GIFT OF CYCLOPEDIC SCIENCE SIMPLIFIED. SPECTRUM ANALYSIS, Exhibited at the^Royal Polytechnic Institution, London. i. SIMPLE SPECTRUM. 2. SILVER. 3. THALLIUM. CYCLOPEDIC SCIENCE SIMPLIFIED. j. H. PEPPER, Of the Royal Polytechnic Institution, Fellow of the Chemical Society, Associate of the Institution of Civil Engineers ; Author of -various IVorks for Youth, &c. EMBRACING LIGHT REFLECTION AND REFRACTION OF LIGHT LIGHT AND COLOUR SPECTRUM ANALYSIS THE HUMAN EYE POLARIZED LIGHT HEAT THERMOMETRIC HEAT CONDUCTION OF HEAT LATENT HEAT STEAM ELECTRICITY VOLTAIC, GALVANIC, OR DYNAMICAL ELECTRICITY MAGNETISM ELECTRO-MAGNETISM, MAGNETO-ELEC . TRICITY, THERMO-ELECTRICITY DIA-MAGNETISM WHEATSTONE'S TELEGRAPHS PNEUMATICS THE AIR-PUMP THE DIVING-BELL ACOUSTICS THE EDUCATION OF THE EAR CHEMISTRY ELEMENTS WHICH ARK NOT METALLIC THE METALS WITH NUMEROUS ILLUSTRATIONS. BE7ISFD EDIT70JJT, FREDERICK WARNE AND CO., BEDFORD STREET, COVENT GARDEN. NEW YORK : SCRIBNER, WELFORD, AND ARMSTRONG. , , INTRODUCTION. T N the Author's earlier works, written only for the youthful student * in Science, a promise was made that other books, to be regarded as a series of steps in Science, should be forthcoming. It is with this view that the present volume is offered; and as the general reader in fact, "the Public" has not the time or the inclination to study the very minute and laborious works of GMELIN, WATT, MILLER, and other learned authors, it is hoped that the facts contained in this more advanced but still elementary work will be found sufficiently attractive to stimulate, at all events, the would-be philosopher to further reading, and especially to perform correct scien- tific experiments. Brevity and simplicity have been carefully attended to in the following pages; and when other authors are quoted, the writer has preferred giving their exact language, instead of altering and para- phrasing words, which frequently detracts from the sense of the passage. The reader will find portions of valuable papers written by 256328 vi INTRODUCTION. FARADAY, DANIELL, WHEATSTONE, BREWSTER, TYNDALL, CROOKES, BROWNING, SIEMENS, NOAD, STEWART, TAIT, MARLOYE, and others, with a brief summary of Photography by JOHN SPILLER, Esq. In a work like this, including such a multiplicity of subjects, the kind indulgence of the reader is invoked for any errors that the most painstaking supervision may have permitted to pass. I dedicate this work, with all kindly feelings, to those students at Harrow, . Eton, Hayleybury, and Cheam, to whom, under the auspices of the Rev. Drs. Vaughan, v Goodford, Hawtrey, Butler, Bradby, and Tabor, I have addressed many lectures on Science. I believe and trust that those lectures have not been alto- gether unfruitful ; but that, they have aided in the establishment of regular science classes for the present generation, instead of the desultory lectures at rare intervals to which custom formerly con- demned the teachers of popular science. JOHN HENRY PEPPER. CONTENTS. LIGHT. Page LIGHT, AND THE ETHER SUPPOSED TO PERVADE THE WHOLE UNIVERSE I CORPUSCULAR THEORY OF LIGHT 2 EXPERIMENTS WITH BLACKED ALUMINIUM Disc 4 SOURCES OF LIGHT 8 HEAT A SOURCE OF LIGHT 10 LIGHT THE FREQUENT ATTENDANT OF ELECTRICAL PHENOMENA 10 CHEMICAL COMBINATION A SOURCE OF LIGHT n Is MECHANICAL FORCE TO BE REGARDED AS A TRUE SOURCE OF LIGHT? 12 THE DIFFUSION OF LIGHT 14 MODIFICATIONS THAT LIGHT MAY UN- DERGO 20 THE REFLECTION OF LIGHT 21 THE GHOST ILLUSION 23 IMAGES FORMED BY SILVERED MIRRORS 27 THE KALEIDOSCOPE 31 THE JAPANESE MAGIC MIRROR 35 BROWNING'S DESCRIPTION OF THE SIL- VERED GLASS REFLECTING TELE- SCOPES 41 THE REFRACTION OF LIGHT 49 DIOPTRICS 49 REFRACTION OF LIGHT THROUGH PLANE GLASS 53 REFRACTION OF PARALLEL RAYS OF LIGHT BY CONVEX SURFACES 53 REFRACTION OF PARALLEL RAYS BY CONCAVE SURFACES 54 OTHER FORMS OF LENSES.... . 54 L I G H T -continued. Page OPTICAL INSTRUMENTS WHOSE PROPERTIES DEPEND ON RE- FRACTION" 55 THE SIMPLE AND COMPOUND MICRO- SCOPE AND TELESCOPE 55 THE CAMERA OBSCURA 57 THE HUMAN EYE 64 THE STEREOSCOPE 68 PROFESSOR WHEATSTONE'S REFLECTING STEREOSCOPE 68 DIRECTIONS FOR USING THE STEREO- SCOPE 71 PERSISTENCE OF VISION 7 i LIGHT AND COLOUR 86 SPECTRUM ANALYSIS 86 ABERRATION AND ACHROMATISM 86 PHYSICAL PROPERTIES OF THE SPEC- TRUM 91 THE DARK OR FIXED LINES IN THE SOLAR SPECTRUM 92 HOW TO USE THE SPECTROSCOPE 95 SPHERICAL ABERRATION 103 THE DISPERSION OF LIGHT, OR CHRO- MATIC ABERRATION 104 THE INTERFERENCE OF LIGHT... 106 COLOURS OF THIN PLATES 106 DOUBLE REFRACTION AND THE POLARIZATION OF LIGHT in POLARIZATION BY REFLECTION AND BY SIMPLE REFRACTION 114 POLARIZATION BY THE TOURMALINE 116 CONTENTS. HEAT. Page THERMOMETRIC HEAT 123 THE COMMON EFFECTS OF HEAT 127 AMOUNT OF EXPANSION IN SOLIDS, LIQUIDS, AND GASES 130 THE EXPANSION OF LIQUIDS 132 THE THERMOMETER 135 THE PYROMETER 139 THE EXPANSION OF GASES 142 CONDUCTION 147 "POTENTIAL" FORCE 151 "ACTUAL" FORCE, OR "ENERGY" 151 LATENT HEAT .. ... 160 H E A ^continued. Page ENERGY OR HEAT 161 CAPACITY FOR HEAT 164 STEAM 172 THE STEAM ENGINE 179 DESCRIPTION OF THE STEAM ENGINE ... 182 EVAPORATION 193 HYGROMETRY 194 RADIATION 196 TRANSMISSION OF HEAT 200 THE CONVERSION OF LIGHT RAYS INTO HEAT RAYS, AND VICE VERSA, BY CHANGE OF RfiFRANGIBILITY 2O2 ELECTRICITY. ELECTRICITY, FRICTIONAL OR STATICAL THE ELECTROSCOPE.... THEORIES OF ELECTRICITY 213 EXPERIMENTS WITH THE ELEC- TROSCOPE 214 ELECTRICAL MACHINES 220 ELECTRICAL ATTRACTION AND REPULSION GOVERNED BY CERTAIN LAWS 227 THE ELECTRIC WELL 234 ELECTRICAL INDUCTION 236 THE ELECTROPHORUS 245 THE LEYDEN JAR 247 EXPERIMENTS WITH THE ELECTRICAL MACHINE, THE LEYDEN JAR, AND LE.YDEN BATTERY 255 ELECTRICIT "Y continued. THE HYDRO-ELECTRIC MACHINE 272 SUMMARY OF THE LAWS OF ELEC- TRICAL ACCUMULATION 279 LATERAL DISCHARGE 284 VOLTAIC, GALVANIC, OR DYNAMI- . CAL ELECTRICITY 285 DYNAMICAL ELECTRICAL PHENOMENA OBTAINED FROM THE VOLTAIC BAT- TERY 311 "FARADAY'S RESEARCHES" 315 ON A NEW MEASURER OF VOLTA-LEC- TRICITY ; 318 OHM'S LAW 326 THE RHEOSTAT OF WHEATSTONE 329 THE CALORIFIC EFFECTS OF THE VOLTAIC CURRENT 335 THE ELECTRIC TORPEDO 341 THE ELECTRIC LAMP 344 MAGNETISM. THE MAGNET 349 DIA-MAGNETISM 364 ELECTRO-MAGNETISM 3 7 c MAGNETIS TH- continued. MAGNETO-ELECTRICITY 378 INDUCTION BY CURRENT ELEC- TRICITY 378 CONTENTS. XI MAGNETIS ^.-continued. THERMO-ELECTRICITY .... Page .. 389 WHEATSTONE'S TELEGRAPHS 392 IMPROVEMENTS IN ELECTRIC TELE- GRAPHS, AND IN APPARATUS CON- NECTED THEREWITH 407 MAGNETIS TUL-coutmued. Page SIR CHARLES WHEATSTONE'S LAST TELEGRAPHIC APPARATUS 413 THE ATLANTIC TELEGRAPH CABLE 421 THE DIFFERENTIAL RESISTANCE MEA- SURER 423 ON THE CONSERVATION OF FORCE 430 PNEUMATICS. PNEUMATICS THE AIR-PUMP THE DIVING-BELL 433 434 442 EXPERIMENTS WITH THE AIR- PUMP ... PNEUMATICS THE BAROMETER ADMIRAL FITZROY'S WEATHER GUIDE. WATER-PUMPS THE PNEUMATIC LEVER 450 452 459 467 ACOUSTICS. ACOUSTICS 472 MARLOYE'S INTRODUCTION TO CHEVAL- LIER'S CATALOGUE 474 ON THE EDUCATION OF THE EAR 480 CONSIDERATIONS ON SOUND 484 PROJECT OF STUDY CONCERNING THE ACOUSTICS OF PUBLIC BUILDINGS ... 489 "ON THE SOUNDS PRODUCED BY FLAME IN TUBES, &c." 502 VIBRATIONS OF STRINGS, RODS, PLATES, AND COLUMNS OF AIR 506 LONGITUDINAL VIBRATIONS OF STRINGS 507 LONGITUDINAL VIBRATIONS OF RODS ... 508 VIBRATING PLATES 509 ACOUSTIC ^continued. TRANSVERSE VIBRATIONS OF BLADES AND RODS 509 LONGITUDINAL VIBRATIONS OF COLUMNS OF AIR 510 EMBOUCHURES 512 THE REFLECTION, REFRACTION, &c., OF SOUNDS THE TRANSMISSION OF SOUNDS THROUGH GASEOUS, LIQUID, AND SOLID MEDIA TRANSMISSION OF SOUND THROUGH LIQUIDS 520 TRANSMISSION OF SOUND THROUGH SOLID CONDUCTORS 520 513 .... 517 CHEMISTRY. CHEMISTRY 527 OLEOGRAPHY : BEING A PROCESS FOR THE UTILIZATION OF TOMLINSON'S CO- HESION FIGURES 531 CHEMISTR Y continued. EXHIBITING COHESION FIGURES TO A LECTURE AUDIENCE 533 CHEMICAL ACTION ... ... 537 Xll CONTENTS. CHEMISTR Y -continued. Page NOMENCLATURE 539 ELEMENTS WHICH ARE NOT ME- TALLIC 544 OXYGEN .' 544 THE PROPERTIES OF OXYGEN 548 REMARKS 549 OZONE 549 NITROGEN 55i HYDROGEN 552 NITROGEN AND HYDROGEN, AMMONIA... 566 THE HALOGENS CHLORINE 568 IODINE 570 THE ART OF PHOTOGRAPHY 571 BROMINE 575 FLUORINE 576 CARBON 577 " ON THE PRESSURE CAVITIES IN TOPAZ, BERYL, AND DIAMOND, AND THEIR BEARING ON GEOLOGICAL THEORIES" 581 COMPOUNDS OF CARBON WITH OXYGEN 582 CARBONIC OXIDE 586 COMPOUNDS OF CARBON WITH HYDROGEN 586 BORON 587 SILICON 589 How GEMS ARE MANUFACTURED 592 SELENIUM ". 594 SULPHUR 595 COMPOUNDS OF SULPHUR WITH HYDROGEN 599 PHOSPHORUS 600 RED PHOSPHORUS 605 THE METALS. TELLURIUM 608 ARSENIC 609 SOURCES OF ARSENIC 609 PHYSICAL QUALITIES OF ARSENIC 609 CHEMICAL PROPERTIES OF ARSENIC 610 ANTIMONY 613 SOURCES WHENCE DERIVED 613 PHYSICAL PROPERTIES OF ANTIMONY ... 614 CHEMICAL PROPERTIES OF ANTIMONY ... 614 BISMUTH ... 614 CHEMISTS, Y continued. Page SOURCES WHENCE DERIVED 614 PHYSICAL PROPERTIES OF BISMUTH 615 CHEMICAL PROPERTIES OF BISMUTH 615 CLASSIFICATION OF THE METALS 616 CLASS I. POTASSIUM 617 SOURCES WHENCE DERIVED 617 PHYSICAL PROPERTIES OF POTASSIUM ... 618 CHEMICAL PROPERTIES OF POTASSIUM... 618 SODIUM 619 SOURCES WHENCE DERIVED 619 PHYSICAL PPOPERTIES OF SODIUM 619 CHEMICAL PROPERTIES OF SODIUM 620 RUBIDIUM 620 CESIUM 620 LITHIUM i 62* AMMONIUM 621 CLASS II. CALCIUM STRONTIUM BARIUM ALUMINIUM , MAGNESIUM. CLASS III. 622 622 623 ... 623 CLASS IV. 626 ZINC 629 SOURCES WHENCE DERIVED 629 PHYSICAL PROPERTIES OF ZINC 630 CHEMICAL PROPERTIES OF ZINC 6x> CADMIUM 630 CLASS V. IRON 631 SOURCES WHENCE DERIVED 631 PHYSICAL PROPERTIES OF IRON 637 CHEMICAL PROPERTIES OF IRON 637 MANGANESE 638 COBALT 6 3 3 NICKEL 639 CHROMIUM 639 URANIUM 639 INDIUM.... ... 6 4 o CONTENTS. Xlll CHEMISTR t continued, CLASS VI. TIN .. Page 640 TITANIUM 640 NIOBIUM 640 TANTALUM 640 CLASS VII. TUNGSTEN 641 CLASS VIII. ARSENIC, ANTIMONY, AND BIS- MUTH 641 CHEMISTS, 'X continued. CLASS IX. Page LEAD 642 THALLIUM 642 CLASS X. SILVER 6 44 COPPER 648 MERCURY 650 CLASS XL PLATINUM 650 GOLD 651 How JEWELLERY is MAIJE BY MACHIN- ERY 652 ORGANIC CHEMISTRY. ORGANIC ANALYSIS , 657 DR RICHARDSON'S EXPERIMENTS IN OR- GANIC DECOMPOSITION .. ... 660 ORGANIC CHEMISTRY-r*/zf. EXPOSURE OF ANIMAL SUBSTANCES TO WATER GAS AT A HIGH TEMPERA- TURE 660664 ON LIGHT, AND THE ETHER SUPPOSED TO PERVADE THE WHOLE UNIVERSE. ABOUT two hundred years ago Descartes, Hook, and Huygens, three of the most celebrated mathematicians of their day, entertained the idea that light was propagated by the vibrations and undulations of a subtile elastic fluid called ether, which not only filled infinite space, but was con tained in all solid, fluid, and gaseous bodies. The immortal Newton, who was opposed to this theory, or at least created one of his own, usually called the Corpuscular Theory of Light, appears to have entertained the opinion (according to Enfield) that " All fixed bodies, when heated beyond a certain degree, emit light and shine ; and this emission is performed by the vibrating motion of their parts." " The heat of a warm room is conveyed through a vacuum by the vibration of a much subtiler medium than air, which, after the air is drawn out, remains in the vacuum. " It is by the vibrations of this medium that light is refracted and reflected, and heat communicated. This medium is exceedingly more elastic and active, as well as subtile, than the air ; it readily pervades all bodies, and is by its elastic force expanded through the heavens. Its density is greater in free and open space than in compact bodies, and increases as it recedes from them. This medium, growing densei and denser perpetually as it passes from the celestial bodies, may, by its elastic force, cause the gravity of those great bodies towards one another, and of their parts towards the bodies. Vision, hearing, and animal motion may be performed by the vibrations of this sub- tile elastic fluid or ether." \LIGHT. These opinions would seem to show that Newton believed all emanations of particles of light were attended by the undulations of an ethereal medium accompanying it in its passage. The theory, however, generally ascribed to him is, that rays of light are small corpuscles emitted with exceeding celerity, travelling at about the rate of one hundred and eighty-two thousand miles per second ; and these rays of light, falling upon the eye, excite vibrations in the tunica retina, which, being propagated along the solid fibres of the optic nerve to the brain, cause the sense of sight. Could Newton, who insisted so much on the importance of experimenting before enunciating a theory, have been acquainted with the highly interesting experiments connected with the inflection or diffraction of light, he would not have opposed the notion of an analogy between the phenomena of light and sound when he says : " The waves, pulses, or vibrations of the air, wherein sound consists, are manifestly inflected, though not so considerably as the waves of water ; and sounds are propagated with equal ease through crooked tubes and through straight lines ; but light was never known to move in any curve, nor to inflect itself ad umbram" This decided statement is directly contradicted by actual experiment, because light can be bent into or towards the shadow. The corpuscular theory fails to explain that which is easily understood by the undulatory theory, and by analogy to waves of water or air, that two rays of light may come together in a special manner and produce darkness, just as two waves of water may interfere with each other and form a smooth surface, or two waves of sound produce silence. Dismissing the theory of Newton as we might pass by the venerable ruins of some ancient edifice, with mingled interest and regret, we may return to the consideration of the ether supposed to fill all space. The great Dr. Franklin, in a letter dated 23rd April, 1752, throws out the suggestion that all the phenomena of light may be more conveniently solved by supposing universal space filled with a subtile elastic fluid, which when at rest is not visible, but whose vibrations affect that fine sense in the eye as those of air do the grosser organs of the ear. Thornbury, Mitchell, and others, endeavoured to prove the materiality cf light by showing that the corpuscles had a power of momentum which might affect other and very light substances. Could this fact have been really ascer- tained, there would be nothing more to say against Newton's hypothesis ; but their experiments were illusory and useless. On the other hand, the supporters of the undulatory theory have within the last three years performed the most elaborate and exact experiments to try to prove the real existence of the ether. Mr. Balfour Stewart, F.R.S., superintendent of Kew Observatory, and Pro- fessor P. G. Tait, M.A., of Edinburgh, whilst leaving other scientific men to make their own deductions from the results they obtained, have called attention to the subject by a paper read before the Royal Society in June, 1865, and modestly entitled " On the Heating of a Disc by Rapid Rotation in vacuo." The authors, having obtained certain results in air, were encouraged to construct the apparatus as figured below, Fig. I, wherewith to procure rotation in vacuo. " In this apparatus a slowly revolving shaft is carried up through a barometer tube, having at its top the receiver which is to be exhausted. When the exhaustion has taken place, the shaft connected with the multiplying gear revolves in mercury. The train of toothed wheels causes the disc of alumi- ON LIGHT. nium to revolve 125 times for each revolution of the shaft. The thermo-electric pile, the most delicate thermometer or test of heat, is connected by two wires carried through two holes in the bed-plate of the receiver with a Thompson's reflecting galvanometer needle (an instrument which is described and figured FIG. i. a, Fifs. i and 2, thermo-electric pile with reflecting cone attached ; ab, height 6 in. from bed-plate; a c, length of cone, &c., jj in. ; c d, diameter of the aperture of the reflecting cone 2^ in. ; / h, the disc of aluminum 13 in. diameter; eg, height from bed-plate to centre of the aluminum disc 8 in. ; h e, distance of centre of the thermo-electric pile from the disc of aluminum 8 in. ; m, base containing the multiplying gear; s i s, air-tight glass receiver, ig in. diameter and 16 in. high, covering the whole. 