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Contributions to Molecular Phyiics in the Domain of Ka has 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 remaining, but the greater is the living force ; the farther D is from F, the greater is the sum of the unconsumed tensions, and the less is the living force. Now the principal 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 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, then, with the particles in contact, the sum of the tensions THE CONSTITUTION OF NATURE. 19 between D and r would be a maximum, and the via viva zero. If, in obedience to the repulsion, D moved away from r, vis viva would be generated ; 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 as well as attraction into account, the principle of the conservation of force affirms that the mechanical value of the tensions and vires vivce of the material universe, so far as we know it, 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 en- hancement of the other, the total value of the property remaining unchanged. The considerations here applied to gravity apply equally to chemical affinity. In a mixture of oxygen and hydrogen the atoms exist apart, but by the application 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 towards each other, we have tensions and nothing else. During their motion towards 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. It is possible to reverse these processes, to unlock the embrace of the atoms and replace them in their first positions. But, to accomplish this, as much heat would be required 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 dis- appear, but in so doing they re-endow the atoms of oxygen and hydrogen with tension. The atoms are thus enabled 20 FRAGMENTS OF SCIENCE. to recombine, and when tbey 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 carbonic acid, which is exhaled from our lungs, produced by our fires, and found sparingly diffused everywhere throughout the air. In the leaves of plants the sunbeams also wrench the atoms of carbonic acid 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 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 its approach to 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 motion, and consequently the vis viva, reach 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 comple- mentary 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 THE CONSTITUTION OF NATURE. 21 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 employed more or less loosely by writers on physics. This confusion is happily avoided by the intro- duction of the term ' energy,' which embraces 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 realise itself if all hindrances 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 one possesses no energy. It has neither motion nor power of motion. But the same body sus- pended 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 consists of our old tensions. We, moreover, speak of the conservation of energy, 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 upwards 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 returns, 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 Holznnne of the woodcutters they possess actual energy. The same is true of the mountains 22 FRAGMENTS OF SCIENCE. themselves. As long as the rocks which compose them can fall to a lower level, they possess potential energy, which is converted 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 striking, 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 oscillations of a heated body. An atom is driven against its neighbour, and recoils. The ultimate amplitude of the recoil being attained, the motion of the atom in that direction is checked, and for an instant its energy is all potential. It is then drawn towards its neighbour with accelerated speed ; thus, by attraction, converting its potential into dynamic energy. Its motion in this direction is also finally checked, and again, for an instant, its energy is all potential. It once more retreats, converting, by repulsion, its potential into dynamic energy, till the latter attains a maximum, after waich 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 mole- cular dynamic energy, developed at the moment of colli- sion, is not the equivalent of the sensible dynamic energy destroyed. The true equivalent is this heat, plus the THE CONSTITUTION OF NATUKE. 23 potential energy conferred upon the molecules by the placing of greater distances between them. This mole- cular potential energy is afterwards, 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, metallic bodies more or less sparsely distributed through the earth's crust. These bodies can be oxydised ; and hence they are, so far as they go, stores of potential energy. But the attractions of the great mass of the earth's crust are already satisfied, and from them no fur- ther energy can possibly be obtained. Ages ago the ele- mentary constituents of our rocks clashed together and produced the motion of heat, which was taken up by the aether and carried away through stellar space. It is lost for ever 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. So it is with almost all the other constituents of the earth's crust. They took their present form in obedience to molecular force ; they turned their potential energy into dynamic, 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 working- power in the world consists of attractions, repulsions, and motions. If the attractions and repulsions be so circum- stanced as to be able to produce motion, they are sources 24 FRAGMENTS OF SCIENCE. 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 work- ing-power ; because the body can be moved by the attrac- tion, 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 a force, which holds the earth and weight together. The same remarks apply to attracting atoms and molecules. 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 perform mechanical work. When, for example, two atoms of hydrogen unite with one of oxygen, to form water, the atoms are first drawn towards each other they move, they clash, and then by virtue of their re- siliency, they recoil and quiver. To this quivering motion we give the name of heat. This atomic vibration is merely the redistribution 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 con- verted into anything else. For the atoms, when mutually clasped to form a molecule of water, are held together by the very attraction which first drew them towards 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 is chemical affinity. By the exertion of a certain pull THE CONSTITUTION OF NATURE. 25 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 produces the motion annihilated or changed into anything 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 attrac- tion 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 tremors of heat to raise a weight, which is done through the intermediation of an elastic fluid in the steam-engine, a certain definite portion of the molecular motion is con- sumed. In this sense, and in this sense only, can the heat be said to be converted into gravity ; or, more correctly, into potential energy of gravity. Here the destruction of the heat has created no new attraction ; but the old attraction has conferred upon it a power of exerting a certain definite pull, between the starting-point of the falling weight and 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 anni- hilated, 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, while, in the other case, the power of producing motion has been augmented 26 FRAGMENTS OF SCIENCE. 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 constancy 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 convertibility of natural forces consists solely in trans- formations of dynamic into potential, and of potential into dynamic energy. In no other sense has the convertibility of force any scientific meaning. From the writings and conversation of distinguished men I learned, that the notion of gravity being an out- standing force, entirely inconvertible, was prevalent among them. Hence the origin of the foregoing exposition. Grave errors have, indeed, been entertained, as to what is really intended to be conserved by the doctrine of con- servation. November 1875. II. RADIATION 1865. 1. Visible and Invisible, Radiation. T)ETWEEN the mind of man and the outer world are \J interposed 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 organised. 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 phe- nomena of radiation, 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 incompetent to evoke its power, while others never reach it at all, being absorbed by the humours of the eye. To 4 28 FRAGMENTS OF SCIENCE. all rays 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 in- creasing strength, be sent through a wire of the refractory metal platinum, the wire first becomes sensibly warm to he touch ; for a time its heat augments, still however remaining obscure ; at length we can no longer touch the metal with impunity ; and at a certain definite temper- ature it emits a feeble red light. As the current aug- ments in power the light augments in brilliancy, until finally the wire appears of a dazzling white. The light which it now emits is similar to that of the sun. 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 refracted by the prism, we are by it enabled to separate one class of rays from another. By such prismatic analy- sis Dr. Draper has shown, that when the platinum wire first begins to glow, the light emitted is sensibly 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 raya 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 colours at RADIATION. 29 the same time the platinum wire must be white-hot: the impression of whiteness being in fact produced by the simultaneous action of all these colours on the optic nerve. In the experiment just described we began with a platinum 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 radia- tion 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 tem- perature 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 emis- sion would embrace the rays of all these stages. There can hardly be a doubt that from the sun itself, rays pro- ceed 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 coloured consti- tuents; he formed what is technically called the solar spectrum. Exposing thermometers to the successive colours he determined their heating power, and found it 30 FRAGMENTS OF SCIENCE. to augment from the violet or most refracted end, to the red or least refracted 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 instru- ments 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 various 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 refrang- ible end. Eitter discovered the extension of the spec- trum 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, therefore, of three distinct parts : first, of ultra- red rays of high heating power, but unsuited to the pur- poses of vision ; secondly, of luminous rays which display the succession of colours, 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 aie of the utmost importance to the organic world. KADIATIOX. 31 2. Origin and Character of Radiation. The Aether. When we see a platinum wire raised gradually to a white heat, and emitting in succession all the colou s of the spectrum, we are simply conscious of a series of changes in the condition of ouv own eyes. We do not see the actions in which these successive colours originate, but the mind irresistibly infers that the appearance of the colours corresponds 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 has been 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 sensations 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 vibrations, or the times in which they were executed. But besides intensifying the old vibrations the current gene- rates new and more rapid ones, and when a certain de- finite rapidity has been attained, the wire begins to glow. The colour first exhibited is red, which corresponds to the lowest rate of vibration of which the eye is able to take 32 FRAGMENTS OF SCIEXCE. cognisance. 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 recognises in the wire itself, as concurrent with the visual changes taking 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, c a very brisk agitation of the insen- sible parts of an object,' it is readily conceivable that on touching 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 platinum, 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 vibra- tions of light and heat, as air is fitted for the transmission of sound. This medium is called the luminiferous aether. Every vibration of every atom of our platinum wire raises in this aether a wave, which speeds through it at the rate of 186,000 miles a second. The aether suffers no rupture of continuity at the surface of the eye, the inter-molecular spaces of the various humours are filled with it ; hence the waves generated by the glowing platinum can cross these humours and impinge on the optic nerve at the back of the eye. Thus the sensation of light reduces itself to the communication of motion. Up to this point we deal with pure mechanics ; but the subsequent translation RADIATION. 33 of the shock of the aethereal 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 disturbance, finally stir the sedges on the river's bank, so do the vibrating atoms generate in the surrounding aether undulations, which finally stir the fila- ments of the retina. The motion thus imparted is trans- mitted with measurable, and not very great velocity to the brain, where, by a process which science does not even tend to unravel, the tremor of the nervous matter is con- verted into the conscious impression of light. Darkness might then be defined as aether at rest ; light as aether in motion. But in reality the aether 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 produced by all other stars. It is the ceaseless thrill caused by those distant orbs collectively in the aether, that constitutes what we call the temperature of space. As the air of a room accommodates itself to the require- ments of an orchestra, transmitting each vibration of every pipe and string, so does the inter-stellar aether accommo- date itself to the requirements of light and heat. Its waves mingle in space without disorder, each being u dowed with an individuality as indestructible as if it alone had disturbed the universal repose. All vagueness with regard to the use of the terms radiation and absorption will now disappear. Eadiation is the communication of vibratory motion to the aether ; and when a body is said to be chilled by radiation, as for 34 FRAGMENTS OP SCIENCE. 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 motion, by imparting it to the medium in which they vibrate. On the other hand, the waves of aether 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 aether to the molecules consists the absorption of radiant heat. All the phenomena of heat are in this way re- ducible to interchanges of motion ; and it is purely as the recipients or the donors of this motion, that we our- selves become conscious of the action of heat and cold. 3. The Atomic Theory in reference to the Aether. 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 condition they constitute water. So also chlorine and sodium are elements, the former a pun- gent 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 elementary bodies as little spheres, the molecules of compound bodies must be pictured as groups of such spheres. This is the atomic theory as KADIATIOff. 35 Dalton conceived it. Now if this theory have any founda- tion in fact, and if the theory of an aether 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 aether, and the acceptance of motion from it, the deportment of the uncombined atoms will be different from thdt of the combined. 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 experi ment. 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 necessarily 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 vapours aggregate to liquids and solids, without any change of their chemical nature. We do not yet know how the transmis- sion 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 mole- cules entirely from the bonds of cohesion, and employing them in the gaseous or vaporous form. Let us endeavour to obtain a perfectly clear mental 86 FRAGMENTS OP SCIENCE. image of the problem now before us. Limiting in the first place our enquiries 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 mole- cules, or passing through the inter-molecular spaces with- out apparent hindrance. 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 dif- ferent molecules possess this power in different degrees. The source of waves chosen for these 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 mag- netic 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 .vhat we shall use in these enquiries, 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 tem- perature, no matter how high that may be, there is no current generated. The current is a consequence of a 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. 37 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 - compensating source, be permitted to radiate against the posterior face, this latter radiation will tend to neutralise the former. When the neutralisation 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. Perfectly equal quantities of heat being imparted to the two faces, the needle points to zero. Let any gas be now permitted to enter the exhausted tube ; if the molecules possess any power of intercepting the calorific waves, the equilibrium previously existing will be destroyed, the compensating source will triumph, and a deflection of the magnetic 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 a* unity. 88 FRAGMENTS OF SCIENCE. Radiation through Gases. Name of gas Relatlre absorption 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 Sulphido of hydrogen . . . .2,100 Ammonia 5,460 Olefiantgas 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 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 un- expected manner the influence of chemical combination. Thus the elementary gases, oxygen, hydrogen, and nitro- gen, 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 intercepts 5,460 units, olefiant gas 6,030 units, while sulphurous 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 aether pass without absorption, and these gases are not sensibly changed in temperature by the most powerful calorific rays. The position 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. RADIATION. 39 that exist uncombined in the atmosphere ; but the absorp- tion 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 other states of aggregation. Some of Melloni's results now attained a new significance ; for this celebrated ex- perimenter had found crystals of the element sulphur to be highly pervious to radiant heat ; he had also proved that lamp-black, and black glass, (which owes its blackness to the element carbon) were to a considerable extent transparent to calorific rays of low refrangibility. These facts, harmonising so strikingly with the deportment of the simple gases, suggested further enquiry. Sulphur dis- solved in bisulphide of carbon was found almost perfectly transparent. The dense and deeply-coloured 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 bromine, 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 permitted to do so, it was found that a layer of dissolved iodine, sufficiently opaque to cut off the light of the midday 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- 40 FRAGMENTS OF SCIENCE. centration. Intercepting the luminous portion of his spectrum 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 William Herschel. For to form his spec- trum 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 we please with the latter. Experiments of this character, not only with the iodine solution, but also Avith black glass and layers of lamp-black, were publicly performed at the Eoyal Institution in the early part of 1862, and the effects at the foci of invisible rays, then ob- tained, 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 j lass. 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 intro- duced, 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 ex- posure of the face of the thermo-electric pile to the focus RADIATION. 41 of invisible rays, dashed the needles of a coarse galva- nometer violently aside. It is now found that, on substi- tuting for the face of the thermo-electric pile a com- bustible body, the invisible rays are competent to set that body on fire. 6. Visible and Invisible Rays 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 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-eighth of that produced at the unseen focus of the invisible rays. Exposing his thermometers to the successive colours 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 repre- sent 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 12 FRAGMENTS OF SCIENCE. glance the manner in which the heat was distributed in the solar spectrum. Professor Miiller of Freiburg, with improved instruments, afterwards made similar experi- ments, 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 particular 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 end- less 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 asso- ciated 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 end towards the red, heat soon manifests itself, augmenting as we approach the red. Of all the colours 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 sud- denly and enormously in intensity, until sit 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 spectrum, signs of heat may be detected. Drawing a datum line, and erecting along it per- pendiculars, proportional in length to the thermal in- tensity at the respective points, we obtain the extra- ordinary curve, shown on the adjacent page, which ex- RADIATION 43 *4 FRAGMENTS OF SCIENCE. hibits 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 ODE, representing the luminous radiation. Indeed, the idea forced upon the mind by this diagram is that the light rays are a mere insigni- ficant appendage to the heat-rays represented by the area A B c D, thrown in as it were by nature for the pur- poses of vision. The diagram drawn by Professor Miiller 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 vapour 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 ob- served when the rays from the electric light are permitted to pass through a film of water, which acts upon them as the atmospheric vapour acts upon the rays of the sun. 7. Combustion by Invisible Rays. 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 RADIATION. 45 convergence being rendered clearly visible by the dust always floating in the air. Placing between the lu- minous 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 anything 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 aether 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 upwards, 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 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 having passed through the re- 16 FRAGMENTS OF SCIENCE. ceiver, still possessing sufficient power to ignite the char coal, and thus initiate the attack of the oxygen. If, instead of being plunged in oxygen, the charcoal be sus- pended in vacuo, it immediately glows at the place where the focus falls. 8. Transmutation of Rays : l 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 solution, 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 paper, two holes are pierced in it, corresponding to the images of the two coke points: but falling on a thin plate of carbon in vacuo, or upon a thin sheet of platinised platinum, either in vacuo or in air, radiant heat is converted into light, and the image stamps itself in vivid incandescence upon both the carbon and the metal. Results similar to those obtained with the electric light have also been obtained witli 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 1 I borrow this term from Professor Challis, 'Philosophical Magazine, ToL xii. p. 521. RADIATION. 47 opposite end of the spectrum. The above results constitute a kind of complement to his discoveries. Professor Stokes named the phenomena which he has discovered and in- vestigated Fluorescence ; for the new phenomena here described I have proposed the term Calorescence. He, by the interposition of a proper medium, so lowered the refrangibility of the ultra-violet rays of the spectrum as to render them visible. 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, violet, and ultra-violet ones. Could we, moreover, raise the original source of rays to a sufficiently high temperature, 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 scries of complete images and spectra being thus extracted from the invisible emission of the primitive source. 1 1 On investigating the calorescence produced by r;iys transmitted through glasses of various colours, it was found that in the case of certain specimens of blue glass, the platinum foil glowed with a pink or purplish light. The eff-et was not subjective, and considerations of obvious interest are sug- gested by it. Different kinds of black glass differ notably as to their power of transmitting radiant heat. In thin plates some descriptions tint the sun with a greenish hue : others make it appear a glowing red without any trace of green. The latter are far more diathermic than the former. In fact, carbon when perfectly dissolved, and incorporated with a good white glass, is highly transparent to the calorific rays, and by employing it as an absorbent the phenomena of ' calorescence ' may be obtained, though in a less striking form than with the iodine. The black glass chosen for thermometers, and intended to absorb completely the solar heat, may en- tirely fail in thie object, if the glass in which the carbon is incorporated be 48 FRAGMENTS OP SCIENCE. 9. Deadness of the Optic Nerve to the Calorific Rays. The layer of iodine used in the foregoing experiments intercepted the rays 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 humours of the eye the albumen of the humours might coagulate along the line of the rays. If, on the contrary, no such high absorp- tion 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 could be done. The eye was therefore caused to approach the dark focus, no defence, in the first instance, being provided ; 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 afterwards by the retina. Eemoving 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 moment before. The platinum colourless. 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-Jight 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. KADIATION. 49 became red-hot. No sensible damage was done to the eye by this experiment; no impression of light was produced; the optic nerve was not even conscious of heat. But the humours of the eye are known to be highly impervious to the invisible calorific rays, and the question therefore arises, ' Did the radiation in the foregoing expe- riment reach the retina at all ? ' The answer is, that the rays were in part transmitted to the retina, and in part absorbed by the humours. 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 in- visible 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 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 distance of a mile, its light at this distance being less than i0<0 J, i000 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 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 lay. 60 FBAGMENTS OF SCIENCE. to be multiplied by 1,500x20,000,000, or by thirty- thousand millions. Thus the thirty thousand millionth part of the invisible radiation from the electric light, re- ceived by the retina at the distance of a foot, would, if slightly changed in character, be amply sufficient to pro- voke vision. Nothing could more forcibly illustrate that special relationship supposed 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 greatei energy, whose periods of recurrence are not in unison with its own. 10. Persistence of Rays. 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 vas increased. Thus, in Dr. Draper's experiments, the rise of temperature that generated the orange, yellow, green, and blue augmented the intensity of the red. 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 intensity 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 currents of air ; through an orifice in the globe the rays could pass from the spiral and fall afterwards upon a thermo-electric 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 : RADIATION. 5) Appearance Eneiyy of of spiral obscure radiation Dark 1 Dark, but hotter 3 Dark, but still hotter .... 5 Dark, but still hotter . . . .10 Feeble red . . . . . .10 Dull red . .--..,. ^.- . . 25 Eed . . . . . . .37 Full red 62 Orange Bright orange Yellow . White . Intense white 144 202 276 440 Thus the augmentation of the electric current, which raises the wire from its primitive dark condition to an intense white heat, exalts at the same time the energy of the obscure radiation, until at the end it is fully 440 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 spectrum, it may be proved that a ray once emitted con- tinues to be emitted with increased energy as the tem- perature is augmented. The platinum spiral, so often referred to, being raised to whiteness by an electric current, a brilliant spectrum 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 59 FRAGMENTS OF SCIENCE. Appearance Energy of of spiral obscure rays Full red 27 Orange 60 Yellow 93 White 122 Here, as in the former case, the dark and bright ra- diations reached their maximum together; as the one augmented, the other augmented, until at last the energy of the obscure rays of the particular refrangibility here chosen, became 122 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 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 oxyhydrogen jet, pours out invisible rays with aug- mented 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 incan- descence. It also holds good for phosphorus burning in oxygen. Every gush of dazzling light has associated with it a gush of invisible radiant heat, which far tran- scends 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 vapours. In such cases when the curve representing the radiant energy of the body is constructed, the obscure radiation towers up- wards like a mountain, the luminous radiation resembling a mere spur at its base. From the very brightness of the light of some of the fixed stars we may infer the intensity KADIATION. 53 of that dark radiation, which is the precursor and insepa- rable 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 permitted 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, whatever 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 aether 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 attained a certain amplitude, the mutual jostling of the atoms pro- duces quicker tremors, and the light-giving waves follow us the necessary product of the heat-giving ones. 11. Absorption of Radiant Heat by Vapours and Odours. We commenced the demonstrations brought forward in this lecture by experiments on permanent gases, and 54 FRAGMENTS OF SCIENCE. we have now to turn our attention to the vapours of volatile liquids. Here, as in the case of the gases, vast differences have been proved to exist between various kinds of molecules, as regards their power of intercepting the calorific waves. While some vapours allow the waves a comparatively free passage, the minutest bubble of other vapours, introduced into the tube already employed for gases, causes a deflection of the magnetic needle. Assum- ing the absorption effected by air, at a pressure of one atmosphere, to be unity, the following are the absorptions effected by a series of vapours at a pressure of -^tk of an atmosphere : Name of vapour 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 vapour in this list ; and acetic ether the most opaque ; -gLth of an atmosphere of the former, however, produces 47 times the effect of a whole atmosphere of air, while -g^th of an atmosphere of the latter produces 612 times the effect of a whole atmosphere of air. Eeducing dry air to the pres- sure of the acetic ether here employed, and comparing them then together, the quantity of wave-motion inter- cepted by the ether would be many thousand times that intercepted by the air. Any one of these vapours discharged into the free atmo- sphere, in front of a body emitting obscure rays, intercepts more or less of the radiation. A similar effect is pro- duced by perfumes diffused in the air, though their at- tenuation is known to be almost infinite. Carrying, for example, a current of dry air over bibulous paper, moist- RADIATION. 55 ened 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 odour, in each case, being carried by a current of dry air into the tube already employed for gases and vapours : 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 Aniseed 372 Thus the absorption by a tube full of dry air being 1, that of the odour of patchouli diffused in it is 30, that of lavender 60, that of rosemary 74, whilst that of aniseed amounts to 372. It would be idle to speculate on the quantities of matter concerned in these actions. 12. Aqueous Vapour 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 50 FRAGMENTS OF SCIENCE. as it does upon the surface of the ocean, receives from it a continual supply of aqueous vapour. It would be an error to confound clouds or fog or any visible mist with the vapour of water : this vapour is a perfectly impal- pable gas, diffused, even on the clearest days, throughout the atmosphere. Compared with the great body of the air, the aqueous vapour it contains is of almost infini- tesimal amount, 99^ out of every 100 parts of the atmo- sphere being composed of oxygen and nitrogen. In the absence of experiment, we should never think of ascribing to this scant and varying constituent any important in- fluence 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 vapour exerts 100 times the action of the air itself, would certainly be an understatement of the fact. The peculiar qualities of this vapour, and the cir- cumstance 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 vapour with an atom of either of the main constituents of our atmosphere, I am not prepared to say how many thou- sand times the action of the former exceeds that of the latter. But it must be borne in mind that these large numbers depend, in part, on the extreme feebleness of the air ; the power of aqueous vapour seems vast, because that of the air with which it is compared is infinitesimal. Absolutely considered, however, this substance, notwith- standing 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 or 20 feet of RADIATION. 67 the earth's surface. This must evidently be of the utmost consequence to the life of the world. Imagine the super- ficial molecules of the earth trembling with the motion of heat, and imparting it to the surrounding aether ; this motion would be carried rapidly away, and lost for ever to our planet, if the waves of aether had nothing but the air to contend with in their outward course. But the aqueous vapour takes up the motion of the aethereal 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 philo- sophers have speculated on the influence of an atmospheric envelope. De Saussure, Fourier, M. Pouillet and Mr. Hopkins have, one and all, enriched scientific literature with contributions on this subject, but the considerations which these eminent men have applied to atmospheric air, have, if my experiments be correct, to be transferred to the aqueous vapour. 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 maximum 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 maxi- mum and minimum is greatest where the air is driest. In the plains of India, on the heights of the Himalaya, in central Asia, in Australia wherever drought reigns, we have the heat of day forcibly contrasted with the chill of night. In the Sahara itself, when the sun's rays cease to impinge on the burning soil, the temperature runs rapidly down to freezing, because there is no vapour over- head to check the calorific drain. And here another instance might be added to the numbers already known, 68 FRAGMENTS OF SCIENCE. in which nature tends as it were to check her own excess. By nocturnal refrigeration, the aqueous vapour of the air is condensed to water on the surface of the earth ; and, as only the superficial portions radiate, the act of condensa- tion makes water the radiating body. Now experiment proves that to the rays emitted by water, aqueous vapour is especially opaque. Hence the very act of condensation, consequent on terrestrial cooling, becomes a safeguard to the earth, imparting to its radiation that particular cha- racter which renders it most liable to be prevented from escaping into space. It might however be urged that, inasmuch as we Derive all our heat from the sun, the selfsame covering which protects the earth from chill must also shut out the solar radiation. 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 sub- stance which absorbs the one must necessarily absorb the other. Through a layer of water, for example, one tenth of an inch in thickness, the sun's rays are trans- mitted with comparative freedom ; 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 freedom through the aqueous vapour of the air : the absorbing power of this substance being mainly exerted upon the heat that endeavours 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. RADIATION. 59 13. Liquids and their Vapours in relation to Radiant Heat. The deportment here assigned to atmospheric vapoui nas been established by direct experiments on air taken from the streets and parks of London, from the downs of Epsom, from the hills and sea-beach of the Isle of Wight, and also by experiments on air in the first instance dried, and afterwards rendered artificially humid by pure dis- tilled 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 inter- cept the waves of heat, was carefully determined. The vapours of the liquids were next taken, in quantities pro- portional to the quantities of liquid, and the power of the vapours to intercept the waves of heat was also deter- mined. Commencing with the substance which exerted the least absorptive power, and proceeding onwards to the most energetic, the following order of absorption was ob- served : Liquids Vapours 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 change their relative powers of absorption. Nothing GO j-RAGMENTS OF SCIENCE. could more clearly prove that the act of absorption de- pends upon the individual molecule, which equally asserts its power in the liquid and the gaseous state. We may assuredly conclude from the above table that the position of a vapour is determined by that of its liquid. Now at the very foot of the list of liquids stands water, signalising itself above all others by its enormous power of absorption. And from this fact, even if no direct experiment on the vapour of water had ever been made, we should be en- titled to rank that vapour as our most powerful absorber of radiant heat. Its attenuation, however, diminishes its action. It has been proved that a shell of air two inches in thickness surrounding our planet, and saturated with the vapour of sulphuric ether, would intercept 35 per cent, of the earth's radiation. And though the quantity of aqueous vapour necessary to saturate air is much less than the amount of sulphuric ether vapour which it can sustain, it is still extremely probable that the estimate already made of the action of atmospheric vapour within 10 feet of the earth's surface, is under the mark ; and that we are indebted to this wonderful substance, to an extent not accurately determined, but certainly far beyond what has hitherto been imagined, for the temperature now existing at the surface of the globe. 14. Reciprocity of Radiation and Absorption. Throughout the reflections which have hitherto occu- pied 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 vapour were intercepted by those molecules in various degrees. In all cases it was -the transference of motion from the aether to the comparatively quiescent molecules of the gas or EADIAT10N. (31 vapour. We have now to change the form of our concep- tion, and to figure 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 vibra- ting, and generating in the surrounding aether undulations which speed through it with the velocity of light. Our object now is to enquire whether the act of chemical com- bination, 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 vapours to the same temperature, and then examine their power of discharging the motion thus imparted to them upon the aether 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 while deflection of a magnetic needle connected with the pile declared the energy of the radiation. By this mode of experiment it was proved that the selfsame molecular arrangement which renders a gas a powerful absorber, renders it in the same degree a power- ful radiator that the atom or molecule which is com- petent to intercept the calorific waves is, in the same degree, competent to generate them. Thus, while the atoms of elementary gases proved themselves unable to emit any sensible amount of radiant heat, the molecules of compound gases were shown to be capable of power- fully disturbing the surrounding aether. By special modes 32 FRAGMENTS OF SCIENCE. of experiment the same was proved to hold good for the vapours of volatile liquids, the radiative power of every vapour 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 dynami- cally 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 vapour, then the warmed air will communicate heat by contact to the vapour, which will be thus enabled to radiate. Thus the molecules of the vapour convert into the radiant form the heat imparted dynamically to the atoms of the air. By this process, which I have called Dynamic Radiation, the radiative power of both vapours and gases has been de- termined, and the reciprocity of their radiation and ab- sorption proved. 2 In the excellent researches of Leslie, De la Provostaye and Desains, and Balfour Stewart, the reciprocity of radiation and absorption, as regards solid bodies, has been variously illustrated ; while the labours, theoretical and experimental, 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 vapours, which have been hitherto thought inaccessible to experiments of this kind, are proved to exhibit the duality of radiation and 1 See page 14 for a definition of vis viva. 2 When heated air imparts its motion to another gas or vapour, the transference of heat is accompanied by a change of vibrating period. The Dynamic Badiation cf vapours is rendered possible by the transmutation of vibrations. KADIATION. 63 absorption, the influence of chemical combination on both being exhibited in the most decisive and extraordinary way. 1 5. Influence of Vibrating Period and Molecular .Form. Physical Analysis of the Human Breath. In the foregoing experiments with gases and vapours 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 rays of light differ from those of invisible heat, only in point of period, the former failing to affect the retina because their periods of recur- rence are too slow. Hence, in some way or other the transparency of our gases and vapours depends upon the periods of the waves which impinge upon them. What is the nature of this dependence ? The admirable re- searches of Kirchhoff help us to an answer. The atoms and molecules of every gaa have certain definite rates of oscillation, and those waves of aether are most copiously absorbed whose periods of recurrence synchronise with the periods of the molecules amongst which they pass. Thus, when we find the invisible rays absorbed and the visible ones transmitted by a layer of gas, we conclude that the oscillating periods of the gaseous molecules co- incide 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 aether impinge upon molecules whose periods of vibration coincide with the recurrence of the undulations, the timed 64 FRAGMENTS OP SCIENCE. strokes of the waves, the vibration of the molecules augments, 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 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. This is actually the case when the waves of the visible spectrum pass through a transparent gas or vapour. There is here no sensible transference of motion from the aether to the molecules ; in other words, there is no sensible ab- sorption of heat. One striking example of the influence of period may be here 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 number, 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 Hume of carbonic oxide, where hot carbonic acid con- stitutes 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, carbonic acid absorbs twice as much as olefiant gas. This wonderful 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 RADIATION. 66 flame freely transfer their motion to the molecules which synchronise with them. Thus it is that the tenth of an atmosphere 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. 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. Badiant 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, while under my direction, made this determination. The absorption produced by the breath freed from its moisture, but retaining its carbonic acid, was first determined. Car- bonic 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 analysed che- mically 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 fl-33 5-22 It is thus proved that in the quantity of aethereal motion which it is competent to take up, we have a practical measure 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 30 FRAGMENTS OF SCIENCE. importance, is not competent to account for the whole of the observed facts. The aether, 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 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, because the group is able to lay a vastly more powerful hold upon the aether than the single atoms can. The cavities and indentations of a molecule com- posed of spherical atoms may be one cause of this aug- mented hold. Another, and probably very potent one may be, that the vibrations, being those of the constituent atoms of the molecule, are generated in highly condensed aether, which acts like condensed air upon sound. But whatever may be the fate of these attempts to visualise 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 con- dition of the aether within the molecules, by which the aether is disturbed. 1 6. Summaiy an I 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 combination on radiation and absorption, were considered and experimentally illustrated. Through the transparent elementary gases radiant heat was found to pass as through a vacuum, while many of the compound RADIATION. 67 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 bisulphide 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, and permitting the calorific rays beyond the red to pass freely through it. This sub- stance 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 refrangibility, the invisible rays of the electric light were rendered visible, and all the colours 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- eighth 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 gradu- ally to intense incandescence, are strengthened together; intense dark heat being an invariable 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 importance to us is due to the fact, that their periods are attuned to the special requirements of the eye. Among the vapours of volatile liquids vast differences were also found to exist, as regards their powers of absorption. "We followed various molecules from a state of liquid to a state of gas, and found, in both states of 88 FRAGMENTS OF SCIENCE. aggregation, the power of the individual molecules equally asserted. The position of a vapour as an absorber of radiant heat was shown to be determined by that of the liquid from which it is derived. Reversing our con- ceptions, and regarding the molecules of gases and vapours 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 selfsame chemical act which rendered a body competent to intercept the waves of aether, rendering it competent, in the same degree, to generate them. Per- fumes were next subjected to examination, and, notwith- standing 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 vapours the aqueous vapour 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 from nightly chill ; and thus the aqueous vapour of our air, attenuated as it is, checks the drain of terrestrial heat, and saves the surface of our planet from the refrigeration which would assuredly accrue, were no such substance in- terposed 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 enquiries in the domain of Radiation, and my aim throughout has been to raise in your minds distinct physical images of the various processes involved RADIATION. 69 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 experience 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 mole- cules, 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 exer- cising 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 enquire, * 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 70 FRAGMENTS OF SCIENCE. intangible 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 aether as in the waves of the sea ; the latter derived as they are from winds, which in their turn are derived from the sun are, indeed, nothing more than the heaped-up motion of the former. It is the calorific 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 parts, the sea waves, which cannot subside without producing heat, finally resolve themselves into waves of aether, 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 another resounds. Now in alter- ing 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. III. ON RADIANT HEAT IN RELATION TO THE COLOUR AND CHEMICAL CONSTITUTION OF BODIES, 1866. ONE of the most important functions of physical science, 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 in- finitely small in comparison with the vast region accessible to thought which lies beyond them. From a few observa- tions of a comet, when it comes within the range of his telescope, an astronomer can calculate its path in regions which no telescope can reach : and in like manner, by means of data furnished in the narrow world of the senses, we make ourselves 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 afterwards 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 72 FRAGMENTS OF SCIENCE. tuning-fork, for example, moulds the air around it into undulations 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 rendering those sonorous vibrations not only tangible but visible ; and it was not until numberless ex- periments of this kind had been executed, that the scien- tific investigator abandoned himself wholly, and without a shadow of misgiving, 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 pro- duces the agitation ? Newton, you know, supposed minute particles to be shot through the humours of the eye against the retina, which he supposed to hang 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 RADIANT HEAT AND ITS RELATIONS. 