth, 12mo. 390 pages* Price, $1.75 
 
 THIS is the latest and most popular of the works of this in- 
 trepid and accomplished English thinker. The American edition 
 of the work is the latest, and contains, in addition to the English 
 edition, Professor Huxley's recent masterly address on " Spon- 
 taneous Generation," delivered before the British Association for 
 the Advancement of Science, of which he was president. 
 The following is from an able article in the Independent : 
 
 The " Lay Sermons, Addresses, and Reviews " is a book to be read 
 by every one who would keep up with the advance of truth as well by 
 those who are hostile as those who are friendly to his conclusions. In 
 it, scientific and philosophical topics are handled with consummate abil- 
 ity. It is remarkable for purity of style and power of expression. No- 
 where, in any modern work, is the advancement of the pursuit of that 
 natural knowledge, which is of vital importance to bodily and mental 
 well-being, so ably handled. 
 
 Professor Huxley is undoubtedly the representative scientific man of 
 the age. His reverence for the right and devotion to truth have estab- 
 lished his leadership of modern scientific thought. He leads the beliefs 
 and aspirations of the increasingly powerful body of the younger men of 
 science. His ability for research is marvellous. There is possible no more 
 equipoise of judgment than that to which he brings the phenomena of 
 Nature. Besides, he is not a mere scientist. His is a popularized phi- 
 losophy ; social questions have been treated by his pen in a manner most 
 masterly. In his popular addresses, embracing the widest range of top- 
 ics, he treads on ground with which he seems thoroughly familiar. 
 
 There are those who hold the name of Professor Huxley as synony- 
 mous with irreverence and atheism. Plato's was so held, and Galileo's, 
 and Descartes's, and Newton's, and Faraday ? g. There can be no greater 
 mistake. No man has greater reverence for the Bible than Huxley. No 
 one more acquaintance with the text of Scripture. He believes there is 
 definite government of the universe ; that pleasures and pains are distrib- 
 uted in accordance with law ; and that the certain proportion of evil 
 woven up in the life even of worms will help the man vho thinks to bear 
 his own share with courage. 
 
 In the estimate of Professor Huxley's future influence upon science, 
 his youth and health form a large element. He has just passed his forty- 
 fifth year. If God spare his life, truth can hardly fail to be the gainer 
 from a mind that is stored with knowledge of the laws of the Creator's 
 operations, and that has learned to love all beauty and hate && nleness of 
 Nature and art. 
 
SPENCERS SYSTEM OF PHILOSOPHY. 
 
 THE PHILOSOPHY OF EVOLUTION. 
 
 By HERBERT SPENCER. 
 
 This great system of scientific thought, the most original and important men- 
 tal undertaking of the age, to which Mr. Spencer has devoted his life, is now well 
 advanced, the published volumes being: First Principles^ The Principles of Bi- 
 ology, two volumes, and The Principles of Psychology, vol. i., which will bo 
 shortly printed. 
 
 This philosophical system differs from all its predecessors in being solidly 
 based on the sciences of observation and induction ; in representing the order 
 and course of Nature ; in bringing Nature and man, life, mind, and society, under 
 one great law of action ; and in developing a method of thought which may serve 
 for practical guidance in dealing with the affairs of life. That Mr. Spencer is the 
 man for this great work will be evident from the following statements : 
 
 " The only complete and systematic statement of the doctrine of Evolution 
 with which I am acquainted is that contained in Mr. Herbert Spencer's ' System 
 of Philosophy ; ' a work which should be carefully studied by all who desire to 
 know whither scientific thought is tending." T. H. HUXLEY. 
 
 " Of all our thinkers, he is the one who has formed to himself the largest new 
 scheme of a systematic philosophy." Prof. MASSON. 
 
 "If any individual influence is visibly encroaching on Mills in this country, it 
 is bis." Ibid. 
 
 " Mr. Spencer is one of the most vigorous as well as boldest thinkers that 
 English speculation has yet produced." Jomr STUART MILL. 
 