1 2 ON LIGHT. in the article on Electricity in this work). The outside of the thermo-electric pile and its attached cone was wrapped round with wadding and cloth, so as to be entirely unaffected by currents of air. " During these experiments the disc of aluminium was rotated rapidly for half a minute, and a heating effect was, in consequence of the rotation, recorded by the thermo-electric pile (an instrument described fully in the article on Electricity). " To obviate the objection that the electric currents which take place in a revolving metallic disc might alter the zero of the galvanometer, the position of the line of light was read before the motion began, and immediately after it ceased, the difference being taken to denote the heating effect produced by the rotation. "The thermometric value of the indications given by the galvanometer was found in this way : The disc was removed from its attachment and laid upon a mercury bath of known temperature. It was then attached to its spindle again, being in this position exposed to the pile, and having a temperature higher than that of the pile by a known amount. The deflection produced by this exposure being divided by the number of degrees by which the disc was hotter than the pile, gives at once the value in terms of the galvanometric scale of a heating of the disc equal to i on Fahrenheit's scale. " The disc of aluminium being blackened with a' coating of lampblack, ap- plied by negative photographic varnish, and rock salt inserted in the cone, the following results were obtained : No. of No. of observations Time at Heat indications set. in each set. full speed. Fahrenheit. I. 3 30 0-85 II. 4 30 0-87 III. 4 30 o'8i IV. 3 30 075 " To ascertain whether the radiant heat recorded was derived from the rock salt, or from heated air, or from the surface of the disc, the next series of experiments were tried. EXPERIMENTS WITH BLACKED ALUMINIUM Disc WITHOUT ROCK SALT. No. of No. of observations Time at Heat indications set. in each set. full speed. Fahrenheit. V. 3 30 0-92 VI. 3 30 0-93 " With certain modifications of the above experiments it was satisfactorily proved that the effect was not due to heating of the rock salt, or to radiation from heated air ; it must therefore be due to the disc of aluminium, which seemed to have rubbed against some matter which remained in the receiver after the air was removed. The question being, was this ether?" The authors further state that, " i. It may be due to the air which cannot be entirely got rid of. " 2. It is possible that visible motion becomes dissipated by an etherial medium in the same manner and possibly to nearly the same extent as molecular motion, or that motion which constitutes heat. " 3. Or, the effect may be due partly to air and partly to ether. " Not to leave the matter wholly undecided, it was suggested by Professors ON LIGHT. Maxwell and Graham that there is another effect of afr, viz., fluid friction, the coefficient for which they believe to be independent of the tension. " It would appear, however, that the fluid friction of hydrogen is much less than that of atmospheric air, so that were the heating effect due to fluid fric- tion it ought to be less in a hydrogen vacuum. An experiment proved that the heating effect due to rotation in a hydrogen vacuum was 22*5, while in an air vacuum it was 23*5, and the authors are inclined to consider these numbers as sensibly the same, and that the experiment indicates that the effect is not due to fluid friction ; at the same time they do not suppose that their experiments have yet conclusively decided the origin of this heating effect, but they hope to elicit the opinions of those interested in the subject, which may serve to direct their future research." These experiments are more satisfactory than any previously tried, and, taken in conjunction with other facts, such as the temporary phosphorescence of certain bodies by what is termed insolation or irradiation, or the action of light in reducing certain salts to their metallic state, or the elaborate and beautiful effects obtainable from thin films of solid, fluid, and gaseous bodies, or the action of crystallized bodies on polarized light, they do altogether impress the reasoning faculties with a conviction that a vibrating motion accompanies the production of all light, which can only be propagated by the communication of these vibrations or tremblings to a medium, itself a? subtile, rare, and exquisite as the delicate mechanism that sets it in motion. Starting with the proposition that all sources of light and luminous bodies, like musical instruments, must first vibrate, it is not difficult to understand by analogy how these vibrations may travel at the rate of 182,000 miles per second, in straight lines, called rays. FIG. 3. A, tuning-fork struck on the leaden cone B, capped with leather, and applied to the end of the rod c, whilst the other end is held against the sounding-board D. A tuning-fork emitting sound might by analogy represent a source of light like the sun, whilst a long rod communicating with it would stand in the place of the theoretical ether, propagating the undulations from the sun through a space of 92^ millions of miles, and if the other end of the rod communicates with the sounding-board of a guitar, the audible sound obtained might com- pare with the light falling on the earth and becoming apparent by radiation. ON LIGHT. The conversion of a continued series of mechanical impulses into waves is beautifully shown by taking hold of the end of a long vulcanized india rubber tube filled with sand, and having attached one end to the ceiling or other con- venient place, it is easy by a jerk to produce the appearance of a wave, which travels distinctly from the hand to the ceiling ; at the same time it demonstrates the progressive nature of the wave or undulation, and as the portion held by the operator cannot move from his hand to the ceiling, it shows how the eye is deceived whilst looking at the motion of waves of water. Every wave in water is propagated by the rising and falling of that which has preceded it, and not because the volume of water representing the wave travels bodily from the spot where it is first noticed to the shore where it breaks. FIG. 4. The Vulcanized Tube attached to the ceiling, and tlirown into protuberance or waves by the hand of the operator. Dr. Tyndall has shown, by a modification of Dr. Young's experiments with vibrating strings upon which light is thrown, a number of very beautiful effects. A silvered cord attached to the iron arm of a curved spring band, one end of which is made to vibrate by an electro-magnet, displays the divisions of the cords into wave-like figures most perfectly when the cord is illuminated by the lime or, better still, the electric light. (Figs. 5 and 6, p. 7.) Using the brilliant light as before, a still more perfect and admirable experi- ment may be conducted by attaching one end of a bright silvered chain to a hook screwed into a vertical whirling table, and the other to a proper stand. The chain being horizontal and the wheel vertical, it may be swung into one long wave, or, by a still more rapid rotation, can be divided into three, four, or more. The links of the chain flash in the light, and produce the most pleasing effects. It must be remembered that if cords, chains, water, air, &c., can assume a wave-like motion, the wonderful tension and elasticity of the hypothetical ether ON LIGHT. 8 ON LIGHT. would permit the latter to adapt itself to the most complicated movements almost with the rapidity of thought. The very spiral, spindle-like, or cork- screw motion observable in the chain and cord affords a -good idea of the mechanism of the propagation of light, as the movement of e ach molecule of ether is always perpendicular to the path of the ray or wave of light. The astonishing rapidity of the periodic movements of the non-gravitating molecules of ether becomes apparent, when it is stated that to produce white light five hundred millions of millions of vibrations of the ether, 1,000,000,000,000 X 500 must occur in every second of time. Or, taking the coloured rays at the extremities of the solar spectrum, viz., the red ray and the violet, the former demands the recurrence of four hundred and fifty-eight millions of millions, 1,000,000.000.000 X 458 ; and the latter, the violet, a still larger number, and greater rapidity of vibration, six hundred and ninety-nine millions of millions, 1,000,000,000,000 X 699 per second. The coloured rays of light are supposed, according to the undulatory theory, to be distinguished from each by the breadths of the different waves, just as the sound of a sti inged instrument may vary according to the diameter and thickness of the strings. A tightly-stretched thin cord vibrating would be the parallel to violet light. It is an axiom that, " The rapidity of vibration is inversely proportional to the length and diameter of the string, and propor- tional to the square root of the tension" A thicker cord 'not so tightly stretched would be the oarallel to red light. SOURCES OF LIGHT. At the various instrument-makers cases containing four or five tubes, filled with white powders and hermetically sealed, are to be obtained. When the tubes are observed in a dark room (and, of course, before exposure to light), ihey are invisible ; if, however, a piece of magnesium wire is now burnt close to the tubes, they will be found to shine in the dark and to emit various coloured rays of faint light. To this curious effect is given the name of phos- phorescence ; and when the same result is obtained by exposing the tubes to the light of the sun, the resulting phenomenon is denominated phosphorescence after insolation, z'.^., after exposure to the sun. The chemical substances which possess the property of developing light after exposure to light are called phosphori, and the best are the diamond, Bolognian phosphorus, or Bologna stone, made from sulphate of baryta, which occurs in nature as a mineral, and is called heavy spar or barytine. It is prepared by heating this mineral with charcoal to a dull red heat, or by the process of Margraf, in which the mineral is powdered, mixed with flour, and made red hot ; or more amusingly by the process of Daguerre, who uses a marrow-bone for his crucible, and, after it is freed from fat and thoroughly dried, fills it with heavy spar, powdered in any ;z0;/-metallic mortar. The bone is now closed with a clay lute, and inclosed in an iron tube, which is surrounded with fine clay, and the whole exposed for three hours to a red heat in a furnace. The sub- stance which produces the effect is a sulphuret of barium. In the same manner strontian phosphorus is obtained from ccelestin. Canton's phosphorus is prepared by exposing a mixture of three parts of sifted and calcined oyster-shells and one part of flowers of sulphur to a strong SOURCES OF LIGHT. fire for one hour. There are also many other phosphori ; amongst these may be enumerated Osann's phosphori, Wach's phosphori, Homberg's phosphorus, Baldwin's phosphorus, and many kinds of fluor spar. The phosphorescence of these various bodies, unlike that of the curious element phosphorus, is produced independently of any chemical change ; and if inclosed in sealed glass tubes, and excluded from light, they may retain the property of showing phosphorescence for many years, whilst the light emitted from phosphorus is due to the slow oxidation of this element ; and if this is arrested, by placing it in water, or in any gas, like nitrogen, the light is no FiG. 7. The Phosphorescent Tubes. longer produced. Upon what principle, then, is it possible to explain the cause of the emission of light after exposing phosphori to the sun or any brilliant artificial light? The most rational theory which can be suggested is, that the undulations of light convey their own vibratory motions to the phosphori, just as one musical instrument may cause another to vibrate symphathetically with it, and phosphorescence is observed so long as the substance continues to vibrate. In a dark room, and without a constant accession or supply of vibratory power, the light becomes fainter and fainter, until it is no longer capable of affecting the eye ; the vibratory power, like any other mechanical motion, must come to an end when cut off from its source of power, when, as in this case, it is removed from the greater vibratory power, the sun or the burning magnesium, which originally set it in motion. This opinion is further confirmed when we take into account the large number of- substances which may become phos- phorescent in a tolerably high degree. If this property was confined to a few bodies, the theory might not be so applicable; but if it is agreed beforehand io ON LIGHT, that any particles may become luminous if they are capable of entering into that state of vibration which we suppose belongs to the sun and artificial sources of light, then it can be understood why the following organic or inorganic substances are all considered to enjoy in a limited degree the pro- perty of phosphorescence after exposure to the sun : crystallized boracic acid, sal ammoniac, sulphate of potash, nitre, crystallized carbonate, borate, or sulphate of soda, rock salt, witherite, radiating heavy spar from Bologna, marienglas, fibrous gypsum, alabaster, artificial sulphate of lime, common fluor spar, crystallized sulphate of magnesia, crystallized alum, arsenious acid, pharmacolite, freshly prepared flowers of zinc, sulphate of mercury, tartar, benzoic acid, loaf sugar, sugar of milk, bleached wax, white paper (especially when it has been heated almost to burning), yellow and red paper, which are nearly as phosphorescent as white paper, egg-shells, corals, snails, pearls, bones, teeth, ivory, leather, and skins of men and animals, tartaric acid, also seeds, grain, flour, starch, crumbs of bread, gum arabic, feathers, cheese, yolk of egg, muscular flesh, tendons, isinglass, glue, horn, all well dried ; moreover, the albumen of trees, bleached linen, bleached cotton yarn, and other bleached vegetable fibres. The above is only a small instalment of the different chemical bodies and common substances which Gmelin enume- rates when he speaks of those things which become phosphorescent by irradia- tion. Phosphorescence may also be further developed by heat, mechanical force, and crystallization, all of which are modes of motion, and suggest the setting up of a vibratory effect, resulting in the production of light. Chemical action, another mode of motion, is concerned in the phosphorescence of live animals and putrifying animal matter, and also in the production of the same effect in living and decaying plants. HEAT A SOURCE OF LIGHT. When iron is heated to a temperature of 635 Fahrenheit, it emits a dull red light, visible only in a darkened room. If the heat is further increased to 903 Fahrenheit, a bright red light is apparent, visible in a chamber fairly illumi- nated. The light attains a greater intensity at the moment the iron is heated to 1000 Fahrenheit. Thus, by the progressive increase of the heat of the iron, what is called a dull red, a pale red, and a white heat is obtained. By increasing the heat of a solid body a development of light or incandescence is obtainable. LIGHT THE FREQUENT ATTENDANT OF ELECTRICAL PHENOMENA. The intense and dazzling brightness of lightning has been known to cause temporary and permanent blindness. The immense electric spark, the result of the discharge of thousands of acres of charged clouds, will probably be more closely imitated than ever by an enormous induction coil, now being constructed by Mr. Apps for the Royal Polytechnic, which is calculated to give a spark 5 ft. in length, the usual length being from 5 to 18 in., or, in very rare cases, 2 ft. At the moment of discharge the electricity may develop light, heat, magnetical, mechanical, and chemical effects. Here is a correlation of forces that might well excuse Oersted in proposing a theory of light in which he regards light as the result of electric sparks. SOURCES OF LIGHT. ii FIG. 8. The Inductorium of Mr, Apps, giving sparks 18 in. in length. CHEMICAL COMBINATION A SOURCE OF LIGHT. Finely divided lead or iron shaken from a tube into the air or oxygen oxidizes rapidly, burns, and emits light. Finely powdered antimony unites rapidly with chlorine gas, and glows with the intensity of light whilst the FIG. 