73 lies entirely beyond the reach of our senses. The waves 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 in- tangible aether, all the phenomena of optics are accounted for, with a fulness, and clearness, and collusiveness, which leave no desire of the intellect unsatisfied. 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 towards the earth ; he considered the precession of the equinoxes, the ebb and flow of the tides, and found all ex- plained by the law of gravitation. He therefore regarded this law as established, and the verdict of science subse- quently confirmed his conclusion. On similar, and, if possible, on stronger grounds, we found our belief in the existence of the universal aether. It explains facts far more various and complicated than those on which Newton based his law. If a single phenomenon could be pointed out which the aether is proved incompetent to explain, we should have to give it up ; but no such phenomenon 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 in its grasp, not only our plane- tary system, but the immeasurable heavens themselves. 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 nothings 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 74 FRAGMENTS OF SCIENCE. images of processes which, though mighty in the aggre- gate beyond all conception, are so minute individually as to elude all observation. It is the waves of air excited by a tuning-fork which render its vibrations audible. It is the waves of aether sent forth from those lamps over- head which render them luminous to us ; but 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 helpful, the conceptions of each being expanded, strengthened, and denned by the conceptions of the other. The aether which conveys the pulses of light and heat not only fills celestial space, swathing suns, and planets, and moons, but it also encircles the atoms of which these bodies are composed. It is the motion of these atoms, and not that of any sensible parts of bodies, that the aether conveys ; it is this motion that constitutes the objective cause of what, in our sensations, are light and heat. An atom, then, sending its pulses through the aether, resembles a tuning-fork sending its pulses through the air. Let us look for a moment at this thrill- ing medium, 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 agitating the aether : some are good radiators, others are bad radiators ; which means that some are so constituted RADIANT HEAT AND ITS RELATIONS. 75 as to communicate their motion freely to the aether, producing therein powerful undulations ; while others are unable thus to communicate their motion, but glide through the medium without materially disturbing its re- pose. Eecent experiments have proved that elementary bodies, except under certain anomalous conditions, belong to the class of bad radiators. An atom, vibrating in the aether, resembles a naked tuning-fork vibrating in the air. The amount of motion communicated to the air by the thin prongs is too small to evoke at any distance the sensation of sound. But if we permit the atoms to combine chemi- cally and form molecules, the result, in many cases, is an enormous change in the power of radiation. The amount of aethereal disturbance, produced by the combined atoms of a body, may be many thousand times that produced by its constituent atoms when uncombined. The effect is roughly typified by a tuning-fork when connected with its resonant case. The fork and its case swing as a compound system, and the vibrations which were before inaudible, are now the source of a musical sound so power- ful, 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 colour of light. Taking a slice of white light from the sun, or from an electric lamp, and causing the light to pass through an arrangement of prisms, it is decomposed. We have the effect obtained by Newton, who first unrolled the solar beam into the splendours of the solar spectrum. At one end of this spectrum we have red light, at the other, violet; and between those extremes lie the other prismatic colours. As we advance along the spectrum from the red to the violet, the pitch of the light if I 76 FRAGMENTS OF SCIENCE. may use the expression heightens, the sensation of violet being produced by a more rapid succession of impulses than that which produces the impression 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 colour 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 vapour of silver you observe that it consists 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 pro- duce 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 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. We may project upon a white screen the beautiful stream of green light from which these bands were derived. This luminous stream is the incandescent vapour of silver. The rates of vibration of the atoms of that vapour are as rigidly fixed as those of two tuning-forks ; and to what- ever height the temperature of the vapour may be raised, the rapidity of its vibrations, and consequently its colour, which wholly depends upon that rapidity, remain un- changed. The vapour of water, as well as the vapour of silver, has its definite periods of .vibration, and these are such as to disqualify the vapour when acting freely as such, from KADIANT HEAT AND ITS KELATIONS. 77 being raised to a white heat. The oxyhydrogen flame, for example, consists of hot aqueous vapour. It is scarcely visible in the air of this room, and it would be still less 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 burn- ing 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. ; while the temperature of the oxyhydrogen flame is 6,000 Fahr. A piece of platinum is heated to vivid redness, at a distance of two inches beyond the visible termination of the flame. The vapour which produces incandescence is here abso- lutely dark. In the flame itself the platinum is raised to dazzling whiteness, and is even pierced by the flame. When this flame impinges on a piece of lime, we have the dazzling Drummond light. But the light is here due to the fact that when it impinges upon the solid body, the vibrations excited 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 motion to the molecules on which they impinge, or will they glide round the molecules, through the intermole- cular 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 cases, you hear them loudly 78 FRAGMENTS OF SCIENCE. Bounding the same musical note. Stopping one of the forks, I throw the other into strong vibration, and bring that other near the silent fork, but not into contact with it. Allowing them to continue in this position for four or five seconds, and then stopping the vibrating fork, the sound has not ceased. The second fork has taken up tho vibrations of its neighbour, and is now sounding in its turn. Dismounting one of the forks, and permitting the other to remain upon its stand, I throw the dismounted fork into strong vibration. You cannot hear it sound. Detached from its stand, the amount of motion which it can communicate to the air is too small to be sensible at any distance. When the dismounted fork is brought 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 the motion should thus transfer itself through the air it is necessary that the two forks should be in perfect unison. If a morsel of wax not larger than a pea be placed on one of the forks, it is rendered thereby powerless io 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 infinitesimal space. But just as it has com- pleted this small vibration, another pulse is ready to strike it. Thus, the impulses add themselves together. In the five seconds during which the forks were held near each other, the vibrating fork sent 1,280 waves against its neighbour, 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 all. Another curious illustration of the influence of syn- RADIANT HEAT AND ITS RELATIONS. 79 chronism on musical vibrations, is this : Three small gas- flames are inserted into three glass tubes of different lengths. Each of these flames can be caused to emit a musical note, the pitch of which is determined by the 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. With an instrument called a syren, a powerful musical note, of increasing pitch, can be pro- duced. Beginning with a. note of low pitch, and ascending gradually to a higher one, we finally attain the note of the flame in the longest tube. The moment it is reached, the flame bursts into song. The other flames are still silent within their tubes. But by urging the instrument on to higher notes, the second flame is started, and the third alone remains. A still higher note starts it also. Thus, as the sound of the syren rises gradually in pitch, it awakens every flame in passing, by striking it with a series of waves whose periods of recurrence are similar to its own. Now the wave-motion from the syren is in part taken up by the flame which synchronises 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. Let 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 fork, while this cold carbonic acid resembles the silent one. What is the consequence ? Through the synchronism of the hot and cold gas, transmission of the radiant heat of the former through the latter is prevented. The cold gas 80 FRAGMENTS OF SCIENCE. is intensely opaque to the radiation from this par- ticular 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 deter- mine the substances of which the sun, 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 synchronism exist- ing between the vibrations of the atoms from which the rays, or more correctly the ivaves, issue, and those of the atoms on which they impinge. To its incompetence to emit white light, aqueous vapour adds incompetence to absorb white light. It can~ not, for example, absorb the luminous rays of the sun, though it can absorb the non-luminous rays of the earth. This incompetence of the vapour to absorb luminous rays is shared by water and ice in fact, by all really trans- parent 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 bodies without disturbing the molecular rest. A purely luminous beam, however intense may be its heat, is sensibly incom- petent to melt the smallest particle of ice. We can, for example, converge a powerful luminous beam upon a sur- face covered with hoar frost, with out 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 rays which have no sunshine whatever in them. The luminous 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 RADIANT HEAT AND ITS RELATIONS. 81 at all absorbed, and hence they cannot produce fusion. But a body of powerful dark rays is emitted by the sun ; and it is these that cause the glaciers to shrink and the snows to disappear ; it is they that fill the banks of the Arve and Arveyron, and liberate from their frozen captivity the Rhone and the Rhine. Placing a concave silvered mirror behind the electric light its rays are converged to a focus of dazzling bril- liancy. Placing in the path of the rays, between the light and the focus, a vessel of water, and introducing at the focus a piece of ice, the ice is not melted by the concen- trated beam. Matches, at the same place, are ignited, and wood is set on fire. The powerful heat, then, of this luminous beam is incompetent to melt the ice. On withdrawing the cell of water, the ice immediately liquefies, and the water trickles from it in drops. Re- introducing 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 little 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 ser- rated, but there is no deviation from the six-rayed type. We have here, in fact, the crystallisation 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 82 FRAGMENTS OF SCIENCE. the exquisite atomic structure of the substance with which Nature every winter roofs our ponds and lakes. Numberless effects, apparently anomalous, might be adduced in illustration of the action of these lightless rays. These two powders, for example, are both white, and undistinguishable from each other by the eye. The luminous rays of the sun are unabsorbed by both from such rays these powders acquire no heat ; still one of them, sugar, is heated so highly by the concentrated beam of the electric lamp, that it first smokes and then violently inflames, while the other substance, salt, is barely warmed at the focus. Placing two perfectly transparent liquids in test-tubes 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 anomalies 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. A substance, as many of you know, has been discovered, by which these dark rays may be detached from the total emission of the electric lamp. This ray-filter is a liquid, 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. When these invisible rays are brought to a focus, at a distance of several feet from the electric lamp, the dark rays form an invisible image of their source. By proper means, this image may be transformed into a visible one of dazzling brightness. It might, moreover, be shown, if time permitted, how, out of those perfectly dark rays, could be extracted, by a process of transmutation, all the colours of the solar spectrum. It might also be proved RADIANT HEAT AND ITS RELATIONS. 83 that those rays, powerful as they are, and sufficient to fuse many metals, can be permitted to enter the eye, and to break upon the retina, without producing the least luminous impression. The dark rays being thus collected, you see nothing at their place of convergence. With a proper thermo- meter it could be proved that even the air at the focus is just as cold as the surrounding air. And mark the conclusion to which this leads. It proves the aether at the focus to be practically detached from the air, that the most violent aethereal motion may there exist, without the least aerial motion. But, though you see it not, there is sufficient heat at that focus to set London on fire. The heat there is competent to raise iron to a temper- ature 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 radiation 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, phosphorus, when placed at the dark focus. It is impos- sible to ignite there a fragment of amorphous phosphorus. But ordinary phosphorus is a far quicker combustible, and its deportment towards radiant heat is still more impressive. It may be exposed to the intense radiation of an ordinary fire without 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 red heat, without ignition. Notwithstanding the energy of the aethereal waves here concentrated, notwithstanding the extremely inflammable character of the elementary body exposed to their action, 84 FRAGMENTS OF SCIENCE. the atoms of that body refuse to partake of the motion of the powerful waves of low refrangibility, and consequently cannot be affected by their heat. The knowledge we now possess will enable us to analyse 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 placed bits of cloth of various colours upon snow, exposed them to direct sunshine, and found that they sank to different depths in the snow. The black cloth sank deepest, the white did not sink at all. Franklin inferred from this 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. One of these two cards is coated with a very dark powder, and the other with a perfectly' white one. I place the pow- dered surfaces before a 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 then most highly heated ? It requires no thermometer to answer this question ? Simply pressing the back of the card, on which the white powder is strewn, against the cheek or forehead, it is found intolerably hot. Placing the dark card in the same position, it is found 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 radia- tion from the flame, but the white bulb has absorbed most, and its mercury stands much higher than that of the other thermometer. This experiment might be varied in a hundred ways : it proves that from the darkness of a RADIANT HEAT AND ITS RELATIONS. 85 body you can draw no certain conclusion regarding its power of absorption. The reason of this simply is, that colour gives us intel- ligence of only one portion, and that the smallest one, of the rays impinging on the coloured body. Were the rays all luminous, we might with certainty infer from the colour 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 colour teaches us nothing. A body may be highly transparent to the one class of rays, and highly opaque to the other. 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 extreme imperviousness to the invisible ones ; while the dark powder was chosen on ac- count of its extreme transparency to the invisible, and its extreme opacity to the visible, rays. In the case of fhe radiation from our fire, about 9 8 per cent, of the whole emis- sion consists of invisible rays ; the body, therefore, which was most opaque to these triumphed as an absorber, though that body was a white one. And here it is worth while to consider the manner in which we obtain from natural facts what may be called their intellectual value. Throughout the processes of Nature we have interdependence and harmony ; and the main value of physics, considered as a mental dis- cipline, consists in the tracing out of this interdependence, and the demonstration of this harmony. The outward and visible phenomena are the counters of the intel- lect ; 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 the phenomena. Let us endeavour, then, to extract from the experiment of Franklin all that it can yield, calling 86 FEAGMENTS OF SCIENCE. to our aid the knowledge which our predecessors have already stored. Let us imagine two pieces of cloth of the same texture, the one black and the other white, placed upon sunned snow. Fixing our attention on the white piece, let us enquire whether there is any reason to expect that it will sink in the snow at all. There is knowledge at hand which enables us to reply at once in the negative. There is, on the contrary, reason to expect that, after a sufficient exposure, the bit of cloth will be found on an eminence instead of in a hollow ; that in- stead of a depression, we shall have a relative elevation of the bit of cloth. For, as regards the luminous rays of the sun, the cloth and the snow are alike powerless ; the one cannot be warmed, nor the other melted, by such rays. The cloth is white and the snow is white, because their confusedly mingled fibres and particles are incom- petent to absorb the luminous rays. Whether, then, the cloth will sink or not depends entirely upon the dark rays of the sun. Now the substance which absorbs these dark rays with the greatest avidity is ice, or snow, which is merely ice in powder. Hence, 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, its shield must, in due time, be left behind, perched upon an eminence like a glacier-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 same as that of the white. For our object being to compare the effects of colour, we must, in order to study this effect in its RADIANT HEAT AND ITS RELATIONS. 87 purity, preserve all the other conditions constant. Let us then suppose the black cloth to be obtained from the dye- ing 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 favour 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 complete analysis of Franklin's experiment. Throughout this discourse the main stress has been laid on chemical constitution, as influencing most power- fully the phenomena of radiation and absorption. With regard to gases and vapours, and to the liquids from which these vapours are derived, it has been proved by the most varied and conclusive experiments that the acts of radia- tion and absorption are molecular that they depend upon chemical, and not upon mechanical, condition. In at- tempting to extend this principle to solids I was met by a multitude of facts, obtained by celebrated experimenters, which seemed flatly to forbid such an extension. Melloni, for example, had found the same radiant and absorbent power for chalk and lamp-black. MM. Masson and Cour- tepee had 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 when bodies are reduced to an extremely fine state of division, the influence of this state is so powerful 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 88 FRAGMENTS OF SCIENCE. researches an oversight has run, the mere mention of which will show what caution is essential in the operations of experimental philosophy ; while an experiment or two will make clear wherein the oversight consists. Filling a brightly polished metal cube with boiling water, I determine the quantity of heat emitted by two of the bright surfaces. As a radiator of heat one of them far transcends the other. Both surfaces appear to be metallic ; what, then, is the cause of the observed differ- ence in their radiative power ? Simply this : one of the surfaces is coated with transparent gum, through which, of course, is seen the metallic lustre behind ; and this varnish, though so perfectly transparent to luminous rays, is as opaque as pitch, or lamp-black, to non-lumi- nous ones. It is a powerful emitter of dark rays; 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 powders red, their white ones white, and their black ones black, but they saw these colours through the coat of varnish which encircled every particle of their powders. When, therefore, it was concluded that colour had no influence on radiation, no chance had been given to it of asserting its influence ; when it was found that all chemi- cal precipitates radiated alike, it was the radiation from a varnish, common to them all, which showed the observed constancy. Hundreds, perhaps thousands, of experiments on radiant heat have been performed in this way, by various enquirers, but the work will, I fear, have to be RADIANT HEAT AND ITS RELATIONS. 89 done over again. I am not, indeed, acquainted with an instance in which an oversight of so trivial a character has been committed by so many able men in succession, and vitiated so large an amount of otherwise excellent work. Basing our reasonings thus on demonstrated facts, we arrive at the extremely probable conclusion that the en- velope 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 consequences from experimental facts, is an incessant exercise of the student of physical science. But having thus followed, for a time, the light of reason alone through a series of phenomena, and emerged from them with a purely intellectual conclusion, our duty is to bring that conclusion to an experimental test. In this way we fortify our science, sparing no pains and 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 presenting the same physical appearance ; one of them is a compound of mercury, and the other a compound of lead. On two sur- faces of a cube are spread these bright red powders, with- out varnish of any kind. Filling the cube with boiling water, and determining the radiation from the two surfaces, one of them is found to emit thirty-nine units of heat, while the other emits seventy-four. This, surely, is a great dif- ference. Here, however, is a second cube, having two of its surfaces coated with the same powders, the only dif- ference being that 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 90 FRAGMENTS OP SCIENCE. either of them. Excluding all varnish, and comparing white -with white, vast differences are found ; comparing black with black, they are 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 evident that as re- gards the radiation and absorption of non-luminous heat, colour teaches us nothing ; and that even as regards the radiation of the sun, consisting as it does mainly of non- luminous rays, conclusions as to the influence of colour may be altogether delusive. This is the strict scientific 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 analysing the experiment of Franklin black dresses are more potent than white ones as absorbers of solar heat. Thus, in brief outline, have been brought before you a few of the results of recent enquiry. 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 whether those dark rays which clear away the Alpine snows, will ever be applied 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 parti- culars. Still they may have great uses unknown to me ; and when our coal-fields are exhausted, it is possible that a more aethereal race than we are may cook their victuals, and perform their work, in this transcendental way. But is it necessary that the student of science should have his labours tested by their possible practical RADIANT HEAT AND ITS RELATIONS. 91 applications ? What is the practical 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 practical 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 profit- able in itself, and requiring no practical application to jus- tify its pursuit. But while the student of Nature distinctly refuses to have his labours judged by their practical issues, un- less 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 know- ledge, and mechanical skill, have made the telegraph what it is, are deserving of all honour. In fact, they have had their reward, both in reputation and in those more substan- tial benefits which the direct service of the public always carries in its train. But who, I 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, 2 who never in their enquiries 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 Caesar quit his victories for 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 application of their discoveries proves. But science has another function to fulfil, in the storing and the training of the human ' Volta. * Faraday. FEAGMENTS OF SCIENCE. mind ; and I would base my appeal to you on the specimen which has this evening been brought before you, 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. The opening paragraph of this article, as indeed many others in this volume, show that 'the crossing of the boundary of experiment,' the mention of which caused so much commotion last year, is no new heresy of mine. December 1875. Ca/, rv. NEW CHEMICAL REACTIONS PRODUCED BY LIGHT. I- IN 1868 I asked permission of theKoyal Society to draw the attention of chemists to a method of experiment which, though simple, was unknown. It consists in sub- jecting the vapours of volatile liquids to the action of concentrated sunlight, or to the concentrated beam of the electric light. This communication was the imme- diate antecedent of the discourse on ' Dust and Disease ' which follows it in this volume ; and as such is introduced here. Action of the Electric Light. A glass tube 2'8 feet long and of 2'5 inches internal diameter, which had been frequently employed in my re- searches on radiant heat, was supported horizontally. At one end of it was placed an electric lamp, the height and position of both being so arranged, that the axis of the glass tube, and that of the parallel beam issuing from the lamp, were coincident. The tube in the first experi- ments was closed by plates of rock-salt, and subsequently by plates of glass. This tube which, as on former occasions, for the sake of distinction, I call the experimental tube, was connected with an air-pump, and also with a series of drying and other tubes used for the purification of the air. 04 FRAGMENTS OP SCIENCE. A number of test-tubes, like F, fig. 2 (I have used at least fifty of them), were converted into Woulfs flasks. FIG. 2. Each of them was stopped by a cork, through which passed two glass tubes : one of these tubes (a) ended immediately below the cork, while the other (6) descended to the bottom of the flask, being drawn out at its lower end to an orifice about 0'03 of an inch in diameter. It was found necessary to coat the cork carefully with cement. The little flask, thus formed, was partially filled with the liquid whose vapour was to be examined; it was then introduced into the path of the purified current of air. The experimental tube being exhausted, and the cock which cut off the supply of purified air being cautiously turned on, the air en- tered the flask through the tube 6, and escaped by the small orifice at the lower end of fc into the liquid. Through this it bubbled, loading itself with vapour, after which the mixed air and vapour, passing from the flask by the tube a, entered the experimental tube, where they were subjected to the action of light. The power of the electric beam to reveal the existence of anything within the experimental tube, or the im- purities of the tube itself, is extraordinary. When the experiment is made in a darkened room, a tube which in ordinary daylight appears absolutely clean, is often shown by the present mode of examination to be exceedingly filthy. DECOMPOSITION BY LIGHT. 95 The following are some of the results obtained with this arrangement : Nitrite of amyl. The vapour of this liquid was in the first instance permitted to enter the experimental tube, while the beam from the electric lamp was passing through it. Curious clouds were observed to form near the place of entry, which were afterwards whirled through the tube. The tube being again exhausted, the mixed air and vapour were allowed to enter it in the dark. The slightly convergent beam of the electric light was then sent through the tube, from end to end. For a moment the tube was optically empty, nothing whatever was seen within it; but before a second had elapsed a shower of liquid spherules was precipitated on the beam, thus gene- rating a cloud within the tube. This cloud became denser as the light continued to act, showing at some places vivid iridescence. The beam of the electric lamp was now converged so as to form within the tube a cone of rays about eight inches long. The tube was cleansed and again filled in darkness. When the light was sent through it, the precipitation upon the beam was so rapid and intense that the cone, which a moment before was invisible, flashed suddenly forth like a solid luminous spear. The effect was the same when the air and vapour were allowed to enter the tube in diffuse daylight. The cloud, however, which shone with such extraordinary radiance under the electric beam, was invisible in the ordinary light of the laboratory. The quantity of mixed air and vapour within the ex- perimental tube could of course be regulated at pleasure. The rapidity of the action diminished with the attenuation of the vapour. When, for example, the mercurial column associated with the experimental tube was depressed onty 96 FRAGMENTS OP SCIENCE. five inches, the action was not nearly so rapid as when the tube was full. In such cases, however, it was exceedingly interesting to observe, after some seconds of waiting, a thin streamer of delicate bluish-white cloud slowly form- ing along the axis of the tube, and finally swelling so as to fill it. When dry oxygen was employed to carry in the vapour, the effect was the same as that obtained with air. When dry hydrogen was used as a vehicle, the effect was also the same. The effect, therefore, is not due to any interaction between the vapour of the nitrite and its vehicle. This was further demonstrated by the deportment of the vapour itself. When it was permitted to enter the experimental tube unmixed with air or any other gas, the effect was substantially the same. Hence the seat of the observed action is the vapour. This action is not to be ascribed to heat. With refer- ence to the glass of the experimental tube, and the air within the tube, the beam employed in these experiments was perfectly cold. It had been sifted by passing it through a solution of alum, and through the thick double-convex lens of the lamp. When the unsifted beam of the lamp was employed, the effect was still the same ; the obscure calorific rays did not appear to interfere with the result. My object here being simply to point out to chemists a method of experiment which reveals a new and beautiful series of reactions, to them I leave the examination of the products of decomposition. The molecule of the nitrite of amyl is obviously shaken asunder by certain specific waves of the electric beam, forming, doubtless, nitric oxide and other products, of which the nitrate of amyl is probably one. The brown fumes of nitrous acid were also seen to mingle with the cloud within the experimental tube. The nitrate of amyl, being less volatile than the nitrite, DECOMPOSITION BY LIGHT. 97 and not being able to maintain itself in the condition of vapour, would be precipitated as a visible cloud along the track of the beam. In the anterior portions of the tube a sifting of the beam by the vapour occurs, which diminishes the chemical action in the posterior portions. In some experiments the pre- cipitated cloud only extended halfway down the tube. When, under these circumstances, the lamp was shifted so as to send the beam through the other end of the tube, precipitation occurred there also. Action of Sunlight. Solar light also effects the decomposition of the nitrite- of-amyl vapour. On October 10 I partially darkened a small room in the Eoyal Institution, into which the sun shone, permitting the light 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 darkened 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 vapour at the distant end of the tube was in part shielded by that in front, and was also more feebly acted on through the divergence of the rays. On reversing the tube, a second and similar cone was precipitated. Physical Considerations. I sought to determine the particular portion of the white beam which produced the foregoing effects. When, previous to entering the experimental tube, the beam was caused to pass through a red glass, the effect was greatly 08 FRAGMENTS OF SCIENCE. weakened, but not extinguished. This was also the case with various samples of yellow glass. A blue glass being introduced, before the removal of the yellow or the red, on taking the latter away augmented precipitation occurred along the truck of the blue beam. Hence, in this case, the more refrangible rays are the most chemically active. The colour of the liquid nitrite of amyl indicates that this must be the case ; it is a feeble but distinct yellow : in other words, the yellow portion of the beam is most freely transmitted. It is not, however, the transmitted portion of any beam which produces chemical action, but the absorbed portion. Blue, as the complementary colour to yellow, is here absorbed, and hence the more energetic action of the blue rays. This reasoning, however, assumes that the same rays are absorbed by the liquid and its vapour. A solution of the yellow chromate of potash, the colour of which may be made almost, if not altogether, identical with that of the liquid nitrite of amyl, was found far more effective in stopping the chemical rays than either the red or the yellow glass. But of all substances the nitrite it- self is most potent in arresting the rays which act upon its vapour. A layer one-eighth of an inch in thickness, which scarcely preceptibly affected the luminous intensity, sufficed to absorb the entire chemical energy of the con- centrated beam of the electric light. The close relation subsisting between a liquid and its vapour, as regards their action upon radiant heat, has been already amply demonstrated. 1 As regards the nitrite of amyl, this relation is more specific than in the cases hitherto adduced ; for here the special constituent of the beam, which provokes the decomposition of the vapour, is shown to be arrested by the liquid. A question of extreme importance in molecular physics 1 'Phil. Trans.' 1864; and p. 59 of this volume. DECOMPOSITION BY LIGHT. 09 here arises: What is the real mechanism of this absorp- tion, and where is its seat ? l I figure, as others do, a molecule as a group of atoms, held together "by their mutual forces, but still capable of motion among themselves. The vapour of the nitrite of amyl is to be regarded as an assemblage of such molecules. The question now before us is this : In the act of absorp- tion, is it the molecules that are effective, or is it their constituent atoms ? Is the vis viva of the intercepted light-waves transferred to the molecule as a whole, or to its constituent parts ? The molecule, as a whole, can only vibrate in virtue of the forces exerted between it and its neighbour mole- cules. The intensity of these forces, and consequently the rate of vibration, would, in this case, be a function of the distance between the molecules. Now the identical absorption of the liquid and of the vaporous nitrite of amyl indicates an identical vibrating period on the part of liquid and vapour, and this, to my mind, amounts to an experimental demonstration that the absorption occurs in the main within the molecule For it can hardly be supposed, if the absorption were the act of the molecule as a whole, that it could continue to affect waves of the same period after the substance had passed from the vaporous to the liquid state. In point of fact, the decomposition of the nitrite of amyl is itself to some extent an illustration of this in- ternal molecular absorption ; for were the absorption the act of the molecule as a whole, the relative motions of its constituent atoms would remain unchanged, and there would be no mechanical cause for their separation. It is probably the synchronism of the vibrations of one portion of the molecule with the incident waves, that enables the 1 My attention was very forci bly directed to this subject some years ago by a conversation with my excellent friend Professor Clausius. 7 100 FRAGMENTS OF SCIENCE. amplitude of those vibrations to augment, until the chain which binds the parts of the molecule together is snapped asunder. The liquid nitrite of amyl is probably also decomposed by light ; but the reaction, if it exists, is incomparably less rapid and distinct than that of the vapour. Nitrite of amyl has been subjected to the concentrated solar rays until it boiled, and it has been permitted to continue boiling for a considerable time, without any distinctly apparent change occurring in the liquid. I anticipate wide, if not entire, generality for the fact that a liquid and its vapour absorb the same rays. A cell of liquid chlorine now preparing for me will, I imagine, deprive light more effectually of its power of causing chlorine and hydrogen to combine than any other filter of the luminous rays. The rays which give chlorine its colour have nothing to do with this combination, those that are absorbed by the chlorine being the really effec- tive rays. A highly sensitive bulb, containing chlorine and hydrogen, in the exact proportions necessary for the formation of hydrochloric acid, was placed at one end of an experimental tube, the beam of the electric lamp being sent through it from the other. The bulb did not explode when the tube was filled with chlorine, while the explosion was violent and immediate when the tube was filled with air. I anticipate for the liquid chlorine an action similar to, but still more energetic than, that exhibited by the gas. If this should prove to be the case, it will favour the view that chlorine itself is molecular and not monatomic. Production of Sky-Hue by the Decomp sit ion of Nitrite of Amyl. When the quantity of nitrite vapour is considerable, and the light intense, the chemical action is exceedingly DECOMPOSITION BY LIGHT. 101 rapid, the particles precipitated being so large as to U'hiten the luminous beam. Not so, however, when a well-mixed and highly attenuated vapour fills the experi- mental tube. The effect now to be described was first obtained when the vapour of the nitrite was derived from a portion of its liquid, accidentally introduced into the passage through which the dry air flowed into the experimental tube. In this case, the electric beam traversed the tube for several seconds before any action was visible. Decom- position then visibly commenced, and advanced slowly. When the light was very strong, the cloud appeared of a milky blue. When, on the contrary, the intensity was moderate, the blue was pure and deep. In Briicke's im- portant experiments on the blue of the sky and the morn- ing and evening red, pure mastic is dissolved in alcohol, and then dropped into water well stirred. When the pro- portion of mastic to alcohol is correct, the resin is pre- cipitated so finely as to elude the highest microscopic power. By reflected light, such a medium appears bluish, by transmitted light yellowish, which latter colour, by augmenting the quantity of the precipitate, can be caused to pass into orange or red. But the development of colour in the attenuated nitrite-of-amyl vapour, though admitting of the same ex- planation, is doubtless more similar to what takes place in our atmosphere. The blue, moreover, is far purer and more sky-like than that obtained from Briicke's turbid medium. Never, even in the skies of the Alps, have I seen a richer or a purer blue than that attainable by a suitable disposition of the light falling upon the precipitated vapour. In exhausting the tube containing the mixed air and nitrite-of-amyl vapour, it was difficult to avoid explosions under the pistons of the air-pump, similar to those which 102 . FEAGMENTS OP SCIENCE. I have already described as occurring with the vapours of bisulphide of carbon and other substances. Though the quantity of vapour present in these cases must have been infinitesimal, its explosion was sometimes sufficient to de- stroy the valves of the pump. Iodide of Allyl. Among the liquids hitherto sub- jected to the concentrated electric light, iodide of allyl, in point of rapidity and intensity of action, comes next to the nitrite of amyl. With the iodide of allyl I have em- ployed both oxygen and hydrogen, as well as air, as a vehicle, and found the effect in all cases substantially the same. The cloud-column here was exquisitely beautiful. It revolved round the axis of the decomposing beam ; it was nipped at certain places like an hour-glass, and round the two bells of the glass delicate cloud-filaments twisted themselves in spirals. It also folded itself into convolu- tions resembling those of shells. In certain conditions of the atmosphere in the Alps I have often observed clouds of a special pearly lustre ; when hydrogen was made the vehicle of the iodide-of-allyl vapour a similar lustre was most exquisitely shown. With a suitable disposition of the light, the purple hue of iodine-vapour came out very strongly in the tube. The remark already made, as to the bearing of the decomposition of nitrite of amyl by light on the question of molecular absorption, applies here also ; for were the absorption the work of the molecule as a whole, the iodine would not be dislodged from the allyl with which it is combined. The non-synchronism of iodine with the waves of obscure heat is illustrated by its marvellous transparency to such heat. May not its synchronism with the waves of light in the present instance be the cause of its divorce from the allyl? Further experiments on this point are in preparation. Iodide of Isopropyl. The action of light upon the DECOMPOSITION BY LIGHT. 103 vapour of this liquid is, at first, more languid than upon iodide of allyl ; indeed many beautiful reactions may be overlooked, in consequence of this languor at the com- mencement. After some minutes' exposure, however, clouds begin to form, which grow in density and in beauty as the light continues to act. In every experiment hitherto made with this substance the column of cloud filling the experimental tube, was divided into two dis- tinct parts near the middle of the tube. In one experi- ment a globe of cloud formed at the centre, from which, right and left, issued an axis uniting the globe with two adjacent cylinders. Both globe and cylinders were animated by a common motion of rotation. As the action continued, paroxysms of motion were manifested; the various parts of the cloud would rush through each other with sudden violence. During these motions beautiful and grotesque cloud-forms were developed. At some places the nebulous mass would become ribbed so as to resemble the graining of wood ; a longitudinal motion would at times generate in it a series of curved transverse bands, the retarding influence of the sides of the tube causing an appearance resembling, on a small scale, the dirt-bands of the Mer de Glace. In the anterior portion of the tube those sudden commotions were most intense ; here buds of cloud would sprout forth, and grow in a few seconds in to per- fect flower-like forms. The cloud of iodide of isopropyl had a character of its own, and differed materially from all others that I had seen. A gorgeous mauve colour was observed in the last twelve inches of the tube ; the vapour of iodine was present, and it may have been the sky-blue scattered by the precipitated particles which, mingling with the purple of the iodine, produced the mauve. As in all other cases here adduced, the effects were proved to be due to the light ; they never occurred in darkness. The forms assumed by some of those actinic clouds, 104 FRAGMENTS OF SCIENCE. in consequence of rotations and other motions, due to differences of temperature, are perfectly astounding. I content myself here with a meagre description of one more of them. The tube being filled with the sensitive mixture, the beam was sent through it, the lens at the same time being so placed as to produce a cone of very intense light. Two minutes elapsed before anything was visible ; but at the end of this time a faint bluish cloud appeared to hang itself on the most concentrated portion of the beam. Soon afterwards a second cloud was formed five inches farther down the experimental tube. Both clouds were united by a slender cord of the same bluish tint as them- selves. As the action of the light continued, the first cloud gradually resolved itself into a series of parallel disks of exquisite delicacy, which rotated round an axis perpen- dicular to their surfaces, and finally blended to a screw surface with an inclined generatrix. This gradually changed into a filmy funnel, from the narrow end of which the ' cord ' extended to the cloud in advance. The latter also underwent slow but incessant modification. It first resolved itself into a series of strata resembling those of the electric discharge. After a little time, and through changes which it was difficult to follow, both clouds pre- sented the appearance of a series of concentric funnels set one within the other, the interior ones being seen through the outer ones. Those of the distant cloud re- sembled claret-glasses in shape. As many as six funnels were thus concentrically set together, the two series being united by the delicate cord of cloud, already re- ferred to. Other cords and slender tubes were afterwards formed, which coiled themselves in delicate spirals around the funnels. Eendering the light along the connecting-cord more ARTIFICIAL SKY. 105 intense, it diminished in thickness and became whiter ; this was a consequence of the enlargement of its particles. The cord finally disappeared, while the funnels melted into two ghost-like films, shaped like parasols. They were barely visible, being of an exceedingly delicate blue tint. They seemed woven of blue air. To compare them with cobweb or with gauze would be to liken them to something infinitely grosser than themselves. In all cases a distant candle-flame, when looked at through the cloud, was sensibly undimmed. 2. ON THE BLUE COLOUR OF THE SKY, AND THE POLARISATION OF SKYLIGHT.' 1869. After the communication of the foregoing brief abstract 'On a new Series of Chemical Reactions produced by Light,' the experiments upon this subject were continued, the number of substances thus acted on being considerably increased. I now beg to direct attention to two questions glanced at incidentally in the abstract referred to the blue colour of the sky, and the polarisation of skylight. Re- serving the historic treatment of the subject for a more fitting occasion, I would merely mention now that these questions constitute, in the opinion of our most eminent authorities, the two great standing enigmas of meteor- ology. Indeed it was the interest manifested in them by Sir John Herschel, in a letter of singular speculative power, addressed to myself, that caused me to enter upon the consideration of these questions so soon. The apparatus with which I work consists, as already stated, of a glass tube about a yard in length, and from 1 In my ' Lectures on Light ' (Longmans), the polarisation of light will be found briefly, but, I trust, clearly explained. 106 FRAGMENTS OF SCIENCE. 