 " One of the acutest metaphysicians of modern times." Ibid. 
 
 " One of our deepest thinkers." Dr. JOSEPH D. HOOKEB. 
 
 It is questionable if any thinker of finer calibre has appeared jji our coun- 
 try." QEOBGE HENRY LEWES. 
 
 "He alone, of all British thinkers, has organized a philosophy." Ibid. 
 
 " He is as keen an analyst as is known in the history of philosophy ; I do not 
 except either Aristotle or Kant." GEORGE RIFLE Y. 
 
 " If we were to give our own judgment, we should say that, since Newton, 
 there has not in England been a philosopher of more remarkable speculative and 
 ystematizing talent than (in spite of some errors and some narrowness) Mr. Her- 
 bert Spencer." Londor* Saturday Review. 
 
 " We cannot refrain from offering our tribute of respect to one who, whether 
 for the extent of his positive knowledge, or for the profundity of his speculative 
 insight, has already achieved a name second to none in the whole range of Eng- 
 lish philosophy, and whose works will worthily sustain the credit of EnglUk 
 thought in the present generation." Westminster Beview. 
 
D. APPLETO& & CO.'S PUBLICATIONS. 
 
 ON 
 
 THE ORIGIN OF SPECIES 
 
 BY 
 
 Means of Natural Selection ; 
 
 OR, 
 
 THE PRESERVATION OF FAVORED RACES 
 
 IN TUB 
 
 STRUGGLE FOE LIFE. 
 
 BY 
 
 ID-A.IVWXN', .A.. M. 
 
 One Volume. 12mo. Cloth. $2.00. 
 
 " His first point is to show that species are in many cases not well 
 defined, and that the whole order of natural history seems to be in a 
 state of mutation, by reason of constant variations. Thus even under 
 domestication, important changes may be introduced by intercrossing, 
 by selection of the best individuals for propagation, by crossing parents 
 marked by however slight, but favorable peculiarities. 
 
 "His second point is what he terms the universal and necessary 
 struggle for existence. This follows from the high geometrical ratio 
 of increase common to all beings. If there were no catastrophes, 
 any one of the existing species would be sufficiently numerous in a 
 few thousand years to cover the whole earth, to the exclusion of every- 
 thing else. 
 
 " His third point is to prove that this struggle is directed by the 
 law of natural selection. Even the races of domestic animals may be 
 constantly improved and modified by choosing the best individuals 
 for propagation. Nature brings the same discipline to bear upon the 
 whole domain of animal and vegetable life. She seizes at once upon 
 any slight variation that is favorable, and perpetuates it ; in the uni- 
 versal pressure, any variation that is injurious is immediately extin- 
 guished." 
 
THE DESCENT OF MAN, 
 
 AND 
 
 SELECTION IN RELATION TO SEX. 
 
 BY 
 
 CHAS. PAR WIN, M. A., F, E. S, 
 
 Two Vols., 12mo. 
 
 PRICE, ..... $4.00 
 
 In these volumes Mr. Darwin has brought forward all the facts and 
 arguments which science has to offer in favor of the doctrine that man 
 has arisen by gradual development from the lowest point of animal life. 
 He had originally intended this work as a posthumous publication, but 
 the extensive acceptance of the views unfolded in his book on the " Origin 
 of Species " induced him to believe that the public were ripe for the most 
 advanced deductions from his theory of "Natural Selection." Aside from 
 the logical purpose which Mr. Darwin had in view, his work is an original 
 and fascinating contribution to the most interesting portion of natural 
 
 history. 
 
 From the London Spectator, 
 
 "For our part, we find Dr. Darwin's vindication of the origin of man a far more 
 wonderful vindication of Theism than Paley's ' Natural Theology,' though we do 
 not know, so reticent is his style, whether or not he conceives it himself/' 
 From the Citizen and Round Table. 
 
 " Even the charge of atheism, which was so violently urged against Mr. Dar- 
 win, is now rarely heard, and theologians, whose orthodoxy is unquestioned, have 
 ventured to admit that it is possible to believe both in Christianity and the Dar- 
 winian theory at the same time." 
 