9. Blotting-paper upon which the Solution of Phosphorus in Sulphide of Carbon has been poured, and then supported on an iron wire. combination is taking place. A solution of phosphorus in sulphide of carbon, poured upon blotting-paper, soon begins to evolve smoke, produced during the formation of phosphoric acid, and then rapidly and spontaneously catches fire 12 ON LIGHT. by the union of the finely divided phosphorus with the oxygen of the air. The name of Greek modernized into Fenian fire is given to this solution, which should only be made and used in small quantities. Is MECHANICAL FORCE TO BE REGARDED AS A TRUE SOURCE OF LIGHT? Since the numerous experiments made at Shoeburyness with iron plates and heavy guns, it has been ascertained over and over again that heat and fre- quently light are produced at the moment the impact or blow is given by the shot. The mechanical force, in the abstract, may be regarded as the source of light ; but not perhaps directly, as the blow develops heat, and the latter, FIGS. 10 and n. The Shadow Blondin. X^Tt-' Arrangement of Mechanism and Oxy-Hydrogen Light required to produce the effect of the Shadow Blondin. A, the mechanical figure; B', the lime-light ; c, the handles used to produce the movements of the figure. the figure probably, the light. It is found that almost all bodies which acquire phos- phorescence by exposure to the sun, or insolation, or by heat, also become luminous by friction or percussion. Sometimes the light obtained by friction is simply electrical. The sparks from a flint and steel are due to the com- bustion of minute particles of metal accelerated by the heat eliminated at the SOURCES OF LIGHT. FIG. 12. Effect in front of the Curtain. moment the particle is struck off. Mechanical force can only be regarded as an indirect mode of producing light, because heat is first developed ; heat is a source of light. From what has been previously stated, it will be understood that all matter may be divided in relation to light into luminous and non-luminous bodies. The sun or a lighted lamp would represent the former, and the moon with the other planets, or a piece of whitened board, the latter, because our satellite shines by borrowed light from the sun, and not by any inherent self-luminosity; the piece of board will reflect and scatter the rays of light from the lamp, and whilst doing this appears very bright. At the same time the board obstructs the light and casts a shadow behind it, and thus indicates another relation of light to solid matter, called opacity ; the opposite to this property being transparency, whilst the intermediate links between opacity and trans- parency are termed semi-transparency, or opalescence. There are many very amusing effects produced by casting shadows of living or inanimate objects on a transparent disc by the oxy-hydrogen light. (Figs. 10, 1 1, 12.) i 4 ON LIGHT. The shadow pantomimic action of living figures visible on a transparent disc with this strong light, and first introduced by the author at the Poly- technic, has gone the round of nearly all the exhibitions and theatres in London and New York. There still remains, however, something new and amusing even in this hackneyed branch of light. Mr. Walker, jun., constructed a very simple and ingenious piece of mechanism, and giving it the outline of a human figure, produced a good imitation of the bold feats performed by Monsieur Blondin on the high rope. The shadow of the figure only was projected on to the disc by the lime-light, and it simulated all the usual movements, such as standing, walking, dancing, and sitting astride the rope. Indeed it did rather more than the living prototype, for the figure stood on its head, and threw the most unnatural but highly-amusing sommersaults. (Figs. 10, n, 12, pp. 12, 13.) THE DIFFUSION OF LIGHT. A luminous object evolves light from every visible point of its surface, and if a single point of light were placed in the centre of a hollow globe, every portion of the internal area would be equally illuminated. FlG. 13. A Flame in the centre of a circle, throwing out rays in every direction, like the spokes of a wheel. Owing to the manner in which light is distributed and transmitted in straight lines diverging from each other, its intensity diminishes as the square of its distance from the luminous source increases, and it is on this principle that the instruments called photometers, or light-measurers, are constructed. A scale of 20 ft. in length, divided into feet and inches, may be used in con- junction with a box somewhat like a stereoscope, containing two mirrors placed at an angle of 45, and reflecting the rays from the two sources of light which are to be confronted with each other. A candle, one of six to the THE DIFFUSION OF LIGHT. pound, and burning so many grains per minute, is fixed in a nozzle, which slides on the scale. The box, which may also slide or be fixed in the centre of the scale, reflects on one side the light from the lamp or gas-burner which is being tested, on the other it reflects the light of the candle. The experi- ment may be conducted either by placing the lamp and the candle at opposite ends of the scale, and moving the box with the reflectors until the two spots of light are equal ; or, the box being fixed in the centre, and the lamp under examination placed at one end of the scale, the candle may be moved towards the box till the lights are equal, the respective distances from the box being then squared, and the greater number divided by the less, will give the quo- tient which represents the illuminating power of the lamp as compared with the candle. FiGS. 14 and 15. Ritchie's Photometer. Section of the box containing the mirrors A B, AC, openings po, EO, to admit the light which is reflected from the mirrors on to two cncular apertures p p, covered with oiled paper, which are seen and com- pared when looked at from the top at T T. The arrows indicate the direction of the rays from the lamp, and L, the wax candle w c. Example : the distance of the lights from the box being respectively 12 ft. and 3 ft. 12 x 12 = 144 -f 3 x 3 = 9- Quotient, 16. In the practice of photometry the standard used is a candle defined by Act of Parliament " as a sperm candle of six to the pound, burning at the rate of 1 20 grains per hour." This standard would be a very simple one if every candle could be made alike, but it unfortunately happens that the composition and the wick are not always the same, and as important experiments have to be made in various parts of the United Kingdom, it becomes difficult to assimilate and compare them with each other. All authorities on this question have condemned the use of test candles. The credit is due to Mr. Crookes, the editor of the " Chemical News," of devising a standard test lamp-wick and combustible fluid which could be made in every part of the civilized world, and of inventing an improved photometer, in which the phenomena of polarized light are employed. The following is the inventor's description of the apparatus and materials used, commencing with the lamp and its fuel :* "Alcohol of sp. gr. 0.805, and pure benzol boiling at 81 C, are mixed together in the proportion of 5 volumes of the former and I of the latter. This burning fluid can be accurately imitated from description at any future time and in any country, and if a lamp could be devised equally simple and invariable, the light which it would yield would, it is presumed, be invariable. This difficulty the writer has attempted to overcome in the following manner. " Chemical News, ' July i7th, iS68. 1 6 ON LIGHT. " A glass lamp is taken of about 2 ounces capacity, the aperture in the neck being 0*25 inch diameter ; another aperture at the side allows the liquid fuel to be introduced, and, by a well-known laboratory device, the level of the' fluid in the lamp can be kept uniform. The wick-holder consists of a platinum tube I '8 1 in. long, and 0-125 m - internal diameter. The bottom of this is closed with a flat plug of platinum, apertures being left in the sides to allow free access of spirit. A small platinum cup 0.5 in. diameter and I in. deep is soldered round the outside of the tube 0*5 in. from the top, answering the threefold purpose of keeping the wick-holder at a proper height in the lamp, preventing evaporation of the liquid, and keeping out dust. The wick consists of 52 pieces of hard-drawn platinum wire, each o'oi in. diameter and 2 in. long, perfectly straight, and tightly pushed down into the platinum-holder until only o'l in. projects above the tube. The height of the burning fluid in the lamp must be sufficient to cover the bottom of the wick-holder ; it answers best to keep it always at the uniform distance of 175 in. from the top of the platinum wick ; a slight variation of level, however, has not been found to influence the light to an extent appreciable by our present means of photometry. The lamp having the reservoir of spirit thus arranged, the platinum wires parallel, and their projecting ends level, a light is applied, and the flame instantly appears, forming a perfectly shaped cone 1*25 in. in height, the point of maximum brilliancy being 0^56 in. from the top of the wick. The extremity of the flame is perfectly sharp, without any tendency to smoke ; without flicker or move- ment of any kind ; it burns, when protected from currents of air, at a uniform rate of 136 gr. of liquid per hour. The temperature should be about 60 F., although moderate variations on ither side exert no perceptible influence. Bearing in mind Dr. Franklin's observations on the direct increase in the light of a candle with the atmospheric pressure, accurate observations ought only to be taken at one height of the barometer To avoid the inconvenience and delay which this would occasion, a table of corrections should be con- structed for each o'l variation of barometric pressure. " There is no doubt that this flame is very much more uniform than that of the sperm candle sold for photometric purposes. Tested against a candle, considerable variations in relative illuminating power have been observed ; but on placing two of these lamps in opposition, no such variations have been detected. The same candle has been used, and the experiments have been repeated at wide intervals, using all usual precautions to ensure uniformity." The results are thus shown to be due to variations in the candle, and not in the lamp. In Arago's "Astronomy," the author describes his photometer in the fol- lowing words : " I have constructed an apparatus by means of which, upon operating with the polarized image of a star, we can succeed in attenuating its intensity by- degrees exactly calculable after a law which I have demonstrated.'" It is difficult to obtain an exact idea of this instrument from the description given ; but from the drawings it would appear to be exceedingly complicated, and to be different in principle and construction from the one now about to be de- scribed. The present photometer has this in common with that of Arago, as well as with those described in 1853 by Bernard,* and in 1854 by Babinet,f * ' Comptes Rendus,' 1 April 2$, i8<3. t " Proceedings of the British Association," Liverpool Meeting, 1854. THE DIFFUSION OF LIGHT. that the phenomena of polarized light are used for effecting the desired end. But it is believed that the present arrangement is quite new, and it certainly appears to answer the purpose in a way which leaves little to be desired. The instrument will be better understood if the principles on which it is based are first described. " Fig. 1 6 shows a plan of the arrangement of parts, not drawn to scale, and only to be regarded as an outline sketch to assist in the comprehension of general principles. Let D repre- / N/" N sent a source of light. This may be a white disc of ( -J- Y + ) porcelain or paper illuminated by any artificial or na- tural light. C represents a similar white disc likewise illuminated. It is required to compare the photome- trie intensities of D and C. (It is necessary that neither D nor C should contain any polarized light, but that the light coming from them, represented on each disc by the two lines at right angles to each other, forming a cross, should be entirely unpolarized.) Let H represent c c a double-refracting achromatic prism of Iceland spar; x \ x x ^ x this \\ill resolve the disc D into two discs, d and d' ', ( "" Y Hh Yl J polarized in opposite directions ; the plane of d being, V / V_^x V__^/ we will assume, vertical, and that of d' horizontal. The & A prism H will likewise give two images of the disc c ; the image c being polarized horizontally, and c' verti- ; cally. The size of the discs D, c, and the separating power of the prism H are to be so arranged that the vertically polarized image d, and the horizontally po- larized image <:, exactly overlap each other, forming, as shown in the figure, one compound disc, c d, built up of /^ X /" X half the light from D and half that from c. f 3 } if ^ J " The measure of the amount of free polarization V__^_. . \_x present in the disc c d, will give the relative photome- ^ 1G 1O trie intensities of D and C. " The letter I represents a diaphragm with a circular hole in the centre, just large enough to allow the compound disc c d to be seen, but cutting off from view the side discs c' d'. In front of the aperture in I is placed a piece of selenite of appropriate thickness for it to give a strongly-contrasting red and green image under the influence of polarized light. K is a doubly-refracting prism, similar in all respects to H, placed at such a distance from the aperture in I that the two discs into which I appears to be split up are separated from each other, as at^- D. If the disc c d contains no polarized light, the images g r will be white, consisting of oppositely polarized rays of white light ; but if there is a trace of polarized light in c d, the two discs g r will be coloured complementary, the contrast between the green and red being stronger in proportion to the quantity of polarized light in c d. " The action of this arrangement will be readily evident. Let it be supposed in the first place that the two sources of light, D and C, are exactly equal. They will each be divided by H into two discs, d' d and c c', and the two polarized rays of which c d is compounded will also be absolutely equal in intensity, and will neutralize each other and form common light, no trace of free polarization being present. In this case the two discs of light g*D will be colourless. Let it now be supposed that one source of light (D for instance) 2 i8 ON LIGHT. is stronger than the other (c). It follows that the two images d' d will be more luminous than the two images c c', and that the vertically polarized ray d will be stronger than the horizontally polarized ray c. The compound disc c d will therefore shine with partially polarized light, the amount of free polarization being in exact ratio with the photometric intensity of D over c. " In this case the image of the selenite plate in front of the aperture I will be divided by K into a red and a green disc. FIG. 17. " Fig. 17 shows the instrument fitted up. A is the eye-piece (shown in enlarged section at Fig. 3). G B is a brass tube, blacked inside, having a piece, shown separate at D c, slipping into the end B. The sloping sides, D B, B c, are covered with a white reflecting surface (white paper or finely ground porcelain), so that when b c is pushed into the end B, one white surface, D B, may be illuminated (as in Fig. 17) by the candle, and the other surface, B C, by the lamp. If the eye-piece A is removed, the observer, looking down the tube G B, will see at the end a luminous white disc divided vertically into two parts, one half being illuminated by the candle E, and the other half by the lamp F. By moving the candle E, for instance, along the scale, the illumination of the half D B can be varied at will, the illumination of the other half remaining stationary. " The eye-piece A (shown enlarged at Fig. 18) will be understood by reference to Fig. 1 6, the same letters representing similar parts. At L is a lens to collect the rays from B D c, Fig. 17), and throw the image into the proper part of the tube. At M is another lens, so adjusted as to give a sharp image of the two discs into which I is divided by the