2 to 3 inches internal diameter. The vapour to be examined is introduced into this tube in the manner de- scribed in my last abstract, and upon it the condensed beam of the electric lamp is permitted to act, until the neutrality or the activity of the substance has been declared. It has hitherto been my aim to render the chemical action of light upon vapours visible. For this purpose substances have been chosen, one at least of whose pro- ducts of decomposition under light shall have a boiling- point so high, that as soon as the substance is formed it shall be precipitated. By graduating the quantity of the vapour, this precipitation may be rendered of any degree of fineness, forming particles distinguishable by the naked eye, or far beyond the reach of our highest micro- scopic powers. I have no reason to doubt that particles may be thus obtained, whose diameters constitute but a small fraction of the length of a wave of violet light. In all cases when the vapours of the liquids employed are sufficiently attenuated, no matter what the liquid may be, the visible action commences with the formation of a blue cloud. I would guard myself at the outset against all misconception as to the use of this term. The * cloud ' here referred to is totally invisible in ordinary daylight. To be seen, it requires to be surrounded by darkness, it only being illuminated by a powerful beam of light. This blue cloud differs in many important particulars from the finest ordinary clouds, and might justly have assigned to it an intermediate position between such clouds and true vapour. With this explanation, the term ' cloud,' or incipient cloud,' as I propose to employ it, cannot, I think, be mis- understood. I had been endeavouring to decompose carbonic acid ARTIFICIAL SKY. JOT gas by light. A faint bluish cloud, due it may be, or it may not be, to the residue of some vapour previously employed, was formed in the experimental tube. On looking across this cloud through a Nicol's prism, the line of vision being horizontal, it was found that when the short diagonal of the prism was vertical, the quantity of light reaching the eye was greater than when the long diagonal was vertical. When a plate of tourmaline was held between the eye and the bluish cloud, the quantity of light reaching the eye when the axis of the prism was perpendicular to the axis of the illuminating beam, was greater than when the axes of the crystal and of the beam were parallel to each other. This was the result all round the experimental tube. Causing the crystal of tourmaline to revolve round the tube, with its axis perdendicular to the illuminating beam, the quantity of light that reached the eye was in all its positions a maximum. When the crystallographic axis was parallel to the axis of the beam, the quantity of light transmitted by the crystal was a minimum. From the illuminated bluish cloud, therefore, polarised light was discharged, the direction of maximum polarisa- tion being at right angles to the illuminating beam ; the plane of vibration of the polarised light was perpendicular to the beam. 1 Thin plates of selenite or of quartz, placed between the Nicol and the bluish cloud, displayed the colours of polarised light, these colours being most vivid when the line of vision was at right angles to the experimental tube. The plate of selenite usually employed was a 1 This is still an undecided point ; but the probabilities are so much in its favour, and it is in my opinion so much preferable to have a physical image on -which the mind can rest, that I do not hesitate to employ the phraseology in the text. 108 FRAGMENTS OF SCIENCE. circle, thinnest at the centre, and augmenting uniformly in thickness from the centre outwards. When placed in its proper position between the Nicol and the cloud, it exhibited a system of splendidly-coloured rings. The cloud here referred to was the first operated upon in the manner described. It may, however, be greatly improved upon by the choice of proper substances, and by the application, in proper quantities, of the substances chosen. Benzol, bisulphide of carbon, nitrite of amyl, nitrite of butyl, iodide of allyl, iodide of isopropyl, and many other substances may be employed. I will take the nitrite of butyl as illustrative of the means adopted to secure the best result, with reference to the present question. And here it may be mentioned that a vapour, which when alone, or mixed with air in the experimental tube, resists the action of light, or shows but a feeble result of this action, may, when placed in proximity with another gas or vapour, exhibit vigorous, if not violent action. The case is similar to that of carbonic acid gas, which, diffused in the atmosphere, resists the decomposing action of solar light, but when placed in contiguity with chlorophyl in the leaves of plants, has its molecules shaken asunder. Dry air was permitted to bubble through the liquid nitrite of butyl, until the experimental tube, which had been previously exhausted, was filled with the mixed air and vapour. The visible action of light upon the mix- ture after fifteen minutes' exposure was slight. The tube was afterwards filled with half an atmosphere of the mixed air and vapour, and a second half-atmosphere of air which had been permitted to bubble through fresh commercial hydrochloric acid. On sending the beam through this mixture, the tube, for a moment, was optically empty. But the pause amounted only to a small fraction of a ARTIFICIAL SKY. 109 second, a dense cloud being immediately precipitated upon the beam. This cloud began blue, but the advance to whiteness was so rapid as almost to justify the application of the term instantaneous. The dense cloud, looked at perpen- dicularly to its axis, showed scarcely any signs of polari- sation. Looked at obliquely the polarisation was strong. The experimental tube being again cleansed and ex- hausted, the mixed air and nitrite-of-butyl vapour was permitted to enter it until the associated mercury column was depressed ^ of an inch. In other words, the air and vapour, united, exercised a pressure not exceeding -3^ of an atmosphere. Air, passed through a solution of hydro- chloric acid, was then added, till the mercury column was depressed three inches. The condensed beam of the elec- tric light passed for some time in darkness through this mixture. There was absolutely nothing within the tube competent to scatter the light. Soon, however, a superbly blue cloud was formed along the track of the beam, and it continued blue sufficiently long to permit of its thorough examination. The light discharged from the cloud, at right angles to its own length, was perfectly polarised. By degrees the cloud became of whitish blue, and for a time the selenite colours, obtained by looking at it nor- mally were exceedingly brilliant. The direction of maxi- mum polarisation was distinctly at right angles to the illuminating beam. This continued to be the case as long as the cloud maintained a decided blue colour, and even for some time after the pure blue had changed to whitish blue. But, as the light continued to act, the cloud became coarser and whiter, particularly at its centre, where it at length ceased to discharge polarised light in the direction of the perpendicular, while it continued to do so at both its endg 110 FRAGMENTS OF SCIENCE. But the cloud which had thus ceased to polarise the light emitted normally, showed vivid selenite colours when looked at obliquely, proving that the direction of maxi- mum polarisation changed with the texture of the cloud. This point shall receive further illustration subse- quently. A blue, equally rich and more durable, was obtained by employing the nitrite-of-butyl vapour in a still more attenuated condition. Now the instance here cited is re- presentative. In all cases, and with all substances, the cloud formed at the commencement, when the precipitated particles are sufficiently fine, is blue, and it can be made to display a colour rivalling that of the purest Italian sky. In all cases, moreover, this fine blue cloud polarises per- fectly the beam which illuminates it, the direction of polarisation enclosing an angle of 90 with the axis of the illuminating beam. It is exceedingly interesting to observe both the per- fection and the decay of this polarisation. For ten or fifteen minutes after its first appearance the light from a vividly illuminated incipient cloud, looked at perpendicularly, is absolutely quenched by a Nicol's prism with its longer diagonal vertical. But as the sky-blue is gradually ren- dered impure by the introduction of particles of too large a size in other words, as real clouds begin to be formed the polarisation begins to deteriorate, a portion of the light passing through the prism in all its positions. It is worthy of note, that for some time after the cessation of perfect polarisation, the residual light which passes, when the Nicol is in its position of minimum transmission, is of a gorgeous blue, the whiter light of the cloud being ex- tinguished. 1 When the cloud texture has become suffici ently coarse to approximate to that of ordinary clouds, the 1 This shows that particles too Urge to polarise the Llue, polarise perfectly light of lower refrangibility. ARTIFICIAL SKY. Ill rotation of the Nicol ceases to have any sensible effect on the quantity of light discharged normally. The perfection of the polarisation, in a direction per- pendicular to the illuminating beam, is also illustrated by the following experiment : A Nicol's prism, large enough to, embrace the entire beam of the electric lamp, was placed between the lamp and the experimental tube. A few bubbles of air, carried through the liquid nitrite of butyl, were introduced into the tube, and they were fol- lowed by about three inches (measured by the mercurial gauge) of air which had passed through aqueous hydro- chloric acid. Sending the polarised beam through the tube, I placed myself in front of it, my eye being on a level with its axis, my assistant Mr. Cottrell occupying a similar position behind the tube. The short diagonal of the large Nicol was in the first instance vertical, the plane of vibration of the emergent beam being therefore also vertical. As the light continued to act, a superb blue cloud, visible to both my assistant and myself, was slowly formed. But this cloud, so deep and rich when looked at from the positions mentioned, utterly disappeared when looked at vertically downwards, or vertically upwards. Reflection from the cloud was not possible in these direc- tions. When the large Nicol was slowly turned round its axis, the eye of the observer being on the level of the beam, and the line of vision perpendicular to it, entire extinction of the light emitted horizontally occurred when the longer diagonal of the large Nicol was vertical. But now a vivid blue cloud was seen when looked at down- wards or upwards. This truly fine experiment was first definitely suggested by a remark in a letter addressed so me by Professor Stokes. As regards the polarisation of skylight, the greatest stumbling-block has hitherto been, that, in accordance with the law of Brewster, which makes the index of refraction 112 FRAGMENTS OF SCIENCE the tangent of the polarising angle, the reflection which produces perfect polarisation would require to be made in air upon air ; and indeed this led many of our most emi- nent men,Brewster himself among the number, to entertain the idea of aerial molecular reflection. 1 I have, however, operated upon substances of widely different refractive indices, and therefore of very different polarising angles as ordinarily defined, but the polarisation of the beam, by the incipient cloud, has thus far proved itself to be abso- lutely independent of the polarising angle. The law of Brewster does not apply to matter in this condition, and it rests with the undulatory theory to explain why. Whenever the precipitated particles are sufficiently fine, no matter what the substance forming the particles may be, the direction of maximum polarisation is at right angles to the illuminating beam, the polarising angle for matter in this condition being invariably 45. Suppose our atmosphere surrounded by an envelope im- pervious to light. But with an aperture on the sunward side, through which a parallel beam of solar light could enter and traverse the atmosphere. Surrounded by air 1 ' The cause of the polarisation is evidently a reflection of the sun's light upon something. The question is on what? Were the angle of maximum polarisation 76, we should look to water or ice as the reflecting body, however inconceivable the existence in a cloudless atmosphere, and a hot summer's day of unevaporated molecules (particles ?), of water. But though we were once of this opinion, careful observation has satisfied us that 90, or thereabouts, is the correct angle, and that therefore whatever be the body on which the light has been reflected, if polarised by a single reflection, the polarising 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 ! ' (Sir John Hersehel, ' Meteorology," par. 233.) Any particles, if small enough, will produce both the colour and the polarisation of the sky. But is the existence of small water-particles on a hot summer's day in the higher regions of our atmosphere inconceivable? It is to be remembered that the oxygen and nitrogen of the air behave as a vacuum to radiant heat, the exceedingly attenuated vapour of the higher atmosphere being therefore in practical contact with the cold of spaco. ARTIFICIAL SKY. 113 not directly illuminated, the track of such a beam through the air would resemble that of the parallel beam of the electric lamp through an incipient cloud. The sunbeam would be blue, and it would discharge laterally light in precisely the same condition as that discharged by the in- cipient cloud. In fact, the azure revealed by such a beam would be to all intents and purposes that which I have called a 'blue cloud.' Conversely our 'blue cloud' is, to all intents and purposes, an artificial sky. 1 But, as regards the polarisation of the sky, we know that not only is the direction of maximum polarisation at right angles to the track of the solar beams, but that at certain angular distances, probably variable ones, from the sun, ' neutral points,' or points of no polarisation, exist, on both sides of which the planes of atmospheric polarisation are at right angles to each other. I have made various observations upon this subject which are reserved for the present ; but, pending the more complete examination of the question, the following facts bearing upon it may be submitted. The parallel beam employed in these experiments tracked its way through the laboratory air, exactly as sun- beams are seen to do in the dusty air of London. I have reason to believe that a great portion of the matter thus floating in the laboratory air consists of organic particles, 1 The opinion of Sir John Herschel, connecting the polarisation and the blue colour of the sky is verified by the foregoing results. ' The more the subject [the polarisation of skylight] is considered,' writes this eminent philosopher, ' the more it -will be found beset with difficulties, and its ex- planation when arrived at will probably be found to carry with it that of the blue colour of the sky itself, and of the great quantity of light it actually does send down to us.' ' We may observe, too,' he adds, ' that it is only where the purity of the sky is most absolute that the polarisation is developed in its highest degree, and that where there is the slightest per- ceptible tendency to cirrus it is materially impaired." This applies word for word to our ' incipient clouds.' 114 FRAGMENTS OF SCIENCE. which are capable of imparting a perceptibly bluish tint to the air. These also showed, though far less vividly, all the effects of polarisation obtained with the incipient clouds. The light discharged laterally from the track of the illuminating beam was polarised, though not perfectly, the direction of maximum polarisation being at right angles to the beam. The horizontal column of air, thus illuminated, was 1 8 feet long, and could therefore be looked at very obliquely. At all points of the beam, throughout its entire length, the light emitted normally was in the same state of polar- isation. Keeping the positions of the Nicol and the selenite constant, the same colours were observed through- out the entire beam, when the line of vision was perpen- dicular to its length. I then placed myself near the end of the beam, as it issued from the electric lamp, and, looking through the Nicol and selenite more and more obliquely at the beam, observed the colours fading until they disappeared. Aug- menting the obliquity the colours appeared once more, but they were now complementai~y to the former ones. Hence this beam, like the sky, exhibited a neutral point, on opposite sides of which the light was polarised in planes at right angles to each other. Thinking that the action observed in the laboratory might be caused, in some way, by the vaporous fumes dif- fused in its air, I had the light removed to a room at the top of the Eoyal Institution. The track of the beam was seen very finely in the air of this room, a length of 14 or 15 feet being attainable. This beam exhibited all the effects observed with the beam in the laboratory. Even the uncondensed electric light falling on the floating matter showed, though faintly, the effects of polarisation. When the air was so sifted as to entirely remove the visible floating matter, it no longer exerted any sensible AKTIFICIAL SKY. 115 action upon the light, but behaved like a vacuum. The light is scattered by particles, not by molecules or atoms. By operating upon the fumes of chloride of ammo- nium, the smoke of brown paper, and tobacco-smoke, I had varied and confirmed in many ways those experiments on neutral points, when my attention was drawn by Sir Charles Wheatstone to an important observation communi- cated to the Paris Academy in 1860 by Professor Govi, of Turin. 1 M. Govi had been led to examine a beam of light sent through a room in which was diffused the smoke of incense, and tobacco-smoke. His first brief communication stated the fact of polarisation by such smoke ; but in his second communication he announced the discovery of a neutral point in the beam, at the opposite sides of which the light was polarised in planes at right angles to each other. But unlike my observations on the laboratory air, and unlike the action of the sky, the direction of maximum polarisation in M. Govi's experiment enclosed a very small angle with the axis of the illuminating beam. The ques- tion was left in this condition, and I am not aware that M. Govi or any other investigator has pursued it further. I had noticed, as before stated, that as the clouds formed in the experimental tube became denser, the polar- isation of the light discharged at right angles to the beam became weaker, the direction of maximum polarisation becoming oblique to the beam. Experiments on the fumes of chloride of ammonium gave me also reason to suspect that the position of the neutral point was not con- stant, but that it varied with the density of the illumin- ated fumes. The examination of these questions led to the follow- 1 ' Comptes Kandus,' tome li. pp. 360 and 669. 116 FRAGMEOTS OP SCIENCE. ing new and remarkable results : The laboratory being well filled with the fumes of incense, and sufficient time being allowed for their uniform diffusion, the electric beam was sent through the smoke. From the track of the beam polarised light was discharged ; but the direction of maximum polarisation, instead of being perpendicular, now enclosed an angle of only 12 or 13 with the axis of the beam. A neutral point, with complementary effects at oppo- site sides of it, was also exhibited by the beam. The angle enclosed by the axis of the beam, and a line drawn from the neutral point to the observer's eye, measured in the first instance 66. The windows of the laboratory were now opened for some minutes, a portion of the incense-smoke being permitted to escape. On again darkening the room and turning on the light, the line of vision to the neutral point was found to enclose, with the axis of the beam, an angle of 63. The windows were again opened for a few minutes, more of the smoke being permitted to escape. Measured as before, the angle referred to was found to be 54. This process was repeated three additional times ; the neutral point was found to recede lower and lower down the beam, the angle between a line drawn from the eye to the neutral point and the axis of the beam falling succes- sively from 54 to 49, 43 and 33. The distances, roughly measured, of the neutral point from the lamp, corresponding to the foregoing series of observations, were these : 1st observation . 2 feet 2 inches. 2nd 2 6 3rd 2 10 4th 3 2 5th 3 7 6th * 6 ARTIFICIAL SKY. 117 At the end of this series of experiments the direction of maximum polarisation had again become normal to the beam. The laboratory was next filled with the fumes of gun- powder. In five successive experiments, corresponding to five different densities of the gunpowder-smoke, the angles enclosed between the line of vision to the neutral point, and the axis of the beam, were 63, 50, 47, 42, and 38 respectively. After the clouds of gunpowder had cleared away the laboratory was filled with the fumes of common resin, rendered so dense as to be very irritating to the lungs. The direction of maximum polarisation enclosed, in this case, an angle of 12, or thereabouts, with the axis of the beam. Looked at, as in the former instances, from a posi- tion near the electric lamp, no neutral point was observed throughout the entire extent of the beam. When this beam was looked at normally through the selenite and Nicol, the ring-system, though not brilliant, was distinct. Keeping the eye upon the plate of selenite, and the line of vision perpendicular, the windows were opened, the blinds remaining undrawn. The resinous fumes slowly diminished, and as they did so the ring-system became paler. It finally disappeared. Continuing to look in the same direction, the rings revived, but now the colours were complementary to the former ones. The neu- tral point had passed me in its motion down the beam, consequent upon the attenuation of the fumes of resin. With the fumes of chloride of ammonium substantially the same results were obtained. Sufficient, however, has been here stated to illustrate the variability of the position of the neutral point. 1 1 Brewster has proved the variability of the position of the neutral point for skylight with the sun's altitude, a result obviously connected with the foregoing experiments. 118 FRAGMENTS OP SCIENCE. Some of the clouds formed in the experiments on the chemical action of light are, as already stated, aston- ishing as to shape. The experimental tube is often divided into segments of dense cloud, separated from each other by nodes of finer matter. Looked at normally, as many as four reversals of the plane of polarisation have been found, in passing from node to segment, and from seg- ment to node. With the fumes diffused in the laboratory, on the contrary, there was no change in the polarisation along the normal, for here the necessary differences of cloud-texture did not exist. By a puff of tobacco-smoke, or of condensed steam, blown into the illuminated beam, the brilliancy of the selenite colours may be greatly augmented. But with different clouds two different effects are produced. Let the ring-system observed in the common air be brought to its maximum strength, and then let an attenuated cloud of chloride of ammonium be thrown into the beam at the point looked at; the ring-system flashes out with augmented brilliancy, but the character of the polarisation remains unchanged. This is also the case when phosphorus, or sulphur, is burned underneath the beam, so as to cause the fine particles of phosphoric acid or of sulphur to rise into the light. With the sulphur- fumes the brilliancy of the colours is exceedingly intensi- fied ; but in none of these cases is there any change in the character of the polarisation. But when a puff of aqueous cloud, or of the fumes of hydrochloric acid, hydriodic acid, or nitric acid is thrown into the beam, there is a complete reversal of the selenite tints. Each of these clouds twists the plane of polarisa- tion 90. On these and kindred points experiments are still in progress. 1 1 Sir John Herschel suggested to me that this change of the polar- isation from positive to negative may indicate a change from polarisation ARTIFICIAL SKY. 119 Almost all liquids have motes in them sufficiently nu- merous to polarise sensibly the light, and \ r ery beautiful effects may be obtained by simple artificial devices. When, for example, a cell of distilled water is placed in front of the electric lamp, and a thin slice of the beam is permitted to pass through it, scarcely any polarised light is discharged, and scarcely any colour produced with a plate of selenite. But if a bit of soap be agitated in the water above the beam, the moment the infinitesimal particles reach the light the liquid sends forth laterally almost per- fectly polarised light ; and if the selenite be employed, vivid colours flash into existence. A still more brilliant result is obtained with mastic dissolved in a great excess of alcohol. The selenite rings, in fact, constitute an extremely delicate test as to the quantity of individually invisible particles in a liquid. Commencing with distilled water, for example, a thick slice of light is necessary to make the polarisation of its suspended particles sensible. A much thinner slice suffices for common water; while, with Briicke's precipitated mastic, a slice too thin to pro- duce any sensible effect with most other liquids, suffices to bring out vividly the selenite colours. 3. 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 de- fect of light in relation to that space. By altering the illumination, either of the object itself or of its environ- ment, we alter the appearance of the object. Take the case of clouds floating in the atmosphere with patches of by reflection to polarisation by refraction. This thought repeatedly occurred to me while looking at the effects ; but it will require much following up before it emerges into clearness. 120 FRAGMENTS OP SCIENCE. blue between them. Anything that changes the illumina- tion of either alters the appearance of both, that appear- ance depending, as stated, upon differential action. Now the light of the sky, being polarised, may, as the reader of the foregoing pages knows, be in great part quenched by a Nicol's prism, while the light of a common cloud, being un polarised, cannot be thus extinguished. Hence the possibility of very remarkable variations, not only in the aspect of the firmament, which is really changed, but also in the aspect of the clouds, which have that firmament as a blackground. 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 brilliancy 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 light behind it. When a reddish cloud at sunset chances to float in the region of maxi- mum polarisation, the quenching of the surrounding light causes it to flash with a brighter crimson. Last Easter eve the Dartmoor sky, which had just been cleansed by a snow-storm, wore a very wild appearance. Bound the horizon it was of steely brilliancy, while reddish cumuli and cirri floated southwards. When the sky was quenched behind them these floating masses seemed like dull embers suddenly 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 splendour. The side of the Weiss- horn seen from the Bel Alp, being turned from the ARTIFICIAL SKY. 121 sun, was tinted mauve; but I wished to observe one of the* rose-coloured 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 distinct peaks and buttresses of the Dom, in addition to its dominant head all covered with pure snow were reddened by the light of sunset. The shoulder of the Alphubel was similarly coloured, while the great mass of the Fletschorn was all a-glow, and so was the snowy spine of the Monte Leona. Looking at the Weisshorn through the Nicol,the glow of its protuberance was strong or weak according to the posi- tion of the prism. The summit also underwent striking changes. In one position of the prism it exhibited a pale white against a dark background ; in the rectangular position 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 wonderful 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 mountain sprang forth with astonish- ing solidity and detachment 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 position of maximum polarisation. By a little practice with the Nicol it was easy to render the extinction of the light, or its restora tion, 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 Alphubel, glowed as if set suddenly on fire. This was immediately dimmed by turning the Nicol through an angle of 90. 122 FRAGMENTS OF SCIENCE. 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 remarkably the distinctness of the mountains. On the morning of August 24 similar effects were finely 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 very nearly perpendicular to the solar beams, the effects on this mountain were most striking. The grey summit of the Matterhorn, at the same time, could scarcely be distinguished from the opalescent 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 production of these effects the only things changed are the sky behind, and the luminous haze in front of the mountains; that these are changed because the light emitted from the sky and from the haze is plane polarised light, and that the light from the snows and from the mountains, being sensibly unpolarised, is not directly affected by the Nicol. It will also be understood that it is not the interposition of the haze as an opaque body that renders the mountains indistinct, but the light of the haze which dims and bewilders the eye, and thus weakens the definition of objects seen through it. These results have a direct bearing upon what artists call ' aerial perspective.' As we look from the summit of Mont Blanc, or from a lower elevation, at the serried crowd of peaks, especially if the mountains be darkly coloured 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 moun- tains as to distance unmistakable. When this haze is ARTIFICIAL SKY. 123 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 polarised. When this happens, aerial perspective is abolished, and mountains very differently distant 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 Khone valley, and between both we have the blue haze referred to, throwing the distant mountains far away. But at certain hours of the day the haze may be quenched, and then the Massa ridge and the mountains beyond the Ehone 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 colour, 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 manner, and is subject to the same laws, as the firmament itself. We live in the sky, not under it. These points were further elucidated by the deport- ment of the selenite plate, with which the readers of the foregoing pages are so well acquainted. On some of the sunny days of August the haze in the valley of the Ehone, as looked at from the Bel Alp, was very remark- able. Towards evening the sky above the mountains op- posite to my place of observation yielded a series of the most splendidly-coloured 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 colours were not much diminished in brilliancy. I should estimate the distance 124 FKAGMENTS OF SCIENCE. across the valley, as the crow flies, to the opposite mountain, at nine miles ; so that a body of air of this thickness can, under favourable circumstances, produce chromatic effects of polarisation 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, coming purely from superficial reflection, is always of the same colour as the light which falls upon the landscape ; the other part reaches 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 dis- tinctive colours. The white light of the sun enters all substances to a certain depth, and is partially ejected by internal reflection ; each distinct substance absorbing and reflecting the light, in accordance with the laws of its own molecular constitution. Thus the solar light is sifted by the landscape, which appears in such colours and vari- ations of colour as, after the sifting process, reach the observer's eye. Thus the bright green of grass, or the darker colour of 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. Under certain circum- stances, it may be quenched by a Nicol's prism, and we then obtain the true colour 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 glimmering field of maize exhibits the most extraordinary variations when looked at through the rotating Nicol. Thoughts and questions like those here referred to ARTIFICIAL SKY. 126 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 moun- tain. I scanned the whole of the sky with my Nicol. Both alone, and in conjunction with the selenite, it pro- nounced the perpendicular to the solar beams to be the direction of maximum polarisation. But at no portion of the firmament was the polarisation complete. The artificial sky produced in the experiments recorded in the preceding pages could, in this respect, be rendered more perfect than the natural one ; while the gorgeous 'residual blue' which makes its appearance when the polarisation 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 extinction of the brilliancy. With certain substances, however, artificially treated, this dull residue may also be obtained. 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 referred, the alternations of light and darkness re- sembled the play of sheet lightning behind the moun- tains. 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 definition under the operation of the prism. 126 FRAGMENTS OF SCIENCE. V. ON DUST AND DISEASE 1870. Experiments on Dusty Air. OLAK light, in passing through a dark room, reveala its track by illuminating the dust floating in the air. ' The sun,' says Daniel Culverwell, ' discovers atomes, though they be invisible by candle-light, and makes them dance naked in his beams.' In my researches on the decomposition of vapours by light, I was compelled to remove these 'atomes' and this dust. It was essential that the space containing the vapours should embrace no -visible thing that no substance capable of scattering light in the slightest sensible degree should, at the outset of an experiment, be found in the wide ' experimental tube ' in which the vapour was enclosed. For a long time I was troubled by the appearance there of floating matter, which, though invisible in diffuse daylight, was at once revealed by a powerfully condensed beam. Two U-tubes were placed in succession in the path of the air, before it entered the liquid whose vapour was to be carried into the experimental tube. One of the U-tubes contained fragments of glass wetted with con- centrated sulphuric acid ; the other, fragments of marble wetted with a strong solution of caustic potash. 1 To my astonishment, the air of the Eoyal Institution, sent 1 The apparatus is figured and described at p. lf>4. OX DUST AND DISEASE. 127 through these tubes at a rate sufficiently slow to dry it, and to remove its carbonic acid, carried into the experi- mental tube a considerable amount of mechanically sus- pended 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. I tried to intercept this floating matter in various ways; and on October 5, 1868, prior to sending the air through the drying apparatus, it was carefully permitted to pass over the tip of a spirit-lamp flame. The floating matter no longer appeared, having been burnt up by the flame. It was therefore organic matter. I was by no means prepared for this result ; having previously thought that the dust of our air was, in great part, inorganic and non-combustible. ! I had constructed a small gus-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, ensured the practical contact of the dust with the incandescent metal. The air of the labo- ratory was permitted to enter the experimental tube, sometimes through the cold, and sometimes through the heated, tube of platinum. In the first column of the 1 According to an analysis kindly furnished to me by Dr. Percy, the dust collected from the walls of the British Museum contains fully 50 per cent, of inorganic matter. I have every confidence in the results of this distinguished 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 pr^sente une remarque : la poussiere que 1'on trouve a la surface de tous ies corps est soumise constamment a des courants d'air, qui doivent soulever Bes particules Ies plus legeres, au nombre desquelles se trouvent, sang doute, de preference Ies corpuscules organises, oeufs ou spores, moins lourds generalement que Ies particules minerales.' * Pasteur was, I believe, the first to employ such a tube. 128 FRAGMENTS OF SCIENCE. following fragment of a long table the quantity of air operated on is expressed by the depression of the mercury gauge of the air-pump. In the second column the con- dition of the platinum tube is mentioned, and in the third the state of the air in the experimental tube. Quantity of air State of platinum tube State of experimental tube 15 inches . . Cold . . Full ot particles. 30 . . . Ked-hot . . Optically empty. The phrase l optically empty ' shows that when the conditions of perfect combustion were present, the floating matter totally disappeared. In a cylindrical beam, which strongly illuminated the dust of the laboratory, I 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 placing the flame at some distance below the beam, the same dark masses stormed upwards. They were blacker than the blackest smoke 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 and was thus answered : A red-hot poker was placed underneath the beam: from it the black wreaths also ascended. A large hydrogen flame was next employed, and it produced those whirling masses of dark- ness, far more copiously than either the spirit-flame or poker. Smoke was therefore out of the question. 1 1 In none of the public rooms of the United States where I had the honour to lecture was this experiment made. The organic dust was too scanty. Certain rooms in England the Brighton Pavilion, for example also lack the necessary conditions. ON DUST AND DISEASE. 129 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 compe- tent to scatter its light. When the flame was piaced 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 substi- tuted 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, unseen, 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 illumination. It is not, however, necessary to burn the particles to produce a stream of darkness. Without actual combustion, currents may be generated which shall displace the floating matter, and appear dark amid the surrounding brightness. I noticed this effect first on placing a red-hot copper ball below 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 pro- duced. 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 cir- cuit. Beginning with a feeble current the temperature of the wire was gradually augmented ; but long 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 purified spectrum. Eight and left of this dark vertical band the floating matter rose upwards, bounding definitely 130 FRAGMENTS OF SCIENCE. the non-luminous 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 oonvecticn current of pure air therefore passed up- wards 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 below that of combustion. But when the platinum wire is intensely heated, the floating matter is not only displaced, but destroyed. I stretched a wire about 4 inches long through the air of an ordinary glass shade resting on cotton-wool, which also sur- rounded the rim. The wire being raised to a white heat by an electric current, the air expanded, and some of it was forced through the cotton-wool. When the current was interrupted, and the air within the shade cooled, the returning air did not carry motes along with it, being fil- tered by the wool. At the beginning of this experiment the shade was charged with floating matter; at the end of half an hour it was optically empty. On the wooden base of a cubical glass shade measuring 1 1 ^ 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 five minutes there was a sensible diminution of the matter, and in ten minutes it was totally consumed. Oxygen, hydrogen, nitrogen, carbonic acid, so prepared as to exclude all floating particles, produce, when poured ON DUST AND DISEASE. 131 or blown into the beam, the darkness of stellar space. Coal-gas does the same. An ordinary glass shade, placed in the air with its mouth downwards, permits the track of the beam to be seen crossing it. When coal-gas or hydro- gen is permitted to enter the shade by a tube reaching to its top, the gas gradually fills the shade from above down- wards. As soon as it occupies the space crossed by the beam, the luminous track is abolished. Lifting the shade so as to bring the common boundary of gas and air above the beam, the track flashes forth. After the shade is full, if it be inverted, the pure gas passes upwards like a black smoke among the illuminated particles. The Germ Theory of Contagious Disease. There is no respite to our contact with the floating matter 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 portion ? It was some time ago the current belief that epidemic 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 by which itself had been assailed. 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, being apparently competent to propagate indefinitely its own decay. Why should not a bit of rotten malaria act in a similar manner within the human frame? In 1836 a very won- derful reply was given to this question. In that year Cagniard de la Tour discovered the yeast-plant, a living 132 FRAGMENTS OP SCIENCE. organism, 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 strik- ing 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 supplied solely with calcined air, putrefaction never sets in. Putrefaction, therefore, he affirmed to be caused, not by the air, but by something which could be destroyed by a sufficiently high temperature. The results of Schwann were confirmed by the independent experi- ments of Helmholtz, Ure, and Pasteur, while other methods, pursued by Schultze, and by Schroeder and Dusch, led to the same result. But as regards fermenta- tion, the minds of chemists, influenced probably by the great authority of Gay-Lussac, fell back upon the old no- tion of matter in a state of decay. It was not the living yeast-plant, but the dead or dying parts of it, which, as- sailed by oxygen, produced the fermentation. This notion was finally exploded by Pasteur. He proved the real ' ferments ' to be organised beings which find in the re- puted 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 favoured by Linnaeus, that epidemic diseases may be due to germs which float in the atmosphere, enter the body, and produce disturbance by the development within the body of parasitic life. The strength of this theory consists in the perfect parallelism of the phenomena of contagious disease with those of life. As a planted acorn gives birth to an oak. competent to produce a whole crop ON DUST AND DISEASE. 133 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 diseases literally plant their seeds, grow, and shake abroad new germs, which, meeting in the human body their proper food and temperature, finally takes possession of whole populations. There is nothing to my knowledge in pure chemistry which resembles the power of self-multiplica- tion 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 contagious disease reproduces itself as rigidly as if it were (as Miss Nightingale puts it) dog or cat. Parasitic Diseases of Silkworms. Pasteur's Researches. It is admitted on all hands that some diseases are the product of parasitic growth. Both in man and in lower creatures, the existence of such diseases has been demon- strated. I am enabled to lay before you an account of an epidemic of this kind, thoroughly investigated and successfully combated by M. Pasteur. For fifteen years a plague had raged among the silkworms of France. They had sickened and died in multitudes, while those that suc- ceeded in spinning their cocoons furnished only a fraction of the normal quantity of silk. In 1853 the silk culture of France produced 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 further augmentation. The weight of the cocoons produced in 1853 was 26,000,000 kilogrammes; in 1865 it had fallen to 4,000,000, the fall entailing, in a single year, a loss of 100,000,000 francs. The country chiefly smitten by this calamity happened to be that of the celebrated chemist Dumas, now perpetual 134 FKAGMENTS OF SCIENCE. 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 silkworm, and he urged his inexperience in reply to his friend. But Dumas knew too well the qualities needed for such an enquiry to accept Pasteur's reason for declining it. ' Je mets,' said he, l 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.' Pam- phlets 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 Silkworm,' 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, 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 confidence in their blind guides. In 1863 the French Minister of Agriculture signed an agreement to pay 500,000 francs for the use of a remedy, which its pro- moter declared to be infallible. It was tried in twelve different departments of France, and found perfectly use- less. In no single instance was it successful. It was under these circumstances 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 important department in France, and it was the most sorely smitten by the plague. The silkworm had been previously attacked by mus- ON DUST AND DISEASE. 135 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. Wafted by winds they often sowed the disease in places far removed from the centre of infection. Muscardine is now said to be very rare, a deadlier malady having taken its place. A frequent outward sign of this new disease are the black spots which cover the silkworms ; 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 Gruerin Meneville noticed in the blood of silkworms vibratory corpuscles, which he supposed to be endowed with independent life. Filippi proved him wrong, and showed that the motion of the corpuscles was the well-known Brownian motion. But Filippi himself committed the error of supposing the cor- puscles to 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 Vittadiani founded, in 1859, a practical method of distinguishing healthy from diseased eggs. The test often proved fallacious, and it was never exten- sively applied. These living 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 automatically through the motions of spinning, without any material to work upon. Its organs, instead of being filled with the clear viscous liquid of the silk, are packed 136 FRAGMENTS OF SCIENCE. to distension by the corpuscles. On this feature of the plague Pasteur fixed his entire attention. The cycle of the silkworm's life is briefly this : From the fertile egg comes the little worm, which grows, and casts its skin. This process of moulting is repeated two or three times at sub- sequent 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- corpuscles 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 grows, the corpus- cles 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 cor- puscles 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 qiii avaient paru dpnis longtemps en Italic, dont les OX DUST AND DISEASE. 137 resultats montraient 1'inutilite de mes efforts, et I'impossi- bilite d'arriver a un resultat pratique dans la direction que je m'etais engage. Que mon ignorance fut grande au sujet des recherches sans nombre qui avaient paru depuis quinze annees.' Pasteur heard the buzz, but he con- tinued his work. In choosing the eggs intended for in- cubation, the cultivators selected those produced in the successful ' educations ' 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, however, easy to make the cultivators accept new guidance. To strike their imagination, and if pos- sible determine their practice, Pasteur hit upon the ex- pedient of prophecy. In 1866 he inspected, at St. Hippolyte-du-Fort, fourteen different parcels of eggs in- tended for incubation. Having examined a sufficient number of the moths which produced these eggs, he wrote out the prediction of what would occur 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 pre- diction and the observed facts. Many of the groups had perished totally ; 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 average crop was obtained. Now, the parcels of eggs here referred to were considered healthy by their owners. They had been hatched and tended in the firm hope that the labour expended on them would prove remunerative. The application of the moth-test for a few 138 FRAGMENTS OP SCIENCE. minutes in 1866, would have saved the labour 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 pro- duction 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, and followed out with admirable skill and completeness the various modes by which the plague was propagated. From moths perfectly free from corpuscles he obtained healthy worms, and se- lecting 10, 20, 30, 50, as the case might be, he in- troduced into the worms the corpusculous matter. It was first permitted to accompany the food. Let us take a single example out of many. Eubbing up a small corpusculous worm in water, he smeared the mixture over the mulberry-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 infection. These constituted his ' lot temoign,' his standard of comparison. On April 16, 1868, he thus infected thirty worms. Up to the 23rd they remained quite well. On the 25th they seemed well, but on that day corpuscles were found in the intes- tines of two of them. On the 27th, or eleven days after the infected repast, two fresh worms were examined, and not only was the intestinal canal found in each case invaded, but the silk organ itself was 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 bulk of the healthy ones. On May 2 ON DUST AND DISEASE. 139 a worm which had just finished its fourth moulting was examined. Its whole body was so filled with the parasite as to excite astonishment that it could live. The disease advanced, the worms died and were examined, and on May 11 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 meal had poisoned them all. The standard lot, on the contrary, spun their fine cocoons, two only of their moths being proved to contain any trace of the parasite, which had doubtless 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 counted the growing number of corpuscles seen in the field of his microscope from day to day. After a conta- gious 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 in the field of his microscope. He then varied the mode of infection. He inoculated healthy worms with the corpusculous matter, and watched the consequent growth of the disease. He proved that 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 demon- strated the spread of infection by the simple association of healthy and diseased worms. By their claws and their dejections, the diseased worms spread infection. It was no hypothetical infected medium no problematical pythogenic gas that killed the worms, but a definite organism. The question of infection at a distance was also examined, and its existence demonstrated. As might 140 FKAGMEXTS OF SCIENCE. be expected from Pasteur's antecedents, the investigation was exhaustive, 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 the important question on which he was engaged : Place (he says) the most skilful educator, even the most expert microscopist, in presence of large educations which present the symptoms described in our experiments ; his judgment will necessarily be erroneous if he confines himself to the knowledge which preceded my researches. The worms will not present to him the slightest spot of p^brine ; 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 chrysalides and the moths, thence to pass into the eggs and smite with sterility 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 corpusculous repast; an effect more or less prompt according to the epoch of life of the worm that has eaten the poisoned food. Pasteur describes in detail his method of securing healthy eggs. It is nothing less than a mode of restor- ing to France her ancient silk husbandry. The justifica- tion of his work is to be found in the reports which reached him of the application and the unparalleled suc- cess of his method, while editing his researches for final publication. In both France and Italy his method has been pursued with the most surprising results. But it was an up-hill fight which led to this triumph. ' Ever,' ON DUST AND DISEASE. 