 From the Charleston Courier. 
 
 "No one can rise from an ordinarily attentive consideration of Mr. Darwin's 
 treatise, without being impressed, not only' with the extent and depth of the 
 knowledge which he has attained upon the subject under treatment, and his long, 
 unwearied labor in collecting facts, but also with his possession of qualities 
 equally rare the true scientific temper, the transparent candor, and the truth- 
 seeking soberness, with which he expresses to you his conclusions, and the pro- 
 cesses oy which he reaches them. 
 
 " Whether you like his discourse or not though you may refuse to acquiesce 
 in his conclusions still you are compelled to bear your witness, that this man 
 has not been laboring to find facts to support a preconceived theory, but that the 
 theory is the irrepressible outgrowth of his accumulated facts." 
 From the Evening Bulletin. 
 
 " This theory is now indorsed by many eminent scientists, who at first com- 
 bated it, including Sir Charles Lyell, probably the most learned of living geolo- 
 gists, and even by a class of Christian divines like Dr. McCosh, who think that 
 certain theories of cosmogony, like the nebular hypothesis and the law of evolu- 
 tion, may be accepted without doing violence to faith." 
 
 Sent free, by mail, to any address in the U. S., on receipt of the price. 
 
 D. APPLETON & CO., Publishers. 
 
Works published by D. Appleton db Co. 
 
 HEAT, 
 
 CONSIDERED AS A MODE OF MOTION, 
 
 Being a Course of Twelve Lectures delivered before the 
 Royal Institution of Great Britain. 
 
 BY JOHtf TTODALL, F. K. 8., 
 
 reoFESSOB or NJLTTTBA.I, PHILOSOPHY IN THE EOYAI. INSTITUTION AUTHOK yr nu 
 "GLAOTEBS OF THE ALPS," ETC. 
 
 With One Hundred Illustrations. Svo, 480 pages. Price, $2. 
 
 Prom the American Journal of Science. With all the skill which has 
 made Faraday the master of experimental science in Great Britain, Professor Tyndall 
 enjoys the advantage of a superior general culture, and is thus enabled to set forth his 
 philosophy with all the graces of eloquence and the finish of superior diction. "With a 
 simplicity, and absence of technicalities, which render his explanations lucid to un- 
 scientific minds, and at the same time a thoroughness and originality by which he in- 
 structs the most learned, he unfolds all the modern philosophy of heat. His work takes 
 rank at once as a classic upon the subject 
 
 KTew York Times. Professor Tyndall's course of lectures on heat is one of the 
 most beautiful illustrations of a mode of handling scientific subjects, which is com- 
 paratively new, and which promises the best results, both to science and to literature 
 generally ; we mean tile treatment of subjects in a style at once profound and popu- 
 lar. The title of Professor Tyndall's work indicates the theory of heat held by him, 
 and indeed the only one now held by scientific men it is a mode of motion. 
 
 Boston Journal. He exhibits the curious and beautiful workings of nature iu 
 ft most delightful manner. Before the reader particles of water lock themselves or fly 
 asunder with a movement regulated like a dance. They form themselves into liquid 
 flowers with fine serrated petals, or into rosettes of frozen gauze ; they bound upward 
 in boiling fountains, or creep slowly onward in stupendous glaciers. Flames burst into 
 music and sing, or cease to sing, as the experimenter pleases, and metals paint them- 
 selves upon a screen in dazzling hues as the painter touches his canvas. 
 
 New York Tribune. The most original and important contribution that has 
 yet been made to the theory and literature of thermotics. 
 
 Scientific American. The work is written in a charming style, and is the 
 most valuable contribution to scientific literature that has been published in many 
 years. It is the most popular exposition of the dynamical theory of heat that has yet 
 appeared. The old material theory of heat may be said to be defunct. 
 