prism K. The part N is an adaptation of Arago's polarimeter ; it consists of a series of thin plates of glass capable of moving round the axis of the tube, and furnished with a pointer and graduated arc. By means of this pile it is possible to partially polarize the rays coming from the illuminated discs in one or the other direction, and thus bring to the neutral state the partially polarized beam c d (Fig. 16), so as to get the images g D free from colour. It is so adjusted that when at the zero point it produces an equal effect on both discs. THE DIFFUSION OF LIGHT. " The action of the instrument is as follows. The standard lamp being placed on one of the supporting pillars which slide along the graduated stem (Fig. 17), it is adjusted to the proper height, and moved along the bar to a convenient distance, depending on the intensity of the light to be measured ; the whole length being a little over 4 ft., each light can be placed at a distance of 24 in. from the disc. The flame is then sheltered from currents of air by black screens placed round, and the light to be compared is fixed in a similar way on the other side of the instrument. The whole should be placed in a dark room, or surrounded with non-reflecting screens ; and the eye must also be protected from direct rays from the two lights. On looking through the eye-piece two bright discs will be seen, probably of diffe- rent colours. Supposing E represents the stan- dard flame, and F the light to be compared with it, the latter must now be slid along the scale until the two discs of light, seen through the eye- piece, are about equal in tint. Equality of illu- mination is easily obtained ; for, as the eye is observing two adjacent discs of light, which pass rapidly from red-green to green-red, through a neutral point of no colour, there is no difficulty in hitting this point with great precision. It has been found most convenient not to attempt to get absolute equality in this manner, but to move the flame to the nearest inch on one side or the other of equality. The final adjustment is now effected at the eye-end, by turning the polarimeter one way or the other up to 45, until the images are seen without any trace of colour. This will be found more accurate than the plan of relying entirely on the alteration of the distance of the flame along the scale; and, by a series of experi- FIG. 1 8. mental adjustments, the value of every angle through which the bundle of plates is rotated can be ascertained once for all, when the future calculations will present no difficulty. Squaring the number of inches between the flames and the centre will give their approximate ratios ; and the number of degrees the eye-piece rotates will give the number to be added or subtracted in order to obtain the necessary accuracy. " The delicacy of the instrument is very great. With two lamps, each about 24 in. from the centre, it is easy to distinguish a movement of one of them to the extent of i-ioth of an inch to or fro ; and by using the polarimeter, an accuracy considerably exceeding that can be attained. " The employment of a photometer of this kind enables us to compare lights of different colours with one another, and leads to the solution of a problem which, from the nature of their construction, would be beyond the powers of the instruments in general use. So long as the observer, by the eye alone, has to compare the relative intensities of two surfaces respectively 22 20 ON LIGHT. illuminated by the lights under trial, it is evident that unless they are of the same tint it is impossible to obtain that absolute equality of illumination in the instrument which is requisite for a comparison. By the unaided eye one cannot tell which is the brighter half of a paper disc illuminated on one side with a reddish, and on the other with a yellowish light ; but, by using the above-described photometer, the problem becomes practicable. For instance, on reference to Fig. 16, suppose the disc D were illuminated with light of a reddish colour, and the disc C with greenish light, the polarized discs d 1 ' d would be reddish, and the discs c' c greenish, the central disc c d being of the tint formed by the union of the two shades. The analysing prism K, and the selenite disc I, will detect free polarization in the disc c d, if it be coloured, as readily as if it were white ; the only difference being that the two discs of light g r cannot be brought to a uniform white colour v/hen the lights from D and C are equal in intensity, but will assume a tint similar to that of c d. When the contrasts of colour between D and C are very strong when, for instance, one is a bright green and the other scarlet there is some difficulty in estimating the exact point of neutrality ; but this only diminishes the accuracy of the comparison, and does not render it impossible, as it would be according to other systems. " No attempt has been made in these experiments to ascertain the exact value of the standard spirit-flame in terms of the Parliamentary sperm candle. Difficulty was experienced in getting two lots of candles yielding light of equal intensities ; and when their flames were compared between themselves and with the spirit-flame, variations of as much as 10 per cent, were some- times observed in the light they gave. Two standard spirit-flames, on the other hand, seldom showed a variation of I per cent., and had they been more carefully made they would not have varied 0*1 per cent. " This plan of photometry is capable of far more accuracy than the present instrument will give. It can scarcely be expected that the first instrument of the kind, roughly made by an amateur workman, should possess equal sensi- tiveness with one in which all the parts have been skilfully made with special adaptation to the end in view." MODIFICATIONS THAT LIGHT MAY UNDERGO. 1. In the same medium of the same density rays of light undergo no change. 2. When rays of light pass out of one medium into another, or into one of a different density, they may undergo the following modifications : 3. The rays of light may rebound from the surface of a solid, fluid, or gaseous body, and are then .said to be reflected, the rebounding being denominated Reflection. 4. A ray of light, after passing into a substance, may be bent from its natural course, or Refracted. 5. A ray of light may be split into two portions when it enters certain bodies, such as Iceland spar, and each portion of the light possesses distinct properties. 6. A ray of light may be so checked in its passage that a portion may be lost or absorbed. 7. A ray of light, by reflection, refraction, double refraction, and absorption, may acquire new properties, and become what is termed Polarized Light. THE REFLECTION OF LIGHT. 21 THE REFLECTION OF LIGHT. Catoptrics is the name given to all effects produced by reflection. It is a word taken from the Greek /caTOTnyHKos, " belonging to a mirror," and whilst the laws which govern the reflection of light are remarkably simple, they give rise to a most interesting series of phenomena. Premising that the incident rays , are those which fall on the surface, and that those sent off are called reflected T&yS) it is soon ascertained ist, that the incident and re- flected rays always lie in the same plane, i.e., if the incident ray falls in a perpendicular plane or direc- tion, the reflected one will also be in the same plane or direction ; and the like reasoning applies to the horizontal position. 2nd, the in- cident and reflected rays always form equal angles, or when light falls upon any surface, whether plane or curved, the angle of re- K R fe the reflecting surfaces A'B is the incident ray ; of B c, the reflected ray ; A B p, the angle of incidence; c B p, the angle of reflection. flection is equal to the angle incidence. The luminous rays may be parallel to each other, like the lines in a copy- book, or they may be divergent when they spread out in the same manner as the sticks of a fan, or convergent when they gradually approach each other, and end in a point like a spear-head. FIG. 20. Reflection of Parallel or Equi-distant Rays. R R R, the parallel rays incident on a plane or flat surface at T, and reflected in lines at equal distances from each other. The rays of the sun are nearly parallel with each other, and will illustrate this fact. 22 ON LIGHT. FIG. 21. Parallel rays falling on a concave mirror, M M, converge or come to a focus or fireplace at F. FIG. 22. Reflection of parallel rays from a convex mirror. The rays which are reflected become divergent* and are shown on the ceiling. THE REFLECTION OF LIGHT, A very large number of the waves of light are lost when they fall even upon the most perfectly polished metallic mirrors ; thus light reflected from a clear and bright surface of metallic mercury at an angle of 78 5' loses nearly one quarter, and only 754 rays out of 1000 are reflected. A transparent substance, like glass, reflects more light from the second sur- face than the first ; and if the former is coated with an amalgam of tin and mercury, the brilliancy of the reflection of the second or coated surface over- powers that of the first, although if a candle is held opposite the best quick- silvered mirror two images are apparent. In the production of illusory effects by reflection from the surface of glass, the image reflected from the surface of the first surface interferes with the second ; but this may be prevented, as shown to the author by a friend, by coating the first side with a very delicate film of collodion or varnish, such as is used for photographic purposes. Thus the intensity of the reflection of the second surface is increased by a coat- ing of amalgam, whilst the intensity of the reflection from the first surface is reduced by coating it with a substance like collodion, having an absorptive rather than a reflecting power on light. Where objects are reflected from either glass or silvered glass plane mirrors, they appear to come from the back, and the image is as far behind the glass -as the real object is before it. It is this physical truth that increases so amaz- ingly the effect of what is familiarly called "The Ghost Illusion." The spectator looking, at the image does not observe the glass which has pro- duced it, because the former is so far in advance of the latter. Had this FlG. 23. physical fact in Catoptics been remem- o, the real object reflected from the glass A B, ft bered, many scientific men would have R '. to ^ e eye at E ; s - behind ^ glass '. sooner discovered the secret of the illu- sion by looking in front of the image for the glass or reflecting surface. The same truth is still more apparent when divergent or convergent rays are traced out in their reflections from a plane surface of glass. To cause the image or ghost to appear, the lights are alternately thrown or. or cut off the real figure. (See Fig. 24, p. 24.) This mode of showing the ghost has to be modified when the angles of vision are so different as seen from the pit, boxes, and gallery of a theatre. Then it is advisable to sink a stage a few feet below the regular stage, and to arrange a board at the same angle as the glass, on which the living figures recline. The latter method allows only certain movements to be exhibited, and is called the "spectroscope" and " phantoscope " by travelling showmen who exhibit the ghost. One of the prettiest stories which can be illustrated with this illusion is that called " The Knight watching his Armour," and as many persons have where the image appears to come from, and if the whole distance from E to s o is measured, it will be found equal to E R, R o. ON LIGHT. seen it at the Polytechnic, and doubtless might wish to entertain others with this popular illusion the little tale is added as a sequel to the contrivance itself. FIG. ^.Exhibition of the " Ghost " at the Polytechnic, being a section of the stage in the large Theatre. . A, the real figure ; B, lime-light ; c c, looking-glass ; D D, plate glass ; G, the spectral image or ghost, which would appear much farther behind the glass D D; s, spectators. KNIGHT WATCHING HIS ARMOUR. N.B. The spectral image described appears at all places marked with a star. * The following is told of a knight, called Hubert de Burgh, who won his spurs on Flodden Field : King James was so pleased with his deeds of valour, that he promised to dub him knight, on the following morning ; but told him that he would have to go through the ancient ordeal of watching his armour throughout the previous night. Sir Hubert started with helmet and corselet to the church. Before entering, he met his lady-love, fair Agnes, and telling her of his good fortune, begged that their wedding might take place on the first day he wore his golden spurs. The maiden consented, and told him that she would also watch with him in spirit throughout the night, and bade him beware of the many temptations held out by the evil spirits to all warriors during the period of their watching. After a loving farewell, Hubert commenced his duties. And now, for the first time, does he feel fatigue from his hard day's fighting; but remembering the caution that he must neither eat, drink, nor sleep during his vigils, he continues to watch and fast until the break of morn : sitting down, he thinks of his good fortune in winning the prize so much coveted by all true warriors. Whilst buried in thought he hears the sound of approaching foot-steps, and THE REFLECTION OF LIGHT. feels that the time has come when he needs all his energy to keep his armour pure from evil touch. On looking up he beholds a Benedictine monk * standing near, watching him most carefully. " Peace be with you," says the monk. " Amen, father," replies the knight. " My son," continues the friar, "thou hast acted nobly this day, and deservest the honours our gracious sovereign is about to confer on thee ; thou hast had a weary day, and needest rest and sustenance ; sleep awhile, and I will keep custody over these true steel arms." " Nay, father," said Hubert, "my duty is to watch, and not take deputy for this all-important work, neither will my instructions permit me to eat or sleep." " My son," replies the priest, " as a brother of our holy order, I absolve thee 26 ON LIGHT. of this heavy charge, and will keep watch ; and in that same capacity I bid thee drink. See ! here is a cup of right good wine which will much relieve thee." "Father," said Hubert, "sorry am I that mistrust enters my mind; I like not to break the solemn right, and though I would gladly accept thy proffered gift, I dare not, without you make the sign of your order over the wine." The monk for some time hesitated, but at length in an angry tone replied " Fool! drink or starve; what care I for such a coward loon?" " Now, by St. Peter," ejaculates Hubert, " these sound not like a good priest's words ; thou wearest the dress without the sign of thy calling. Who art thou ? Answer quickly, or this good sword shall make short work of thy disguised body." Grasping his sword, he advances towards the friar, who, with a fiendish laugh, vanishes from before him, and is gone. Hubert felt it must have been an evil spirit who sought to destroy him, and with firmer determination to resist, he again returns to his weary task. Some time elapsed, when there comes before him, gliding out of the darkness, a beauteous syren,* who speaks kindly and fairly to him of his great prowess and feats of arms. She tells him she is an inhabitant of fairy-land in fact, their queen that she loves him fondly, and beseeches him to come to their fairy home, where he shall reign supreme. She pictures to him the delight of being always young and gay of being master of countless hosts flying through the night amidst the stars prince of all the land ; and in such strains does she pour forth her eloquence, that he flies with her in fancy through the realms she so beautifully describes ; but the thought of his fair Agnes, and the promise made, recalls him to his duty, and slowly advancing towards his armour, he lays his hand on the left side of his corselet, saying, "If thou be a spirit of evil, thus do I destroy thy charm." The temptress gives one faint sigh, and vanishes from his view. Hubert, now relieved from a second temptation, watches with renewed vigilance ; he now knows that the morn is not far distant that morn which blesses him doubly, by giving him the name of knight and a fair bride. The thought of Agnes causes him pain : " So soon shall I be forced to leave her, to seek a fortune which I have not;" and for the first time he knew what it was to wish for wealth. Whilst deep in thought how he should increase his little store of treasure, a stately man, dressed in the garb of a wealthy merchant, stands before him and questions him upon the sadness of his looks.