141 he say .4, ' since the commencement of these researches, I have been exposed to the most obstinate and unjust con- tradictions ; but I have made it a duty to leave no trace of these conflicts in this book.' And in reference to parasitic diseases, generally, he uses the following weighty words : ' II est au pouvoir de I'homme de faire dis- paraitre de la surface du globe les maladies parasitaires, si, comme c'est ma conviction, la doctrine des generations spontanees est une chimere.' Pasteur dwells upon the ease with which an island like Corsica might be absolutely isolated from the silkworm epidemic. And with regard to other epidemics, Mr. Simon describes an extraordinary case of insular exemp- tion, for the ten years extending from 1851 to 1860. Of the 627 registration districts of England, one only had an entire escape from diseases which, in whole or in part, were prevalent 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 because 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 insu- lar. It was the district of the Scilly Isles ; to which it was most improbable that any febrile contagion should come from without. And its escape is an approximative proof that, at least for those ten years, no contagium of measles, nor any contagium of scarlet-fever, nor any con- tagium 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 districts, the district of the Scilly Isles was one. A second parasitic disease of silkworms, called in France la flacherie, co-existent with pebrine, but quite distinct from it, has also been investigated by Pasteur. Enough, 142 FRAGMENTS OF SCIENCE. however, has been said to send the reader interested in these questions to the original volumes for further in- formation. To one important practical point M. Pasteur, in a letter to myself, 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, et des principes que j'etablis touchant la maladie des vers a soie. Beaucoup de ces colonies pourraient cultiver le mftrier 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 prospdrit 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 cha- racter 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 attributed the cause of the disease.' Those acquainted with our 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 ill-smelling drains. According to them, the matter of epidemic 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 ON DUST AND DISEASE. 143 such diseases, than they do in the spontaneous generation of mice. Pasteur, for example, found that pebrine had been known for an indefinite time as a disease among silk- worms. The development of it which he combated was merely the expansion of an already existing power the bursting into open conflagration of a previously smoul- dering 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 prevent, 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 in har- mony with the method of Nature. We sometimes hear diphtheria spoken of as if it were a new disease of the last twenty years ; but Mr. Simon tells me that about three centuries ago tremendous epidemics of it began to rage in Spain (where it was named Garrotillo)., and soon afterwards in Italy ; and that since that time the disease has been well known to all successive generations of doctors. In or about 1758, for instance, Dr. Starr, of Liskeard, in a communication to the Eoyal Society, particularly described the disease, with all the characters which have recently again become familar, but under the name of morbus strangulatorius, as then severely epidemic in Cornwall. This fact is the more interesting, as diph- theria, in its more modern reappearance, 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 north-western parts of India. Let me here state an item of my own experience. When 144 FRAGMENTS OF SCIENCE. I was at the Bel Alp last year the English chaplain received letters informing him of the breaking out of scarlet-fever among his children. He lived, if I remember 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, and thus deflect him from the truth, while those of another school would deny to a drain, however foul, the power of pro- ducing a specific disease. After close enquiry he recol- lected that a hobby-horse had been used both by his boy and another who, a short time previously, had passed thi ough scarlet-fever. Drains and cesspools, indeed, are by no means in such evil odour 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. But, if the germs be present, defective drains and cesspools become the potent distributors of disease and death. Corrupted air may promote an epidemic, but cannot produce 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 epidemic dis- ease has been transplanted to the place where it newly appears. With a clearness and conclusiveness not to be surpassed, Dr. William Budd has traced such diseases from place to place ; showing how they plant themselves, at distinct foci, among populations subjected to the same atmospheric influences, just as grains of corn might ON DUST AXD DISEASED J An ed* t c a /. Le carried in the pocket and sown. Hildebrand, 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 visitant une malade attaquee de scarlatine, et que je portai de Vienne en Podolie, sans 1'avoir mis depuis plus d'un an et demi, me communiqua, des que je fus arrive, cette maladie con- tagieuse, que je repandis ensuite dans cette province, ou 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 disease, being of a very mild character, had been quite overlooked ; 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 housekeeper at length cleared up the mystery. The young lady, on her arrival, had expressed a wish to occupy a room in an isolated tower. Her desire was granted ; 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 disease, merely because in special cases the parentage may be indistinct ? Those best acquainted with atomic nature, and who are most ready to admit, as regards even 140 FRAGMENTS OP SCIENCE. higher things than this, the potentialities of matter, will be the last 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. As already stated, the germ theory of putre- faction was started by Schwann, but the illustrations of this theory adduced by Professor Lister are of such public 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 appeared 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 extra- ordinary chemical changes under the influence of an excessively minute quantity of a substance which, regarded chemically, 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 atmospheric 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 instrument, 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 accumulated 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 there was no absolute OX DUST AND DISEASE- 147 aecurity of immunity from bad consequences. However care- fully 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 abcess, 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 some- thing from without. Inflammation of the punctured wound, even supposing it to have occurred, would not explain the pheno- menon. For mere inflammation, whether acute or chronic, though it occasions the formation of pus, does not induce putre- faction. 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 instru- ment in oil, and the subsequent precautions, prevented the en- trance 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 pro- tected situation will fail to be wiped off when the instrument is *hrust through the tissues. Then when the trocar is withdrawn, 9 148 FRAGMENTS OF SCIENCE. 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 dis- lodged when the tube is taken out, and left behind in the cavity. The germ theory tells us that these particles 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 effect produced, compared with the minuteness and the inert- ness, 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 putre- fying 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 move- ments. 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 peculiarities of living struc- tures that they possess extraordinary powers of effecting 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 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 1 ' Introductory Lecture before the University of Edinburgh.' ON DUST AND DISEASE. 149 wound they shall be killed as they fall. With this in view 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 himself, has been, that even in the midst of abominations too shocking to be mentioned here, and in the neighbourhood of wards where death was rampant from pyaemia, erysipelas, and hospital gan- grene, he was able to keep his patients absolutely free from these terrible scourges. Let me here recommend to your attention Professor Lister's 'Introductory Lecture before the University of Edinburgh,' which I have already quoted ; his paper on ' The Effect of the Anti- septic System of Treatment on the Salubrity of a Surgical Hospital ; ' and the article in the ' British Medical Journal ' of January 14, 1871. 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 acquainted 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 putrefaction. Professor Lister, as far as I know, was the first to give a philosophical interpretation of this feet, which he describes 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 150 FRAGMENTS OF SCIENCE. freely mixed with air, undergoes no decomposition. The air ia sometimes pumped into the pleural cavity in such abundance that, making its way through the wound in the pleura costalis, it inflates the cellular tissue of the whole body. Yet this occasions no alarm to the surgeon (although if the blood in the pleura were to putrefy, it would infallibly occasion dangerous suppu- rative 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-passages, 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 hypothesis farther 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, moreover, that this ' something ' is not a vapour nor a gas, nor indeed a molecule of any kind, but a particle. 1 The term 'particulate' has been used in the Keports of the Medical Department of the Privy Council to describe this supposed constitution of contagious matter ; and 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 treatment of particles methods are available which it would be futile to apply to molecules. 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. ON DUST AND DISEASE. 16] Application of Luminous Beams to these Researches. My own interference with this great question, while sanctioned by eminent names, has been also an object of varied and ingenious attack. On this point I will only say that when angry feeling escapes from behind the intellect, where it may be useful as an urging force, and places itself athwart the intellect, it is liable to produce all manner of delusions. Thus my censors, for the most part, have levelled their remarks against positions which were never assumed, and against claims which were never made. The simple history of the matter is this : During the autumn of 1868 I was much occupied with the obser- vations referred to at the beginning of this discourse. For fifteen years it had been my habit to make use of floating dust to reveal the paths of luminous beams through the air; but until 1868 I did not intentionally reverse the process, and employ a luminous beam to reveal and examine the dust. In a paper presented to the Eoyal Society in December, 1869, the observations which in- duced me to give more special attention to the question of spontaneous generation, and the germ theory of epi- demic disease, are thus described : The Floating Matter of the Air. Prior to the discovery of the foregoing action (the chemical action of light upon vapours, Fragment IV.), 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 free within from suspended matter. Neither condition is, however, easily attained. For however well the tubes might be washed and polished, and however 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 152 FRAGMENTS OF SCIENCE. some impurity. All chemical processes, 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 revealed by the poweifully condensed electric beam. The floating motes resembled minute particles of liquid which had been carried mechanically from the U-tubes 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 passage through the caustic potash and sulphuric acid. This, however, was the case ; the motes really came from without. They also passed with freedom through a variety of ethers and alcohols. In fact, it requires long-con- tinued action on the part of an acid first to wet the motes and afterwards 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 therefore 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 afterwards, 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 care, but not without a great deal of searching out of disturbances, the experimental tube, even when filled with air or vapour, contains nothing competent to scatter the light. The space within it has the aspect 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 154. S S' is the glass experimental 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 ON DUST AND DISEASE. 163 with the other end S' we have the flask F, containing the liquid whose vapour 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 t t', con- taining a tolerably tightly fitting plug of cotton-wool. To save the air-pump gauge from the attack of such vapours as act on mercury, as also to facilitate observation, a separate barometer tube was employed. Through the cork which stops the flask F two glass tubes, a and Z>, pass air-tight. The tube a ends immediately under the cork ; the tube /;, 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 turned carefully 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 vapour, it finally passes into the experimental tube, where it is submitted to examination. The electric lamp L placed at the end of the experimental tube furnishes the necessary beam. The facts here forced upon my attention had a bearing too evident to be overlooked. The inability of air which had been filtered through, cotton-wool to generate animal- cular life, had been demonstrated by Schroeder and Pasteur : here, the cause of its impotence was rendered evident to the eye. The experiment proved that no sensible amount of light was scattered by the molecules of the air ; that the scattered light always arose from sus- pended particles ; and the fact that the removal of these abolished simultaneously the power of scattering light and of originating life, obviously detached the life-origi- nating power from the air, and fixed it on something sus- pended in the air. Gases of all kinds passed with freedom through the plug of cotton- wool ; hence the tiling whose 164 FEAGMEMS OF SCIENCE. ON DUST AND DISEASE. 155 removal by the cotton-wool rendered the gas impotent, could not itself have been matter in the gaseous condition. It at once occurred to me that the retina, protected as it was, in these experiments, from all extraneous light, might be converted into a new and powerful instrument of demonstration in relation to the germ theory. But the observations also revealed the danger incurred in experiments of this nature ; showing that without an amount of care far beyond that hitherto bestowed upon them, such experiments left the door open to errors of the gravest description. It was especially manifest that the chemical method employed by Schultz in his experi- ments, and so often resorted to since, might lead to the most erroneous consequences ; that neither acids nor alkalies had the power of rapid destruction hitherto as- cribed to them. In short, the employment of the lumin- ous beam rendered evident the cause of success in experi- ments rigidly conducted like those of Pasteur ; while it made equally evident the certainty of failure in experi- ments less severely and less skilfully carried out. Dr. Bennetts Experiments. But I do not wish to leave an assertion of this kind without illustration. Take, then, 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 solution of caustic potash, the other sulphuric acid. All the bent tubes 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 1 'British Medical Journal,' 13, pt. ii. 1868. 156 FRAGMENTS OF SCIENCE. containing sulphuric acid, and also through a bulb con- taining 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 efficacy. 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 Dr. Bennett's apparatus, the very result? which he 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 iu them ; whilst it re- quired from four to nine months to develop fungi in the others. In one of the eight, moreover, even after this interval no fungi appeared. In a second series of ex- periments there was a similar exception. In a third series the cork stoppers used in the first and second series were abandoned, and glass stoppers employed. 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 negative cases, the prepared air was forced into the infusion when it was boiling hot. Dr. Bennett made a 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 conclu- sion ? Not that the boiling hot liquid, employed in his ON DUST AND DISEASE. 167 first experiments, had destroyed such germs as had run the gauntlet of his apparatus ; but that air which, pre- vious to being sealed up, had been exposed to a tempera- ture 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 sub- jected 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 por- tion of each year in a medium of less density than that which Dr. Bennett describes as scarcely meriting the name of air. The inhabitants of the higher Alpine chalets, with their flocks and herds, and the grasses which support these, do the same ; while the chamois rears its kids in air rarer still. Insect life, moreover, is sometimes exhibited with monstrous prodigality at Alpine heights. 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 infusion which had been boiled and per- mitted 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 his experiments the conclusion to which they so obviously point. On them, on the contrary, he founds a defence of the doctrine of spontaneous generation, and a general theory of sponta- neous development. So strongly was he impressed with 158 FRAGMENTS OF SCIENCE. 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 evidence of the spontaneous origin of such fungi. And he accounts for the absence of life in many of his experi- ments by an hypothesis which will not bear a moment's examination. But, knowing that organic particles may pass unscathed through alkalies and acids, the results of Dr. Bennett are precisely what ought under the circum- stances to be expected. Indeed, their harmony with the conditions now revealed is a proof of the honesty and accuracy with which they were executed. The caution exercised by Pasteur both in the exe- cution of his experiments, and in the reasoning based upon them, is perfectly evident to those who, through the practice of severe experimental enquiry, have rendered themselves competent to judge of good experimental work. He found germs in the mercury used to isolate his air. He was never sure that they did not cling to the instru- ments he employed, or to his own person. Thus when he opened his hermetically sealed flasks upon the Mer de Glace, he had his eye upon the file used to detach the drawn-out necks of his bottles ; and he was careful to stand to leeward when each flask was opened. Using these precau- tions, he found the glacier air incompetent, in nineteen cases out of twenty, to generate life ; while similar flasks, opened amid the vegetation of the lowlands, were soon crowded with living things. M. Pouchet repeated Pas- teur's experiments in the Pyrenees, adopting the precau- tion of holding his flasks above his head, and obtaining a different result. Now great care would be needed to render this procedure a real precaution. The luminous beam at once shows us its possible effect. Let smoking brown paper be placed at the open mouth of a glass ON DUST AND DISEASE. 159 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 under- neath the shade, a vertical wind of surprising violence, considering the small elevation of temperature, rises from the hand, displacing by comparatively dark air the illumi- nated smoke. Unless special care were taken such a wind would rise 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, would be annulled. Let me now direct attention to another result of Pasteur, the cause and significance of which are at once revealed by the luminous beam. He prepared twenty-one flasks, each containing a decoction of yeast, filtered and clear. He boiled the decoction so as to destroy what- ever 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 courtyard 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 deve- loped. In all the others organisms speedily appeared. Now here is an experiment conducted in Paris, on which we can throw obvious light in London. Causing our luminous beam to pass through a large flask filled with the air of this room, and charged with its germs and its dust, the beam is seen crossing the flask from side to side. But here is another similar flask, which cuts a clear gap out of the beam. It is 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 emained quiet for some time. Acting upon this obvious suggestion I set aside three other flasks, filled, in the first instance, with mote-filled air. They are now optically 160 FRAGMENTS OF SCIENCE. empty. Our former experiments proved that the life- producing particles attach themselves to the fibres of cotton-wool. In the present experiment the motes have been brought by gentle air-currents, established by slight differences of temperature within our closed vessels, into contact with the interior surface, to which they adhere. The air of these flasks has deposited its dust, germs and all, and is practically free from suspended matter. I had a chamber erected, the lower half of which is of wood, its upper half being enclosed by four glazed window-frames. It 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 February 6 it 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 cham- ber as it did in the air of the laboratory. The chamber was examined almost daily ; a perceptible diminution of the floating matter being noticed as time advanced. At the end of a week the chamber was optically empty, exhi- biting no trace of matter competent to scatter the light. Such must have been the case in the stagnant caves of the Paris Observatory. "Were our electric beam sent through the air of these caves its track would be invisible ; thus showing the indissoluble association of the scattering of light by air and its power to generate life. I will now turn to what seems to me a more interest- ing 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 memories. 1 Why air,' said he, ' introduced into the pleural cavity, through a wounded lung, should have such wholly different effects from that entering through a permanently open wound, penetrating from without, was to me a complete ON DUST AND DISEASE. 161 mystery, till I heard of the germ theory of putrefaction, when it at once occurred to me that it was only natural that the air should be filtered of germs by the air-passages, one of whose offices is to arrest inhaled particles of dust, and prevent them from entering the air-cells.' Here is a surmise which bears the stamp of genius, but which needs verification. If, for the words ' it is only natural ' we were authorised to write ' it is perfectly cer- tain,' the demonstration would be complete. Such de- monstration is furnished by experiments with a beam of light. One evening, towards the close of 1869, while pouring various pure gases across the dusty track of a luminous beam, the thought occurred to me of using my breath instead of the gases. I then noticed, for the first time, the extraordinary darkness produced by the expired air, towards the end of the expiration. Permit me to re- peat the experiment in your presence. I fill my lungs with ordinary air and breathe through a glass tube across the beam. The condensation of the aqueous vapour of the breath is shown by the formation of a luminous white cloud of delicate texture. We abolish this cloud by drying the breath previous to its entering the beam ; or, still more simply, by warming the glass tube. The luminous track of the beam is for a time uninterrupted by the breath, "because the dust returning from the lungs makes good, in great part, the particles displaced. After a time, however, an obscure disk appears in the beam, the darkness of which increases, until finally, towards the end of the expiration, 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 thus proved to be 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 FRAGMENTS OF SCIENCE. 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. 1 Application of Luminous Beams 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 enquiries conducted with that instrument. In micro- scopic examination 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 illuminated, its general condition being revealed, by the scattered light. 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 particles of which are so small as wholly to elude the microscope, may, when examined by the method alluded to, produce not only sensible, but striking, effects upon the eye. We will apply the method, in the first place, to an experiment of M. Pouchet intended to prove conclusively that animalcular life is developed in cases where no ante- cedent germs could possibly exist. He produced water from the combustion of hydrogen in air, justly arguing that no germ could survive the heat of a hydrogen flame. But he overlooked the fact that his aqueous vapour was condensed in the air, and was allowed as water to 1 Dr. Burden Sanderson draws attention to the important observation of Brauell, which shows that the contagium of a pregnant animal, suffering from splenic fever, is not found in the blood of the fcetus ; the placental apparatus acting as a filter, and holding back the infective particles. ON DUST AND DISEASE. 103 trickle through the air. Indeed the experiment is one of a number by which workers like M. Pouchet are differen- tiated from workers like Pasteur. I will show you some water, produced by allowing a hydrogen flame to play upon a polished silver condenser, formed by the bottom of a silver basin, containing ice. The collected liquid is pellucid 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 produce a continuous luminous cone. In passing through the air the water loaded itself with this matter; and the deportment of such water could obviously have no influence in deciding this great question. We are invaded with dirt not only in the air we breathe, but in the water we drink. To prove this I take the bottle of water intended to quench your lecturer's thirst ; which, in the track of the beam, simply reveals itself as dirty water. And this water is no worse than the other London waters. Thanks to the kindness of Professor Frankland, I have been furnished with speci- mens 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 sus- pended matter ? The grosser matter, undoubtedly, but not the more finely divided matter. Water may be passed any number of times through bibulous paper, it will continue laden with fine matter. Water passed through the charcoal filter of Lipscomb's, or through the filters of the Silicated Carbon Company, has its grosser matter removed, but it is thick with fine matter. Nine- tenths of the light scattered by these suspended particles is perfectly polarised in a direction at right angles to the beam, and this release of the particles from the ordinary law of polarisation is a demonstration of their smallness. I should say by far the greater number of the particles 164 FRAGMENTS OF SCIENCE. concerned in this scattering are wholly beyond the range of the microscope, and no ordinary filter can intercept such particles. It is next to impossible, by artificial means, to produce a pure water. Mr. Hartley, for exam- ple, some time ago distilled water while it was surrounded by hydrogen, but the water was not free from floating matter. It is so hard to be clean in the midst of dirt. In water from the Lake of Geneva, which has remained long without being stirred, we have an approach to the pure liquid. I have a bottle of it here, which 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 pre- cautions are required to obtain this degree of purity. The following apparatus has been 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 clean glass bulb. In the funnel is placed a block of the most transparent ice, and over the funnel a glass receiver. This is first exhausted and refilled several times with air, 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 surface, and also to 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. The water obtained in this way is the purest hitherto obtained. Still I should hesitate to call it absolutely pure. When ON DUST AND DISEASE. 165 condensed light is sent through it, the track of the beam is not invisible, but of the most exquisitely delicate blue. This blue is purer than that of the sky, so that the matter which produces 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 suspended in the water. But when we remember that this perfection of blue is approached gradually through stages of less perfect blue ; and when we con- sider that a blue in all respects 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 purification. The evidence, I think, points distinctly to the conclusion that could we push the process of purification still farther, even this last delicate trace of blue would disappear. 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 exceedingly clean recipient and a natural filter, and from which we can obtain water extremely free from me- chanical impurities. 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 to the entry of the rain into the chalk, where any organic impurity which the water may carry in is soon oxidised 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 rainfall, instead of washing the surface and collecting in streams, sinks into the fissured chalk and IG6 FRAGMENTS OF SCIENCE. percolates through it. When this formation is suitably tapped, we obtain water of exceeding briskness and purity. A large glass globe, filled with the water of a well near Tring shows itself to be 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. 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, not only that its purity is unrivalled, but that its quantity is practically inexhaustible. Data sufficient to test this are now, I believe, in existence ; the number of wells sunk in the chalk being so considerable, and the quantity of water which they yield so well known. But this water, so admirable as regards freedom from mechanical impurity, labours under the disadvantage of being rendered very hard by the carbonate of lime which it holds in solution. The chalk-water in the neighbour- hood of Watford contains about seventeen grains of car- bonate 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 ; and it furs our boilers, because the lime held in solution is precipi- tated by boiling. If the water be used cold, its hardness must be neutralised at the expense of soap, before it will give a lather. These are serious objections to the use of chalk-water in London. But they are successfully met by the demonstration that such water can be softened inexpen- sively, 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 ON LUST AND DISEASE. 167 softened for the supply of the city of Canterbury ; and at the Chiltern Hills it is softened for the supply of Tring and Aylesbury. Caterham also enjoys the luxury. I have visited all these places, and made myself acquainted with the works. At Canterbury there are three reservoirs covered in and protected, by a concrete roof and layers of pebbles, both from the summers heat and the winter's cold. Each reservoir can hold 120,000 gallons of water. Adjacent to these reservoirs are others containing pure slaked lime the so-called ' cream of lime.' These being filled with water, the lime and water are thoroughly mixed by air forced in by an engine through apertures in the bottom of the reservoir. The water soon dissolves all the lime it is capable of dis- solving. The mechanically suspended lime is then allowed to subside to the bottom, leaving a perfectly transparent lime-water behind. The softening process is this : Into one of the empty reservoirs is introduced a certain quantity of the clear lime-water, and after this about nine times the quantity of the chalk- water. The transparency imme- diately disappears the mixture of the two clear liquids becoming thickly turbid, through the precipitation of carbonate of lime. The precipitate is permitted to sub- side. It is crystalline and heavy, and in about twelve hours a layer of pure white carbonate of lime is formed at the bottom of the reservoir, with a water of extra- ordinary beauty and purity overhead. A few days ago I pitched some halfpence into a reservoir sixteen feet deep at the Chiltern Hills. This depth hardly dimmed the coin. Had I cast in a pin, it could 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 168 FKAGMENTS OF SCIENCE. hottest summer it is cool, its temperature being twenty degrees above the freezing point ; and it does not freeze in winter if conveyed in proper pipes. The reservoirs are covered ; a leaf cannot blow into them, and no surface contamination can reach the water. It passes direct from the main into the house tap ; no cisterns are employed, and the supply is always fresh and pure. This is the kind of water which is supplied to the fortunate people of Tring, Caterham, and Canterbury. The foregoing article, as far as it relates to the theory which ascribes epidemic disease to the development of low parasitic life within the human life, was embodied in a discourse delivered before the Eoyal Institution in January 1870. In June 1871, after a brief reference to the polarisation of light by cloudy matter, I ventured to recur to the subject in these terms: What is the practical uses of these curiosities ? If we exclude the interest attached to the observation of new facts, and the enhancement of that interest through the knowledge that facts often become the exponents of laws, these curiosities are- in themselves worth little. They will not enable us to add to our stock of food, or drink, or clothe?, or jewel- lery. But though thus shorn of all usefulness in them- selves, they may, by carrying thought into places which it would not otherwise have entered, become the antecedents of practical consequences. In looking, for example, at our illuminated dust, we may ask ourselves what it is. How does it act, not upon a beam of light, but upon our own organisations ? The question then assumes a prac- tical character. We find on examination that this dust is organic matter in part living, in part dead. There are among it particles of ground straw, torn rags, smoke, the pollen of flowers, the spores of fungi, and the germs of other things. But what have they to do with the OX DUST AM) DISEASE. 16 animal economy ? Let me give you an illustration to which my attention has been lately drawn by Mr. George Henry Lewes, who writes to me thus : * I wish to direct your attention to the experiments of Von Eecklingshausen should you happen not to know them. They are striking confirmations of what you say of dust and disease. Last spring, when I was at his laboratory in Wiirzburg, I examined with him blood that had been three weeks, a month, and five weeks, out of the body, preserved in little porcelain cups under glass shades. This blood was living and growing. Not only were the Amoeba-like movements of the white corpuscles present, but there were abundant evidences of the growth and development of the corpuscles. I also saw a frog's heart still pulsating which had been removed from the body (I forget how many days, but certainly more than a week). There were other examples of the same persistent vitality, or absence of putrefaction. Von Kecklings- hausen did not attribute this to the absence of germs germs were not mentioned by him ; but when I asked him how he represented the thing to himself, he said the whole mystery of his operation consisted in keeping the blood free from dirt. The instruments employed were raised to a red heat just before use ; the thread was silver thread and was similarly treated ; and the porcelain cups, though not kept free from air, were kept free from currents. He said he often had failures, and these he attributed to particles of dust having escaped his precautions.' Professor Lister, who has founded upon the removal or destruction of this ' dirt' great and numerous improve- ments in surgery, tells us the effect of its introduction into the blood of wounds. He informs us what would happen with the extracted blood should the dust get at it. The blood would putrefy and become fetid ; and when you examine more closely what putrefaction means, you find 170 FRAGMENTS OF SCIENCE. the putrefying substance swarming with organic life, the germs of which have been derived from the air. "We are now assuredly in the midst of practical matters ; and with your permission I will refer once more to a ques- tion which has recently occupied a good deal of public attention. As regards the lowest forms of life, the world is divided, and has for a long time been divided, into two parties, the one affirming that we have only to submit absolutely dead matter to certain physical conditions, to envolve from it living things ; the other (without wishing to set bounds to the power of matter) affirming that, in our day, life has never been found to rise independently of pre-existing life. I belong to the party which claims life as a derivative of life. The question has two factors the evidence, and the mind that judges of the evidence ; and it may be purely a mental set or bias on my part that causes me throughout this long discussion, to see, on the one side, dubious facts and defective logic, and on the other side firm reasoning and a knowledge of what rigid experimental enquiry demands. But, judged of practically, what, again, has the question of Spontaneous Generation to do with us ? Let us see. There are numerous diseases of men and animals that are demonstrably the products of parasitic life, and such diseases may take the most terrible epidemic forms, as in the case of the silkworms of France in our day. Now it is in the highest degree important to know whether the parasites in question are sponta- neously developed, or are wafted from without to those afflicted with the disease. The means of prevention, if not of cure, would be widely different in the two cases. But this is not all. Besides these universally admitted cases, there is the broad theory, now broached and daily growing in strength and clearness daily, indeed, gaining more and more of assent from the most successful workers and profound thinkers of the medical profession itself ON DUST AND DISEASE. 17J the theory, namely, that contagious disease, generally, is of this parasitic character. Had I any cause to regret having introduced this theory to your notice more than a year ago, that regret should now be expressed. I would certainly renounce in your presence whatever leaning towards the germ theory my words might then have betrayed. But since the time referred to I have heard or read nothing which shakes my conviction of the truth of the theory. Let me briefly state the grounds on which its supporters rely. From their respective viruses you may plant typhoid fever, scarlatina, or small-pox. What is the crop that arises from this husbandry? As surely as a thistle rises from a thistle seed, as surely as the fig comes from the fig, the grape from the grape, the thorn from the thorn, so surely does the typhoid virus increase and multiply into typhoid fever, the scarlatina virus into scarlatina, the small-pox virus into small-pox. What is the conclusion that suggests itself here? It is this: That the thing which we vaguely call a virus is to all intents and purposes a seed : that, excluding the notion of vitality, in the whole range of chemical science you cannot point to an action which illustrates this perfect parallelism with the pheno- mena of life this demonstrated power of self-multiplica- tion and reproduction. The germ theory alone accounts for the phenomena. In cases of epidemic disease, it is net on bad air or foul drains that the attention of the physician of the future will primarily be fixed, but upon disease germs, which no bad air or foul drains can create, but Avhich may be pushed by foul air into virulent energy of reproduction. You may think I am treading on dangerous ground, that I am putting forth views that may interfere with salutary prac- tice. No such thing. If you wish to learn the impotence of medical practice in dealing with contagious diseases, you have only to refer to a recent Harveian oration by 10 172 FEAGMENTS OF SCIENCE. Dr. Gull. Such diseases defy the physician. They must run their course, and the utmost that can be done for them is careful nursing. And this, though I do not specially insist upon it, would favour the idea of their vital origin. For if the seeds of contagious disease be themselves living things, it may be difficult to destroy either them or their progeny, without involving their living habitat in the same destruction. It has been said, and it is sure to be repeated, that I am quitting my own metier, in speaking of these things. Not so. I am dealing with a question on which minds accustomed to weigh the value of experimental evidence are alone competent to decide, and regarding which, in its present condition, minds so trained are as capable of form- ing an opinion as regarding the phenomena of magnetism or radiant heat. 'The germ theory of disease,' it has been said, 'appertains to the biologist and the physician.' Granted. But where is the biologist or physician, whose researches, in connection with this subject, could for one instant be compared to those of the chemist Pasteur ? It is not the philosophic members of the medical profession who are dull to the reception of truth not originated with- in the pale of the profession itself. I cannot better con- clude this portion of my story than by reading to you an extract from a letter addressed to me some time ago by Dr. William Budd, of Clifton, to whose insight and energy the town of Bristol owes so much in the way of sanitary improvement. ' As to the germ theory itself,' writes Dr. Budd, * that is a matter on which I have long since made up my mind. From the day when I first began to think of these subjects, I had never had a doubt that the specific cause of conta- gious fevers must be living organisms. ' It is impossible, in fact, to make any statement bear- ing upon the essence or distinctive characters of these ON DUST AND DISEASE. 173 fevers, without using terms which are of all others the most distinctive of life. Take up the writings of the most violent opponent of the germ theory, and, ten to one, you will find them full of such terms as " propagation," "self-propagation," " reproduction," " self-multiplication," and so on. Try as he may if he has anything to say of those diseases which is characteristic of them he cannot evade the use of these terms, or the exact equivalents to them. While perfectly applicable to living things, these terms express qualities which are not only inapplicable to common chemical agents, but, as far as I can see, actually inconceivable of them.' Cotton-wool Respirator. Once, then, established within the body, this evil form of life, if you will allow me to call it so, must run its course. Medicine as yet is powerless to arrest its progress, and the great point to be aimed at is to prevent its access to the body. It was with this thought in my mind that I ventured to recommend, more than a year ago, the use of cotton-wool respirators in infectious places. I would here repeat my belief in their efficacy if properly con- structed. But I do not wish to prejudice the use of these respirators, by connecting them indissolubly with the germ theory. There are too many trades in England where life is shortened and rendered miserable by the introduction of matters into the lungs which might be kept out of them. Dr. Grreenhow has shown the stony grit deposited in the lungs of stonecutters. The black lungs of colliers is an- other case in point. In fact, a hundred obvious cases might be cited, and others that are not obvious might be added to them. We should not, for example, think that printing implied labours where the use of cotton-wool "espirators might come into play ; but the fact is that the 174 FRAGMENTS OP SCIENCE. dust arising from the sorting of the type is very destructive of health. I went some time ago into a manufactory in one of our large towns, where iron vessels are enamelled by coating them with a mineral powder, and subjecting them to a heat sufficient to fuse the powder. The organisation of the establishment was excellent, and one thing only was needed to make it faultless. In a large room a number of women were engaged covering the vessels. The air was laden with the fine dust, and their faces appeared as white and bloodless as the powder with which they worked. By the use of cotton-wool respirators these women might be caused to breathe air as free from suspended matter as that of the open street. Over a year ago a Lancashire seedsman wrote to me, stating that during the seed season his men suffered horribly from irritation and fever, so that many of them left his service. He asked for help, and I gave him my advice. At the conclusion of the sea- son, this year, he wrote to inform me that he had folded a little cotton-wool in muslin, and tied it in front of the mouth ; and that with this simple defence he had passed through the season in comfort, and without a single com- plaint from his men. Against the use of such a respirator the obvious objec- tion arises, that it becomes wet and heated by the breath. While I was casting about for a remedy for this a friend for- warded to me from Newcastle a form of respirator invented by Mr. Carrick, a hotel-keeper at Glasgow, which, by a slight modification, may be caused to meet the case perfectly. The respirator, with its back in part removed, is shown in fig. 4. Under the partition of wire-gauze q r, is a space intended by Mr. Carrick for medicated substances,' and which may be filled with cotton-wool The mouth is placed against the aperture 0, which fits closely round the lips, and the filtered air enters the mouth through a light valve V, which is lifted by the act ON DUST AND DISEASE. 175 of inhalation. During exhalation this valve closes ; the breath escapes by a second valve, v, into the open air. Fio. 4. The wool is thus kept dry and cool ; the air in passing through it being filtered of everything it holds in suspension. Fireman's Respirator. We have thus been led by our first unpractical experi- ments into a thicket of practical considerations. But another step is possible. Admiring, as I do, the bravery of our firemen, and hearing that smoke was a more serious enemy than flame itself, I thought of devising a fireman's respirator. Our fire-escapes are each in charge of a single man, and it would be of obvious importance to place it in the power of each of those men to penetrate through the densest smoke, into the recesses of a house, and there to rescue those who would otherwise be suffocated or burnt. Cotton-wool, which so effectually arrested dust, was first tried; but, though found soothing in certain gentle kinds of smoke, it was no match for the pungent fumes of a resinous fire, which evolves a most abominable smoke. For the purpose of catching the 176 FRAGMENTS OF SCIENCE. atmospheric germs, M. Pouchet spread a film of glycerine on a plate of glass, urged air against the film, and ex- amined the dust which stuck to it. The moistening of the cotton-wool with this substance was a decided improve- ment ; still the respirator only enabled us to remain in dense smoke for three or four minutes, after which the irritation became unendurable. Eeflection suggested that, in combustion so imperfect as the production of dense smoke implies, there must be numerous hydro- carbons produced, which, being in a state of vapour, would be very imperfectly arrested by the cotton-wool. These, in all probability, were the cause of the residual irritation ; and if these could be removed, a practically perfect res- pirator might possibly be obtained. I state the reasoning exactly as it occurred to my mind. Its result will be anticipated by many present. All bodies possess the power of condensing, in a greater or less degree, gases and vapours upon their surfaces, and when the condensing body is very porous, or in a fine state of division, the force of condensation may produce very remarkable effects. Thus, a clean piece of platinum-foil placed in a mixture of oxygen and hydrogen so squeezes the gases together as to cause them to combine ; and if the experiment be made with care, the heat of combination may raise the platinum to bright redness. The prompt- ness of this action is greatly augmented by reducing the platinum to a state of fine division. A pellet of spongy platinum,' for instance, plunged into a mixture of oxygen and hydrogen, causes the gases to explode instantly. In virtue of its extreme porosity, a similar power is possessed by charcoal. It is not strong enough to cause the oxygen and hydrogen to combine like the spongy platinum, but it so squeezes the more condensable vapours, and q,cts with such condensing power upon the oxygen of the air, as to bring both within the combining distance, thus enabling ON DUST AND DISEASE. 177 the oxygen to attack and destroy the vapours in the pores of the charcoal. In this way, effluvia of all kinds may be virtually burnt up ; and this is the principle of the excel- lent charcoal respirators invented by Dr. Stenhouse. Armed with one of these, you may go into the foulest- smelling places without having your nose offended. But, while powerful to arrest vapours, the charcoal res- pirator is ineffectual as regards smoke. The smoke-parti- cles get freely through the respirator. With a number of such respirators, tested downstairs, from half a minute to a minute was the limit of endurance. This might be exceeded by Faraday's simple method of emptying the lungs completely, and then filling them before going into a smoky atmosphere. In fact, each solid smoke particle is itself a bit of charcoal, and carries on it, and in it, its little load of irritating vapour. It is this, far more than the particles of carbon themselves, that produces the irri- tation. Hence two causes of offence are to be removed : the carbon particles which convey the irritant by adhesion and condensation, and the free vapour which accompanies the particles. The moistened cotton-wool I knew would arrest the first ; fragments of charcoal I hoped would stop the second. In the first fireman's respirator, Mr. Carrick's arrangement of two valves, the one for inhalation, the other for exhalation, are preserved. But the portion of it which holds the filtering and absorbent substances, is prolonged to a depth of four or five inches (see fig. 5). Under the partition of wire-gauze q r at the bottom of the space which fronts the mouth is placed a layer of cotton-wool, c, moistened with glycerine ; then a thin layer of dry wool, cf ; then a layer of charcoal fragments ; and finally a second thin layer of dry cotton-wool. The succession of the layers may be changed without prejudice to the action. A wire-gauze cover, shown in plan below 3g. 5, keeps the substances from falling out of the respi- 178 FRAGMENTS OF SCIENCE. rator. A layer of caustic lime has been added for the absorption of carbonic acid; but in the densest smoke FIG. 5. fete. that we have hitherto employed, it has not been found ne- cessary, nor is it shown in the figure. In a flaming build- ON DUST AND DISEASE. 