 Louisville Democrat. This is one of the most delightful scientific works w 
 hsTe ever met. The lectures are so full of life and spirit that we can almost imagine 
 the lecturer before us, and see his brilliant experiments in every stage of their progress. 
 lli theory is so carefully and thoroughly explained that no one can fail to understand 
 tt. Such books as these create a love for science. 
 
 Independent. Professor Tyndairs expositions and experiments are remarkably 
 thoughtful, ingenious, clear, and convincing; portions of the book have almost tb 
 interest of a romance, so startling are the descriptions and elucidations. 
 
Wwks of Herbert Spencer published by D. Appielon & C*. 
 A NEW SYSTEM OF PHILOSOPHY. 
 
 FIRST PRINCIPLES. 
 
 & VoL Large 12mo. 515 Pages. Price $2 50. 
 
 CONTENTS : 
 PAKT FIRST. The Unknowable. 
 
 Ciiaptei i. Religion and Science; IL Ultimate Religious Ideas; IIL 
 Ultimate Scientific Ideas ; IV. The Relativity of all Knowledge ; V Th 
 Reconciliation. 
 
 PART SECOND. Laws of the Knowable. 
 
 I. Laws in General; IL The Law of Evolution; IIL The same con- 
 tinued; IV. The Causes of Evolution; V. Space, Tune, Matter, Motion, and 
 Force ; VL The Indestructibility of Matter ; VII. The Continuity of Motion ; 
 VIIL The Persistence of Force ; IX. The Correlation and Equivalence of 
 Forces; X. The Direction of Motion ; XI. The Rhythm of Motion; XII. The 
 Conditions Essential to Evolution ; XIII. The Instability of the Homoge- 
 neous ; XIV. The Multiplication of Effects ; XV. Differentiation *nd Inte- 
 gration ; XVI. Equilibration ; XVII. Summary and Conclusion. 
 
 In the first part of this work Mr. Spencer defines the province, limits, and 
 relations of religion and science, and determines the legitimate scope of 
 philosophy. 
 
 In part second he unfolds those fundamental principles which have been 
 arrived at within the sphere of the knowable ; which are true of all orders 
 of phenonema, and thus constitute the foundation of all philosophy. The 
 law of Evolution, Mr. Spencer maintains to be universal, and he has here 
 worked it out as the basis of his system. 
 
 These First Principles are the foundation of a system of Philosophy 
 bolder, more elaborate, and comprehensive perhaps, than any other which 
 na* been hitherto designed in England. British Quarterly Review. 
 
 A work lofty in aim and remarkable hi execution. Cornhill Mayadnt. 
 
 In the works of Herbert Spencer we have the rudiments of a positive 
 Theology, and an immense step toward the perfection of the science of Psy- 
 chology. Christian Examiner. 
 
 If we mistake not, m spite of the very negative character of his own ro- 
 Kilts, he has foreshadowed some strong arguments for tlte doctrine of a post 
 tire Christian Theology. New Englander. 
 
 As far as tte frontiers of knowledge, where the intellect may go, there ft 
 to living man whose guidance may more safely be trusted. 
 
Work* <tf Herbert Spencer published by D. AppUtcn & Co. 
 Tiie Fhilosopliy of Herbert Spencer. 
 
 THE 
 
 PRINCIPLES OF BIOLOGY 
 
 Vol. I. 475 pages. (Now in press.) 
 
 CONTENTS: 
 PART I. THE DATA OP BIOLOGY. 
 
 L Organic Matter. II. The actions of Forces on Organic Matter. III. The 
 re-actions of Organic Matter on Forces. IV. Proximate Definition of 
 Life. V. The Correspondence between Life and its Circumstances. 
 
 VI. The Degree of Life varies as the Degree of Correspondence. 
 
 VII. The Scope of Biology. 
 
 PART II. THE INDUCTIONS OF BIOLOGY. 
 
 L Growth. II. Development. III. Function. IV. Waste and Repair. 
 V. Adaptation. VI. Individuality. VII. Genesis. VIII. Heredity. 
 