* " For one so young," said his visitor, " should ne'er be sad." " Good sir," replies Hubert, "thou seemest kindly in thy manner, so will I tell thee of my only grief. To-morrow, by the will of ur good king, I put on the golden spurs of knighthood, I wed a noble lady, whom I shall drag down to my own level of poverty ; though the world has given me an honoured name, still do I lack the wealth to keep my wife in station that befits her, and calm reflection tells me I did wrong to take her promise, and so, sir, do I feel sad." " Beshrew me, but thou art a noble youth," replies the merchant " noble in thought as well as deed, and if it had been ordained that I was blessed with such a son, he should not long need wealth." "Ah!" said Hubert, "fate has not given me a parent's love, care, or assist- ance ; my mother died when I was yet a babe, and ere many months my father followed her, dying as a noble soldier should, upon the battle-field." " Stay, said the merchant, " may I again question thee as to whether him THE REFLECTION OF LIGHT. 27 you spoke of was the noble Ralph de Burgh, one of my most true and honest friends?" " 'T is so," said Hubert, " and if my father sought to win your friendship, pray extend the same good fellowship to his son." " That I will, right willingly," returns the merchant. " Stay," continued he, " methinks you said you needed gold nay, turn not away I have enough, too much for an old and childless man. Say, let me aid thee. I ask v it as a favour; nay, I command it, as your father's friend. Here, take this purse to meet your most urgent wants, and to-morrow shalt thou revel in as great wealth as any son of our noble houses. Nay, I will take no denial," Hubert, who had been struggling within himself as to his right to take the proffered gift, at last rises to approach the stranger, when he imagines he hears sweet music passing through the air. He stops Lo listen, and fancies he hears a well-known voice exclaim, " Beware ! keep to your trust, 't is almost morn." Amazed, he steps back, and sees his fair Agnes beckoning him away,* and waving the merchant back, who, with a frown and disappointed look, fades into the darkness. The maiden said, " Dear Hubert, thy task is finished ; for see, the morn is breaking. Farewell ; we meet again at noon, never to be parted. I said I would watch over thee in spirit; say, have I performed the task?" As the warrior is about to embrace his beloved, she disappears from before him. The first tint of the morning sun soon glistened upon his helmet ; so this true knight had watched from eve till sunrise to guard his armour from all evil spirits. IMAGES FORMED BY SILVERED MIRRORS. Soon after the novelty of " the Ghost " had waned, another illusion was pre- sented to the public called "Proteus; or, We are Here, but not Here," Mr. Thomas Tobin and the author being co-inventors. A large and handsome box, like a huge sentry-box on wheels, and raised from the floor so that the spec- tators could see under, over, and all round it, is wheeled on to the platform (Fig. 26). On being opened it appeared to be well lighted from the top by an ordinary railway carriage lamp, and, of course, seemed to be perfectly empty. The assistant being now invited to enter the box, the door is closed and locked, and, after a few minutes have elapsed, is re-opened, when a skeleton appeared to be standing in the very place where the living being had been formerly observed (Fig. 27.) Again the door is closed, and the next time it is opened the skeleton has vanished, and the assistant walks out of the box with a carpet bag. The person explaining the apparatus now goes in, and sounds the walls all round with his knuckles ; and, while doing this, the door is suddenly closed, and being as quickly opened, he is found to have disappeared, again to appear after the door is once more closed and opened. This illusion is produced by two plane silvered mirrors, folding into the sides of the box, and when open forming together an angle of 45. The mirrors when open reflect the two sides of the box, and, as already explained, they appear behind the mirrors, and cause the spectator to suppose that he is looking at an empty box. In the angle formed by the mirrors the skeleton is concealed and brought out when required, and in the same place the assistant and lecturer are alternately hidden. Thus a box can be constructed in which the most elaborate tricks of the Davenport Brothers may be performed. 28 ON LIGHT. FIG. 26. FIG. 27. In the accompanying drawings, Fig. 26 is an exterior, and Fig. 27 an interior view, and Fig. 28 a horizontal section of the box or chamber above referred to. The sides may be made of wood, or papier mache, or sheet iron, but the former is preferred. a a are two doors hung at the angles of the box, and capable of closing on the post d or of lying back in a recess in the sides, as shown on the right-hand side of the box. These doors a a have crlass mirrors on the sides ffff, and a fresco or design at the upper part of the box or chamber suitable for the illusion to be represented. The post d is set at the junction of the lines bisecting the angles of the back and sides. The box or chamber as shown is rectangular. If, for conve- nience or for the purpose of any particular re- presentation, the box or chamber is desired to be wider at the front than at the back, the post will still be placed at the junction of the two lines bisecting the angles made by the back and two sides, but any considerable departure from the rectangular form would be found inconvenient, b is a door of clear thick plate glass ; c is the external door. A lamp is hung at the top of the post d to light and assist in ventilating FIG. 28. THE REFLECTION OF LIGHT. 29 the box by promoting an upward current, and a mat or rug is placed at the bottom of the box or chamber. The same co-inventors, by placing a silvered-glass mirror at an angle, and thrown back from the spectators, produced some very popular illusions, one of which, called " The Modern Delphic Oracle," may be thus described. The curtain being raised, a person dressed in the garb of an ancient Athenian nobleman walks through and out of the entrance to a temple, across which a curtain rolls as he passes. Walking in front, he throws incense on a Crazier of charcoal, and invokes Socrates to appear. The curtain now rolls back and Elevation showing the appearance presented by the illusion called "The Modern Delphic Oracle." discloses the head of the sage floating in the air, the proof of its solidity being that it casts a shadow on the wall behind. The Greek asks Socrates whether the words he spoke on the occasion of his memorable trial accurately expressed his real convictions whether the purpose of his life was as pure as we have been taught to believe. The sage replies : 'It was my purpose ever to control The stormy passions that perturb the soul ; Averse from idle pomp and wealth,' to find The only lasting treasure in the mind. The truth I learned without reward to teach. And show the falsehood hid by forms of speech; The voice that warned within me to obey That safest guide when doubtful was my way. I learned to live as one prepared to die, And calmly met my fate when death drew nigh j Rejoiced to quit this troubled world, and rest Immortal in the regions of the blest ! " * * Written by John Oxenford, Esq, ON LIGHT. The curtain once more rolls before the entrance, and as it is re-opened to allow the Athenian to pass through, the head has vanished, and nothing but the bare walls are apparent. This illusion is performed with the aid of a large silvered mirror, which is placed at an angle across the small chamber in which the head appears, and being perforated in the centre, the head of the actor is thrust through the hole, whilst the rest of the large mirror conceals his body, and, reflecting only the top of the room, painted to represent the back of the temple, induces the spectator to suppose he is looking at a head suspended in an empty room. FIG. 30. Transverse section- A B, the silvered mirror; c, the hole through which the actor thrusts his head; B D, the ceiling painted is reflected in the mirror, and appears behind the head at H H. The mirror is carefully supported on a framework on wheels, and can be rolled out of the way when the actor representing the Athenian walks through in coming out and returning to the temple. The exhibition of the Ghost at the Polytechnic took London by surprise as a novelty. It is, however, evident from the next diagram, copied from "Robinson's Recreative Memoirs," published in 1831, that he approached very near to the arrangements necessary to produce reflected images from plane surfaces. In the first place, Robertson remarks, it is necessaay to take care that the angles of the mirror must not exceed 20. You may try in vain to increase this angle by increasing the size of the mirrors a, b, c, which reci- procally cause the rays to pass through the opening where a double-convex lens is placed. Thus to obtain an image of the same size as the object say 6 ft. high it is necessary to place the figure 18 ft. distant' from the mirror r, and to use a lens of 9 ft. focus, to have the image 1 8 ft. on the other side of THE REFLECTION OF LIGHT. Flc., which were either fixed or placed loosely in a cell at the end of the instru- ment. " When this idea was carried into execution, and the reflectors placed in the tube and filled up on the preceding principle, the kaleidoscope in its simple form was completed. " When the kaleidoscope was brought to this degree of perfection, it was impossible not to perceive that it would prove of the highest service in all the ornamental arts, and would at the same time become a popular instrument for the purposes of rational amusement. With these views, I thought it advisable to secure the exclusive property of it by a patent. But, in consequence of one of the patent instruments having been exhibited to some of the London opticians, the remarkable properties of the kaleidoscope became known before any number of them could be prepared for sale. " According to the computation of those who were best able to form an opinion on the subject, no fewer than 200,000 instruments were sold in London and Paris during three months. " In order to construct the kaleidoscope in its most simple form, we must procure two reflectors about 5, 6, 7, or 8 in. long. These reflectors may be either rectangular plates, or plates shaped like those in Fig. 32, having their broadest ends, A o, B O, from I to 2 in. wide, and their narrowest ends, a E, b E, half an inch wide. " If the reflectors are of glass, the newest plate glass should be used. The plate glass may be either quicksilvered or not, and its posterior surface may be ground, or covered with black wax, or black varnish, or anything else that reverses its reflecting power. THE REFLECTION OF LIGHT. 33 FIG. 32. " The proper application of the objects at the end of the reflectors is now the only step which is required to complete the simple kaleidoscope. The most simple method consists in bringing the tube about half an inch beyond the ends of the reflectors. A circular piece of thin glass of the same diameter as the tube is then pushed into the tube so as to touch the reflectors. The pieces of coloured glass being laid upon this piece of glass when the tube is FIG. 33. The Oxy-hydrogen Kaleidoscope as made by Mr. Darker. Key pattern, produced from a key. held in a vertical position, another disc, having its outer surface ground with fine emery, is next placed above the glass fragments, being prevented from pressing upon them by a ring of brass, and is kept in its place by burnishing down the end of the tube." Such are the instructions given by Sir David Brewster for the manufacture of the ordinary kaleidoscope ; he also speaks of the application of the instrument to the magic lantern, but as the details were not sufficiently complete to enable any one to throw the kaleidoscopic figure 3 ON LIGHT. on the disc, the author was induced to urge Mr. Darker, of Paradise Street, Lambeth, to persevere in the adjustment of the mirrors, lenses, and lighting until perfection was obtained. During the Christmas of 1866 the oxy-hydrogen kaleidoscope was exhibited daily at the Polytechnic with the greatest success, and by its means the principle of the instrument could be better understood. FIG. 34. a, Figures obtained by putting a single figure, such as key, into the apparatus; b, c t other figures produced by using the light only with 'an empty slide. It is chiefly by the adjustment of the light that the original angular opening is gradually multiplied by reflection eight times, and eight distinct sectors or divisions become visible on the disc. When the tip of the finger is now inserted, eight single reflections or four double ones are the result, and by thrusting in all the fingers the curious figures shown at , Fig. 35, are obtained. Not only are transparent bodies, such as glass, exhibited with success, but any opaque object will produce the most distinct and symmetrical figures on the FIG. 35. Figures obtained on the screen from the oxy-hydrogen kaleidoscope with pins and needles, D; the fingers, E; and F, a comb. screen; in Fig. 35 the pattern ? is chiefly produced with a cell containing only pins and needles. If glass be used, it should always be broken from coloured glass rods with the hammer, in order to secure the conchoidal fracture, as the wedge-shaped figures give gradual tones of colour, which are very pleasing to the eye, and produce fair imitations of the colours and grouping of rubies, emeralds., and sapphires when projected on the screen. THE REFLECTION OF LIGHT. 35 A gentleman, who saw these and other patterns, and especially some obtained by using ferns and other natural objects, was so pleased that he stated it was his intention to have an oxy-hydrogen kaleidoscope fitted up in his calico- printing establishment, in order to assist the artist who designed the patterns ; and he stated that, although they had long used the ordinary kaleidoscope for this purpose, the oxy-hydrogen one gave a much better notion of the effect required to be produced, and would enable the manufacturer to select and decide upon the best patterns for commercial purposes. The phenomena of light produced by reflection, and the instruments which have been constructed to demonstrate these effects, are too numerou& to be detailed here, so that two or three examples must suffice. The property of reflection is affected more by the condition of the surface than by the physical nature of the substance used as a reflector. The kaleidoscope reflectors em- ployed by Mr. Darker are made of the best plate glass, coated with metallic silver, and it is extremely difficult to prevent a slight deposit of moisture upon them. The watery particles greatly impair the kaleidoscopic figures, and demonstrate how thoroughly the power of reflection depends on the state of the surface, as this exquisitely thin film of moisture interferes with the perfect illumination of the kaleidoscopic figure. FIG. -fiBack of the Japanese Mirror. THE JAPANESE MAGIC MIRROR. Some mirrors made in Japan have a very curious property. The back is usually ornamented with Japanese characters, also with flowers, vases, &c. ; the front is polished in the usual manner, like any other metallic speculum, and, if carefully examined, with or without a magnifying power, betrays nothing more than the highly polished surface of the alloy, which appears to be com- posed chiefly of tin and copper. When, however, the mirror is held in the highly divergent rays emitted from an oxy-hydrogen light, it not only reflects on to a disc the surface of the polished disc, but likewise all the Japanese characters, vases, and flowers, which are in relievo on the back of the mirror. 