179 ing, indeed, the mixture of air with the smoke never permits the carbonic acid to become so dense as to be irrespirable; but in a place where the gas is present in undue quantity, the fragments of lime would materially mitigate its action. In a small cellar-like chamber with a stone flooring and stone walls, the first experiments were made. We placed there furnaces containing resinous pine-wood, lighted the wood, and, placing over it a lid which pre- vented too brisk a circulation of the air, generated dense volumes of smoke. With our eyes protected by suitable glasses, my assistant and I have remained for half an hour and more in smoke so dense and pungent that a single inhalation, through the undefended mouth, would be perfectly unendurable. We might have prolonged our stay for hours. Having thus far perfected the instru- ment, I wrote to the chief officer of the Metropolitan Fire Brigade, asking him whether such a respirator would be of use to him. His reply was prompt ; it would be most valuable. He had, however, made himself acquainted with every contrivance of the kind in this and other countries, and had found none of them of any practical use. He offered to come and test it here, or to place a room at my disposal in the City. At my request he came here, accompanied by three of his men. Our small room was filled with smoke to their entire satisfaction. The three men went successively into it, and remained there as long as Captain Shaw wished them. On coming out they said that they had not suffered the slightest in- convenience ; that they could have remained all day in the smoke. Captain Shaw then tested the respirator with the same result, and he afterwards took great interest in the perfecting of the instrument. Various ameliorations and improvements have recently 180 FRAGMENTS OF SCIENCE. been introduced into the smoke respirator. The hood of Captain Shaw has been improved upon by the simple and less expensive mouthpiece of Mr. Sinclair ; and this, in its turn, has been simplified and improved by my assistant Mr. John Cottrell. The respirator is now in considerable demand, and it has already done good practical service. Care is, however, necessary in moistening the wool with glycerine. It must be carefully teazed, so that the indi- vidual fibres may be moistened, and clots must be avoided. I cannot recommend the layers of moistened flannel which, in some cases, have been used instead of cotton- wool : nothing equals the wool, when carefully treated. An experiment made last year brought out very con- spicuously the necessity of careful packing, and the enor- mous comparative power of resisting smoke irritation possessed by our firemen, and the able officer who com- mands them. Having heard from Captain Shaw that, in some recent very trying experiments, he had obtained the best effects from dry cotton-wool, and thinking that I could not have been mistaken in my first results, which proved the dry so much inferior to the moistened wool and its associated charcoal, I proposed to Captain Shaw to bring the matter to a test at his workshops in the City. He was good enough to accept my proposal, and thither I went on May 7, 1874. The smoke was generated in a confined space from wet straw, and it was certainly very dia- bolical. At this season of the year I am usually somewhat shorn of vigour, and therefore not in the best condition for severe experiments ; still I wished to test the matter in my own person. With a respirator which had been in use some days previously, and which was not carefully packed, I followed a fireman into the smoke, he being provided with a dry-wool respirator. I was compelled to quit the place in about three minutes, while the fireman remained there for six or seven minutes. ON DUST AND DISEASE. 181 I then tried his respirator upon myself, and found that with it I could not remain more than a minute in the smoke ; in fact the first inhalation provoked coughing. Thinking that Captain Shaw himself might have lungp more like mine than those of his fireman, I proposed that we should try the respirators together ; but he informed me that his lungs were very strong. He was, however, good enough to accede to my request. Before entering the den a second time I repacked my respirator, with due care, and entered the smoke in company with Captain Shaw. I could hear him breathe long slow inhalations ; his labour was certainly greater than mine, and after the !apse of seven minutes I heard him cough. In seven and a half minutes he had to quit the place, thus proving that his lungs were able to endure the irritation seven times as long as mine could bear it. I continued in the smoke, with hardly any discomfort, for sixteen minutes, and cer- tainly could have remained in it much longer. The advantage arising from the glycerine was thus placed beyond question. During this time I was in a condition to render very material assistance to a person in danger of suffocation. Helmholtz on llay Fever. In my lecture on Dust and Disease in 1870, I referred to an experiment made by Helmholtz upon himself which strikingly connected hay fever with animalcular life. About a year ago I received from Professor Binz of Bonn a short, but important paper, embracing Helmholtz's account of his observation, to which Professor Binz has added some remarks of his own. The paper, being mainly intended for English medical men, was published in English, and though here and there its style might be amended, I think it better to publish it unaltered here. 182 FRAGMENTS OF SCIENCE. From what I have observed (says Professor Binz) of re- cent English publications on the subject of hay fever, I am led to suppose that English authorities are inaccurately ac- quainted with the discovery of Professor Helmholtz, as far back as 1868, of the existence of uncommon low organisms in the nasal secretions in this complaint, and of the possibility of arresting their action by the local employ- ment of quinine. I therefore purpose to republish the letter in which he originally announced these facts to my- self, and to add some further observations on this topic. The letter is as follows : ! 4 1 have suffered, as well as I can remember, since the year 1847, from the peculiar catarrh called by the English " hay fever," the speciality of which consists in its attack- ing its victims regularly in the hay season (myself between May 20 and the end of June), that it ceases in the cooler weather, but on the other hand quickly reaches a great intensity if the patients expose themselves to heat and sunshine. An extraordinarily violent sneezing then sets in, and a strongly corrosive thin discharge, with which much epithelium is thrown off. This increases, after a few hours, to a painful inflammation of the mucous mem- brane and of the outside of the nose, and excites fever with severe headache and great depression, if the patient cannot withdraw himself from the heat and the sunshine. In a cool room, however, these symptoms vanish as quickly as they come on, and there then only remains for a few days a lessened discharge and soreness, as if caused by the loss of epithelium. I remark, by the way, that in all my other years I had very little tendency to catarrh or catching cold, while the hay fever has never failed during the twenty-one years of which I have spoken, and has never attacked me earlier or later in the year than the ' Of. VirchoVs ' Archiv.' vol. xlvi. p. 100. ON DUST AND DISEASE. 183 times named. The condition is extremely troublesome, and increases, if one is obliged to be much exposed to the sun, to an excessively severe malady. 1 The curious dependence of the disease on the season of the year suggested to me the thought that organisms might be the origin of the mischief. In examining the secretion I regularly found, in the last five years, certain vibrio-like bodies in it, which at other times I could not observe in my nasal secretion. . . . They are very small, and can only be recognised with the immersion-lens of a very good Hartnack's microscope. It is characteristic of the common isolated single joints that they contain four nuclei in a row, of which two pairs are more closely united. The length of the joints is 0'004 millimetre. Upon the warm objective-stage they move with moderate activity, partly in mere vibration, partly shooting back- wards and forwards in the direction of their long axis ; in lower temperatures they are very inactive. Occasionally one finds them arranged in rows upon each other, or in branching series. Observed some days in the moist chamber, they vegetated again, and appeared somewhat larger and more conspicuous than immediately after their excretion. It is to be noticed that only that kind of secretion contains them which is expelled by violent sneezings ; that which drops slowly does not contain any. They stick tenaciously enough in the lower cavities and recesses of the nose. ' When I saw your first notice respecting the poisonous action of quinine upon infusoria, I determined at once to make an experiment with that substance, thinking that these vibrionic bodies, even if they did not cause the whole illness, still could render it much more unpleasant through their movements and the decompositions caused by them. For that reason I made a neutral solution of sulphate of quinine, which did not contain much of the 184 FRAGMENTS OF SCIENCE. salt (1'800), but still was effective enough, and caused moderate irritation on the mucous membrane of the nose. I then lay flat on my back, keeping my head very low, and poured with a pipette abou four cubic centimetres into both nostrils. Then I turned my head about in order to let the liquid flow in all directions. 1 The desired effect was obtained immediately, and re- mained for some hours ; I could expose myself to the sun without fits of sneezing and the other disagreeable symp- toms coming on. It was sufficient to repeat the treatment three times a day, even under the most unfavourable cir- cumstances, in order to keep myself quite free. 1 There were then no such vibrios in the secretion. If I only go out in the evening, it suffices to inject the quinine once a day, just before going. After continuing this treatment for some days the symptoms disappear completely, but if I leave off they return till towards the end of June. 'My first experiments with quinine date from the summer of 1867 ; this year (1868) I began at once as soon as the first traces of the illness appeared, and I have thus been able to stop its development completely. ' I have hesitated as yet in publishing the matter, be- cause I have found no other patient 2 on whom I could try the experiment. There is, it seems to me, no doubt, con- sidering the extraordinary regularity in the recurrence and course of the illness, that quinine had here a most quick and decided effect. And this again makes my hypothesis very probable, that the vibrios, even if being no specific form but a very frequent one, are at least the cause of the rapid increase of the symptoms in warm air, as heat excites them to lively action.' 1 There is no foundation for the objection that syringing the nose could not cure the asthma which accompanies hay fover ; for this asthma is only the reflex effect arising from the irritation of the nose. 13. 3 Helmholtz, now Professor of Physics at the University of Berlin, is, although M.D., no medical practitioner.' B. ON DUST AND DISEASE. 185 I should be very glad if the above lines would induce medical men in England the haunt of hay fever to test the observation of Helmholtz. To most patients the ap- plication with the pipette may be too difficult or impos- sible ; I have therefore already suggested the use of Weber's very simple but effective nose-douche. Also it will be advisable to apply the solution of quinine tepid. It can, further, not be repeated often enough that quinine is frequently adulterated, especially with cinchonia, the action of which is much less to be depended upon. Dr. Frickhofer, of Schwalbach, has communicated to me a second case in which hay fever was cured by local application of quinine. 1 Professor Busch, of Bonn, autho- rises me to say that he succeeded in two cases of ' catarrhus aestivus ' by the same method : a third patient was obliged to abstain from the use of quinine, as it produced an un- bearable irritation of the sensible nerves of the nose. In the autumn of 1872 Helmholtz told me that his fever was quite cured, and that in the meantime two other patients had, by his advice, tried this method, and with the same success. 1 Cf. Virchow's 'Archiv.' (1870), vol. li. p. 176. 188 FRAGMENTS OF SCIENCE. VI. VOYAGE TO ALGERIA TO OBSERVE THE ECLIPSE. 1870. THE opening of the Eclipse Expedition was not pro- pitious. Portsmouth, on December 5, 1870, was swathed by a fog, which was intensified by smoke, and traversed by. a drizzle of fine rain. At six P.M. I was on board the ' Urgent.' On Tuesday morning the weather was too thick to permit of the ship's being swung and her compasses calibrated. The Admiral of the port, a man of very noble presence, came on board. Under his stimulus the energy which the weather had damped appeared to become more active, and soon after his de- parture we steamed down to Spithead. Here the fog had so far lightened as to enable the officers to swing the ship. At three P.M. on Tuesday, December 6, we got away, gliding successively past Whitecliff Bay, Bembridge, Sandown, Shanklin, Ventnor, and St. Catherine's Light- house. On "Wednesday morning we sighted the Isle of Ushant, on the French side of the Channel. The northern end of the island has been fretted by the waves into de- tached tower-like masses of rock of very remarkable ap- pearance. In the Channel the sea was green, and opposite Ushant it was a brighter green. On Wednesday evening we committed ourselves to the Bay of Biscay. The roll of the Atlantic was full, but not violent. There had been scarcely a gleam of sunshine throughout the day, but the VOYAGE TO ALGERIA. 187 cloud-forms were fine, and their apparent solidity impres- sive. On Thursday morning the green of the sea was displaced by a deep indigo blue. The whole of Thursday we steamed across the bay. We had little blue sky, but the clouds were again grand and varied cirrus, stratus, cumulus, and nimbus, we had them all. Dusky hair-like trails were sometimes dropped from the distant clouds to the sea. These were falling showers, and they some- times occupied the whole horizon, while we steamed across the rainless circle which was thus surrounded. Sometimes we plunged into the rain, and once or twice, by slightly changing our course, avoided a heavy shower. From time to time perfect rainbows spanned the heavens from side to side. At times a bow would appear in fragments, showing the keystone of the arch midway in air, and its two buttresses on the horizon. In all cases the light of the bow could be quenched by a Nicol's prism, with its long diagonal tangent to the arc. Sometimes gleaming patches of the firmament were seen amid the clouds. When viewed in the proper direc- tion, the gleam could be quenched by a Nicol's prism, a dark aperture being thus opened into stellar space. At sunset on Thursday the denser clouds were fiercely fringed, while through the lighter ones seemed to issue the glow of a conflagration. On Friday morning we sighted Cape Finisterre the extreme end of the arc which sweeps from Ushant round the Bay of Biscay. Calm spaces of blue, in which floated quietly scraps of cumuli, were behind us, but in front of us was a horizon of portentous darkness. It continued thus threatening throughout the day. Towards evening the wind strengthened to a gale, and at dinner it was difficult to preserve the plates and dishes from destruction. Our thinned company hinted that the rolling had other consequences. It was very wild when we went to bed. I slumbered and slept but after 188 FRAGMENTS OF SCIENCE. some time was rendered anxiously conscious that my body had become a kind of projectile, with the ship's side for a target. I gripped the edge of my berth to save myself from being thrown out. Outside, I could hear somebody say that he had been thrown from his berth, and sent spinning to the other side of the saloon. The screw laboured violently amid the lurching ; it incessantly quitted the water, and, twirling in the air, rattled against its bearings, and caused the ship to shudder from stem to stern. At times the waves struck us, not with the soft impact which might be expected from a liquid, but with the sudden solid shock of battering-rams. * No man knows the force of water,' said one of the officers, ' until he has experienced a storm at sea.' These blows followed each other at quicker intervals, the screw rattling after each of them, until, finally, the delivery of a heavier stroke than ordinary seemed to reduce the saloon to chaos. Furniture crashed, glasses rang, and alarmed enquiries immediately followed. Amid the noises I heard one note of forced laughter; it sounded very ghastly. Men tramped through the saloon, and busy voices were heard aft, as if something there had gone wrong. I rose, and not without difficulty got into my clothes. In the after-cabin, under the superintendence of the able and energetic navigating lieutenant, Mr. Brown, a group of blue-jackets were working at the tiller-ropes. These had become loose, and the helm refused to answer the wheel. High moral lessons might be gained on shipboard, by observing what steadfast adherence to an object can accomplish, and what large effects are heaped up by the addition of infinitesimals. The tiller-rope, as the blue- jackets strained in concert, seemed hardly to move ; still it did move a little, until finally, by timing the pull to the lurching of the ship, the mastery of the rudder was obtained. I had previously gone on deck. Eound the VOYAGE TO ALGERIA. 189 saloon-door were a few members of the eclipse party, who seemed in no mood for scientific observation. Nor did I ; but I wished to see the storm. I climbed the steps to the poop, exchanged a word with Captain Toynbee, the only member of the party to be seen on the poop, and by his direction made towards a cleat not far from the wheel. 1 Bound it I coiled my arms. With the exception of the men at the wheel, who stood as silent as corpse?, I was alone. I had seen grandeur elsewhere, but this was a new form of grandeur to me. The 'Urgent' is long and narrow, and during our expedition she lacked the steady- ing influence of sufficient ballast. She was for a time practically rudderless, and lay in the trough of the sea. I could see the long ridges, with some hundreds of feet between their crests, rolling upon the ship perfectly parallel to her sides. As they approached, they so grew upon the eye as to render the expression < mountains high ' intelligible. At all events, there was no mistaking their mechanical might, as they took the ship upon their shoulders, and swung her like a pendulum. The deck sloped sometimes at an angle which I estimated at over forty-five degrees ; wanting my previous Alpine practice, I should have felt less confidence in my grip of the cleat. Here and there the long rollers were tossed by interference into heaps of greater height. The wind caught their crests, and scattered them over the sea, the whole surface of which was seething white. The aspect of the clouds was a fit accompaniment to the fury of the ocean. The inoon was almost full at times concealed, at times re- vealed, as the scud flew wildly over it. These things ap- pealed to the eye, while the ear was filled by the groaning of the screw and the whistle and boom of the storm. 1 The cleat is a T-shaped mass of metal employed for the fastening oi ropes. 190 FRAGMENTS OP SCIENCE. Nor was the outward agitation the cnly object of interest to me. I was at once subject and object to myself, and watched with intense interest the workings of my own mind. The ' Urgent ' is an elderly ship. She had been built, I was told, by a contracting firm for some foreign Government, and had been diverted from her first purpose when converted into a troop-ship. She had been for some time out of work, and I had heard that one of her boilers, at least, needed repair. Our scanty but excellent crew, moreover, did not belong to the ' Urgent,' but had been gathered from other ships. Our three lieutenants were also volunteers. All this passed swiftly through my mind as the steamer shook under the blows of the waves, and I thought that probably no one on board could say how much of this thumping and straining the i Urgent ' would be able to bear. This uncertainty caused me to look steadily at the worst, and I tried to strengthen myself in the face of it. But at length the helm laid hold of the water, and the ship was got gradually round to face the waves. The rolling diminished, a certain amount of pitching taking its place. Our speed had fallen from eleven knots to two. I went again to bed. After a space of calm, when we seemed crossing the vortex of a storm, heavy tossing re- commenced. I was afraid to allow myself to fall asleep, as my berth was high, and to be pitched out of it might be attended with bruises, if not with fractures. From Friday at noon to Saturday at noon we accomplished sixty- six miles, or an average of less than three miles an hour. I overheard the sailors talking about this storm. The ' Urgent,' according to those that knew her, had never previously experienced anything like it. 1 1 There is, it will be seen, a fair agreement between these impressions and those so vigorously described by a scientific correspondent of the 'Times.' VOYAGE TO ALGERIA. 191 All through Saturday the wind, though somewhat sobered, blew dead against us. The atmospheric effects were exceedingly fine. The cumuli resembled mountains in shape, and their peaked summits shone as white as Alpine snows. At one place this resemblance was greatly strengthened by a vast ai'ea of cloud, uniformly.illuminated, and lying like a n&ve below the peaks. From it fell a kind of cloud-river strikingly like a glacier. The horizon at sunset was remarkable spaces of brilliant green between clouds of fiery red. Eainbows had been frequent through- out the day, and at night a perfectly continuous lunar bow spanned the heavens from side to side. Its colours were feeble ; but, contrasted with the black ground against which it rested, its luminousness was extraordinary. Sunday morning found us opposite to Lisbon, and at midnight we rounded Cape St. Vincent, where the lurch- ing seemed disposed to recommence. Through the kind- ness of Lieutenant Walton, a cot had been slung for me. It hung between a tiller-wheel and a flue, and at one A.M. I was roused by the banging of the cot against its boun- daries. But the wind was now behind us, and we went along at a speed of eleven knots. We felt certain of reaching Cadiz by three. But a new lighthouse came in sight, which some affirmed to be Cadiz Lighthouse, while the surrounding houses were declared to be Cadiz itself. Out of deference to these statements, the navigating lieutenant changed his course, and steered for the place. A pilot came on board, and he informed us that we were before the mouth of the Guadalquivir, and that the light- house was that of Cipiona. Cadiz was still some eighteen miles distant. We steered towards the city, hoping to get into the harbour before dark. But the pilot was snapped up by another vessel, and we did not get in. We beat about during the night, and in the morning found ourselves 192 FRAGMENTS OP SCIENCE. about fifteen miles from Cadiz. The sun rose behind the city, and we steered straight into the light. The three- towered cathedral stood in the midst-, round which swarmed apparently a multitude of chimney-stacks. A nearer approach showed the chimneys to be small turrets. A pilot was taken on board ; for there is a dangerous shoal in the harbour. The appearance of the town as the sun shone upon its white and lofty walls was singularly beautiful. We cast anchor ; some officials arrived and demanded a clean bill of health. We had none. They would have nothing to do with us ; so the yellow quaran- tine flag was hoisted, and we waited for permission to land the Cadiz party. After some hours' delay the English consul and vice-consul came on board, and with them a Spanish officer ablaze with gold lace and decorations. Under slight pressure the requisite permission had been granted. We landed our party, and in the afternoon weighed anchor. Thanks to the kindness of our excellent paymaster, I was here transferred to a roomier berth. Cadiz soon sank beneath the sea, and we sighted in succession Cape Trafalgar, Tarifa, and the revolving light of Ceuta. The water was very calm, and the moon rose in a quiet heaven. She swung with her convex surface downwards, the common boundary between light and shadow being almost horizontal. A pillar of reflected light shimmered up to us from the slightly rippled sea. 1 had previously noticed the phosphorescence of the water, but to-night it was stronger than usual, especially among the foam at the bows. A bucket let down into the sea brought up a number of the little sparkling organisms which caused the phosphorescence. I caught some of them in my hand. And here an appearance was observed which was new to most of iis, and strikingly beautiful to all. Standing at the bow and looking forwards, at a distance of forty or fifty yards from the ship, a number of luminous VOYAGE TO ALGERIA. 193 streamers were seen rushing towards us. On nearing the vessel they rapidly turned, like a comet round its peri- helion, placed themselves side by side, and, in parallel trails of light, kept up with the ship. One of them placed itself right in front of the bow as a pioneer. These comets of the sea were joined at intervals by others. Sometimes as many as six at a time would rush at us, bend with extra- ordinary rapidity round a sharp curve, and afterwards keep us company. I leaned over the bow, and scanned the streamers closely. The frontal portion of each of them revealed the outline of a porpoise. The rush of the creatures through the water had started the phospho- rescence, every spark of which was converted by the motion of the retina into a line of light. Each porpoise was thus wrapped in a luminous sheath. The phospho- rescence did not cease at the creature's tail, but was carried many porpoise-lengths behind it. To our right we had the African hills, illuminated by the moon. Gibraltar Eock at length became visible, but the town remained long hidden by a belt of haze. Through this at length the brighter lamps struggled. It was like the gradual resolution of a nebula into stars. As the in- tervening depth became gradually less, the mist vanished more and more, and finally all the lamps shone through it. They formed a bright foil to the sombre mass of rock above them. The sea was so calm and the scene so lovely that Mr. Huggins and myself stayed on deck till the ship was moored, near midnight. During our walking to and fro a striking enlargement of the disk of Jupiter was ob- served, whenever the heated air of the funnel came be- tween us and the planet. On passing away from the heated air, the flat dim disk would immediately shrink to a luminous point. The effect was one of visual persistence. The retinal image of the planet was set quivering in all azimuths by the streams of heated air, describing in 194 FRAGMENTS OP SCIENCE. quick succession minute lines of light, which summed themselves to a disk of sensible area. At six o'clock next morning, the gun at the Signal Station on the summit of the rock, boomed. At eight the band on board the i Trafalgar ' training-ship, which was in the harbour, struck up the national anthem ; and im- mediately afterwards a crowd of mite like cadets swarmed up the rigging. After the removal of the apparatus belonging to the Gibraltar party we went on shore. Winter was in England when we left, but here we had the warmth of summer. The vegetation was luxuriant palm- trees, cactuses, and aloes, all ablaze with scarlet flowers. A visit to the Governor was proposed, as an act of necessary courtesy, and I accompanied Admiral Ommaney and Mr. Huggins to ' the Convent,' or Government House. We sent in our cards, waited for a time, and were then con- ducted by an orderly to his Excellency. He is a fine old man, over six feet high, and of frank military bearing. He received us and conversed with us in a very genial manner. He took us to see his garden, his palms, his shaded promenades, and his orange-trees loaded with fruit, in all of which he took manifest delight. Evidently ' the hero of Kars' had fallen upon quarters after his own heart. He appeared full of good nature, and engaged us on the spot to dine with him that day. We sought the town-major for a pass to visit the lines. While awaiting his arrival I purchased a stock of white glass bottles, with a view to experiments on the colour of the sea. Mr. Huggins and myself, who wished to see the rock, were taken by Captain Salmond to the library, where a model of Gibraltar is kept, and where we had a capital preliminary lesson. At the library we met Colonel Maberly, a courteous and kindly man, who gave us good advice regarding our excursion. He sent an orderly with us to the entrance of the lines. The orderly handed us over to an intelligent Irishman, who was directed to show VOYAGE TO ALGERIA. 195 us everything that we desired to see, and to hide nothing from us. We took the ' upper line,' traversed the galleries hewn through the limestone; looked through the embra- sures, which opened like doors in the precipice, towards the hills of Spain ; reached St. George's Hall, and went still higher, emerging on the summit of one of the noblest cliffs I have ever seen. Beyond were the Spanish lines, marked by a line of white sentry-boxes ; nearer were the English lines, less conspicuously indicated ; and between both was neutral ground. Behind the Spanish lines rose the conical hill called the Queen of Spain's Chair. The general aspect of Spain from the rock is bold and rugged. Doubling back from the galleries, we struck upwards towards the crest, reached the Signal Station, where we indulged in ' shandy- gaff ' and bread and cheese. Thence to O'Hara's Tower, the highest point of the rock. It was built by a former Governor, who, forgetful of the laws of terrestrial curva- ture, thought he might look from the tower into the port of Cadiz. The tower is riven, and it may be climbed along the edges of the crack. We got to the top of it ; thence descended the curious Mediterranean Stair a zigzag, mostly of steps down a steeply falling slope, amid palmetto brush, aloes, and prickly pear. Passing over the Windmill Hill, we were joined at the ' Governor's Cottage ' by a car, and drove afterwards to the lighthouse at Europa Point. The tower was built, I believe, by Queen Adelaide, and it contains a fine dioptric apparatus of the first order, constructed by Messrs. Chance, of Birmingham. At the appointed hour we were at the Convent. During dinner the same genial traits which appeared in the morning were still more conspicuous. The freshness of the Governor's nature showed itself best when he spoke of his old antagonist in arms, MouraviefF. Chivalry in war is consistent with its stern prosecution, 11 196 FRAGMENTS OF SCIENCE. These two men were chivalrous, and after striking the last blow became friends for ever. Our kind and cour- teous reception at Gibraltar is a thing to be remembered with pleasure. On December 15 we committed ourselves to the Mediterranean. The views of Gibraltar with which we are most acquainted represent it as a huge ridge ; but its aspect, end on, both from the Spanish lines and from the other side, is truly noble. There is a sloping bank of sand at the back of the rock, which I was disposed to regard simply as the debris of the limestone. I wished to let myself down upon it, but had not the time. My friend Mr. Busk, however, assures me that it is silica, and that the same sand constitutes the adjacent neutral ground. There are theories afloat as to its having been blown from Sahara. The Mediterranean throughout this first day, and indeed throughout the entire voyage to Oran, was of a less deep blue than the Atlantic. Possibly the quantity of organisms may have modified the colour. At night the phosphorescence was startling, breaking with the suddenness of a snapped spring along the crests of the waves formed by the port and starboard bows. Its strength was not uniform. Having flashed brilliantly for a time, it would in part subside, and afterwards regain its vigour. Several large phosphorescent masses of weird appearance also floated past. On the morning of the 16th we sighted the fort and lighthouse of Marsa el Kibir, and beyond them the white walls of Oran lying in the bight of a bay, sheltered by dominant hills. The sun was shining brightly ; during our whole voyage we had not had so fine a day. The wisdom which had led us to choose Oran as our place of observation seemed demonstrated. A rather excitable pilot came on board, and he guided us in behind the Mole, which had suffered much damage "last year from an unex VOYAGE TO ALGERIA. 197 plained outburst of waves from the Mediterranean. Both port and bow anchors were cast in deep water. With three huge hawsers the ship's stern was made fast to three gun-pillars fixed in the Mole ; and here for a time the ' Urgent ' rested from her labours. M. Janssen, who had rendered his name celebrated by his observations of the eclipse in India in 1868, when he showed the solar flames to be eruptions of incandescent hydrogen, was already encamped in the open country about eight miles from Oran. On December 2 he had quitted Paris in a balloon, with a strong young tailor as his assistant, had descended near the mouth of the Loire, seen M. Gambetta, and received from him en- couragement and aid. On the day of our arrival his en- campment was visited by Mr. Huggins, and the kind and courteous Engineer of the Port drove me subsequently, in his own phaeton, to the place. It bore the best repute as regards freedom from haze and fog, and commanded an open outlook ; but it was inconvenient for us on account of its distance from the ship. The place next in repute was the railway station, between two and three miles distant from the Mole. It was inspected, but, being enclosed, was abandoned for an eminence in an adjacent garden, the property of Mr. Hinshelwood, a Scotchman who had settled some years previously as an Esparto merchant in Oran. 1 He, in the most liberal manner, placed his ground at the disposition of the party. Here the tents were pitched, on the Saturday, by Captain Salmond and his intelligent corps of sappers, the instruments being erected on the Monday under cover of the tents. Close to the railway station runs a new loopholed wall of defence, through which the highway passes into the open country. Standing on the highway, and looking 1 Esparto is a kind of grass now much used in the manufacture of paper, 198 FRAGMENTS OP SCIENCE. southwards, about twenty yards to the right is a small bastionet, intended to carry a gun or two. Its roof I thought would form an admirable basis for my telescope, while the view of the surrounding country was unimpeded in all directions. The authorities kindly allowed me the use of this bastionet. Two men, one a blue-jacket named Elliot, and the other a marine named Hill, were placed at my disposal by Lieutenant "Walton ; and, thus aided, on Monday morning I mounted my telescope. The instru- ment was new to me, and some hours of discipline were spent in mastering all the details of its manipulation. Mr. Huggins joined me, and we visited together the Arab quarter of Oran. The flat-roofed houses appeared very clean and white. The street was filled with loiterers, and the thresholds were occupied by picturesque groups. Some of the men were very fine. We saw many straight, manly fellows who must have been six feet four in height. They passed us with perfect indifference, evincing no anger, suspicion, or curiosity, hardly caring in fact to glance at us as we passed. In one instance only during my stay at Oran was I spoken to by an Arab. He was a tall, good-humoured fellow, who came smiling up to me, and muttered something about ' les Anglais.' The mixed population of Oran is picturesque in the highest degree : the Jews, rich and poor, varying in their costumes as their wealth varies ; the Arabs more picturesque still, and of all shades of complexion the negroes, the Spaniards, the French, all grouped together, and each preserving their own individuality, formed a picture intensely interesting to me. On Tuesday, the 20th, I was early at the bastionet. The night had been very squally. The sergeant of the sappers took charge of our key, and on Tuesday morning Elliot went for it. He brought back the intelligence that the tents had been blown down, and the instruments over- VOYAGE TO ALGERIA. 199 turned. Among these was a large and valuable equato- rial from the Royal Observatory, Greenwich. It seemed hardly possible that this instrument, with its wheels and verniers and delicate adjustments, could have escaped uninjured from such a fall. This, however, was the case; and during the day all the overturned instruments were restored to their places, and found to be in practical working order. This and the following day were devoted to incessant schooling. I had come out as a general star- gazer, and not with the intention of devoting myself to the observation of any particular phenomenon. I wished to see the whole the first contact, the advance of the moon, and the successive swallowing up of the solar spots, the breaking of the last line of crescent by the lunar mountains into Bailey's beads, the advance of the shadow through the air, the appearance of the corona and pro- minences at the moment of totality, the radiant streamers of the corona, the internal structure of the flames, a glance through a polariscope, a sweep round the landscape with the naked eye, the reappearance of the solar limb through Bailey's beads, and, finally, the retreat of the lunar shadow through the air. I was provided with a telescope of admirable definition, mounted, adjusted, packed, and most liberally placed at my disposal by Mr. Warren De La Rue. The telescope grasped the whole of the sun, and a considerable portion of the space surrounding it. But it would not take in the extreme limits of the corona. For this I had lashed on to the large telescope a light but powerful instrument, constructed by Ross, and lent to me by Mr. Huggins. I was also furnished with an excellent binocular by Mr. Dallmeyer. In fact, no man could have been more efficiently supported. It required a strict parcelling out of the interval of totality to embrace in it Ihe entire series of observations. These, while the sun remained 200 FRAGMENTS OF SCIENCE. visible, were to be made with an unsilvered diagonal eye- piece, which reflected but a small fraction of the sun'a light, this fraction being still further toned down by a dark glass. At the moment of totality the dark glass was to be removed, and a silver reflector pushed in, so as to get the maximum of light from the corona and pro- minences. The time of totality was distributed as fol- lows : 1. Observe approach of shadow through the air: totality. 2. Telescope . . . .30 seconds. 3. Finder .... 30 seconds. 4. Double image prism . .15 seconds. 5. Naked eye . . . .10 seconds. 6. Finder or binocular . . 20 seconds. 7. Telescope . . . .20 seconds. 8. Observe retreat of shadow. In our rehearsals Elliot stood beside me, watch in hand, and furnished with a lantern. He called out at the end of each interval, while I moved from telescope to finder, from finder to polariscope, from polariscope to naked eye, from naked eye back to finder, from finder to telescope, abandoning the instrument finally to observe the retreating shadow. All this we went over twenty times, while looking at the actual sun, and keeping him in the middle of the field. It was my object to render the repetition of the lesson so mechanical as to leave no room for flurry, forgetfulness, or excitement. Volition was not to be called upon, nor judgment exercised, but a well-beaten path of routine was to be followed. Had the opportunity occurred, I think the programme would have been strictly carried out. But the opportunity did not occur. For several days the weather had been ill-natured. "We had wind so strong as to render the hawsers at the stern of the ' Urgent ' as rigid as iron, and to destroy the navigating lieutenant's sleep. "We had clouds, a thunder-storm, and some rain. VOYAGE TO ALGERIA. 201 Still the hope was held out that the atmosphere would cleanse itself, and if it did we were promised air of extraordinary limpidity. Early on the 22nd we were all at our posts. Spaces of blue in the early morning gave us some encouragement, but all depended on the re- lation of these spaces to the surrounding clouds. Which of them were to grow as the day advanced ? The wind was high, and to secure the steadiness of my instrument I was forced to retreat behind a projection of the bastionet, place stones upon its stand, and, further, to avail myself of the shelter of a sail. My practised men fastened the Bail at the top, and loaded it with boulders at the bottom. It was tried severely, but it stood firm. The clouds and blue spaces fought for a time with varying success. The sun was hidden and revealed at in- tervals, hope oscillating in synchronism with the changes of the sky. At the moment of first contact a dense cloud intervened; but a minute or two afterwards the cloud had passed, and the encroachment of the black body of the moon was evident upon the solar disk. The moon marched onward, and I saw it at frequent intervals ; a large group of spots were approached and swallowed up. Subsequently I caught sight of the lunar limb as it cut through the middle of a large spot. The spot was not to be distinguished from the moon, but rose like a mountain above it. The clouds, when thin, could be seen as grey scud drifting across the black surface of the moon ; but they thickened more and more, and made the intervals of clearness scantier. During these moments I watched with an interest bordering upon fascination the march of the silver sickle of the sun across the field of the telescope. It was so sharp and so beautiful. No trace of the lunar limb could be observed beyond the sun's boundary. Here, indeed, it could only be relieved by the corona, which was utterly cut off by the dark glass. The blackness of 202 FRAGMENTS OF SCIENCE. the moon beyond the sun was, in fact, confounded with the blackness of space. Beside me was Elliot with the watch and lantern, while Lieutenant Archer, of the Koyal Engineers, had the kind- ness to take charge of my note-book. I mentioned, and he wrote rapidly down, such things as seemed worthy of remembrance. Thus my hands and mind were entirely free ; but it was all to no purpose. A patch of sunlight fell and rested upon the landscape some miles away. It was the only illuminated spot within view. But to the north-west there was still a space of blue which might reach us in time. Within seven minutes of totality an- other space towards the zenith became very dark. The atmosphere was, as it were, on the brink of a preci- pice ; it was charged with humidity, which required but a slight chill to bring it down in clouds. This was fur- nished by the withdrawal of the solar beams ; the clouds did come down, covering up the space of blue on which our hopes had so long rested. I abandoned the telescope and walked to and fro, like a caged leopard. As the moment of totality approached, the descent towards darkness was as obvious as a falling stone. I looked towards a distant ridge, where the darkness would first appear. At the moment a fan of beams, issuing from the hidden sun, was spread out over the southern heavens. These beams are bars of alternate light and shade, pro- duced in illuminated haze by the shadows of floating cloudlets of varying density. The beams are practically parallel, but by an effect of perspective they appear diver- gent, having the sun, in fact, for their point of converg- ence. The darkness took possession of the ridge referred to, lowered upon M. Janssen's observatory, passed over the southern heavens, blotting out the beams as if a sponge had been drawn across them. It then took possession of three spaces of blue sky in the south-eastern atmosphere. VOYAGE TO ALGERIA. 203 I again looked towards the ridge. A glimmer as of day- dawn was behind it, and immediately afterwards the fan of beams, which had been for more than two minutes absent, revived. The eclipse of 1870 had ended, and, as far as the corona was concerned, we had been defeated. Even in the heart of the eclipse the darkness was by no means perfect. Small print could be read. In fact, the clouds which rendered the day a dark one, by scat- tering light into the shadow, rendered the darkness less intense than it would have been had the atmosphere been without cloud. In the more open spaces I sought for stars, but could find none. There was a lull in the wind before and after totality, but during the totality the wind was strong. I waited for some time on the bastionet, hoping to get a glimpse of the moon on the opposite border of the sun, but in vain. The clouds continued, and some rain fell. The day brightened somewhat after- wards, and, having packed all up, in the sober twilight Mr. Crookes and myself climbed the heights above the fort of Vera Cruz. From this eminence we had a very noble view over the Mediterranean and the flanking African hills. The sunset was remarkable, and the whole outlook exceedingly fine. The able and well-instructed medical officer of the 1 Urgent,' Mr. Goodman, observed the following tempera- tures during the progress of the eclipse : Hour Dpg. Hour Deg. 11.45 . . 56 12.43 . . 51 11.55 . 55 1.5 .. 52 12.10 . . 54 1.27 . . 53 12.37 . . 53 1.44 . . 56 12.39 . . 52 2.10 . . 57 The minimum temperature occurred some minutes after totality, when a slight rain fell. The wind was so strong on the 23rd that Captain Henderson would not venture out. Guided by Mr. 204 FRAGMENTS OP SCIENCE. Goodman, I visited a cave scooped into a remarkable stratum of shell-breccia, and, thanks to my guide, secured specimens. Mr. Busk informs me that a precisely similar breccia is found at Gibraltar, at approximately the same level. During the afternoon, Admiral Ommaney and myself drove to the fort of Marsa el Kibir. The forti- fication is of ancient origin, the Moorish arches being still there in decay, but the fort is now very strong. About four or five hundred fine-looking dragoons were looking after their horses, waiting for a lull to enable them to embark for France. One of their officers was wander- ing in a very solitary fashion over the fort. We had some conversation with him. He had been at Sedan, had been taken prisoner, but had effected his escape. He shook his head when we spoke of the termination of the war, and predicted its long continuance. There was bitterness in his tone as he spoke of the charges of treason so lightly levelled against French commanders. The green waves raved round the promontory on which the fort stands, smiting the rocks, breaking into foam, and jump- ing, after impact, to a height of a hundred feet and more into the air. On our return our vehicle broke down through the loss of a wheel. The Admiral went on board, while I hung long over the agitated sea. The little horses of Oran well merit a passing word. Their speed and endurance, which are both heavily drawn upon by their drivers, are extraordinary. The wind sinking, we lifted anchor on the 24th. For some hours we went pleasantly along; but during the afternoon the storm revived, and it blew heavily against us all the night. When we came opposite the Bay of Almeria, on the 25th, the captain turned the ship, and steered into the bay, where, under the shadow of the Sierra Nevada, we passed Christmas night in peace. Next morning ' a rose of dawn ' rested on the snows of the ad- VOYAGE TO ALGERIA. 205 jacent mountains, while a purple haze was spread over the lower hills. I had no notion that Spain possessed so fine a range of mountains as the Sierra Nevada. The height is considerable, but the form also is such as to get the maximum of grandeur out of the height. We weighed anchor at eight A.M., passing for a time through shoal water, the bottom having been evidently stirred up. The adjacent land seemed eroded in a remarkable manner. It has its floods, which excavate these valleys and ravines, and leave those singular ridges behind. Towards evening I climbed the mainmast, and, standing on the cross-trees, saw the sun set amid a blaze of fiery clouds. The wind was strong and bitterly cold, and I was glad to slide to the deck along a rope, which stretched from the mast-head to the ship's side. That night we cast anchor beside the Mole of Gibraltar. On the morning of the 27th, in company with two friends, I drove to the Spanish lines, with the view of seeing the rock from that side. It is an exceedingly noble mass. The Peninsular and Oriental mail-boat had been sig- nalled and had come. Heavy duties called me homeward, and by transferring myself from the ' Urgent' to the mail- steamer I should gain three days. I hired a boat, rowed to the steamer, learned that she was to start at one, and returned with all speed to the ' Urgent.' Making known to Captain Henderson my wish to get away, he expressed doubts as to the possibility of reaching the mail-steamer in time. With his accustomed kindness, he, however, placed a boat at my disposal. Four hardy fellows and one of the ship's officers jumped into it ; my luggage, hastily thrown together, was tumbled in afterwards, and we were immediately on our way. We had nearly four miles to row in about twenty minutes ; but we hoped the mail- boat might not be punctual. For a time we watched her anxiously; there was no motion; we came nearer, but the 206 FRAGMENTS OP SCIENCE. flags were not yet hauled in. Tiie men put forth all their strength, animated by the exhortations of the officer at the helm. The roughness of the sea rendered their efforts to some extent nugatory : still we were rapidly ap- proaching the steamer. At length she moved, punctual almost to the minute, at first slowly, but soon with quick- ened pace. We turned to the left, so as to cut across her bows. Five minutes' pull would have brought us up to her. The officer waved his cap and I my hat. ' If they could only see us, they might back to us in a moment.' But they did not see us, or if they did, they paid no at- tention to us. I returned to the ' Urgent,' discomfited, but grateful to the fine fellows who had wrought so hard to carry out my wishes. Glad of the quiet, in the sober afternoon I took a walk towards Europa Point. The sky darkened and heavy squalls passed at intervals. Private theatricals were at the Convent, and the kind and courteous Governor had sent cards to the eclipse party. I failed in my duty in not going. St. Michael's Cave is said to rival, if it does not outrival, the Mammoth Cave of Kentucky. On the 28th Messrs. Crookes, Carpenter, and myself, guided by a military policeman who understood his work, explored the cavern. The mouth is about 1,100 feet above the sea. We zigzagged up to it, and first were led into an aperture in the rock, at some height aoove the true entrance of the cave. In this upper cavern we saw some tall and beauti- ful stalactite pillars. The water drips from the roof charged with bicar- bonate of lime. Exposed to the air, the carbonic acid partially escapes, and the simple carbonate of lime, which is hardly at all soluble in water, deposits itself as a solid, forming stalactites and stalagmites. Even the exposure of chalk or limestone water to the open air partially softens it A specimen of the Eedbourne wacer exposed VOYAGE TO ALGERIA. 