 IX. Variation. X. Genesis, Heredity, and Variation. XI. Classifica- 
 tion. XII. Distribution. 
 
 PART III. THE EVOLUTION OP Luna. 
 
 I. Preliminary. II. General Aspects of the Special-creation-hypothesis. 
 III. General Aspects of the Evolution-hypothesis. IV. The Arguments 
 from Classification. V. The Arguments from Embryology. VI. The 
 Arguments from Morphology. VII. The Arguments from Distribution. 
 VIII. How is Organic Evolution caused ? IX. External Factors. 
 
 X. Internal Factors. XI. Direct Equilibration. XII. Indirect Equili 
 bration. XIII. The Cooperation of the Factors. XIV. The Converg 
 ence of the Evidences. 
 
 All these works are rich In materials for forming intelligent opinions, even where 
 we are unable to agree with those put forth by the author. Much may be learned from 
 Jhcm in departments in which our common Educational system is very deficient. The 
 active citizen may derive from them accurate systematized information concerning hia 
 highest duties to society, and the principles on which they arc based. He may gaio 
 dearer notions of the value and bearing of evidence, and be better able to distinguish 
 between facts and inferences. He may find common things suggestive of wiser thought 
 nay, we will venture to say of truer emotion than before. By giving us fuller rcali- 
 tations of liberty and justice his vritings will tend to increase onr self-reliance in the 
 ireat emergency of civilization to which we have been summoned Atlantic Monthly 
 
D. APPLETON & CO.'JS PUBLICATIONS. 
 THE 
 
 Correlation and Conservation of Forces. 
 
 WITH AN 
 
 . NTBODUCTION AND BKIEF BIOGEAPHICAL NOTIOES 
 By EDWARD L. YOUMANS, M.D. 12rao, 490 pages. 
 
 CONTENTS. 
 
 L By W. R. GROVE. The Correlation of Physical Forces. 
 IL By Prof. HELMHOLTZ. The Interaction of Natural Forces. 
 HI. By J. R. MATER. 1. Remarks on the Forces of Inorganic Nature. 
 
 2. On Celestial Dynamics. 
 
 3. On the Mechanical Equivalent of Her t. 
 
 IV. By Dr. FARADAY. Some Thoughts on the Conservation of Forces. 
 V. By Prof. LIEBIO. The Connection and Equivalence of Forces. 
 VI. By Dr. CARPENTER. The Correlation of the Physical and Vital Forces 
 
 "This work is a very welcome addition to our scientific literature, and will be 
 particularly acceptable to those who wish to obtain a popular, but at the same time 
 precise and clear view of what Faraday justly calls the highest law in physical science, 
 the principle of the conservation offeree. Sufficient attention has not been paid to the 
 publication of collected monographs or memoirs upon special subjects. Dr. Youmans' 
 work exhibits the value of such collections in a very striking manne% and we earnestly 
 hope his excellent example may be followed in other branches of science." American 
 Journal of Science. 
 
 " It was a happy thought which suggested the publication of this volume. The 
 question is often asked, and not so easily answered, What are the new doctrines of the 
 Correlation and Conservation of Forces? In this volume we have the answer, and 
 with the reasons of its chief expounders ; those who are ignorant on that theme, can 
 thus question the original authorities." Neio Englander. 
 
 "We here have the original expositions of the new Philosophy of Forces, accompa- 
 nied by an excellent exposition of both the expositions and the expositors; the whole 
 will be a rare treat to the lovers of advancing scientific thought." Methodist 
 Quarterly Review. 
 
 " This is, perhaps, the most remarkable book of the age. We have here the latest 
 discoveries, and the highest results of thought concerning the nature, laws, and con- 
 Bections of the forces of the universe. No higher or more sublime problem can engage 
 the intellect of man than is discussed by these doctors of science intent alone on aniv 
 tog at the truth." Detroit Free Press. 
 
 'This work presents a praiseworthy specimen of complete and faithful authorship, 
 ad it* publication at this time will form an epoch in th experience of many thinking 
 mind a." ibune. 
 
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