3 2 ON LIGHT, FIG. 37. Reflection from the front or bright side of the Japanese Mirror. We have in the above experiment a scientific puzzle that is somewhat difficult to explain. May it be supposed that much of the success of the effect obtained is due to the nature of the alloy used in the casting of the mirror ? The figures in relief on the back of the mirror, during the operation of casting, must first enter the mould in the liquid state : are these first and quickly congealed before the whole mass of metal ? and does the minute difference in* the molecular condition of the metal produced by a greater rapidity of cooling, extend through the thin metal to the front and polished side ? Would careful heating and anfiealing destroy the effect ? Whatever may be the method employed, it is certain that the figures reflected from the surface are wholly invisible, and cannot be observed in the strongest light, and with a good magnifying-glass. In all cases where metals are inlaid with other metals the lines where the metals join are distinctly visible, and there- fore it cannot be supposed that the Japanese mirror is made in this manner. Are the mirrors cast in a double mould, one side of which is in intaglio and the other in relievo, and after being cast do they grind down the sides of the mirror in which the figures are sunk, until they get a plain surface, which is then polished, leaving the other side and back of the mirror with the figures in relief? The pattern die, conferred on both sides of the metal whilst soli- THE REFLECTION OF LIGHT. 37 difying, might still further determine the molecular difference. It is a curious circumstance that the Chinese mirrors, made in imitation of the Japanese mirrors, do not answer the purpose, the former being much heavier than the latter. Whatever may be the secret of success, it is certain that this is only another instance of the remarkable ingenuity of the Japanese workers in metal. Sir D. Brewster explains the apparent anomaly by suggesting that the design on the back is dexterously reproduced by careful engraving, which is so lightly done that the figures traced are quite invisible after the mirror is brought to the highest degree of polish, and it is only by submitting the mirror to a powerful light, and casting the reflection of the surface on a wall, that the design becomes apparent. The concealment of the most delicate engraving, unless done in some way by Barton's ruling-machine, would be extremely difficult, if not impossible. The Japanese know nothing of the machine with which Barton ruled his steel patterns, and even if they did the reflected patterns would give evidence of colour, which is not the case. In the " Journal of the Asiatic Society," vol. i., page 242, there is a very clever paper, by James Prinseps, on " The Magic Mirrors of Japan." He says: " The Japanese mirror is a slightly convex disc of bell metal, about 6 in. in diameter, and a quarter of an inch in thickness on the edge, ground and po- lished on the convex face, and covered with a thin coating of silver to give it a white colour. (Fig. 38, p. 39.) " The back of the mirror is deeply curved or indented, with ornamental work in circles and festoons, and it bears an inscription in the Japanese character in high relief upon what may be termed the tympanum of the disc ; in the centre there is a projecting knob, perforated laterally to receive a string for suspending the mirror. The metal is highly sonorous when struck as a bell, and is so soft as easily to be indented or scratched on contact with any hard substance. I found its composition to be Copper 80 Tin 20 100 with no traces 01 silver or arsenic, and a very slight indication of zinc*" Mr. Prinseps then describes the curious property of the mirror, similar in effect to those already mentioned and illustrated at Fig. 37, p. 36, He then proceeds to discuss the cause of this seeming anomaly. " It then occurred that the various parts of the Japanese mirror might be of different density, supposing the pattern to be made by stamping, and that either the rays of light might be more forcibly repelled by the denser metal than by the lighter, or that parts of the surface would acquire different degrees of polish, sufficient to cause the illusion, although imperceptible to the eye. But in such case the thin parts, from being the hardest, should give the stronger reflection. " This supposition was also overthrown by experiment. A disc of silver, having been annealed at a red heat so as to be quite soft, was stamped on the back with a circular ring, deeply indented, so as to harden the silver in that part only. The opposite surface was then ground and polished, when it was found to give a clear and uniformly reflected spectrum. " Another and, I believe, the true explanation is suggested by the well-known 3 8 ON LIGHT. phenomenon of the reflection from a brass button, which every school-boy has remarked when sporting his Sunday ' blue coat with metal buttons' in the sun- shine of his tutor's parlour-window. The button throws a radiated irregular image on the wall, exhibiting two bright concentric circles, one on the edge and another about one-third within it, and there is generally a bright spot in the centre : all of this seems but the picture of the stamp on the back of the button : the radii resemble, and indeed coincide with, the letters of 'superfine' or 'trebly gilt' inscribed within a double circle, and the central spot represents the shank. There can be little doubt that the principle is in this case precisely that of the Japanese mirror ; and, on a cursory view, the surface looks equally smooth and unsuspicious. On minute examination, however, of several buttons, I found them to be by no means plane ; their general surface is slightly convex; there is a hollow in the centre and a projection in the position of the inscrip- tion behind, caused no doubt by the blow necessary in stamping it. The polish is probably given by a rotary motion, and consequently does not remove these very small irregularities. To follow up the experimental investigation, I selected one of the buttons which gave a good image, ground it on a flat hone, and polished it: all of the magical figures vanished in a moment, and a plain, bright disc appeared in their stead. Here, then, may be a key to the mystery of the mirror: the deception is entirely produced by irregularities on the sur- face, which are rendered the less perceptible to the eye because the surface is convex instead of being plane. But it may be objected that the two circles which appear bright in the reflected spectrum of the button represent the indented or thin parts of the metal, whereas the thick parts of the Japanese mirror are those which will appear illuminated. A short analysis of the facts in either case will readily explain to what these discrepancies are attributable ; but it will be necessary to have recourse to a diagram. " Let A B, Fig. 38, be a plain mirror upon which the rays of light R impinge ; they will be reflected uniformly to R', forming a clear image. Now let A B c D E F G be another reflecting surface, having two convexities, B C, E F, and one concavity in the centre D (the condition nearly of the brass button). In this case the light reflected from the outer concave flexures of the protruding portion of the surfaces B c, E F, will converge in the foci b c and ^/respectively, at distances corresponding to the radius of their curvature ; the effect will, of course, be visible within wide limits of the actual focus. In most of the buttons, however, the central depression is so great that it collects the rays in a focus, d t a few inches only in front of the surface ; and when the spectrum is thrown farther off, the rays crossing from two less distinct luminous foci, d' d', it follows from analogy that the thin parts or tympanum of the Japanese mirror are slightly convex with reference to the rest of the reflecting surface, which may have been caused either by the ornamental work being stamped or partially carved with the hammer and chisel on its back, or, what is more probable, that part of the metal was by this stamping rendered harder, so that in po- lishing it was not worn away to the same extent." Since the above was written, an English brass-finisher appears to have dis- covered the secret. Taking ordinary brass, he finds that any figure stamped upon it with a proper die, and ground down and polished, will not reflect the figure impressed by the die; but if the process with the same die is repeated three times, so that the figure intended to be projected from the surface is stamped three times in the same place, and subsequently ground down and polished after each stamping, then a molecular difference is established between THE REFLECTION OF LIGHT. 39 FIG. 38. the stamped and unstamped parts, which is not apparent to the eye, but is shown directly the surface so acted on is used for reflecting light. There can be no doubt that, until the magic lantern was invented, the only optical apparatus used by persons who pretended to wield the " magic art " consisted of plane and concave mirrors. The memoirs of Monsieur E. J. Robertson, published in Paris in 1831, disclose some amusing applications of surfaces that reflect light, and he describes' how the magician Nostrodamus deceived the politic Marie de Medicis, and pretended to show the astute queen the king for whom the throne of the Bourbons was destined. He states that Marie de Medicis, disquieted by apprehensions regarding the succession to the throne of France, went to consult Nostrodamus. This dealer in miracles by the use of plain mirrors produced the effect shown in Fig. 39, p. 40. La Boite Magique. The magic box is another amusing example of the same kind, only in this case a concave mirror is employed instead of a plane one. This experiment, Robertson declares, is charming, and having, he says, told a lady the secret of several illusions which pleased her greatly, he happened to be staying with the same individual in the country, at the time that a most agreeable gentleman was paying his court to her; the latter said to her lover, " If you do not fear apparitions, I promise you one this evening which may please you. At twelve precisely open the box that you ON LIGHT. FIG. 39. The throne, placed in the first apartment A, is reflected by a mirror concealed in the canopy B, Marie de Medicis beholds the representation of the image in a mirror c, supported by a Cupid. will find on your table, of which this is the key, and my image will come out of the box." This promise seemed only an agreeable kind of banter to her gal- lant, and, though he promised to open the box, he feared to do so, lest he might be made the dupe of some trick. At first he would not touch it, but at last, yielding to curiosity, he opened the box, when the image of his lady-love immediately appeared, with a very grave and composed air ; but she, guessing that the countenance of her gallant must bear a strange a serio-comic, though interesting expression, forgot that silence was necessary, and, burst- ing out into laughter, was thus disco- vered in the adjoining room. FIG. 40. A, concave mirror; the head B, inclined towards c, appears to emerge from D, to an eye placed at EJ the head, B, must be well illuminated, and the mirror in the shadow, so that it may not be visible; G is the wall; at H a box to open, firmly fixed on a table K. The interior of the box is painted black, and of course the \vall which separates the two apartments is open under the table. THE REFLECTION Of LIGHT. 41 The ancients made use of concave mirrors to rekindle the vestal fires. Plutarch says they employed o-Kac/>aa, or dishes, for that purpose. They were, most likely, hemispherical vessels highly polished within. As an illustration of the more refined uses and applications of silvered mirrors, may be quoted the admirable instructions given by Mr. John Browning, of in Minories, for adjusting and using reflectors for astronomical tele- scopes with silvered-glass specula. FIG. 41. MR. BROWNING'S DESCRIPTION OF THE SILVERED GLASS REFLECTING TELESCOPES. These telescopes are of the kind called Newtonian, a form so well known, that it is, perhaps, scarcely necessary to describe it ; but I append a plain diagram (Fig. 41) and brief description, because it will assist in making clearer the instructions I have given further on, of the method of adjusting the instrument. The Newtonian telescope consists of a tube closed at the lower end, which is occupied by a concave mirror, M. The cone of rays reflected from this mirror is again reflected at right angles from the surface of a small plane mirror, m n, mounted at an angle of 45, near the open end of the tube, into the eye-piece, which is exactly opposite.* In reflecting telescopes, as originally constructed, the concave mirror was made of an extremely hard alloy, known as speculum metal. These metallic mirrors possessed several disadvantages, so serious in character -that they have for some time fallen out of general use. The principal defects were the following : 1. From the extreme brittleness of the alloy, they were very liable to fracture, sometimes breaking merely from a sudden change of temperature. 2. From their great weight it was extremely difficult to mount them in such a way as to prevent flexure, the smallest amount of which greatly injured their optical performance. 3. Their greatest drawback, however, consisted in the fact that the surface of the metal, from damp or other causes, sometimes became very rapidly tarnished, and this tarnish could seldom be removed, except by repolishing and, consequently, refiguring the mirror ; and this involved nearly as great an outlay as the purchase of a new speculum, besides incurring the serious risk of a fine figure being irretrievably lost. In the telescope now described, the metallic mirror is replaced by one of * The mirror must not be worked to a spherical, but to a very perfect parabolic curve. 42 ON LIGHT. glass, on the surface of which a coating of pure silver has been deposited by Liebig's process, and described further on. These glass mirrors are not at all injuriously affected by change of tem- perature, and their lightness very considerably reduces their liability to flexure ; indeed, mounted in the manner I shall presently describe, no flexure has ever been observed in them. I may, however, state that I make the discs of the specula, which Mr. With parabolizes for me, out of glass nearly twice the substance of that generally used for the purpose. The coating of pure silver reflects fully one-third more light than the best speculum metal, as the alloy before mentioned is called. But the greatest superiority of silvered glass over metallic mirrors consists in the fact that, should they become tarnished, their brilliancy may readily be restored by gentle friction with soft leather and a little of the finest rouge ; and even should the silver coating become utterly spoiled, it may be easily removed without in any way impairing either the figure or polish of the glass speculum, and a fresh one deposited at a trifling cost, thus making the mirror equal to new ; and this may be repeated indefi- nitely. Should the owner possess a little patience, he may renew the coating himself at the cost of only a few pence. The silvering process is fully described further on. With this alteration these telescopes have latterly gained much ground in the opinion of practical observers well known in the scientific world, who have had considerable experience in working with them. On figuring Specula. About three years since, the Rev. Cooper Key dis- covered a more simple method of parabolizing the surface of specula than any which had hitherto been employed, and by this process he produced two fine specula of 12 in. diameter. The process by which these specula were worked Mr. Key communicated to Mr. G. With, and after having worked by Mr. Key's process until a few months since, Mr. With at length contrived, another plan of working, by which he considers still finer results are with greater certainty secured. The wonderful perfection of Mr. With's specula is now generally admitted, and it is almost certain that they surpass any that have previously been produced. I have great pleasure in stating that specula of Mr. With's para- bolizing are now only to be obtained from me. On mounting Specula. It has elsewhere been suggested that much of the dissatisfaction which has been expressed by those who have used reflectors has arisen from their having been imperfectly mounted. Because specula are much cheaper than achromatic object-glasses, it has been supposed that they could be mounted at proportionately less cost than that incurred in mounting reflectors. This is only true to the extent that cost can be saved by reason of their shorter focal length. It cannot be too strongly enforced that, to give the best performance, reflectors require to be mounted more steadily than refractors, because by a well-known law of optics the effect of any vibration will be multiplied many times. Their tubes must also be carefully arranged, so as to avoid as much as possible the interference of air-currents, which are the bane of reflectors improperly mounted or badly situated. The specula in the telescopes now described are mounted rigidly on a new plan, which ensures permanence in adjustment and prevents flexure. This plan is represented in Fig. 42. The bottom of the speculum A is a carefully prepared plane surface, and the bottom of the inner iron cell B, on which it rests, is also a plane. The THE REFLECTION OF LIGHT. 