207 by Messrs. Graham, Miller, and Hofmann, in a shallow basin, fell from eighteen degrees to nine degrees of hard- ness. The softening process of Clark is virtually a has- tening of the natural process. Here, however, instead of being permitted to evaporate, half the carbonic acid is appropriated by lime, the half thus taken up, as well as the remaining half, being precipitated. The solid pre- cipitate is permitted to sink, and the clear supernatant liquid is limpid soft water. We returned to the real mouth of St. Michael's Cave, which is entered by a wicket. The floor was somewhat muddy, and the roof and walls were wet. We were soon in the midst of a natural temple, where tall columns sprang complete from floor to roof, while incipient columns were growing to meet each other, upwards and downwards. The water which trickles from the stalactite, after having in part yielded up its carbonate of lime, falls upon the floor vertically underneath, and there builds the stalag- mite. Consequently, the pillars grow from above and below simultaneously, along the same vertical. It is easy to distinguish the stalagmitic from the stalactitic portion of the pillars. The former is always divided into short segments by protuberant rings, as if deposited periodi- cally, while the latter presents a uniform surface. In some cases the points of inverted cones of stalactite rested on the centres of pillars of stalagmite. The process of solidification and the architecture were alike beautiful. We followed our guide through various branches and arms of the cave, climbed and descended steps, halted at the edges of dark shafts and apertures, and squeezed our- selves through narrow passages. From time to time we halted, while Mr. Crookes illuminated with ignited mag- nesium wire, the roof, columns, dependent spears, and graceful drapery of the stalactites. Once, coming to a magnificent cluster of icicle-like spears, we helped our- 208 FRAGMENTS OF SCIENCE. selves to specimens. There was some difficulty in detach- ing the more delicate ones, their fragility was so great. A consciousness of vandalism, which smote me at the time, haunts me still ; for, though our requisitions were moderate, this beauty ought not to be at all invaded. Pendent from the roof, in their natural habitat, nothing can exceed their delicate beauty ; they live, as it were, surrounded by organic connections. In London they are curious, but not beautiful. Of gathered shells Emerson writes : I wiped away the weeds and foam, And brought my sea-born treasures home : But the poor, unsightly, noisome things Had left their beauty on the shore, With the sun, and the sand, and the wild uproa The promontory of Gibraltar is so burrowed with caverns that it has been called the Hill of Caves. They are apparently related to the geologic disturbances which the rock has undergone. The earliest of these is the tilting of the once horizontal strata. Suppose a force of torsion to act upon the promontory at its southern ex- tremity near Europa Point, and suppose the rock to be of a partially yielding character ; such a force would twist the strata into screw-surfaces, the greatest amount of twisting being endured near the point of application of the force. Such a twisting the rock appears to have suffered ; but instead of the twist fading gradually and uniformly off, in passing from south to north, the want of uniformity in the material has produced lines of dis- location where there are abrupt changes in the amount of twist. Thus, at the northern end of the rock the dip to the west is nineteen degrees ; in the Middle Hill it is thirty-eight degrees ; in the centre of the South Hill, or Sugar Loaf, it is fifty-seven degrees. At the southern extremity of the Sugar Loaf the strata are vertical, while VOYAGE TO ALGERIA. 209 farther to the south they actually turn over and dip to the east. The rock is thus divided into three sections, separated from each other by places of dislocation, where the strata are much wrenched and broken. These are called the Northern and Southern Quebrada, from the Spanish 4 Tierra Quebrada,' or broken ground. It is at these places that the inland caves of Gibraltar are almost exclusively found. Based on the observations of Dr. Falconer and himself, an excellent and most interesting account of these caves, and of the human remains and works of art which they contain, was communicated by Mr. Busk to the meeting of the Congress of Prehistoric Archaeology at Norwich, and afterwards printed in the * Transactions ' of the Congress. 1 Long subsequently to the operation of the twisting force just referred to, the promontory under- went various changes of level. There are sea-terraces and layers of shell-breccia along its flanks, and numerous caves which, unlike the inland ones, are the product of marine erosion. The Ape's Hill, on the African side of the strait, Mr. Busk informs me has undergone similar disturbances. 1 In the harbour of Gibraltar, on the morning of our departure, I resumed a series of observations on the colour of the sea. On the way out a number of specimens had been collected, with a view to subsequent exami- nation. But the bottles were claret bottles, of doubtful purity. At Gibraltar, therefore, I purchased fifteen white glass bottles, with ground glass stoppers, and at Cadiz, thanks to the friendly guidance of Mr. Cameron, I se- 1 In this essay Mr. Busk refers to the previous labours of Mr. Smith, of Jordan Hill, to whom we owe most of our knowledge of the geology of the rock. 2 No one can rise from the perusal of Mr. Busk's paper without a feeling of admiration for the principal discoverer and indefatigable ex- plorer of the Gibraltar caves, the late Captain Frederick Brome. 210 FRAGMENTS OF SCIENCE. cured a dozen more. These seven-and-twenty bottlea were filled with water, taken at different places between Oran and Spithead.' And here let me express my warmest acknowledgments to Captain Henderson, the commander of H.M.S. ' Urgent,' who aided me in my observations in every possible way. Indeed, my thanks are due to all the officers for their unfailing courtesy and help. The captain placed at my disposal his own coxswain, an intelligent fellow named Thorogood, who skilfully attached a cord to each bottle, weighted it with lead, cast it into the sea, and, after three successive rinsings, filled it under my own eyes. The contact of jugs, buckets, or other vessels was thus avoided ; and even the necessity of pouring out the water, afterwards, through the dirty London air. The mode of examination applied to these bottles has been already described. 1 The liquid is illuminated by a powerfully condensed beam, its condition being revealed through the light scattered by its suspended particles. ' Care is taken to defend the eye from the access of all other light, and, thus defended, it becomes an organ of inconceiv- able delicacy.' Were water of uniform density perfectly free from suspended matter, it would, in my opinion, scatter no light at all. The track of a luminous beam could not be seen in such water. But ' an amount of impurity so infinite- simal as to be scarcely expressible in numbers, and the individual particles of which are so small as wholly to elude the microscope, may, when examined by the method alluded to, produce not only sensible, but striking, effects upon the eye.' The results of the examination of nineteen bottles filled at various places between Gibraltar and Spithead, are here tabulated : 1 On Dust and Disease, pp. 156, 157. VOYAGE TO ALGERIA. 211 No. Locality Colour of Sea Appearance in Luminous Beam 1 2 4 5 Gibraltar Harbour . . . Two mil*-s from Gibraltar Off Cabreta Point . . . Off Cabreta Point . . . Off Tarif a Green . . Clearer green Bright green Black-indigo Undecided. . Thick with fine particles Thick with very fine particles Still thick, but less so Much less thick, very pure Thicker than No. 4 6 7 8 9 10 11 12 13 14 15 16 17 Beyond Tarifa .... Twelve miles from Cadiz Cadiz Harbour .... Fourteen miles from Cadiz Fourteen mileg from Cadiz Between Capes St. Mary and Vincent .... Off the Burlings .... Beyond the Barlings . . Off Cape Finisterre . . Bay of Biscay .... Bay of Biscay .... Off TJshant . Cobalt-blue Yellow-green Yellow-green Yellow-green Bright green Deep indigo Strong green Indigo . . Undecided . Black-indigo Indigo . . Much purer than No. 5 Very thick Exceedingly thick Thick, but less so Much less thick Very little matter, very pure Thick, with fine matter Very little matter, pure Less pure Very little matter, very pure Very fine matter. Iridescent 18 19 Off St. Catherine's . . . Spithead Yellow-green Green . . Exceedingly thick Exceedingly thick Here we have three specimens of water, described as green, a clearer green, and bright green, taken in Gibraltar Harbour, at a point two miles from the harbour, and off Cabreta Point. The home examination showed the first to be thick with suspended matter, the second less thick, and the third still less thick. Thus the green brightened as the suspended matter diminished in amount. Previous to the fourth observation our excellent navi- gating lieutenant, Mr. Brown, steered along the coast, thus avoiding the adverse current which sets in, through the Strait, from the Atlantic to the Mediterranean. He was at length forced to cross the boundary of the At- lantic current, which was defined with extraordinary sharpness. On the one side of it the water was a vivid green, on the other a deep blue. Standing at the bow of the ship, a bottle could be filled with blue water, while at the same moment a bottle cast from the stern could be filled with green water. Two bottles were se- cured, one on each side of this remarkable boundary. In the distance the Atlantic had the hue called ultra- marine ; but looked fairly down upon, it was of almost inky blackness black qualified by a trace of indigo. 212 FRAGMENTS OF SCIENCE. What change does the home examination here reveal ? In passing to indigo, the water becomes suddenly aug- mented in purity, the suspended matter becoming sud- denly less. Off Tarifa, the deep indigo disappears, and the sea is undecided in colour. Accompanying this change, we have a rise in the quantity of suspended matter. Beyond Tarifa, we change to cobalt-blue, the suspended matter falling at the same time in quantity. This water is distinctly purer than the green. We approach Cadiz, and at twelve miles from the city get into yellow-green water ; this the London examination shows to be thick with suspended matter. The same is true of Cadiz harbour, and also of a point fourteen miles from Cadiz in the home- ward direction. Here there is a sudden change from yellow-green to a bright emerald-green, and accompanying the change a sudden fall in the quantity of suspended matter. Between Cape St. Mary and Cape St. Vincent the water changes to the deepest indigo, a further dimi- nution of the suspended matter being the concomitant phenomenon. We now reach the remarkable group of rocks called the Burlings, and find the water between the shore and the rocks a strong green ; the home examination shows it to be thick with fine matter. Fifteen or twenty miles beyond the Burlings we come again into indigo water, from which the suspended matter has in great part dis- appeared. Off Cape Finisterre, about the place where the l Captain ' went down, the water becomes green, and the home examination pronounces it to be thicker. Then we enter the Bay of Biscay, where the indigo resumes its power, and where the home examination shows the greatly augmented purity of the water. A second specimen of water, taken from the Bay of Biscay, held in suspension fine particles of a peculiar kind ; the size of them was such as to render the water richly iridescent. It showed VOYAGE TO ALGERIA. 213 itself green, blue, or salmon-coloured, according to the direction of the line of vision. Finally, we come to our last two bottles, the one taken opposite St. Catherine's lighthouse, in the Isle of Wight, the other at Spithead. The sea at both these places was green, and both speci- mens, as might be expected, were pronounced by the home examination to be thick with suspended matter. Two distinct series of observations are here referred to the one consisting of direct observations of the colour of the sea, conducted during the voyage from Gibraltar to Portsmouth ; the other carried out in the laboratory of the Eoyal Institution. And here it is to be noted that in the home examination I never knew what water was placed in my hands. The labels, with the names of the localities written upon them, had been tied up, all information regarding the source of the water being thus held back. The bottles were simply numbered, and not till all of them had been examined, and described, were the labels opened, and the locality and sea-colour corresponding to the various specimens ascertained. The home observations, therefore, must have been perfectly unbiassed, and they clearly establish the association of the green colour with fine suspended matter, and of the ultramarine colour, and more especially of the black-indigo hue of the Atlantic, with the comparative absence of such matter. So much for mere observation ; but what is the cause of the dark hue of the deep ocean ? l A preliminary remark or two will clear our way towards an explanation. Colour resides'in white light, appearing generally when any consti- 1 A note, written to me on October 22, by my friend Canon Kingsley, contains the following reference to this point: 'I have never seen the Lake of Geneva, but I thought of the brilliant dazzling dark blue of the mid-Atlantic under the sunlight, and its black-blue under cloud, both so solid that one might leap off the sponson on to it without fear; this was to me the most wonderful thing which I saw on my voyages to and from the West Indies." 214 FRAGMENTS OP SCIENCE. tuent of the white light is withdrawn. The hue of a purple liquid, for example, is immediately accounted for by its ac- tion on a spectrum. It cuts out the yellow and green, and allows the red and blue to pass through. The blending of these two colours produces the purple. But while such a liquid attacks with special energy the yellow and green, it enfeebles the whole spectrum. By increasing the thick- ness of the stratum we may absorb the whole of the light. The colour of a blue liquid is similarly accounted for. It first extinguishes the red ; then, as the thickness aug- ments, it attacks the orange, yellow, and green in suc- cession ; the blue alone finally remaining. But even it might be extinguished by a sufficient depth of liquid. And now we are prepared for a brief, but tolerably complete, statement of that action of sea-water upon light, to which it owes its darkness. The spectrum em- braces three classes of rays the thermal, the visual, and the chemical. These divisions overlap each other ; the thermal rays are in part visual, the visual rays in part chemical, and vice versa. The vast body of thermal rays lie beyond the red, being invisible. These rays are attacked with exceeding energy by water. They are absorbed close to the surface of the sea, and are the great agents in evaporation. At the same time the whole spectrum suffers enfeeblement ; water attacks all its rays, but with different degrees of energy. Of the visual rays, the red are first extinguished. As the solar beam plunges deeper into the sea, orange follows red, yellow follows orange, green follows yellow, and the various shades of blue, where the water is deep enough, follow green. Absolute extinction of the solar beam would be the consequence if the water were deep and uniform. If it contained no suspended matter, such water would be as black as ink. A re- flected glimmer of ordinary light would reach us from VOYAGE TO ALGERIA. 215 its surface, as it would from the surface of actual ink ; but no light, hence no colour, would reach us from the body of the water. In very clear and deep sea-water this condition is approximately fulfilled, and hence the extraordinary dark- ness of such water. The indigo, already referred to, is, I believe, to be ascribed in part to the suspended matter, which is never absent, even in the purest natural water ; and in part to the slight reflection of the light from the limiting surfaces of strata of different densi- ties. A modicum of light is thus thrown back to the eye, before the depth necessary to absolute extinction has been attained. An effect precisely similar occurs under the moraines of glaciers. The ice here is ex- ceptionally compact, and, owing to the absence of the internal scattering common in bubbled ice, the light plunges into the mass, where it is extinguished, the per- fectly clear ice presenting an appearance of pitchy black- ness. 1 The green colour of the sea has now to be accounted for ; and here, again, let us fall back upon the sure basis of experiment. A strong white dinner-plate had a lead weight securely fastened to it. Fifty or sixty yards of strong hempen line were attached to the plate. My assistant, Thorogood, occupied a boat, fastened as usual to the davits of the ' Urgent,' while I occupied a second boat nearer the stern of the ship. He cast the plate as a mariner heaves the lead, and by the time it had reached me it had sunk a considerable depth in the water. In all cases the hue of this plate was green ; even when the sea was of the darkest indigo, the green was vivid and pronounced. I could notice the gradual deepening of the 1 I learn from a correspondent that certain Welsh tarns, which are reputed bottomless, have this inky hue. 216 FKAGMENTS OF SCIENCE. colour as the plate sank, but at its greatest depth, even in indigo water, the colour was still a blue-green. 1 Other observations confirmed this one. The ' Urgent ' is a screw" steamer, and right over the blades of the screw was an orifice called the screw-well, through which one could look from the poop down upon the screw. The surface-glimmer, which so pesters the eye, was here in a great measure removed. Midway down, a plank crossed the screw-well from side to side ; on this I placed my- self and observed the action of the screw underneath. The eye was rendered sensitive by the moderation of the light; and, to remove still further all disturbing causes, Lieutenant Walton had a sail and tarpaulin thrown over the mouth of the well. Underneath this I perched my- self and watched the screw. In an indigo sea the play of colour was indescribably beautiful, and the contrast be- tween the water, which had the screw-blades, and that which had the bottom of the ocean, as a background, was extraordinary. The one was of the most brilliant green, the other of the deepest ultramarine. The surface of the water above the screw-blade was always ruffled. Liquid lenses were thus formed, by which the coloured light was withdrawn from some places and concentrated upon others, the water flashing with metallic lustre. The screw- blades in this case played the part of the dinner-plate in the former case, and there were other instances of a similar kind. The white bellies of porpoises showed the green hue, varying in intensity as the creatures swung to and fro be- tween the surface and the deeper water. Foam, at a certain depth below the surface, is also green. In a rough sea the light which has penetrated the summit of a wave sometimes reaches the eye, a beautiful green cap being thus placed upon the wave, even in indigo water. 1 In no case, of course, is the green pure, but a mixture of green and blue. VOYAGE TO ALGEEIA. 217 But how is this colour to be connected with the sus- pended particles ? Take the dinner-plate which showed so brilliant a green when thrown into indigo water. Suppose it to diminish in size, until it reaches an almost microscopic magnitude. It would still behave substan- tially as the larger plate, sending to the eye its modicum of green light. If the plate, instead of being a large coherent mass, were ground to a powder sufficiently fine, and in this condition diffused through the clear sea- water, it would send green light to the eye. In fact, the suspended particles which the home examination reveals, act in all essential particulars like the plate, or like the screw-blades, or like the foam, or like the bellies of the porpoises. Thus I think the greenness of the sea is physi- cally connected with the matter which it holds in sus- pension. We reached Portsmouth on January 5, 1871. Then ended a voyage which, though its main object was not realised, has left behind it pleasant memories, both of the aspects of nature and the kindliness of men. 218 FRAGMENTS OP SCIENCE. VII. NIAGARA. 1872. IT is one of the disadvantages of reading books about natural scenery that they fill the mind with pictures, often exaggerated, often distorted, often blurred, and, even when well drawn, injurious to the freshness of first impres- sions. Such has been the fate of most of us with regard to the Falls of Niagara. There was little accuracy in the estimates of the first observers of the cataract. Startled by an exhibition of power so novel and so grand, emotion leaped beyond the control of the judgment, and gave currency to notions which have often led to disappoint- ment. A record of a voyage in 1535 by a French mariner named Jacques Cartier, contains, it is said, the first printed allusion to Niagara. In 1603 the first map of the district was constructed by a Frenchman named Champlain. In 1648 the Jesuit Eageneau, in a letter to his superior at Paris, mentions Niagara as 'a cataract of frightful height.' l In the winter of 1678 and 1679 the cataract was visited by Father Hennepin, and described in a book dedicated 'to the King of Great Britain.' He gives a drawing of the waterfall, which shows that serious changes have taken place since his time. He describes it as ' a great and pro- 1 From an interesting little book presented to me at Brooklyn by its author, Mr. Holly, some of these data are derived : Hennepin, Kalm, Bake well, Lyell, Hall, and others I have myself consulted. NIAGARA. 219 digious cadence of water, to which the universe does not offer a parallel.' The height of the fall, according to Hennepin, was more than 600 feet. ' The waters,' he says, ' which fall from this great precipice do foam and boil in the most astonishing manner, making a noise more terrible than that of thunder. When the wind blows to the south its frightful roaring may be heard for more than fifteen leagues.' The Baron la Hontan, who visited Niagara in 1687, makes the height 800 feet. In 1721 Charlevois, in a letter to Madame de Maintenon, after referring to the exaggerations of his predecessors, thus states the result of his own observations : i For my part, after examining it on all sides, I am inclined to think that we cannot allow it less than 140 or 150 feet,' a remarkably close estimate. At that time, viz. a hundred and fifty years ago, it had the shape of a horseshoe, and reasons will subsequently be given for holding that this has been always the form of the cataract, from its origin to its present site. As regards the noise of the fall, Charlevois declares the accounts of his predecessors, which, I may say, are repeated to the present hour, to be altogether extravagant, lie is perfectly right. The thunders of Niagara are formi- dable enough to those who really seek them at the base of the Horseshoe Fall ; but on the banks of the river, and particularly above the fall, its silence, rather than its noise, is surprising. This arises, in part, from the lack of resonance ; the surrounding country being flat, and there- fore furnishing no echoing surfaces to reinforce the shock of the water. The resonance from the surrounding rocks causes the Swiss Eeuss at the Devil's Bridge, when full, to thunder more loudly than the Niagara. On Friday, November 1, 1872, just before reaching the village of Niagara Falls, I caught, from the railway train, my first glimpse of the smoke of the cataract. Immediately after my arrival I went with a friend to 220 FKAGMSNTS OF SCIENCE. the northern end of the American Fall. It may be that my mood at the time toned down the impression pro- duced by the first aspect of this grand cascade ; but I felt nothing like disappointment, knowing, from old ex- perience, that time and close acquaintanceship, the gradual interweaving of mind and nature, must powerfully influence my final estimate of the scene. After dinner we crossed to Goat Island, and, turning to the right, reached the southern end of the American Fall. The river is here studded with small islands. Crossing a wooden bridge to Luna Island, and clasping a tree which grows near its edge, I looked long at the cataract, which here shoots down the precipice like an avalanche of foam. It grew in power and beauty. The channel spanned by the wooden bridge was deep, and the river there doubled over the edge of the precipice, like the swell of a muscle, unbroken. The ledge here overhangs, the water being poured out far beyond the base of the precipice. A space, called the Cave of the Winds, is thus enclosed between the wall of rock and the falling water. Goat Island ends in a sheer dry precipice, which connects the American and Horseshoe Falls. Midway between both is a wooden hut, the residence of the guide to the Cave of the Winds, and from the hut a winding staircase, called Biddle's Stair, descends to the base of the precipice. On the evening of my arrival I went down this stair, and wandered along the bottom of the cliff. One well-known factor in the formation and retreat of the cataract was immediately observed. A thick layer of limestone formed the upper portion of the cliff. This rested upon a bed of soft shale, which extended round the base of the cataract. The violent recoil of the watei against this yielding substance crumbles it away, under- mining the ledge above, which, unsupported, eventually breaks off, and produces the observed recession. NIAGARA. 221 At the southern jxtremity of the Horseshoe is a pro- montory, formed by the doubling back of the gorge, ex- cavated by the cataract, and into which it plunges. On the promontory stands a stone building, called the Terrapin Tower, the door of which had been nailed up because of the decay of the staircase within it. Through the kind- ness of Mr. Townsend, the superintendent of Groat Island, the door was opened for me. From this tower, at all hours of the day, and at some hours of the night, I watched and listened to the Horseshoe Fall. The river here if, evidently much deeper than the American branch ; and instead of bursting into foam where it quits the ledge, it bends solidly over, and falls in a continuous layer of the most vivid green. The tint is not uniform ; long stripes of deeper hue alternating with bands of brighter colour. Close to the ledge over which the water rolls, foam is generated, the light falling upon which, and flashing back from it, is sifted in its passage to and fro, and changed from white to emerald-green. Heaps of superficial foam are also formed at intervals along the ledge, and are immediately drawn into long white striaB. 1 Lower down, the surface, shaken by the reaction from below, incessantly rustles into whiteness. The descent finally resolves itself into a rhythm, the water reaching the bottom of the fall in periodic gushes. Nor is the spray uniformly diffused through the air, but is wafted through t in successive veils of gauze-like texture. From all this it is evident that beauty is not absent from the Horseshoe Fall, but majesty is its chief attribute. The plunge of the water is not wild, but deliberate, vast, and fascinating. From the Terrapin Tower, the adjacent arm of the Horse- shoe is seen projected against the opposite. one, midway 1 The direction of the wind with reference to the course of a ship may be inferred with accuracy from the foam-streaks on the surface of the sea. 222 FRAGMENTS OF SCIENCE. down ; to the imagination, therefore, is left the picturing of the gulf into which the cataract plunges. The delight which natural scenery produces in some minds is difficult to explain, and the conduct which it prompts can hardly be fairly criticised by those who have never experienced it. It seems to me a deduction from the completeness of the celebrated Thomas Young, that he was unable to appreciate natural scenery. ' He had really,' says Dean Peacock, * no taste for life in the country ; he was one of those who thought that no one who was able to live in London would be content to live elsewhere.' Well, Dr. Young, like Dr. Johnson, had a right to his delights ; but I can understand a hesitation to accept them, high as they were, to the exclusion of That o'erflowing joy which Nature yields To her true lovers. To all who are of this mind, the strengthening of desire on my part to see and know Niagara Falls, as far as it is possible for them to be seen and known, will be intel- ligible. On the first evening of my visit, I met, at the head of Biddle's Stair, the gmde to the Cave of the Winds. He was in the prime of manhood large, well built, firm and pleasant in mouth and eye. My interest in the scene stirred up his, and made him communicative. Turning to a photograph, he described, by reference to it, a feat which he had accomplished some time previously, and which had brought him almost under the green water of the Horseshoe Fall. ' Can you lead me there to-morrow ?' I asked. He eyed me enquiringly, weighing, perhaps, the chances of a man of light build, and with grey in his whiskers, in such an undertaking. ' I wish,' I added, ' to see as much of the fall as can be seen, and where you lead I will endeavour to follow.' His scrutiny relaxed into a NJAGAEA. 223 smile, and he said, * Very well ; I shall be ready for you to-morrow.' On the morrow, accordingly, I came. In the hut at the head of Biddle's Stair I stripped wholly, and re- dressed according to instructions, drawing on two pairs of woollen pantaloons, three woollen jackets, two pairs of socks, and a pair of felt shoes. Even if wet, my guide assured me that the clothes would keep me from being chilled ; and he was right. A suit and hood of yellow oilcloth covered all. Most laudable precautions were taken by the young assistant who helped to dress me to keep the water out ; but his devices broke down immediately when severely tested. We descended the stair; the handle of a pitchfork doing, in my case, the duty of an alpenstock. At the bottom, the guide enquired whether we should go first to the Cave of the Winds, or to the Horseshoe, remarking that the latter would try us most. I decided on getting the roughest done first, and he turned to the left over the stones. They were sharp and trying. The base of the first portion of the cataract is covered with huge boulders, obviously the ruins of the limestone ledge above. The water does not distribute itself uniformly among these, but seeks for itself channels through which it pours torrentially. We passed some of these with wetted feet, but without diffi- culty. At length we came to the side of a more formidable current. My guide walked along its edge until he reached its least turbulent portion. Halting, he said, * This is our greatest difficulty ; if we can cross here, we shall get far towards the Horseshoe.' He waded in. It evidently required all his strength to steady him. The water rose above his loins, and it foamed still higher. He had to search for footing, amid unseen boulders, against which the torrent rose violently. He struggled and swayed, but he struggled successfully, 224 FRAGMENTS OF SCIENCE. and finally reached the shallower water at the other side. Stretching out his arm, he said to me, ' Now come on.' I looked down the torrent, as it rushed to the river below, which was seething with the tumult of the cataract. De Saussure recommended the inspection of Alpine dangers, with the view of making them familiar to the eye before they are encountered ; and it is a wholesome custom in places of difficulty to put the possibility of an accident clearly before the mind, and to decide beforehand what ought to be done should the accident occur. Thus wound up in the present instance, I entered the water. Even where it was not more than knee-deep, its power was manifest. As it rose around me, I sought to split the torrent by presenting a side to it ; but the insecurity of the footing enabled it to grasp my loins, twist me fairly round, and bring its impetus to bear upon my back. Further struggle was impossible ; and feeling my balance hopelessly gone, I turned, flung myself towards the bank just quitted, and was instantly, as expected, swept into shallower water. The oilcloth covering was a great iucumbrance ; it had been made for a much stouter man, and, standing up- right after my submersion, my legs occupied the centre of two bags of water. My guide exhorted me to try again. Prudence was at my elbow, whispering dissuasion ; but, taking everything into account, it appeared more im- moral to retreat than to proceed. Instructed by the first misadventure, I once more entered the stream. Had the alpenstock been of iron it might have helped me ; but, as it was, the tendency of the water to sweep it out of my hands rendered it worse than useless. I, however, clung to it by habit. Again the torrent rose, and again I wavered ; but, by keeping the left hip well against it, I remained upright, and at length grasped the hand of my leader at the other side. He laughed pleasantly. The NIAGARA. 228 first victory was gained, and he enjoyed it. 'No traveller,' he said, ' was ever here before.' Soon afterwards, by trusting to a piece of drift-wood which seemed firm, I was again taken off my feet, but was immediately caught by a protruding rock. We clambered over the boulders towards the thickest spray, which soon became so weighty as to cause us to stagger under its shock. For the most part nothing could be seen ; we were in the midst of bewildering tumult, lashed by the water, which sounded at times like the cracking of innumerable whips. Underneath this was the deep resonant roar of the cataract. I tried to shield my eyes with my hands, and look upwards ; but the de- fence was useless. The guide continued to move on, but at a certain place he halted, and desired me to take shelter in his lee, and observe the cataract. The spray did not come so much from the tipper ledge, as from the rebound of the shattered water when it struck the bottom. Hence the eyes could be protected from the blinding shock of the spray, while the line of vision to the upper ledges re- mained to some extent clear. On looking upwards over the guide's shoulder I could see the water bending over the ledge, while the Terrapin Tower loomed fitfully through the intermittent spray-gusts. We were right under the tower. A little farther on the cataract, after its first plunge, hit a protuberance some way down, and flew from it in a prodigious burst of spray ; through this we staggered. We rounded the promontory on which the Terrapin Tower stands, and moved, amid the wildest commotion, along the arm of the Horseshoe, until the boulders failed us, and the cataract fell into the profound gorge of the Niagara River. Here the guide sheltered me again, and desired me to look up ; I did so, and could see, as before, the green gleam of the mighty curve sweeping over the upper ledge, 228 FRAGMENTS OF SCIENCE. and the fitful plunge of the water, as the spray between ua and it alternately gathered and disappeared. An emi- nent friend of mine often speaks of the mistake of those physicians who regard man's ailments as purely chemical, to be met by chemical remedies only. He contends for the psychological element of cure. By agreeable emotions, he says, nervous currents are liberated which stimulate blood, brain, and viscera. The influence rained from ladies' eyes enables my friend to thrive on dishes which would kill him if eaten alone. A sanative effect of the same order I experienced amid the spray and thunder of Niagara. Quickened by the emotions there aroused, the blood sped exultingly through the arteries, abolishing introspection, clearing the heart of all bitterness, and enabling one to think with tolerance, if not with tender- ness, on the most relentless and unreasonable foe. Apart from its scientific value, and purely as a moral agent, the play was worth the candle. My companion knew no more of me than that I enjoyed the wildness ; but as I bent in the shelter of his large frame he said, ' I should like to see you attempting to describe all this.' He rightly thought it indescribable. The name of this gallant fellow was Thomas Conroy. We returned, clambering at intervals up and down, so as to catch glimpses of the most impressive portions of the cataract. We passed under ledges formed by tabular masses of limestone, and through some curious openings formed by the falling together of the summits of the rocks. At length we found ourselves beside our enemy of the morning. Conroy halted for a minute or two, scanning the torrent thoughtfully. I said that, as a guide, he ought to have a rope in such a place ; but he retorted that, as no traveller had ever thought of coming there, he did not see the necessity of keeping a rope. He waded in. The struggle to keep himself erect was evident NIAGARA. 227 enough ; he swayed, hut recovered himself again and again. At length hs slipped, gave way, did as I had done, threw himself towards the bank, and was swept into the shallows. Standing in the stream near its edge, he stretched his arm towards me. I retained the pitchfork- handle, for it had been useful among the boulders. By wading some way in, the staff could be made to reach him, and I proposed his seizing it. ' If you are sure,' he replied, * that, in case of giving way, you can maintain your grasp, then I will certainly hold you.' Eemarking that he might count on this, I waded in, and stretched the staff to my com- panion. It was firmly grasped by both of us. Thus helped, though its onset was strong, I moved safely across the tor- rent. All danger ended here. We afterwards roamed soci- ably among the torrents and boulders below the Cave of the Winds. The rocks were covered with organic slime, which could not have been walked over with bare feet, but the felt shoes effectually prevented slipping. We reached the cave and entered it, first by a wooden way carried over the boulders, and then along a narrow ledge, to the point eaten deepest into the shale. When the wind is from the south, the falling water, I am told, can be seen tranquilly from this spot ; but when we were there, a blinding hurricane of spray was whirled against us. On the evening of the same day, I went behind the water on the Canada side, which, after the experiences of the morning, struck me as an imposture. Still even this latter is exciting to some nerves. Its effects upon himself is thus vividly described by Mr. Bakewell, jun. : ' On turning a sharp angle of the rock, a sudden gust of wind met us, coming from the hollow between the fall and the rock, which drove the spray directly in our faces, with such force that in an instant we were wet through. When in the midst of this shower- bath the shock took away my breath : I turned back and 228 FEAGMENTS OF SCIENCE. scrambled over the loose stones to escape the conflict. The guide soon followed, and told me that I had passed the worst part. With that assurance I made a second attempt ; but so wild and disordered was my imagination that when I had reached half way I could bear it no longer.' * To complete my knowledge I desired to see the fall from the river below it, and long negotiations were ne- cessary to secure the means of doing so. The only boat fit for the undertaking had been laid up for the winter; but this difficulty, through the kind intervention of Mr. Townsend, was overcome. The main one was to secure oarsmen sufficiently strong and skilful to urge the boat where I wished it to be taken. The son of the owner of the boat, a finely-built young fellow, but only twenty, and therefore not sufficiently hardened, was willing to go ; and up the river, it was stated, there lived another man who would do anything with the boat which strength and daring could accomplish. He came. His figure and expression of face certainly indicated extraordinary firm- ness and power. On Tuesday, November 5, we started, each of us being clad in oilcloth. The elder oarsman at once assumed a tone of authority over his companion, and struck immediately in amid the breakers below the American Fall. He hugged the cross freshets instead of striking out into the smoother water. I asked him why he did so, and he replied that they were directed outivards, not downwards. The struggle, however, to prevent the bow of the boat from being turned by them, was often very severe. The spray was in general blinding, but at times it disappeared and yielded noble views of the fall. The edge of the cataract is crimped by indentations which 1 'Mag. of Nat Hist.,' 1830, pp. 121, 122. NIAGARA. 229 exalt its beauty. Here and there, a little below the highest ledge, a secondary one juts out ; the water strikes it and bursts from it in huge protuberant masses of foam and spray. We passed Goat Island, came to the Horse- shoe, and worked for a time along the base of it, the boulders over which Conroy and myself had scrambled a few days previously lying between us and the base. A rock was before us, concealed and revealed at intervals, as the waves passed over it. Our leader tried to get above this rock, first on the outside of it. The water, however, was here in violent motion. The men struggled fiercely, the older one ringing out an incessant peal of command and exhortation to the younger. As we were just clearing the rock, the bow came obliquely to the surge ; the boat was turned suddenly round and shot with astonishing rapidity down the river. The men returned to the charge, now trying to get up between the half-concealed rock and the boulders to the left. But the torrent set in strongly through this channel. The tugging was quick and violent, but we made little way. At length, seizing a rope, the principal oarsman made a desperate attempt to get upon one of the boulders, hoping to be able to drag the boat through the channel; but it bumped so violently against the rock, that the man flung himself back and relinquished the attempt. "We returned along the base of the American Fall, running in and out among the currents which rushed from it laterally into the river. Seen from below the American Fall is certainly exquisitely beautiful, but it is a mere frill of adornment to its nobler neighbour the Horseshoe. At times we took to the river, from the centre of which the Horseshoe Fall appeared especially magnificent. A streak of cloud across the neck of Mont Blanc can double its apparent height, so here the green summit of the cataract shining above the smoke of spray 230 FRAGMENTS OF SCIENCE. appeared lifted to an extraordinary elevation. Had Hennepin and La Hontan seen the fall from this position, their estimates of the height would have been perfectly excusable. From a point a little way below the American Fall, a ferry crosses the river, in summer, to the Canadian side. Below the ferry is a suspension bridge for carriages and foot-passengers, and a mile or two lower down is the railway suspension bridge. Between the ferry and the latter the river Niagara flows unruffled ; but at the sus- pension bridge the bed steepens and the river quickens its motion. Lower down the gorge narrows, and the rapidity and turbulence increase. At the place called the ' Whirlpool Rapids ' I estimated the width of the river at 300 feet, an estimate confirmed by the dwellers on the spot. When it is remembered that the drainage of nearly half a continent is compressed into this space, the impetuosity of the river's escape through this gorge may be imagined. Had it not been for Mr. Bierstadt, the distinguished photo- grapher of Niagara, I should have quitted the place without seeing these rapids ; for this, and for his agreeable company to the spot, I have to thank him. From the edge of the cliff above the rapids, we descended, a little I confess to a climber's disgust, in an ' elevator,' because the effects are best seen from the water level. Two kinds of motion are here obviously active, a motion of translation and a motion of undulation the race of the river through its gorge, and the great waves generated by its collision with, and rebound from, the obstacles in its way. In the middle of the river the rush and tossing are most violent ; at all events, the impetuous force of the individual waves is here most strikingly dis- played. Vast pyramidal heaps leap incessantly from the river, some of them with such energy as to jerk their sum- NIAGARA. 231 mits into the air, where they hang suspended as bundles of liquid spherules. The sun shone for a few minutes. At times the wind, coming up the river, searched and sifted the spray, carrying away the lighter drops, and leaving the heavier ones behind. Wafted in the proper direction, rainbows appeared and disappeared fitfully in the lighter mist. In other directions the common gleam of the sun- shine from the waves and their shattered crests was exqui- sitely beautiful. The complexity of the action was still further illustrated by the fact, that in some cases, as if by the exercise of a local explosive force, the drops were shot radially from a particular centre, forming around it a kind of halo. The first impression, and, indeed, the current explana- tion of these rapids is, that the central bed of the river is cumbered with large boulders, and that the jostling, tossing, and wild leaping of the water there, are due to its impact against these obstacles. I doubt this explanation. At all events, there is another sufficient reason to be taken into account. Boulders derived from the adjacent cliffs visibly cumber the sides of the river. Against these the water rises and sinks rhythmically but violently, large waves being thus produced. On the generation of each wave, there is an immediate compounding of the wave- motion with the river-motion. The ridges, which in still water would proceed in circular curves round the centre of disturbance, cross the river obliquely, and the result is that at the centre waves commingle, which have really been generated at the sides. In the first instance, we had a composition of wave-motion with river-motion; here we have the coalescence of waves with waves. Where crest and furrow cross each other, the motion is annulled ; where furrow and furrow cross, the river is ploughed to a greater depth ; and where crest and crest aid each other, we have that astonishing leap of the water which breaks the cohe- 232 FRAGMENTS OF SCIENCE. pion of the crests, and tosses them shattered into the air. From the water level the cause of the action is not so easily seen ; but from the summit of the cliff the lateral generation of the waves, and their propagation to the centre, are perfectly obvious. If this explanation be cor- rect, the phenomena observed at the Whirlpool Eapida form one of the grandest illustrations of the principle of interference. The Nile ' cataract,' Mr Huxley informs me, offers more moderate examples of the same action. At some distance below the Whirlpool Eapids we have the celebrated whirlpool itself. Here the river makes a sudden bend to the north-east, forming nearly a right angle with its previous direction. The water strikes the con- cave bank with great force, and scoops it incessantly away. A vast basin has been thus formed, in which the sweep of the river prolongs itself in gyratory currents. Bodies and trees which have come over the falls, are stated to cir- culate here for days without finding the outlet. From various points of the cliffs above, this is curiously hidden. The rush of the river into the whirlpool is obvious enough ; and though you imagine the outlet must be visible, if one existed, you cannot find it. Turning, however, round the bend of the precipice to the north-east, the outlet comes into view. The Niagara season was over ; the chatter of sight- seers had ceased, and the scene presented itself as one of holy seclusion and beauty. I went down to the river's edge, where the weird loneliness seemed to increase. The basin is enclosed, by high and almost precipitous bunks covered, at the time, with russet woods. A kind of mystery attaches itself to gyrating water, due perhaps to the fact that we are to some extent ignorant of the direc- tion of its force. It is said that at certain points of the whirlpool pine-trees are sucked down, to be ejected mys- teriously elsewhere. The water is of the brightest emerald- NIAGARA. 