43 FIG. 42. speculum is clamped in this cell by the ring G G, and it may be removed from and replaced in the telescope without altering its adjustment. The elastic methods of mounting the speculum, which have hitherto been employed, generally required re-adjustment whenever the speculum had been removed. The reflecting diagonal prism or mirror is mounted in the manner shown in the diagrams Figs. 43 and 44. FIG. 44. In these B B B represent strips of strong chronometer spring steel, placed edgewise towards the speculum, by which the prism or small mirror D is suspended. The mirror, thus mounted, does not produce such coarse rays on bright stars as when it is fixed to a single stout arm ; it is also less liable to vibration, which is very injurious to distinct vision, or to flexure, which interferes with the accuracy of the adjustments. If an observer determines to lay out a given sum in the purchase of a tele- scope, he will find it to his advantage to have a smaller speculum completely mounted, instead of a large speculum imperfectly mounted. With the smaller and perfect instrument he will really do more work, and with much greater comfort and satisfaction to himself. No matter how good a speculum may be, nothing can be told of its performance on difficult double stars if it is mounted on an unsteady stand. 44 ON LIGHT. The alt-azimuth stand, represented in Fig. 45, is entirely of iron. The tube of the telescope is of extremely stout block tin, coloured dark green, the stand being coloured dark chocolate. The body is equipoised, so that it will remain in any position, while the movements are so smooth, and the leverage so arranged, that a star may be followed, even with a power of 300, without screw motions. The instrument can be used on a table, at any window ; and a stand is supplied with it, on which it can be supported at a convenient height when it is used in the open air. This mounting is only adapted for a small-sized speculum, say not exceeding 5 in. in diameter, as, if made of a larger size, it FlG. 45. The small Alt-azimuth. would be so heavy as not to be portable ; while with higher powers than 300, such as specula of 6 in. and above will easily bear, the celestial bodies cannot be followed without screw motions. By fastening the circular foot down on a block of wood of a wedge form, the angle being the complementary angle to the latitude of the place, this stand can very readily, and at a comparatively trifling expense, be made to move equatorially, so that the heavenly bodies can be followed with a single motion of the telescope. Such an arrangement is shown in Fig. 45. A cheaper mounting is shown in Fig. 54. The 4|-inch silvered-glass speculum, with powers from 100 to 150, will divide /3 Orionis. a Lyrae. 8 Geminorum. e Hydras. Ursae Majoris. Bootis. v Ceti. Draconis. The 6| will divide, with powers from 200 to 300 Arietis. a Herculis. Bootis. 32 Orionis. i Equulei. 77 Coronas Borealis. 36 Andromedas. The 8 J, with powers from 300 to 350, in a favourable state of the air, will divide THE REFLECTION OF LIGHT. 45 y- Andromedas. fj, Bootis. These last-named double stars are both under half a second apart, and are so difficult to divide as to have hitherto been considered good work for a 12-inch speculum. TO SILVER GLASS SPECULA. Prepare three standard solutions : Solution A Solution B Solution C 90 grains 4 ounces i ounce 25 ounces ounce Dissolve. Dissolve. Dissolve. Crystals of nitrate of silver Distilled water .... Potassa, pure by alcohol Distilled water .... Milk-sugar, in powder Distilled water 5 ounces Solutions A and B will keep, in stoppered bottles, for any length of time ; solution C must be fresh. The Silvering Fluid. To prepare sufficient for silvering an 8-inch speculum, pour 2 ounces 'of solution A into a glass vessel capable of holding 35 fluid ounces. Add, drop by drop, stirring all the time with a glass rod, as much liquid ammonia as is just necessary to obtain a clear solution of the grey precipitate first thrown down. Add 4 ounces of solution B. The brown-black precipi- tate formed must be just re-dissolved by the addition of more ammonia as before. Add distilled water until the bulk reaches 1 5 ounces, and add, drop by drop, some of solution A, until a grey precipitate, which does not re-dissolve after stirring for three minutes, is obtained, then add 15 ounces more of dis- tilled water. Set this solution aside to settle. Do not filter. When all is ready for immersing the mirror, add to the silvering solution 2 ounces of solution C, and stir gently and thoroughly. Solution C may be filtered. Perfectly pure chemicals may be obtained of Mr. Townson, 89, Bishopsgate Within, London, E.G., and Mr. R. f Thomas, 10, Pall Mall. To prepare the Speculum. Procure a circular block of wood 2 in. thick and 2 in. less in diameter than the speculum. Into this should be screwed three eye-pins at equal distances, as in Fig. 46. To these pins fasten stout whipcord, making a secure loop at the top. Melt some soft pitch in any convenient vessel, and hav- ing placed the wooden block face upwards on a level table, pour on it the fluid pitch, and on the pitch place the back of the speculum, having previously moistened it with a thin film of spirit of turpentine to secure adhesion. Let the whole rest until the pitch is cold. To clean the Speculum. Place the speculum, cemented to the circular block, face upwards, on a level table ; pour on it a small quantity of strong nitric acid, and rub it gently all over the surface with a brush made by plugging a glass tube with pure cotton wool. (Fig. 47.) Having perfectly cleaned the surface and sfdes, wash well with common water, and finally with distilled water. Place the speculum face downwards in a dish containing a little rectified spirit ef wine until the silvering fluid is ready. To immerse the Speculum. Take a circular dish about 3 in. FIG. 47. deep and 2 in. larger in diameter than the speculum. Mix in it FIG. 46. 4 6 ON LIGHT. FIG. 48. the silvering solution and the solution C, and suspend the speculum, face downwards, in the liquid, which may rise about a quarter of an inch up the side of the speculum. When the silvering is completed, remove the speculum from the solution, and immediately wash with plenty of water, using at least two gallons, and finally with a little distilled water. Place the speculum on its edge on blotting- paper to drain and dry. (Fig. 48.) When perfectly dry, polish the film by gently rubbing first with a piece of the softest wash-leather, using circular strokes (Fig. 49), and finally with the addition of a little finest rouge.* A " flat " may be silvered by fastening with pitch to a slice of cork, cleaning as above described, and using as much sil- vering fluid as will form a stratum about half an inch deep beneath the mirror. To separate the Speculum from the Block. Stand the speculum on its side, insert the edge of a sharp half-inch chisel between the wood and glass, adminis- tering two or three gentle blows, and the block and mirror will separate safely and easily. It is preferable to obtain the aid of an assistant in this operation. Any pitch which remains on the back of the mirror may be removed by scraping and a little turpentine. The cost of silvering an 8-inch speculum, exclusive of the cost of alcohol, which may be used over and over again, will not exceed gd., Nitrate of silver being 45. per oz. Potash . . 8d. Milk-sugar . . 2d. n Avoid all excess of ammonia, and be sure to use//m? distilled water. ON WORKING GLASS SPECULA. FIG. 49. FIG. 50. When parallel rays of light are allowed to fall upon the surface of a concave mirror, if the surface be a spherical curve, the rays will not all be reflected to a single point. ,In Fig. 50 it will be seen that the rays A A, falling on the mirror, would be * The silvering will be completed in from 30 to 70 minutes, according to temperature ; Jo minute? will be sufficient in summer. THE REFLECTION OF LIGHT. 47 reflected and form an image at a ; while the rays B B would be reflected and form an image at <, farther from the front of the mirror. If the reflected images were viewed with an eye-piece placed anywhere in front of the mirror, they would not be in focus at the same time, so that only a blurred and indistinct image would be seen. To make the mirror reflect rays falling on all parts of its surface to one point, it is necessary that it should be fashioned into a parabolic curve. FIG. 51. Such a curve is snown in Fig. 51, which maybe considered as a spherical curve, in which the curve has been made deeper or the outer portion flattened. In practice the amount of this difference is so exceedingly minute as to be inappreciable by actual measurement. Sir John Herschel states that the utmost variation of a 4-foot speculum from a spherical curve is less than than one 2i,oooth part of an inch. Yet it is well known that for telescopic use a mirror with a spherical curve is, for the reason just explained, totally useless. In working the glass specula, a disc of hard crown glass, varying in substance from three-quarters of an inch to one and a half inches, according to the size of the speculum for which it is intended, is turned, and polished on the edge. One side of this disc is ground to a truly plane surface. On this side the speculum, when mounted on the writer's plan, rests in its cell. The other side is then ground to a concave spherical curve of such a radius as will produce the desired focus. This spherical curve is converted into a parabolic figure somewhat thus : An iron tool, similar to that on which the spherical curve has been ground, fs covered with a layer of pitch, tempered to a certain consistency. This pitch is warmed, and the speculum being laid upon it makes the pitch assume the same curve. The speculum is then polished on the pitch with rouge. In this polishing the speculum and polisher are not worked together equally all over the surfaces, but the speculum is moved in such a manner that it is polished away most towards the edge, and a parabolic curve is produced. During the process both the speculum and the polisher continually revolve. The diagram of Lord Rosse's machine, with which he figured his speculum 6 ft. in diameter, will give an idea of the action of the speculum and polisher on each other. This machine is represented in Fig. 52; A is the spindle, by turning which the whole machine is set in motion ; H I is the speculum ; K L, the polisher ; B, an excentric which carries the polisher backwards and forwards ; G, another excentric Avhich moves the polisher from side to side slowly during the recipro- ON LIGHT. eating motion. The amount of motion given to the polisher, and the rapidity of rotation of the speculum, can be changed at pleasure. Fig. 52. In Fig. 53 the dotted line represents the spherical curve of the mirror when the polishing is begun, and the continuous line the parabolic curve it assumes when the polishing process is finished. It will be, of course, understood that in all the diagrams these curves are enormously exaggerated. During the graduated polishing the speculum is repeatedly tested, until the desired definition is attained. When completed, if accurately figured, the marginal inch of the speculum should give equally sharp definition with the centre, and have identically the same focus. FIG. 54. In figuring the mirrors of the telescopes herein described, an improved method has been adopted of fashioning the parabolic curve; it is believed this method gives superior results to any hitherto attained.* * The reader who wishes for further information on this subject is referred to Sir John Herschel's work on "The Telescope." THE REFRACTION OF LIGHT. 49 THE REFRACTION OF LIGHT. When a ray of light passes from one medium to another 01 tne same density, and in a perfectly straight line, no alteration of its course takes place ; but if the light passes in an oblique direction, its course is broken or refracted, i.e., bent back from its natural path. To this branch of optics, which includes, perhaps, the widest field of inquiry, and traces the propagation of light through transparent, solid, liquid, and gaseous bodies, has been given the name of DIOPTRICS. To prove that a straight line representing a ray of light is really bent when passing from a rare medium, air, into a denser one, such as water, nothing is easier than to place a bright shilling on the end of an ivory paper-knife, which is inclined in a large empty tumbler. On looking down the paper-knife a straight line only is apparent, terminating with the coin ; but if the tumbler is filled with water whilst the observer is still looking down the flat surface, he FIG. 55. A simple demonstration of the property of Refraction. will notice that at the point of juncture between the air and water a break takes place, and the end of the paper-knife, or all that part immersed, appears to be lifted up or bent upwards from its natural course or direction. If a small pocket-pistol were now aimed at the coin and the bullet discharged it would certainly miss, because every visible object appears to be in a direction repre- sented by a straight line drawn from it to the eye. A straight line ruled to the. shilling would not touch it, the line must be ruled to, or the pistol aimed at, a point nearer to the spectator than the apparent position of the coin. The bending of the ray is governed by certain laws known as " Descartes' Laws.'"' Firstly, Whatever the obliquity of the incident ray, the sine of the incident angle and the sine of the angle of refraction are in a constant ratio for the same two media, but vary with different media. Secondly, The incident and the refracted rays are in the same plane, which is perpendicular to the surface separating the two media. A very complete French apparatus (Fig. 56), described in Ganot's " Physics," 4 5 ON LIGHT. is made for the purpose of proving those laws experimentally,, It consists of a large and carefully graduated circle supported on a tripod stand. In the centre is placed a semi-cylindrical glass vessel filled with water, or any other fluid whose index of refraction it is required to ascertain, so that the level of the fluid coincides with the height of the centre of the circle. From the mirror A, a ray of light is reflected through a hole in the screen B, and falls on the surface of the water at c. Passing through the water, the course of the refracted ray is traced to a screen D, on which the circular image is received. In the various positions of the screens B and D, attached to arms radiating from the centre C, the sines of the angles of incidence and refraction are ob- tained and measured by two graduated rules E F, movable so as to be always horizontal, and therefore perpendicular to the diameter G H. The numbers vary with the positions of the screens, but the sines of the incident and re- fracted rays are in a constant ratio to the same two media, viz., air and water. If the sine of the incident ray is doubled, the sine of the refracted one will increase in the same ratio. When another fluid is put into the trough, a variation in the sines would occur, and it is in this