233 green. The gorge through which it escapes is narrow, and the motion of the river swift though silent. The surface is steeply inclined, but it is perfectly unbroken. There are no lateral waves, no ripples with their breaking bubbles to raise a murmur ; while the depth is here too great to allow the inequality of the bed to ruffle the sur- face. Nothing can be more beautiful than this sloping liquid mirror formed by the Niagara, in sliding from the whirlpool. The green colour is, I think, correctly accounted for in Fragment VI. In crossing the Atlantic I had frequent opportunities of testing the explanation there given. Looked properly down upon, there are portions of the ocean to which we should hardly ascribe a trace of blue ; at the most, a hint of indigo reaches the eye. The water, indeed, is practically black, and this is an indication both of its depth and its freedom from mechani- cally suspended matter. In small thicknesses water i . sensibly transparent to all kinds of light ; but, as the thickness increases, the rays of low refrangibility are first absorbed, and after them the other rays. Where, there- fore, the water is very deep and very pure, all the colours are absorbed, and such water ought to appear black, as no light is sent from its interior to the eye. The approxima- tion of the Atlantic Ocean to this condition is an indica- tion of its extreme purity. Throw a white pebble into such water ; as it sinks it becomes greener and greener, and, before it disappears, it reaches a vivid blue-green. Break such a pebble into fragments, each of these will behave like the unbroken mass ; grind the pebble to powder, every particle will yield its modicum of green ; and if the particles be so fine as to remain suspended in the water, the scattered light will be a uniform green. Hence the greenness of shoal water. You go to bed with the black Atlantic around you. 234 FRAGMENTS OF SCIENCE. You rise in the morning, find it a vivid green, and correctly infer that you are crossing the bank of New- foundland. Such water is found charged with fine matter in a state of mechanical suspension. The light from the bottom may sometimes corne into play, but it is not necessary. A storm can render the water muddy, by rendering the particles too numerous and gross. Such a case occurred towards the close of my visit to Niagara. There had been rain and storm in the upper-lake regions, and the quantity of suspended matter brought down quite extinguished the fascinating green of the Horseshoe. Nothing can be more superb than the green of the Atlantic waves, when the circumstances are favourable to the exhibition of the colour. As long as a wave remains unbroken no colour appears ; but when the foam just doubles over the crest, like an Alpine snow-cornice, under the cornice we often see a display of the most exquisite green. It is metallic in its brilliancy. But the foam is necessary to its production. The foam is first illuminated, and it scatters the light in all directions ; the light which passes through the higher portion of the wave alone reaches the eye, and gives to that portion its matchless colour. The folding of the wave, producing, as it does, a series of longitudinal protuberances and furrows which act like cylindrical lenses, introduces variations in the intensity of the light, and materially enhances its beauty. We have now to consider the genesis and proximate destiny of the Falls of Niagara. We may open our way to this subject by a few preliminary remarks upon erosion. Time and intensity are the main factors of geologic change, and they are in a certain sense con- vertible. A feeble force acting through long periods, and an intense force acting through short ones, may produce approximately the same results. To Dr. Hooker I have NIAGARA. 236 been indebted for some specimens of stones, the first examples of which were picked up by Mr. Hnckworth on the shores of I/yell's Bay, near Wellington, in New Zealand. They were described by Mr. Travers in the ' Transactions of the New Zealand Institute.' Unacquainted with their origin, you would certainly ascribe their forms to human workmanship. They resemble knives and spear-heads, being apparently chiselled off into facets, with as much attention to symmetry as if a tool, guided by human in- telligence, had passed over them. But no human instru- ment has been brought to bear upon these stones. They have been wrought into their present shape by the wind- blown sand of I/yell's Bay. Two winds are dominant here, and they in succession urged the sand against opposite sides of the stone ; every little particle of sand chipped away its infinitesimal bit of stone, and in the end sculptured these singular forms. 1 The Sphynx of Egypt is nearly covered up by the sand of the desert. The neck of the Sphynx is partly cut across, not, as I am assured by Mr. Huxley, by ordinary weathering, but by the eroding action of the fine sand 1 ' These stones, which have a strong resemblance to works of human art, occur in great abundance, and of various sizes, from half-an-inch to several inches in length. A large number were exhibited showing the various forms, which are those of wedges, knives, arrow-heads, &c., and all with sharp cutting edges. ' Mr. Travers explained that, notwithstanding their artificial appearance, these stones were formed by the cutting action of the wind-driven sand, as it passed to and fro over an exposed boulder-bank. He gave a minute account of the manner in which the varieties of form are produced, and re- ferred to the effect which the erosive action thus indicated would have on railway and other works executed on sandy tracts. Dr. Hector stated that although, as a group, the specimens on the table could not well be mistaken for artificial productions, still the forms are so peculiar, and the edges, in a few of them, so perfect, that if they were dis- covered associated with human works, there is no doubt that they would have been referred to the so-called " stone period." ' Extracted from the Minutes of the Wellington Philosophical Society, February 9, 1869. 236 FRAGMENTS OF SCIENCE. blown against it. In these cases Nature furnishes us with hints which may he taken advantage of in art ; and this action of sand has been recently turned to extraordinary account in the United States. When in Boston, I was taken by Mr. Josiah Quincey to see the action of the sand-blast. A kind of hopper containing fine silicious sand was connected with a reservoir of compressed air, the pressure being variable at pleasure. The hopper ended in a long slit, from which the sand was blown. A plate of glass was placed beneath this slit, and caused to pass slowly under it ; it came out perfectly depolished, with a bright opalescent glimmer, such as could only be produced by the most careful grinding. Every little particle of sand urged against the glass, having all its energy concentrated on the point of impact, formed there a little pit, the depolished surface consisting of innumerable hollows of this description. But this was not all. By protecting certain portions of the surface, and exposing others, figures and tracery of any required form could be etched upon the glass. The figures of open iron-work could be thus copied ; while wire-gauze placed over the glass produced a reticulated pattern. But it required no such resisting substance as iron to shelter the glass. The patterns of the finest lace could be thus reproduced ; the delicate filaments of the lace itself offering a sufficient protection. All these effects have been obtained with a simple model of the sand-blast devised by my assistant. A fraction of a minute suffices to etch upon glass a rich and beautiful lace pattern. Any yielding substance may be employed to protect the glass. By immediately diffusing the shock of the particle, such sub- stances practically destroy the local erosive power. The hand can bear, without inconvenience, a sand-shower which would pulverise glass. Etchings executed on glass with suitable kinds of ink are accurately worked out by tho NIAGARA. 237 sand-blast. In fact, within certain limits, the harder the surface, the greater is the concentration of the shock, and the more effectual is the erosion. It is not necessary that the sand should be the harder substance of the two ; corundum, for example, is much harder than quartz ; still, quartz-sand can not only depolish, but actually blow a hole through a plate of corundum. Nay, glass may be depolished by the impact of fine shot ; the grains in this case bruising the glass, before they have time to flatten, and turn their energy into heat. And here, in passing, we may tie together one or two apparently unrelated facts. Supposing you turn on, at the lower part of a house, a cock which is fed by a pipe from a cistern at the top of the house, the column of water, from the cistern downwards, is set in motion. By turning off the cock, this motion is stopped ; and when the turning off is very sudden, the pipe, if not strong, may be burst by the internal impact of the water. By distribu- ting the turning of the cock over half a second of time, the shock and danger of rupture may be entirely avoided. We have here an example of the concentration of energy in time. The sand-blast illustrates the concentration of energy in space. The action of flint and steel is an illustration of the same principle. The heat required to generate the spark is intense; and the mechanical action, being moderate, must, to produce fire, be in the highest degree con- centrated. This concentration is secured by the collision of hard substances. Calc-spar will not supply the place of flint, nor lead the place of steel, in the production of fire by collision. With the softer substances, the total heat produced may be greater than with the hard ones, but, to produce the spark, the heat must be intensely localised. But we can go far beyond the mere depolishing of glass ; indeed, I have already said that quartz-sand can wear a hole througb corundum. This leads me to express 238 FRAGMENTS OF SCIENCE my acknowledgments to General Tilghman, 1 who is the inventor of the sand-blast. To his spontaneous kindness I am indebted for some beautiful illustrations of his process. In one thick plate of glass a figure has been worked out to a depth of |ths of an inch. A second plate, |ths of an inch thick, is entirely perforated. Through a circular plate of marble, nearly half an inch thick, open work of the most intricate and elaborate description has been executed. It would probably take many days to per- form this work by any ordinary process ; with the sand- blast it was accomplished in an hour. So much for the strength of the blast ; its delicacy is illustrated by a beautiful example of line engraving, etched on glass by means of the blast. 2 This power of erosion, so strikingly displayed when sand is urged by air, renders us better able to conceive its action when urged by water. The erosive power of a river is vastly augmented by the solid matter carried along with it. Sand or pebbles, caught in a river vortex, can wear away the hardest rock ; ' potholes ' and deep cylin- drical shafts being thus produced. An extraordinary in- stance of this kind of erosion is to be seen in the Val Tournanche, above the village of this name. The gorge at Handeck has been thus cut out. Such waterfalls were once frequent in the valleys of Switzerland ; for hardly any valley is without one or more transverse barriers of resist- ing material, over which the river flowing through the 1 The absorbent power, if I may use the phrase, exerted by the indus- trial arts in the United States, is forcibly illustrated by the rapid transfer of men like Mr. Tilghman from the life of the soldier to that of the civilian. General McClellan, now a civil ngineer, whom I had the honour of fre- quently meeting in New York, is a most eminent example of the same kind. At the end of the war, indeed, a million and a half of men were thus drawn, in an astonishingly short time, from military to civil life. It is obvious that a nation with these tendencies can have no desire for war. 2 The sand-blast will be in operation this year at the Kensington International Exhibition. NIAGARA. 239 valley once fell as a cataract. Near Pontresina, in the Engadin, there is such a case ; a hard gneiss being there worn away to form a gorge, through which the river from the Morteratsch glacier rushes. The barrier of the Kirchet above Meyringen is also a case in point. Behind it was a lake, derived from the glacier of the Aar, and over the }>arrier the lake poured its excess of water. Here the rock, being limestone, was in great part dissolved ; but added to this we had the action of the sand particles carried along by the water, each of which, as it struck the rock, chipped it away like the particles of the sand-blast. Thus, by solution and mechanical erosion, the great chasm of the Fiensteraarschlucht was formed. It is demonstrable that the water which flows at the bottoms of such deep fissures once flowed at the level of what is now their edges, and tumbled down the lower faces of the barriers. Almost every valley in Switzerland furnishes examples of this kind ; the untenable hypothesis of earthquakes, once so readily resorted to in accounting for these gorges, being now for the most part abandoned. To produce the Canons of Western America, no other cause is needed than the in- tegration of effects individually infinitesimal. And now we come to Niagara. Soon after Europeans had taken possession of the country, the conviction appears to have arisen that the deep channel of the river Niagara below the falls had been excavated by the cataract. In Mr. Bakewell's ' Introduction to Geology,' the prevalence of this belief has been referred to ; it is expressed thus by Professor Joseph Henry in the * Transactions of the Albany Institute : ' l 'In viewing the position of the falls, and the features of the country round, it is impossible not to be impressed with the idea that this great natural raceway has been formed by the continued action of the irresistible Quoted by Bakewell. 240 FRAGMENTS OP SCIENCE. Niagara, and that the falls, beginning at Lewiston, have, in the course of ages, worn back the rocky strata to their present site.' The same view is advocated by Sir Charles Lyell, by Mr. Hall, by M. Agassiz, by Professor Kamsay, indeed by most of those who have inspected the place. A connected image of the origin and progress of the cataract is easily obtained. Walking northward from the village of Niagara Falls by the side of the river, we have to our left the deep and comparatively narrow gorge, through which the Niagara flows. The bounding cliffs of this gorge are from 300 to 350 feet high. We reach the whirlpool, trend to the north-east, and after a little time gradually resume our northward course. Finally, at about seven miles from the present falls, we come to the edge of a declivity, which informs us that we have been hitherto walking on table-land. At some hundreds of feet below us is a comparatively level plain, which stretches to Lake Ontario. The declivity marks the end of the precipitous gorge of the Niagara. Here the river escapes from its steep mural boundaries, and in a widened bed pursues its way to the lake which finally receives its waters. The fact that in historic times, even within the memory of man, the fall has sensibly receded, prompts the question, How far has this recession gone ? At what point did the ledge which thus continually creeps backwards begin its retrograde course 9 To minds disciplined in such researches the answer has been, and will be At the precipitous de- clivity which crossed the Niagara from Lewiston on the American to Queenston on the Canadian side. Over this transverse barrier the united affluents of all the upper lakes once poured their waters, and here the work of erosion began. The dam, moreover, was demonstrably of sufficient height to cause the river above it to submerge Goat Island ; and this would perfectly account for the finding by Sir Charles Lyell, Mr. Hall, and others, in the sand and gravel NIAGARA. 241 of the island, the same fluviatile shells as are now found in the Niagara Eiver higher up. It would also account for those deposits along the sides of the river, the discovery of which enabled Lyell, Hall, and Kamsay to reduce to demonstration the popular belief that the Niagara once flowed through a shallow valley. The physics of the problem of excavation, which I made clear to my mind before quitting Niagara, are revealed by a close inspection of the present Horseshoe Fall. We see evidently that the greatest weight of water bends over the very apex of the Horseshoe. In a passage in his excellent chapter on Niagara Falls, Mr. Hall alludes to this fact. Here we have the most copious and the most violent whirling of the shattered liquid; here the most powerful eddies recoil against the shale. From this por- tion of the fall, indeed, the spray sometimes rises without solution of continuity to the region of clouds, becoming gradually more attenuated, and passing finally through the condition of true cloud into invisible vapour, which is sometimes reprecipitated higher up. All the phenomena point distinctly to the centre of the river as the place of greatest mechanical energy, and from the centre the vigour of the fall gradually dies away towards the sides. The Horseshoe form, with the concavity facing downwards, is an obvious and necessary consequence of this action. Eight along the middle of the river the apex of the curve pushes its way backwards, cutting along the centre a deep and comparatively narrow groove, and draining the sides as it passes them. 1 Hence the remarkable discrepancy between the widths of the Niagara above and below the Horseshoe. All along its course, from Lewiston Heights to its present position, the form of the fall was probably that of a horse- 1 In the discourse the excavation of the centre and drainage of< the sides action was illustrated by a model devised by my assistant, Mr. John Cottrell. 242 FRAGMENTS OF SCIENCE. shoe ; for this is merely the expression of the greater depth, and consequently greater excavating power, of the centre of the river. The gorge, moreover, varies in width, as the depth of the centre of the ancient river varied, being narrowest where that depth was greatest. The vast comparative erosive energy of the Horseshoe Fall comes strikingly into view when it and the American Fall are compared together. The American branch of the upper river is cut at a light angle by the gorge of the Niagara. Here the Horseshoe Fall was the real excavator. ft cut the rock, and formed the precipice, over which the American Fall tumbles. But since its formation, the ero- sive action of the American Fall has been almost nil, while the Horseshoe has cut its way for 500 yards across the end of Goat Island, and is now doubling back to ex- cavate its channel parallel to the length of the island. This point, which impressed me forcibly, has not, I have just learned, escaped the acute observation of Professor Eamsay. 1 The river bends ; the Horseshoe immediately ac- commodates itself to the bending, and will follow implicitly the direction of the deepest water in the upper stream. The flexibility of the gorge, if I may use the term, is de- termined by the flexibility of the river channel above it. Were the Niagara centre above the fall sinuous, the gorge would obediently follow its sinuosities. Once suggested, no doubt geographers will be able to point out many examples of this action. The Zambesi is thought to present a great difficulty to the erosion theory, because of the sinuosity of the chasm below the Victoria Falls. But, assuming the basalt to be of tolerably uniform texture, had the river 1 His words are : ' Where the body of water is small in the American Fall, the edge has only receded a few yards (where most eroded) during the time that the Canadian Fall has receded from the north corner of Gait Island to the innermost curve of the Horseshoe Fall.' Quarterly Journal of Geological Society, May 1859. NIAGARA.. 243 been examined before the formation of this sinuous channel, - the present zigzag course of the gorge below the fall could, I am persuaded, have been predicted, while the sounding of the present river would enable us to predict the course to be pursued by the erosion in the future. But not only has the Niagara Eiver cut the gorge ; it lias carried away the chips of its own workshop. The ehale, being probably crumbled, is easily carried away. But at the base of the fall we find the huge boulders already described, and by some means or other these are removed down the river. The ice which fills the gorge in winter, and which grapples with the boulders, has been regarded as the transporting agent. Probably it is so to some extent. But erosion acts without ceasing on the abutting points of the boulders, thus withdrawing their support and urging them gradually down the river. So- lution also does its portion of the work. That solid matter is carried down is proved by the difference of depth between the Niagara Kiver and Lake Ontario, where the river enters it. The depth falls from 72 feet to 20 feet, in consequence of the deposition of solid matter caused by the diminished motion of the river. 1 The accompanying highly instructive map has been reduced from one published in Mr. Hall's ' Geology of New York.' It is based on surveys executed in 1842, by Messrs. Gibson and Evershed. The ragged edge of the American Fall north of Goat Island marks the amount of erosion which it has been able to accomplish, while the Horseshoe Fall was cutting its way southward across the end of Goat Island to its present position. The American Fall is 168 feet high, a precipice cut down, not by itself, but by the Horseshoe Fall. The latter in 1842 was 159 feet high, and, as shown by the map, is already turning 1 Near the mouth of the gorge at Queenston, the depth, according to the Admiralty Chart, is 180 feet ; well within the gorge it is 132 feet. 13 COAT IS LAND NIAGAKA. 246 eastward, to excavate its gorge along the centre of the upper river, p is the apex of the Horseshoe, and T marks the site of the Terrapin Tower, with the promontory ad- jacent, round which I was conducted by Conroy. Pro- bably since 1842 the Horseshoe has worked back beyond the position here assigned to it. In conclusion, we may say a word regarding the proxi- mate future of Niagara. At the rate of excavation assigned to it by Sir Charles Lyell, namely, a foot a year, five thou- sand years or so will carry the Horseshoe Fall far higher than Goat Island. As the gorge recedes it will drain, as it has hitherto done, the banks right and left of it, thus leaving a nearly level terrace between Goat Island and the edge of the gorge. Higher up it will totally drain the American branch of the river ; the channel of which in due time will become cultivable land. The American Fall will then be transformed into a dry precipice, forming a simple continuation of the cliffy boundary of the Niagara. At the place occupied by the fall at this moment we shall have the gorge enclosing a right angle, a second whirlpool being the consequence of this. To those who visit Niagara a few millenniums hence I leave the verification of this prediction. All that can be said is, that if the causes now in action continue to act, it will prove itself literally true. 246 FRAGMENTS OP SCIENCE VIII. LIFE AND LETTERS OF FARADAY. 1870. 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 lather was a smith; that he was born at Newington Butts in 1791; that he ran 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 apprenticeship he found his appetite for knowledge pro- voked and strengthened by the books he stitched and covered. Thus he grew in wisdom and stature to his year of legal manhood, when he appears in the volumes before us as a writer of letters, which reveal his occupa- tion, acquirements, and tone of mind. His correspondent was Mr. Abbott, a member of the Society of Friends, who, with a forecast of his correspondent'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 FARADAY. 247 proposition that no new enquiry 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 toy 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 ac- cept 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 of self-control became a second nature to him at lagt, and lent serenity to his later years. In October 1812 he was engaged by a Mr. De la Roche as a journeyman bookbinder ; but the situation did not suit him. His master appears to have been an aus- tere and passionate man, and Faraday was to the last degree sensitive. All his life he continued so. He suf- fered at times from dejection ; and a certain grimness, too, pervaded his moods. ' At present,' he writes to Abbott, ' J. 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 con- ceive 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 affections and lusts of him who har- 248 FRAGMENTS OF SCIENCE. hours it, but is often a cause of disturbed digestion to his friends. About three months after his engagement with De la Roche, Faraday quitted him and bookbinding together. He had heard Davy, ccpied his lectures, and written to him, entreating to be released from Trade, which he hated, and enabled to pursue Science. Davy recognised the merit of his correspondent, kept his eye upon him, and, when occasion offered, drove to his door and sent in a letter, offering him the post of assistant in the labora- tory of the Royal Institution. He was engaged 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 valuable.' Faraday derived great profit from this little association. In the laboratory he had a discipline sturdier still. Both Davy and himself were at this time frequently cut and bruised by explosions of chlo- ride 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 receiver 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 forehead 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 whicn, compounded with his rare intel- lectual power, made him a great experimental philosopher. FAEADAT. 249 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 organ of vision. Late in life I have heard him say that he could never fully understand an experiment until he had seen it. But he did not confine himself to experi- ment. He aspired to he a teacher, and reflected and wrote upon the method of scientific exposition. ' A lec- turer,' he observes, * should appear easy and collected, un- daunted and unconcerned : ' still ' his whole behaviour should evince respect for his audience.' These recommend- ations were afterwards 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 hope- lessly unintelligible, he suddenly evoked a certain reckless- ness of thought, and, without halting to extricate his bewil- dered 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 vigour 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 im- pose 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 4 Bibliotheque Britannique,' had been the first to make the English chemist's labours known abroad. He wel- comed Davy to his country residence in 1814. Both were 250 FRAGMENTS OF SCIENCE. sportsmen, and they often went out shooting together. On these occasions P'araday charged Davy's gun while De la Eive charged his own. Once the Genevese philosopher found 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 servaut. On enquiiy De la Eive was somewhat shocked to find that the soi-disant domestique was really preparateur in the laboratory 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. Rumour states that a dinner in honour 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 f on de laboratoire. 1 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 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 friendship 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 condensed abstract. It was at the desire of Dr. Bence Jones that I asked him to do so. The rumour of a banquet at Geneva illustrates the tendency to sub- stitute for the youth of 1814 the Faraday of later years. FARADAY. 251 state of transition.' And again : ' Nothing is more diffi- cult and requires more caution than philosophical deduc- tion, nor is there anything more adverse to its accuracy than fixity of opinion.' Not that lie was wafted about by every wind of doctrine; but that he united flexibility with his strength. In striking contrast with this intel- lectual expansiveness was 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, and 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 en- tries 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 be- come 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 262 FRAGMENTS OF SCIENCE. moment's alloy of this evening's happiness occurred. Everything 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 myself,' he writes to her in a subsequent letter, ' to convey most perfectly my affec- tion 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 cha- racter 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, fot 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 afterwards, 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 ap- pearing in these volumes, which justify his opinion that Davy in those days had become jealous of Faraday. This, which is the prevalent belief, is also reproduced in an excellent article in the March number of ' Fraser's Maga- zine.' 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 afterwards the acute mind of Wollaston per- ceived that a wire carrying a current ought to rotate round its own axis under the influence of a magnetic pole. In 1821 he tried, but failed, to realise this result in the laboratory of the Royal Institution. Faraday was not present at the moment, but he came in immediately after- FARADAY. 253 wards and heard the conversation of Wollaston and Davy about the experiment. He had also heard a rumour 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 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 that result. 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 impos- sible 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 possessed. Eumour and fact had connected the name of Wollaston with these supposed interactions between magnets and currents. When, therefore, Faraday in October published his successful experiment, without any allusion to Wollaston, general, though really un- grounded, criticism followed. I say ungrounded because, firstly, Faraday's experiment 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 authorised 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. Wai-burton and others thereupon affirmed that Wollaston's ideas had been 264 FRAGMENTS OF SCIENCE. 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 whispers to me, are, I suspect, spoken aloud among scientific men.' He might have written explanations and defences, but he went straighter to the point. He wished to see the prin- cipals face to face to plead his cause before them person- ally. There was a certain vehemence in his desire to do this. He saw Wollaston, he saw Davy, he saw Warburton ; and I am inclined to think that it was the irresistible candour and truth of character which these viva voce defences re- vealed, 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 analysed 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 by 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 decomposition was proved by Faraday to be chlorine liquefied by its own pressure. On the day of its discovery he communicated this result to Dr. Paris. Davy, on being informed of it, instantly liquefied another gas in the same way. Having struck thus into Faraday's enquiry, 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 estrangement on this account is not rare ; nor was Davy wise enough to discern that Faraday had passed FAEADAY. 256 the mere assistant stage, and become a discoverer. It is now hard to avoid magnifying this error. But had Fara- day died or ceased to work at this time, or had his subse- quent 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 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 EoyaJ Institution, and I trust that you will not only do something good and honourable 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 Eoyal Society is, I am persuaded, to be ascribed. These matters are touched upon with perfect candour, 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 rumours which I would willingly scatter for ever. The pupil's magnitude, and the splendour of his position, are too great and absolute to need as a foil the humiliation of his master. Brothers in intellect, Davy and Faraday, however, could never have become brothers in feeling ; their characters were too unlike. Davy loved the pomp and circumstance of fame ; Faraday the inner con- sciousness 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 dignifying inward force. In one great par- ticular they agreed. Each of them could have turned his science to immense commercial profit, but neither of them 256 FRAGMENTS OF SCIENCE. did so. The noble excitement of research, and the delight of discovery, constituted 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. The first volume of the ' Life and Letters ' reveals to us the youth who was to be father to the man. Skil- ful, 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 Rota- tions, and it planted in him more daring ideas of a similar kind. Magnetism 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 experiments on this subject began. He had been forti- fied by previous trials, which, though failures, had be- gotten instincts directing him towards 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 November he had discovered and colli- gated a multitude of the most wonderful and unexpected phenomena. He had generated currents by currents ; currents by magnets, permanent and transitory ; and he afterwards generated currents by the earth itself. Arago's * Magnetism of Rotation,' 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, illustration of the great principle of Magneto- electric Induction. Nothing equal to this, in the way FABADAY. 257 of pure experimental enquiry, 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. Re- nouncing professional work, from which at this time he might have derived an income of many thousands a year, he poured his whole momentum into his researches. He was long entangled 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 Decom- position in his hands. If his discovery of Magneto-elec- tricity 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 Conduction, Induction, and Mode of Propagation. He discovered and illustrated the prin- ciple of Inductive capacity ; and, turning to theory, he asked himself how electrical attractions 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 experi- ments 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. 1 In a very remarkable paper published in Poggendorff 's ' Annalen ' fot 1857, Werner Siemens accepts and develops Faraday's theory of Molecular Induction. 258 FRAGMENTS OP SCIENCE. Faraday had heard it stated that henceforth physical discoveries 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 acquaint- ance with the nature, present condition, and prospective vastness of the field of physical enquiry. The tendency of natural science doubtless is to bring all physical phe- nomena 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 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 investiga- tions ' Experimental Eesearches 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 3362. In 1859 he collected and published a fourth volume of papers, under the title, ' Experimental Kesearches in Chemistry and Physics.' Thus the apostle of experiment illustrated its power, and magnified his office. The second volume of the Eesearches embraces memoirs on the Electricity of the G-ymnotus ; 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 Mag- netic Rotations, and Faraday's letters in relation to the controversy it aroused. The contribution of most per- manent value here, is that on the Source of Power in the Voltaic Pile. By it the Contact Theory, pure and simple, was totally overthrown, and the necessity of chemical action on the maintenance of the current demonstrated. FARADAY. 259 The third volume of the Eesearches opens with a memoir entitled ' The Magnetisation of Light, and the Illumination of Magnetic Lines of Force.' It is difficult even now to affix a definite meaning to this title ; but the discovery of the rotation of the plane of polarisation, which it announced, seems pregnant with great results. The writings of William Thomson on the theoretic aspects of the discovery; the excellent electro-dynamic measure- ments of Wilhelm Weber, which are models of experi- mental completeness and skill ; Weber's labours in con- junction with his lamented friend Kohlrausch above all, the researches of Clerk Maxwell on the Electro-magnetic Theory of Light 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 connection was first raised by the discovery here referred to. 1 Faraday himself seemed to cling with particular 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 scien- tific history. Kapidly following it is the discovery of Diamagnetism, or the Eepulsion 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. 1 A letter addressed to me by Professor Weber on March 18 last contains the following reference to the connection here mentioned : ' Die Hoffnung einer solchen Combination ist durch Faraday's Entdeekung der Drehung der Polarisationsebene durch magnetische Directionskraft zuerst, und sodann durch die Uebereinstimmung derjenigen Geschwindigkeit, welche das Verhaltniss der electro-dynamischen Einheit zur electro-sta- tischen ausdriickt, mit der Geschwindigkeit des Lichts angeregt worden ; und niir scheint von alien Versuchen, welche zur Verwirklichung dieser Hoffnung gemacht worden sind, das von Herrn Maxwell gemachte am erfol greichst en .' '260 FRAGMENTS OF SCIENCE. Here he stopped. Seebeck, Becquerel, and others, also touched the discovery. These fragmentary gleams excited a momentary curiosity, and \vere 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 scien- tific truth as give occupation to some of the best intellects of our age. The salient quality of Faraday's scientific character reveals itself from beginning to end of these volumes : a union of ardour 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 philo- sophy. The brain may be filled with that philosophy ; but without the energy and insight which this man pos- sessed, 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 express- ing itself after its own fashion, and acknowledging no mediator between it and Nature. The second volume of the ' Life and Letters,' like the first, is a 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, Dumas, Liebig, Melloni, Becquerel, Oersted, Pliicker, Du Bois Eeymond, Lord Melbourne, Prince Louis Napoleon, and many other distinguished men. I FAKADAY. 261 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 unful- filled hopes which spring up in the mind of fertile investi- gators : ' 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 peans that already begin to rise, and will speedily swell into a shout of triumph, astounding even to yourself, than thatofJ.F.W. Herschel.' Faraday's behaviour 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 disappointed hope Melloni waited for the report of the Commissioners, appointed by the Academy of Sciences to examine his labours. 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 Kumford Medal of the Eoyal Society. A sum of money always accompanies this medal ; and the pecuniary help was, at this time, even more essential than the mark of honour to the young refugee. Melloni's gratitude was boundless : ' Et vous, monsieur,' he writes to Faraday, ' qui appar- tenez a line societe a laquelle je n'avais rien offert, vous qui me connaissiez a peine de nom ; vous n'avez pas demande si j'avais des ennemis faibles ou puissants, ni calcule quel en etait le nombre ; mais vous avez parle pour I'oppiime etranger, pour celui qui n'avait pas le moindre droit a tant de bienveillance, et vos paroles ont 262 FRAGMENTS OF SCIENCE. ete accueillies favorablement par des collogues conscien- cieux I Je reconnais bien la des hommes dignes de leur noble mission, les veritables representants de la science d'un pays libre et genereux.' 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 demanded and given was complete. I think it certain that, notwithstanding the very fidl account of this transac- tion given by Dr. Bence Jones, motives and influences were at work which even now are not entirely revealed. The minister 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 her- self 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 Melbourne's defence, and there is a flavour 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-in-law. His bodily vigour 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 Nieson towering over the Lake of Thun, we FARADAY. 263 come upon the charming little scrap which I have else- where 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 anything 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 Staubbach, Giessbach, and of the scenic effects of sky and mountain, are all fine and sympathetic. 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 excur- sion to the ' Hotel des Neufchatelois ; : but indisposition put the project out. From the Fort of Ham, in 1843, Faraday received a letter addressed to him by Prince Louis Napoleon Bona- parte. 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 com- bination to give to a voltaic battery, in order to produce a spark capable of setting fire to powder under water 01 under ground ? ' 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 1 The ' science' has since been applied, \rith astonishing effect, by those who had studied it far more thoroughly than the Emperor of the French. 264 FRAGMENTS OF SCIENCE. 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 Birmingham, 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 pleasantest and happiest thing that I can do. Nothing 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.' Take this, moreover, as indicative of his love for Nature : ' 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 t/iousand 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 to * a very small and despised sect of Christians, known, if known at all, as Sandemanians, 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 quitting his moorings, and his science acted indirectly as the safeguard of his particular FAEADAT. 265 faith. For his investigations so filled his mind as to leave no room for sceptical 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 con- stituents awe, reverence, truth, and love. It is worth enquiring how so profoundly religious a mind, and so great a teacher, would be likely to regard our present 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. 1 First, then,' 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 anything for religion, but I wish to take that part of human nature which 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 principle? 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 266 FRAGMENTS OF SCIENCE. 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 described as the petition of a son, into whose heart God had sent the Spirit of His Son, and who with abso- lute 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 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 character 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 labours of the biographer final. So great a character will challenge reconstruction. In the coming time some sympathetic spirit, with the requisite strength, knowledge, 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, FARADAY. 207 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 labour of love. 14 28 FRAGMENTS OP SCIENCE. IX. THE COPLEY MEDALIST OF 1870. npHIETY years ago Electro- magnetism was looked to as JL a motive power, which might possibly compete with steam. In centres of industry, such as Manchester, at- tempts to investigate and apply this power were numerous. This is shown by the scientific literature of the time. Among others Mr. James Prescot Joule, a resident of Man- chester, took up the subject, and, in a series of papers pub- lished in Sturgeon's ' Annals of Electricity' between 1839 and 1841, described various attempts at the construction and perfection of electro-magnetic engines. The spirit in which Mr. Joule pursued these enquiries is revealed in the following extract : ' I am particularly anxious,' he says, 4 to communicate any new arrangement in order, if possi- ble, to forestall the monopolising designs of those who seem to regard this most interesting subject merely in the light of pecuniary speculation.' He was naturally led to investigate the laws of electro-magnetic attractions, and in 1840 he announced the important principle that the attractive force exerted by two electro-magnets, or by an electro-magnet and a mass of annealed iron, is directly proportional to the square of the strength of the magnet- ising current; while the attraction exerted between an electro-magnet and the pole of a permanent steel magnet, varies simply as the strength of the current. These investigations were conducted independently of, though a THE COPLEY MEDALIST OF 1870. 269 little subsequently to, the celebrated enquiries of Henry, Jacobi, and Lenz and Jacobi, on the same subject. On December 17, 1840, Mr. Joule communicated to the Royal Society a paper on the production of heat by Voltaic electricity. In it he announced the law that the calorific effects of equal quantities of transmitted electricity are proportional to the resistance overcome by the current, whatever may be the length, thickness, shape, or character of the metal which closes the cir- cuit; and also proportional to the square of the quantity of transmitted electricity. This is a law of primary importance. In another paper, presented to, but de- clined by, the Royal Society, he confirmed this law by new experiments, and materially extended it. He also executed experiments on the heat consequent on the passage of Voltaic electricity through electrolytes, and found, in all cases, that the heat evolved by the proper action of any Voltaic current is proportional to the square of the intensity of that current, multiplied by the resistance to conduction which it experiences. From this law he deduced a number of conclusions of the highest importance to electro-chemistry. It was during these enquiries, which are marked throughout by rare sagacity and originality, that the great idea of establishing quantitative relations between Me- chanical Energy and Heat arose and assumed definite form in his mind. In 1843 Mr. Joule read before the meeting of the British Association at Cork a paper ' On the Calorific Effects of Magneto-Electricity, and on the Mechanical Value of Heat.' Even at the present day this memoir is tough reading, and at the time it was written it must have appeared hopelessly entangled. This, I should think, was the reason why Faraday advised Mr. Joule not to submit the paper to the Royal Society. But its drift and results are summed up in these memorable 270 FRAGMENTS OP SCIENCE. words by its author, written some time subsequently : ' In that paper it was demonstrated experimentally, that the mechanical power exerted in turning a magneto-electric machine is converted into the heat evolved by the passage of the currents of induction through its coils ; and, on the other hand, that the motive power of the electro-magnetic engine is obtained at the expense of the heat, due to the chemical .reaction of the battery by which it is worked.' l It is needless to dwell upon the weight and importance oi this statement. Considering the imperfections incidental to a first determination, it is not surprising that the ' mechanical values of heat,' deduced from the different series of ex- periments published in 1843, varied widely from each other. The lowest limit was 587, and the highest 1,026 foot-pounds, for 1 Fahr. of temperature. One noteworthy result of his enquiries, which was pointed out at the time by Mr. Joule, had reference to the exceedingly small fraction of the heat actually converted into useful effect in the steam-engine. The thoughts of the celebrated Julius Eobert Mayer, who was then engaged in Germany upon the same question, had moved independently in the same groove ; but to his labours due reference will be made on a future occa- sion.* In the memoir now referred to, Mr. Joule also announced that he had proved heat to be evolved during the passage of water through narrow tubes ; and he deduced from these experiments an equivalent of 770 foot-pounds, a figure remarkably near the one now ac- cepted. A detached statement regarding the origin and convertibility of animal heat strikingly illustrates the penetration of Mr. Joule, and his mastery of principles, at the period now referred to. A friend had mentioned to 1 Phil. Mag. May, 1845. 2 See the next Fragment THE COPLEY MEDALIST OF 1870. 271 him Haller's hypothesis, that animal heat might arise from the friction of the blood in the veins and arteries. * It is unquestionable,' writes Mr, Joule, * that heat is produced by such friction ; but it must be understood that the mechanical force expended in the friction is a part of the force of affinity which causes the venous blood to unite with oxygen, so that the whole heat of the system must still be referred to the chemical changes. But if the animal were engaged in turning a piece of machinery, or in ascending a mountain, I apprehend that in proportion to the muscular effort put forth for the purpose, a dimi- nution of the heat evolved in the system by a given chemical action would be experienced.' The italics in this memorable passage, written, it is to be remembered, in 1843, are Mr. Joule's own. The concluding paragraph of this British Association paper equally illustrates his insight and precision, regard- ing the nature of chemical and latent heat. ' I had,' he writes, ' endeavoured to prove that when two atoms combine together, the heat evolved is exactly that which would have been evolved by the electrical current due to the chemical action taking place, and is therefore pro- portional to the intensity of the chemical force causing the atoms to combine. I now venture to state more explicitly, that it is not precisely the attraction of affinity, but rather the mechanical force expended by the atoms in falling towards one another, which determines the in- tensity of the current, and, consequently, the quantity of heat evolved ; so that we have a simple hypothesis by which we may explain why heat is evolved so freely in the combination of gases, and by which indeed we may account " latent heat " as a mechanical power, prepared for action, as a watch-spring is when wound up. Suppose, for the sake of illustration, that 8 Ibs. of oxygen and 1 Ib. of hydrogen were presented to one another in the 272 FRAGMENTS OF SCIENCE. gaseous state, and then exploded ; the heat evolved would be about 1 Fahr. in 60,000 Ibs. of water, indicating a me- chanical force, expended in the combination, equal to a weight of about 50,000,000 Ibs. raised to the height of one foot. Now if the oxygen and hydrogen could be presented to each other in a liquid state, the heat of combination would be less than before, because the atoms in com- bining would fall through less space.' No words of mine axe needed to point out the commanding grasp of mole- cular physics, in their relation to the mechanical theory of heat, implied by this statement. Perfectly assured of the importance of the principle which his experiments aimed at establishing, Mr. Joule did not rest content with results presenting such discre- pancies as those above referred to. He resorted in 1844 to entirely new methods, and made elaborate experiments on the thermal changes produced in air during its expan- sion : firstly, against a pressure, and therefore performing work ; secondly, against no pressure, and therefore per- forming no work. He thus established anew the relation between the heat consumed and the work done. From five different series of experiments he deduced five different mechanical equivalents ; the agreement between them being far greater than that attained in his first experiments. The mean of them was 802 foot-pounds. From experiments with water agitated by a paddle-wheel, he deduced, in 1845, an equivalent of 890 foot-pounds. In 1847 he again operated upon water and sperm-oil, agitated them by a paddle-wheel, determined their eleva- tion of temperature, and the mechanical power which produced it. From the one he derived an equivalent of 781'5 foot-pounds; from the other an equivalent of 782*1 foot-pounds. The mean. of these two very close deter- minations is 781*8 foot-pounds. At this time the labours of the previous ten years had THE COPLEY MEDALIST OF 1870. 278 made Mr. Joule completely master of the conditions es- sential to accuracy and success. Bringing his ripened experience to bear upon the subject, he executed in 1849 a series of 40 experiments on the friction of water, 50 experiments on the friction of mercury, and 20 experi- ments on the friction of plates of cast-iron. He deduced from these experiments our present mechanical equivalent sf heat, justly recognised all over the world as 'Joule's equivalent.' There are labours so great and so pregnant in conse- quences, that they are most highly praised when they are most simply stated. Such are the labours of Mr. Joule. They constitute the experimental foundation of a principle of incalculable moment, not only to the practice, but still more to the philosophy of Science. Since the days of Newton, nothing more important than the theory, of which Mr. Joule is the experimental demonstrator, has been enunciated. I have omitted all reference to the numerous minor papers with which Mr. Joule has enriched scientific litera- ture. Nor have I alluded to the important investigations which he has conducted jointly with Sir William Thom- son. But sufficient, I think, has been here said to show that, in conferring upon Mr. Joule the highest honour of the Koyal Society, the Council paid to genius not only a well-won tribute, but one which had been fairly earned twenty years previously. 274 FRAGMENTS OF SCIENCE. X. THE COPLEY MEDALIST OF 1871. DR. JULIUS ROBERT MAYER was educated for the medical profession. In the summer of 1840, as he himself informs us, he was at Java, and there observed that the venous blood of some of his patients had a singularly bright red colour. The observation riveted his attention; he reasoned upon it, and came to the conclusion that the brightness of the colour was due to the fact that a less amount of oxidation sufficed to keep up the temperature of the body in a hot climate than in a cold one. The darkness of the venous blood he regarded as the visible sign of the energy of the oxidation. It would be trivial to remark that accidents such as this, appealing to minds prepared for them, have often led to great discoveries. Mayer's attention was thereby drawn to the whole question of animal heat. Lavoisier had ascribed this heat to the oxidation of the food. ' One great principle,' says Mayer, ' of the physiological theory of combustion, is that under all circumstances the same amount of fuel yields, by its perfect combustion, the same amount of heat ; that this law holds good even for vital processes ; and that hence the living body, notwithstand- ing all its enigmas and wonders, is incompetent to generate heat out of nothing.' But beyond the power of generating internal heat, the animal organism can also generate heat outside of itself. THE COPLEY MEDALIST OF 1871. 275 A blacksmith, for example, by hammering can heat a nail, and a savage by friction can warm wood to its point of ignition. Now, unless we give up the physiological axiom that the living body cannot create heat out of nothing, ' we are driven,' says Mayer, * to the conclusion that it is the total heat generated within and without that is to be regarded as the true calorific effect of the matter oxidised in the body.' From this, again, he inferred that the heat generated externally must stand in a fixed relation to the work expended in its production. For, supposing the organic processes to remain the same ; if it were possible, by the mere alteration of the apparatus, to generate different amounts of heat by the same amount of work, it would follow that the oxidation of the same amount of material would sometimes yield a less, sometimes a greater, quantity of heat. ' Hence,' says Mayer, ' that a fixed relation subsists between heat and work, is a postulate of the physiological theory of combustion.' This is the simple and natural account, given subse- quently by Mayer himself, of the course of thought started by his observation in Java. But the conviction once formed, that an unalterable relation subsists between work and heat, it was inevitable that Mayer should seek to express it numerically. It was also inevitable that a mind like his, having raised itself to clearness on this important point, should push forward to consider the relationship of natural forces generally. At the beginning of 1842 his work had made considerable progress ; but he had become physician to the town of Heilbronn, and the duties of his profession limited the time which he could devote to purely scientific enquiry. He thought it wise, therefore, to secure himself against accident, and in the spring of 1842 wrote to Liebig, asking him to publish in . his Annalen ' a brief preliminary notice of the work then 276 FRAGMENTS OF SCIENCE. accomplished. Liebig did so, and Dr. Mayer's first paper is contained in the May number of the * Annalen ' for 1842. Mayer had reached his conclusions by reflecting on the complex processes of the living body ; but his first step in public was to state definitely the physical principles on which his physiological deductions were to rest. He begins, therefore, with the forces of inorganic nature. He finds in the universe two systems of causes which are not mutually convertible ; the different kinds of matter and the different forms of force. The first quality of both he affirms to be indestructibility. A force cannot become nothing, nor can it arise from nothing. Forces are con- vertible but not destructible. In the terminology of his time, he then gives clear expression to the ideas of poten- tial and dynamic energy, illustrating his point by a weight resting upon the earth, suspended at a height above the earth, and actually falling to the earth. He next fixes his attention on cases where motion is apparently des- troyed, without producing other motion ; on the shock of inelastic bodies, for example. Under what form does the vanished motion maintain itself? Experiment alone, says Mayer, can help us here. He warms water by stirring it ; he refers to the force expended in overcoming friction. Motion in both cases disappears; but heat is generated, and the quantity generated is the equivalent of the motion destroyed. ' Our locomotives,' he observes with extra- ordinary sagacity, 'may be compared to distilling ap- paratus : the heat beneath the boiler passes into the motion of the train, and is again deposited as heat in the axles and wheels.' A numerical solution of the relation between heat and work was what Mayer aimed at, and towards the end of his first paper he makes the attempt. It was known that a definite amount of air, in rising one degree in tempera- THE COPLEY MEDALIST OF 1871. 277 ture, can take up two different amounts of heat. If its volume be kept constant, it takes up one amount : if its pressure be kept constant it takes up a different amount. These two amounts are called the specific heat under con- stant volume and under constant pressure. The ratio of the first to the second is as 1 : 1-421. No man, to my know- ledge, prior to Dr. Mayer, penetrated the significance of these two numbers. He first saw that the excess 0-421 was not, as then universally supposed, heat actually lodged in the gas, but heat which had been actually con- sumed by the gas in expanding against pressure. The amount of work here performed was accurately known, the amount of heat consumed was also accurately known, tnd from these data Mayer determined the mechanical equivalent of heat. Even in this first paper he is able to direct attention to the enormous discrepancy between the theoretic power of the fuel consumed in steam-engines, and their useful effect. Though this paper contains but the germ of his further labours, I think it may be safely assumed that, as regards the mechanical theory of heat, this obscure Heil- bronn physician, in the year 1842, was in advance of all the scientific men of the time. Having, by the publication of this paper, secured him- self against what he calls ' Eventualitaten,' he devoted every hour of his spare time to his studies, and in 1845 published a memoir which far transcends his first one in weight and fulness, and, indeed, marks an epoch in the history of science. The title of Mayer's first paper was, * Remarks on the Forces of Inorganic Nature.' The title of his second great essay was, ' Organic Motion in its Connection with Nutrition.' In it he expands and illus- trates the physical principles laid down in his first brief paper. He goes fully through the calculation of the mechanical equivalent of heat. He calculates the per- 278 FKAGMENTS OP SCIENCE. formances of steam-engines, and finds that lOOlbs. of coal, in a good working engine, produce only the same amount of heat as 95 Ibs. in an un working one ; the 5 Ibs. dis- appearing having been converted into work. He deter- mines the useful effect of gunpowder, and finds nine per cent, of the force of the consumed charcoal invested on the moving ball. He records observations on the heat generated in water agitated by the pul ping-engine of a paper manufactory, and calculates the equivalent of that heat in horse-power. He compares chemical combination with mechanical combination the union of atoms with the union of falling bodies with the earth. He calculates the velocity with which a body starting at an infinite distance would strike the earth's surface, and finds that the heat generated by its collision would raise an equal weight of water 17,356 C. in temperature. He then determines the thermal effect which would be produced by the earth itself falling into the sun. So that here, in 1845, we have the germ of that meteoric theory of the sun's heat which Mayer developed with such extraordinary ability three years afterwards. He also points to the almost exclusive efficacy of the sun's heat in producing mechanical motions upon the earth, winding up with the profound remark, that the heat developed by friction in the wheels of our wind and water mills comes from the sun in the form of vibratory motion ; while the heat pro- duced by mills driven by tidal action is generated at the expense of the earth's axial rotation. Having thus, with firm step, passed through the powers of inorganic nature, his next object is to bring his prin- ciples to bear upon the phenomena of vegetable and animal life. Wood and coal can burn ; whence come their heat, and the work producible by that heat ? From the immeasurable reservoir of the sun. Nature has proposed to herself the task of storing up the light which THE COPLEY MEDALIST OF 1871. 279 streams earthward from the sun, and of casting into a permanent form the most fugitive of all powers. To this end she has overspread the earth with organisms which, while living, take in the solar light, and by its consump- tion generate forces of another kind. These organisms are plants. The vegetable world, indeed, constitutes the instrument whereby the wave-motion of the sun is changed into the rigid form of chemical tension, and thus prepared for future use. With this prevision, as shall subsequently be shown, the existence of the human race itself is insepar- ably connected. It is to be observed that Mayer's utter- ances are far from being anticipated by vague statements regarding the ' stimulus ' of light, or regarding coal as ' bottled sunlight.' He first saw the full meaning of De Saussure's observation of the reducing power of the solar rays, and gave that observation its proper place in the doctrine of conservation. In the leaves of a tree, the carbon and oxygen of carbonic acid, and the hydrogen and oxygen of water, are forced asunder at the expense of the sun, and the amount of power thus sacrificed is accurately restored by the combustion of the tree. The heat and work potential in our coal strata are so much strength withdrawn from the sun of former ages. Mayer lays the axe to the root of many notions regarding ' vital force ' which were prevalent when he wrote. With the plain fact before us that plants cannot perform the work of reduction, or generate chemical tensions, in the absence of the solar rays, it is, he contends, incredible that these tensions should be caused by the mystic play of the vital force. Such an hypothesis would cut off all investigation ; it would land us in a chaos of unbridled phantasy. ' I count,' he says, ' therefore, upon assent when I state, as an axiomatic truth, that during vital processes the conver- sion only, and never the creation of matter or force occurs.' 280 FRAGMENTS OF SCIENCE. Having cleared his way through the vegetable world, as he had previously done through inorganic nature, Mayer passes on to the other organic kingdom. The physical forces collected by plants become the property of animals. Animals consume vegetables, and cause them to reunite with the atmospheric oxygen. Animal heat is thus produced ; and not only animal heat, but animal motion. There is no indistinctness about Mayer here ; he grasps his subject in all its details, and reduces to figures the concomitants of muscular action. A bowler who imparts to an 8-lb. ball a velocity of 30 feet, con- sumes in the act -fa of a grain of carbon. A man weighing 150 Ibs., who lifts his own body to a height of 8 feet, consumes in the act 1 grain of carbon. In climb- ing a mountain 10,000 feet high, the consumption of the same man. would be 2 oz. 4 drs. 50 grs. of carbon. Boussingault had determined experimentally the addition to be made to the food of horses when actively working, and Liebig had determined the addition to be made in the case of men. Employing the mechanical equivalent of heat, which he had previously calculated, Mayer proves the additional food to be amply sufficient to cover the increased oxidation. But he does not content himself with showing, in a general way, that the human body burns according to definite laws, when it performs mechanical work. He seeks to determine the particular portion of the body con- sumed, and in doing so executes some noteworthy calcula- tions. The muscles of a labourer 150 Ibs. in weight weigh 64 Ibs.; when perfectly desiccated they fall to 15 Ibs Were the oxidation corresponding to that labourer's work exerted on the muscles alone, they would be utterly con- sumed in 80 days. The heart furnishes a still more striking example. Were the oxidation necessary to sustain the heart's action exerted upon its own tissue, it would be utterly THE COPLEY MEDALIST OF 1871. 281 consumed in 8 days. And if we confine our attention to the two ventricles, their action would be sufficient to consume the associated muscular tissue in 3^ days. Here, in his own words, emphasised in his own way, is Mayer's pregnant conclusion from these calculations : ' The muscle is only the apparatus by means of which the conversion of the force is effected ; but it is not the substance consumed in the production of the mechanical effect.'' He calls the blood ' the oil of the lamp of life ; ' it is the slow-burning fluid whose chemical force, in the furnace of the capilla- ries, is sacrificed to produce animal motion. This was Mayer's conclusion twenty-six years ago. It was in complete opposition to the scientific conclusions of his time ; but eminent investigators have since amply veri- fied it. Thus, in baldest outline, I have sought to give some notion of the first half of this marvellous essay. The second half is so exclusively physiological that I do not wish to meddle with it. I will only add the illustration employed by Mayer to explain the action of the nerves upon the muscles. As an engineer, by the motion of his finger in opening a valve or loosing a detent, can liberate an amount of mechanical motion almost infinite com- pared with its exciting cause, so the nerves, acting upon the muscles, can unlock an amount of activity, wholly out of proportion to the work done by the nerves them- selves. As regards these questions of weightiest import to the science of physiology, Dr. Mayer, in 1845, was assuredly far in advance of all living men. Mayer grasped the mechanical theory of heat with commanding power, illustrating it and applying it in the most diverse domains. He began, as we have seen, with physical principles ; he determined the numerical rela- tion between heat and work ; he revealed the source of 282 FKAGMENTS OP SCIENCE. the energies of the vegetable world, and showed the rela- tionship of the heat of our fires to solar heat. He followed the energies which were potential in the vegetable, up to their local exhaustion in the animal. But in 1845 a new thought was forced upon him by his calculations. He then, for the first time, drew attention to the astounding amount of heat generated by gravity where the force has sufficient distance to act through. He proved, as I have before stated, the heat of collision of a body falling from an infinite distance to the earth, to be sufficient to raise the temperature of a quantity of water, equal to the falling body in weight, 1 7,356 0. He also found, in 1845, that the gravitating force between the earth and sun was competent to generate an amount of heat equal to that obtainable from the combustion of 6,000 times the weight of the earth of solid coal. With the quickness of genius he saw that we had here a power sufficient to produce the enormous temperature of the sun, and also to account for the primal molten condition of our own planet. Mayer shows the utter inadequacy of chemical forces, as we know them, to produce or maintain the solar temperature. He shows that were the sun a lump of coal it would be utterly consumed in 5,000 years. He shows the diffi- culties attending the assumption that the sun is a cooling body ; for, supposing it to possess even the high specific heat of water, its temperature would fall 15,000 in 5,000 years. He finally concludes that the light and heat of the sun are maintained by the constant impact of meteoric matter. I never ventured an opinion as to the accuracy of this theory ; that is a question which may still have to be fought out. But I refer to it as an illustration of the force of genius, with which Mayer followed the mechanical theory of heat through all its applications. Whether the meteoric theory be a matter of fact or not, with him abides the honour of proving to demonstration that the THE COPLEY MEDALIST OF 1871. 283 light and heat of suns and stars may be originated and maintained by the collisions of cold planetary matter. It is the man who with the scantiest data could accom- plish all this in six short years, and in the hours snatched from the duties of an arduous profession, that the Eoyal Society, in 1871, crowned with its highest honour. Dr. Mayer had never previously received any mark of recog- nition from the Society. Comparing this brief history with that of the Copley Medalist of 1870, the differentiating influence of ' environ- ment,' on two minds of similar natural cast and endow- ment, comes out in an instructive manner. Withdrawn from mechanical appliances, Mayer fell back upon reflec- tion, selecting with marvellous sagacity, from existing physical data, the single result on which eould be founded a calculation of the mechanical equivalent of heat. In the midst of mechanical appliances, Joule resorted to ex- periment, and laid the broad and firm foundation which has secured for the mechanical theory the acceptance it now enjoys. A great portion of Joule's time was occupied in actual manipulation ; freed from this, Mayer had time to follow the theory into its most abtruse and impres- sive applications. With their places reversed, however, Joule might have become Mayer, and Mayer might have become Joule. It does not lie in the way of these brief articles to enter upon the great developments of the Dynamical Theory, accomplished since Joule and Mayer executed their memorable labcurs. 284 FRAGMENTS OF SCIENCE. XI. ELEMENTARY MAGNETISM. A tECTTTRE TO SCHOOLMASTERS. 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 regu- lated. 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 enquiring 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 endeavours 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 disap- pears, as the connection and mutual dependence of phy- sical powers become more and more manifest : until he is finally led, and that chiefly by the science of which I MAGNETISM. 286 happen this evening to be the exponent, to regard Nature as an organic whole as a body each of whose members sympathises with the rest, changing, it is true, from age to age, but without one real break of continuity, or a single interruption of the fixed relation of cause and effect. The system of things which we call Nature is, how- ever, 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 are taken up by different individuals, and thus receive 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 centrifugal 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 distributor of knowledge, but not a creator, and he fails to attain that vitality of thought, and correct- ness 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 phenomena 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 286 FRAGMENTS OF SCIENCE. and gases, whether in a state of translation or of undula- tion. The science of mechanics is a portion of natural philosophy, though at present so large as to need the ex- clusive attention of him who would cultivate it pro- foundly. Astronomy is the application of physics to the motions of the heavenly bodies, the vastness of the field causing it, however, to he regarded as a department in it- self. 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 compounds. Through their power of separating carbonic acid into its constituents, the solar beams build up the whole vegetable world, and by it the animal world. The touch of the self-same beams causes hydrogen and chlorine to unite with sudden explo- sion, and to form by their combination a powerful acid. Thus physics and chemistry intermingle. Physical agents are, however, 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 the subject of magnetism is to be studied, and how a know- ledge 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 philosophy has accomplished. I might point to those applications 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 photograph, the medical applications of physics, and the million other inlets by which scientific thought filters into practical life. That would be easy compared with the task of informing you how you are to make the study of physics MAGNETISM. 287 the instrument of your own culture ; how you are to possess its facts and make them living seeds which shall take root and grow in the mind, and not lie like dead lumber in the storehouse of memory. This is a task much heavier than the mere cataloguing of scientific achievements ; 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 advise you to get a knowledge of facts from actual observ- ation. 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, provide 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 operation of the force yourself. Half of our book writers describe experiments which they never made, and their descriptions 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 somebody like myself to magnetise 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, and attach your fibre to it. Do it neatly. In the loop place your darning-needle, and bring the two ends or polea, as 288 FRAGMENTS OP SCIENCE. they are called, 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, however, 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 colour 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 stupid ; you are conversing with Nature, and must acquire a certain grace and mastery over her language ; and these practice can alone impart. Let every move- ment be made with care, and avoid slovenliness from the outset. In every one of your experiments endeavour to feel the responsibility of a moral agent. Experiment, as I have said, is the language 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 ex- MAGNETISM. 289 tremity of the magnet attracts one end of the needle, and repels the other. The simple attraction, observed in the first instance, is now replaced by a dual force. Kepeat 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 pen- knife 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 instrument 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 towards the north. Make sure of this, and do not take the statement on my authority. Now take a second darning-needle like the first, and magnetise 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 magnetised upon each other. Take one of them in your hand, and leave the othei suspended ; bring the eye-end of the former near the eye- end of the latter ; the suspended needle retreats : it is repelled. Make the same experiment with the two points; you obtain the same result, the suspended needle is re- pelled. Now cause the dissimilar ends to act on each 290 FRAGMENTS OF SCIENCE. other you have attraction point attracts eye, and eyo attracts point. Prove 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 repulsion is mutual. You have thus demonstrated in the clearest manner the funda- mental law of magnetism, that like poles repel, and that unlike poles attract, each other. You may say that this is all easily understood without doing ; 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 sufficient strength, or employ a thin silk ribbon, and form a loop large enough to hold your magnet. Suspend it; it turns its marked end towards 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 con- venient to determine its north pole yourself, and to mark it with a file. You vary your experiments by causing your magnetised darning-needle to attract and repel your large magnet ; it is quite competent to do so. In mag- netising 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 demon- strate 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 deprecated. A manual dexterity in illustrat- MAGNETISM. 291 ing the interaction of magnetic poles is of the utmost importance at this stage of your progress ; and you must not neglect attaining this 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 endeavour to follow 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, wood, 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 regulation of ships' compasses, you will not fall, as some projectors have done, into the error of screening off the magnetism of the ship by the interposition of such sub- stances. 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 movements visible at a distance, denoting the north and south poles by different colours, 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 hardened ; rendered, in fact, almost as brittle as glass. Six inches of this, magnetised in the manner of the darning-needle, will be better able to carry your paper indexes. Having secured such a strip, you proceed thus : Magnetise a small sewing-needle and determine its 15 292 FRAGMENTS OP SCIENCE. poles ; or, break half an inch, or an inch, off your magnet- ised 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 distri- bution of the magnetism in your strip of steel. Hold the strip upright in your left hand, and cause the test- needle to approach the lower end of your strip ; one end is attracted, the other is repelled. Eaise 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 attracted. 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 opposite end. Supposing the north end of your little needle to be that attracted below, you infer that the entire lower half of your magnetised 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 suggest- iveness the value of an 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 succession to your test-needle, suffices to show that you have not a magnet with a single pole that each half possesses 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 exhibist precisely the same magnetic distribution as the MAGNETISM. 293 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 impossibility ; but does the mind stop there ? No : you follow the breaking process in idea when you can no longer realise it in fact ; your thoughts wander amid the very atoms of your steel, and you con- clude 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 particles. Here, then, is an exhibition of power which we can call forth at pleasure or cause to disappear. We mag- netise our strip of steel by drawing it along the pole of a magnet ; we can demagnetise it, or reverse its magnetism, by properly drawing it along the same pole in the oppo- site direction. 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 magnetised ? The question leads us beyond the region of sense, and into that of imagination. 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 acts necessarily follow, we call the assumption a theory, and, once possessing it, we can not only revive at pleasure facts already known, but we can predict others which we have never seen. Thus, then, in the prosecution of physical science, our powers of observation, memory, imagination, and inference, are all drawn upon. We 294 FRAGMENTS OF SCIENCE. 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 faculty interposes to carry out the principle to its logical consequences. A perfect theory gives dominion over natural facts ; and even an assumption which can only partially stand the test of a comparison with facts, may be of eminent 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 vietf 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 colours ; their mixture produces white. Now I ask you to imagine each of these colours 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 colours being equal to the repulsion of the similar ones. Imagine the two colours mixed so as to produce white, and suppose two strips of wood painted with this white ; what will be their action upon each other ? Suspend 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 repel the red component of your suspended strip ; but then it will attract the green, and, the forces being equal, they neutralise each other. In fact, the least reflection shows you that the strips will be as indifferent to each other as two unmagnetised darning-needles would be under the same circumstances. But suppose, instead of mixing the colours, we painted one half of each strip from centre to end red, and the other half green, it is perfectly manifest that the two MAGNETISM. 295 strips would now behave towards each other exactly as our two magnetised darning-needles the red end would repel the red and attract the green, the green would repel the green and attract the red ; so that, assuming two colours thus related to each other, we could by their mixture produce the neutrality of an unmagnetised body, while by their separation we could produce the duality of action of magnetised bodies. But you have already anticipated a defect in my con- 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 molecule of wood painted green on one face and red on the opposite one. The resultant action of ail the atoms would then exactly resemble the action of a magnet. Here also, if the two opposite colours of each atom could be caused to mix so as to produce white, we should have, as before, perfect neutrality. For these two self-repellent and mutually attractive colours, substitute in your minds two invisible self-repellent and mutually attractive fluids, which in ordinary steel are mixed to form a neutral compound, but which the act of magnetisation separates from each other, placing the op- posite fluids on the opposite faces of each molecule. You have then 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 magnetisation consists in the forcible separation of two fluids which existed in the steel before it was magnetised, but which then neutralised each other by their coalescence. And if you 296 FRAGMENTS OF SCIENCE. 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 rather be the gainer by my own utterance, and by the reaction of your strength. The magnet also is the gainer by the reaction of the body which it magnetises. Look now to your excited piece of steel ; figure each molecule with its opposed fluids spread over its oppo- site faces. How can this state of things be permanent ? 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 neutralise each other? To meet this question philo- sophers 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 magnetised which require the greatest amount of l coercion' to tear their fluids asunder are the very ones which offer the greatest resist- ance to the reunion of the fluids, after they have been once separated. Such kinds of steel are most suited to the formation of permanent magnets. It is manifest, indeed, that without coercive force a permanent magnet would not be at all possible. You have not forgotten, that previous to magnetising your darning-needle both its ends were attracted by your magnet ; 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 MAGNETISM. 297 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 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 magnetised, though not so forcibly as when in actual contact. The nail thus presented to the magnet is itself a temporary magnet. That end which is turned towards 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 between 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 iron under the influ- ence 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 ; repell- 298 FRAGMENTS OF SCIENCE. ing 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 imagina- tion must be quite nimble in picturing these changes. You must be able to see the fluids dividing and reuniting, according as the magnet is 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. You ought to satisfy yourself that they possess the power of placing 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 immensely augmented by an acquaintance with those hidden prin- ciples whereon the facts depend. Still, while you use this theory of magnetic fluids, to track out the pheno- mena and link them together, be sure to tell your pupils that it is to be regarded as a symbol merely, a symbol, moreover, which is incompetent to cover all the facts, 1 but which does good practical service whilst we are waiting for the actual truth. 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 ita own futility. MAGNETISM. 299 This state of excitement into which the annealed iron is thrown by the influence of the magnet, is sometimes called 'magnetisation by influence.' More commonly, however, the magnetism is said to be ' induced ' in the iron, and hence this mode of magnetising is called ' mag- netic 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 unmagnetised darning-needle was attracted in your first experiment; and from this you may at once deduce the consequence that, after the steel has been magnetised, 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 magnetism overbalance the repulsion due to the per- manent 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 farther. 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 300 FRAGMENTS OF SCIENCE. exert no influence on the pole presented to its centre ? Let us see. Let s N, fig. 7, be our magnet, and let n represent a particle of north magnetism placed exactly opposite the middle of the magnet. Of course this is an imaginary case, as you can never in reality thus detach your north magnetism from its neighbour. 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 expressed by the line n m, and the magnitude and direction of the FIG. 7. 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 ra n, which expresses the attraction. Acted upon by two such forces, the particle n 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 towards the magnetic 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 opposite 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 mag- netism, opposite the magnetic equator, its south pole being urged along n , and its north along n p, the little MAGNETISM. 01 needle will be compelled to set itself parallel to the magnet s N. Make the experiment, and satisfy yourselves that this is a true deduction. 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 towards p, and south pole towards 5, 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 act more powerfully on the particle than the more distant one. Let s N, fig. 8, 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 Fro. 8. S iv H N 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 irould lead to the conclusion that the particle would be urged along n q. If we place at n a short magnetic needle, its north pole will be urged along n p, its south 302 FRAGMENTS OF SCIENCE. pole along n g, the only position possible to the needle, thus acted on, being along the line p q, which is no longer parallel to the magnet. Verify this deduction 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 mechanical principles, assign a definite direction to the magnetic needle at every particular place. And substi- tuting, as before, a bit of iron wire for the magnetic needle, the positions of both will be the same. 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, in- stead 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 yon? 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 MAONETIC LINES OP FORCE. Fi-ot* a Photograph by Professor MATB 804 FRAGMENTS OF SCIENCE. 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 mag- netism to the south pole. They are the lines along which the decomposition of the neutral fluid takes place. 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 anything 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 order, and to supply what is needed to render the explanation complete. Do not quit the subject until you thoroughly understand it; and if you are then 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 production of the magnetic curves, the time which we have spent together will not have been spent in vain. In this thorough manner we must master our materials, reason upon them, and, by determined study, attain to clearness of conception. Facts thus dealt with exercise an expansive force upon the boundaries of thought ; they widen the mind to generalisation. We soon recognise a brotherhood between the larger phenomena of Nature and the minute effects which we have observed in our private chambers. Why, we enquire, 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 the middle point of your silk fibre to your magnetic needle ; the needle will thus be uninterfered with by the paper loop, and will enjoy to some extent a power of MAGNETISM. 305 ' 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 towards 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 horizontality ; pass it on towards the south pole, its north end now dips, and directly over the south pole the needle becomes vertical, its north end being now turned downwards. 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 vary- ing from nothing to 90. If we go to the equatorial regions of the earth with a suitably 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 terres- trial magnetic pole the needle sets vertical. The south magnetic pole has not yet been found, but Sir James Eoss discovered the north magnetic pole on June 1, 1831. In this manner we establish 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 direction of the magnetic needle in London, which is called the magnetic meridian, encloses an angle of 24 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 constituents are gradually changing their dis- 306 FRAGMENTS OP SCIENCE. tribution. 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 to enable us to guess, even approximately, 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 needle. I now reverse the poker, bringing its knob undermost ; the knob is now a north pole and attracts the south end of a magnetic needle. Gret such a poker and carefully repeat this experiment ; satisfy yourselves that the fluids shift their position, ac- cording 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 antagonist which opposes the full decomposition of the neutral fluid. The component fluids may be figured as meeting an amount of friction, or possessing an amount of adhesion, which pre- vents them from gliding over the molecules of the poker. Can we assist the earth in this case ? If we wish to re- move 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 fluids to the molecules 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 molecules refuses to MAGNETISM. 307 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. By the use of sensible images, more or less gross, he will first give those whom he teaches definite conceptions, puri- fying 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 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 p. 259, 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 direc- tions 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 currents. He supposed the molecules of a magnetic body to be surrounded by such currents, which, however, in the natural state of the body mutually neutralised each other, on account of their confused grouping. The act i/f magnetisation he supposed to consist in setting these molecular currents parallel to each other ; and, starting from this principle, he reduced all the phenomena of magnet- ism to the mutual action of electric currents. 1 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 ? 308 FRAGMENTS OF SCIENCE. According to Coulomb's and Poisson's theory, the act of magnetisation 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 towards 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 towards 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 them- selves 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 being the case, proves that these molecular currents are not the mechanism by which diamagnetic hiduction 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 actu- ally 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 wculd 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 resist ance.' Diamagnetism and Magne-crystallic Action, p. 136-7. XII. DEATH BY LIGHTNING. T)EOPLE in general imagine, when they think at all JL 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 sensation 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 conscious- ness. 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 experimental arrangement, Helmholtz has de- termined 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, there- fore, 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 belongs a deter- minate molecular arrangement of the brain that every thought or feeling has its physical correlative in that 1 A most admirable lecture on the velocity of nervous transmission has been published by Dr. Du Bois Eeymond in the ' Proceedings of the Royal Institution ' for 1 866, vol. iv. p. 575. 810 FKAGMENTS OF SCIENCE. 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 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-ter.th of a second is demanded for this purpose. Thus, in the case of the whale above supposed, we have first half a second consumed 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 trans- mission 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 responded 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 conductors 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 arrangements 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 cha- racter. 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 DEATH BY LIGHTNING. 3tl might be drawn a 'priori from the experiments of Helm- holtz. Cases of insensibility, moreover, are not uncommon which do not result in death, and after which the persons affected have 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 endures 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 illuminated by an instantaneous flash con- tinues 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 colour-top is familiar to most of us. By this instrument a disk with differently-coloured sectors is caused to rotate rapidly ; the colours 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 colour being clearly seen. Professor Dove has found that a flash of lightning produces the same effect. During a thunder- storm he put a colour-top in exceedingly rapid motion, and found that every flash revealed the top as a motion- less object with its colours distinct. If illuminated solely 812 FEAGMENTS OF SCIENCE. by a flash of lightning, the motion of all bodies on the earth's surface would, as Dove has remarked, appear sus- pended. A cannon-ball, for example, would have its flight apparently arrested, and would seem to hang motionless in space as long as the luminous impression which revealed 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 neighbourhood of Mannneim, being overtaken by rain, placed himself under a tree, beneath which a woman had previously taken shelter. He looked upwards 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- wards 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 me. 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 DEATH BY LIGHTNING. 318 the discharge went through my body. Life was abso- lutely blotted out for a very sensible interval, without a trace of pain. In a second or so consciousness returned ; I saw myself in the presence of the audience and ap- paratus, and, by the help of these external appearances, immediately concluded that I had received the battery discharge. The intellectual 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 my- self 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 painless- ness 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 degree implicated. It is an abrupt stoppage of sensation, unaccompanied by a pang. July 8, 1865. FRAGMENTS OF SCIENCE. XIII. SCIENCE AND THE 'SPIRITS? THEIE refusal to investigate ' spiritual phenomena is often urged as a reproach to scientific men. I kere propose to give a sketch of an attempt to apply to the 4 phenomena ' those methods of enquiry 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 in- vited, or rather entreated, by one of his friends to meet and question them. He had, however, already made their acquaintance, 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 condition of mind. On the contrary, I thought it pro- bable that some physical principle, not evident to the spiritualists themselves, might underlie their manifesta- tions. Extraordinary effects are produced by the accu- mulation of small impulses. Galileo set a heavy pendu- lum 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. Precon- ceived notions can, moreover, vitiate, to an extraordinary degree, the testimony of even veracious persons. Hence SCIENCE AND THE 'SPIRITS/ 315 my desire to witness those extraordinary 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 place at a private residence in the neighbourhood of London. 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 suspicion. 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 downwards, 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. E 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. 16 816 FKAGMEXTS OF SCIENCE. Some time previously I had visited Baron Reichen- bach, 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 ? 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.' /. ' 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.' /. * You are aware of the effects ascribed by Baron Reichenbach to magnets ? ' Medium. ' Yes ; but a magnet makes me terribly ill.' /. 'Am I to understand that, if this room were perfectly dark, you could tell whether it contained a magnet, without being informed of the fact ? ' Medium. 1 1 should know of its presence on entering the room.' /.'How?' Medium. ' I should be rendered instantly ill.' /. ' How do you feel to-day ? ' Medium. ' Particularly well ; I have not been so well for months.' /. ' 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 stam- mered, ' No ; I am not en rapport with you.' / sat at her right hand, and a left-hand pocket, with- in six inches of her person, contained a magnet. Our host here deprecated discussion, as it ' exhausted SCIENCE AND THE 'SPIRITS.' 317 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 re- lated to me by other wonders. A lady present discoursed on spiritual atmospheres, which she could see as beautiful colours when she closed her eyes. I professed myself able to see similar colours, 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 per- formances 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 pre- viously 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 con- tinue 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 c 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 318 FRAGMENTS OF SCIENCE. 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 stetho- scope. 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 myself 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 oscillat- ing 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 touching it, caused it to vibrate, and produce a peculiar buzzing sound. A per- fectly candid and warm-hearted old gentleman 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 explana- tion 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 pro- bably 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 unavail- ing in opposition to theirs. No human power could pre- vent the table from moving when they pulled it. During SCIENCE AND THE 'SPIRITS.' 3J9 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, however, 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. 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 ingemiity 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 consti- tuted the conditions of the phenomena. A rearrangement of our positions was proposed and carried out ; and soon afterwards 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 permission to touch the medium's hand. < Oh I I know I tremble,' was her reply. Throwing one leg across the other, I accidentally nipped a muscle, and produced thereby an involuntary 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 pro- moted it. My attention was promptly drawn to the mo- tion ; and a gentleman beside me, whose value as a witness I was particularly desirous to test, expressed his belief that it was out of the compass of human power to produce 820 FRAGMENTS OF SCIENCE. so strange a tremor. 'I believe,' he added, earnestly. 1 that it is entirely the spirits' work.' ' So do I,' added, with heat, the candid and warmhearted 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 per- plexity ; and, as there was sufficient reason to believe that the disclosure of the secret would simply provoke anger, I kept it to myself. Again a period of conversation intervened, during which the spirits became animated. The evening was confessedly a dull one, but matters appeared to brighten towards its close. The spirits were requested to spell the name by which I am known in the heavenly world. Our host commenced repeating the alphabet, and when he reached the letter