manner the first law is proved. By moving the mirror and screen B, so that the light falls perpendicularly on the surface of the water, the instrument proves the second law, as there cannot then be any angle formed, or sines to record or measure. Supposing the sine of the angle of refraction in the above experiment with air and water to measure 12 in., and the sine of the angle of incidence 16 in., it would follow that in water the sine of the angle of incidence is to the sine of the angle of refraction as 1*336 to I, or as nearly as possible i^ to I. The number i'336, which expresses this ratio for water, is called the index: of re- fraction for water, and sometimes its refractive power. The determination of the refractive powers of various kinds of glass is of great use in the manufacture of achromatic telescopes ; and sometimes the purity of a liquid, and its freedom from adulteration, may be approximately ascertained by taking the index of refraction. In the chapter devoted to the consideration of the reflection of light, it was thought to be the most simple and instructive plan to trace the progress of parallel rays when thrown off from plane, con- cave, or convex surfaces. The forms of refracting bodies, and their action on light, are so numerous and well discussed in the more elaborate works on Dioptrics, that it is mere repetition to quote them all. The laws of refraction being known, and the refractive power of the glass used for experiment FIG. THE REFRACTION OF LIGHT. being ascertained, the mathematician may work out on paper the exact direc- tion of the light passing into or out of the most complicated forms. As an illustration of this mode of investigation, the following instructions are given by Brewster, in order to enable the student to study the refraction of light through one of the most important optical instruments, viz., the Prism. (Fig. 570' An optical prism, a solid having three plane surfaces. A B, A C, called its refracting surfaces ; B C is called the base of the prism. Let ABC (Fig. 58) be a prism of plate glass, whose index of refraction is 1-500, and let H R be a ray of light falling obliquely upon its first surface A B at the point R. Round R, as a centre, and with any radius H R, describe the circle H M b. Through R draw M R N perpendicular to A B, and H m perpen- dicular to M R. The angle H R M will be the angle of incidence of the ray H R, and H m its sine, which in the present case is 1*500. Then, having made a scale in which the distance H m is 1*500, or \\ parts, take one part or unity from the same scale, and having set one foot of the compasses on the circle, some- where about b, move it to different points of the circle till the other foot strikes only one point n of the line R N ; the point b thus found will be that through which the refracted ray passes, R b will be the re- fracted ray, and n^b the angle of refrac- tion, because the sine b 'n of this angle has been made such, that its ratio to H ?/?, the sine of the angle of incidence, is as i to i '500. The ray R b thus refracted will go on in a straight line till it meets the second surface of the prism at R R', when it will again suffer refraction in the direction R b'. In order to determine this direction, make R' H' equal to R H, and, with this distance as radius, describe the circle H' b'. Draw R' N perpendicular to A C, and H' m' perpendicular to R N, and form a scale on which H' m shall be one part, or i 'ooo, and divide it into tenths and hun- dredths. From this scale take in the compasses the index of refraction 1-500 as i^ of these parts ; and, having set one foot somewhere in the line R' n\ move it to different parts of it till the other foot falls upon some part of the circle about b>, taking care that the point b 1 is such, that when one foot of the compasses is placed there, the other foot will touch the line R' $', continued only in one place, join R' b Then, since H' R' vv is the angle of incidence, or the second surface A C and H' m its sine, and since n' &', the sine of the angle b' R w, has been made to have to H' in' the ratio of 1*500 to i, b' R' n' will be the angle of refraction, and R' b' the refracted ray. In the construction of the figure (Fig. 58) the ray H R has been made to fall upon the prism at such an angle that the refracted .ray R R' is equally inclined to the faces A B, A C ; or is parallel to the base B C of the prism ; and it will be found that the angle of incidence H R B is equal to the angle of emergence b' R 7 C. Under these cir- cumstances, we shall find, by working the angle H R B either greater or less than it is in the figures, that the angle of deviation H E D is less than at any other angle of incidence. If we, therefore, place the eye behind the prism at b\ and turn the prism round in the plane BAG, sometimes bringing A towards 42 5 2 ON- LIGHT. the eye and sometimes pushing it from it, we shall easily discover the position when the image of the candle seen in the direction b' D has the least devia- tion. When this position is found, the angles H R B and b' R' C are equal, and R R' is parallel to B C, and perpendicular to A F, a line bisecting the refracting angle B A C of the prism ; but since B A F is known, the angle of refraction B R N is also known ; and the angle of incidence H R B being found by the preceding methods, we may determine the index of refraction for any prism by the following analogy : As the sine of the angle of refraction is to the sine of the angle of incidence, so is unity to the index of refraction ; or the index of refraction is equal to the sine of the angle of incidence divided by the sine of the angle of refraction. By this method we may readily measure the refractive power of all bodies. If the body be solid, it must be shaped into a prism ; and if it is soft or fluid, it must be placed in the angle B A C of a hollow prism, ABC, (Fig. 59) made by cementing together three pieces of plate glass, A B, A C, B C. A very simple hollow prism for this purpose maybe made by fastening FIG. 59. together at any angle two pieces of plate glass, A B, A c, with a bit of wax F. A drop of the fluid may then be placed in the angle at A, when it will be retained by the force of capillary attraction. TABLE OF THE INDICES OF REFRACTION. Vacuum . I "OOOOOO Lens, Crystalline I-384 Air .... 1-000294 Vitrous . 1-339 Albumen . 1*360 Aqueous . 1-336 Alcohol 1-374 Nitrous Oxide Gas . Ammonia Gas. 1-000385 Nitric Acid 1-410 Alum i '45 7 Oxygen . 1-000272 Amber 1-547 Olefiant Gas . 1-000678 Bisulphide of Carbon 1-678* Oil, Olive . 1-470 Carbonic Acid Gas . i -000449 Turpentine. 1-475 Chlorine Gas 1-000772 Castor 1-490 Diamond . 2-439 Cloves 1-535 Ether 1-358 Cassia' 1-641 Fluid Spar 1-434 Phosphorus 2-424 Glass, Flint 1-605 Quartz 1-548 Plate . 1*543 Ruby 1779 Crown . 1*534 Sapphire . 1794 Garnet 1-815 Sulphur 2-115 Hydrogen . 1-000138 Sulphuric Acid Gas . i -000665 Hydrochloric Acid Gas i -000449 Sulphuric Acid . 1-434 Hydrochloric Acid . 1-410 Tabasheer mi Iceland Spar Water I-336 Ordinary ray . 1-654 Solid (Ice) 1-310 Extraordinary ray . 1-483 Zircon 1-961 The course of parallel rays of light through plane, concave, and convex * Used to till prisms for spectrum analysis. THE REFRACTION OF LIGHT. 53 surfaces of glass may now be considered, and they will be found to contrast in the most simple manner with similar-shaped reflecting surfaces. FIG. 60. REFRACTION OF LIGHT THROUGH PLANE GLASS. Let A B (Fig. 60) be a ray of light incident on the upper surface or side of a piece of ordinary plate glass, marked C C, whose other or under side, D D, is parallel to C C. On entering the glass the ray is refracted in the direction B E, and it will be refracted again at its exit from the under side, D D, to the same amount as at its entrance in the line E F ; consequently an eye placed at F would see the ray as if it came from the pint A' along the line F E A 7 . The light has undergone refraction, and an object seen through a window is not seen in its true position ; but, as parallel rays falling upon a plane glass retain their parallel lines after passing through it, the object does not appear to undergo any change unless the two surfaces of the glass are uneven, and not parallel with each other, when distortion takes place. Such an effect is rarely seen now in looking through the windows of good houses, because they are usually glazed with plate glass, the sides of which are nearly parallel. It has already been shown that convex mirrors (page 22) render parallel rays of light divergent ; precisely the reverse occurs with convex refracting surfaces. REFRACTION OF PARALLEL RAYS OF LIGHT BY CONVEX SURFACES. Fig. 6 1 represents a piece of glass cut into the form of a double-convex lens A B, a figure such as would be pro- duced by placing one watch-glass on the edge of another having precisely the same amount of convexity. Let C D be a ray of light falling perpen- dicularly on the refracting surface ^ E and passing straight through the glass, in obedience to the law al- ready enunciated, that a ray of light which falls perpendicularly on a re- fracting surface undergoes no change in its direction, and therefore C D passes through the middle or axis FIG. 61. 54 ON LIGHT. of the crystal lens without deviation from a straight line C D E. The other two rays, F G, H I, falling at an angle on the glass, undergo refraction, and are bent towards and emerge from the other side, and meet at the point E, called the principal focus, or focus for parallel rays. These parallel rays of light are refracted by a double-convex lens, and become convergent, meeting at a point called the focus. On the other hand, if E be considered as the luminous point from which divergent rays are emitted, they become parallel rays when they emerge from the double-convex lens A B. REFRACTION OF PARALLEL RAYS BY CONCAVE SURFACES. Let A B (Fig. 62) be a glass lens, whose two sides are hollowed out, or concave, and C D a ray of light falling perpendicularly on the surface, and therefore passing straight through the lens. F G and H I are two other rays impinging on the surface of the glass at an angle ; these undergo re- fraction, and are bent outwards in the direction F G K and H I K. Thus the property of a concave lens is just the reverse of a concave mirror, the former causing parallel rays of light to become divergent, the latter convergent; and if the rays K K be regarded as convergent rays, they become parallel when emerg- FIG. 62. ing from the concave lens A B. OTHER FORMS OF LENSES. For various optical purposes a variety of lenses, in addition to the prism, the double convex, or the double concave lenses, is required, which may be ground into the following forms : a. A spherical lens, causing parallel rays to become convergent . d. A piano convex lens ; parallel rays become conver- gent ..... THE REFRACTION OF LIGHT. 55 c. A plano-concave lens ; parallel rays become diver- gent . . . . . o . . . d. A meniscus ; parallel rays become convergent . e. A concavo-convex lens ; when the concavity exceeds the convexity, parallel rays become divergent FIG. 63. It is good practice for the student in physics to make careful drawings of the above figures, and to trace the paths of imaginary rays of light through them. The drawings may be varied by supposing the lenses to be made of any of the solid transparent substances whose refracting indices are given in the table at page 52. OPTICAL INSTRUMENTS WHOSE PROPERTIES DEPEND ON REFRACTION. THE SIMPLE AND COMPOUND MICROSCOPE AND TELESCOPE. It follows from the laws of refraction already explained, that when a double- convex lens (Fig. 64) acts on rays proceeding from an object, such as a candle, A B, that, as the rays are not all parallel, they will be collected into a focus A' B' at a distance behind the lens somewhat greater than the focus for parallel rays at E, and that an inverted image of the candle A B will be produced at A" B', which may be received on any white surface. Thus a double-convex lens becomes the most simple microscope which can be used, and it is some- times used for that purpose in the examination of samples of wheat. The ON LIGHT. FIG. 64. cheapest microscope the author has seen is that made by Me Culloch, of Blucher Street, Birmingham, for half-a-crown. It includes a lens made, seemingly, of a filament of glass melted into' a globule, fitted into a brass tube which contains a plate of glass to be used as an object-holder (such as for the eels in paste), and the opposite end of the brass tube is closed with a diaphragm, which can be unscrewed if more light is required. The whole is fitted into a case, and might be made a very amusing companion for young people when they go into the fields ; and if lost, the value is not an alarming consideration. Another marvel of cheap- ness ib a telescope made by Solomon, of Albe- marle Street, at a cost of five shillings. The latter, of course, is not achromatic ; but its definition of distant objects is really excellent, and the work- manship good. In the compound microscope the image A' B" (Fig. 64) is still further magnified, and can be more carefully examined by the addition of an- other double-convex lens, say of an inch focal distance. It is the image formed in the tube of the compound telescope, which may be again magnified by employing a second lens with a very short focus. In these cases the first lens is called Fig. 65. Simple Microscope, the object-glass, and the second the eye-piece or glass. Of late years the most elaborate and per- fect microscopes have been made in this country ; so that England stands unrivalled in this branch of optical instruments, whilst her microscopical societies have contributed largely to our knowledge of those things which cannot be appreciated or examined without the use of these contrivances. in which the Lens is focused bv turning the Screw. C \j FIG. 66. The Compound Telescope. THE REFRACTION OF LIGHT. 57 B, The object-glass, which throws an inverted image into the dark tube ; C is the eye-glass, which magnifies the inverted image. This telescope could only be used for astronomical purposes ; but, by the addition of two other convex lenses at D E, called erecting-glasses, an erect image is obtained. THE CAMERA OBSCURA. A dark chamber into which a double-convex lens is fitted. The invention of this pleasing contrivance has been usually ascribed to Baptista Porta, as it appears in his " Magica Naturalis," lib. xvii., cap. vi., first published at Frank- fort about 1589 or 1591. Fifty years ago the camera obscura was more popular than it is now, and was frequently erected on elevated spots of ground by wealthy individuals, the consequence being that the whole apparatus and the building to which it was attached were most carefully made and adjusted to each other. FIG. 67. Fig. 67 represents a dome or cupola placed over a room erected for the purpose of a camera obscura. The whole dome, which carries the box and containing a mirror placed at an angle over a double-convex lens, may be made to turn round on friction-wheels ; or, what is more simple, the box is made movable in a groove upon the dome, and may be turned with a long rod by a person inside. The box is recommended to be of a cubical form, of about 6 or 7 in. square, and contains a carefully ground plain silvered mirror, which should be made of parallel glass placed diagonally in the box ; the mirror itself should be attached by hinges at the lower end, so that a different angle may be obtained if required. Underneath the mirror, in a round cell at the bottom of the box, is fixed a double-convex lens, about 6 or 8 ft. focus and 4 or 5 in. in diameter; this lens will form, upon a white table ON LIGHT. placed on the floor below, the image of the objects reflected by the mirror above at the focal distance of the lens. FIG. 68.- 772* Prism Camera. D D D, section of a pyramidal box; M, a brass tube open on one side, moving in another tube, and containing the rectangular prism ABC, one side of which, A c, is convex, and the other, c B, concave ; o, the framework to support the sheet of paper. The diameter of the table should be 2\ or 3 ft., and, in order to cor- rect the indistinct images formed at the edge by spherical aberra- tion, it is usual to make the sur- face slightly concave, and to form it of the best plaster of paris or stucco. The table should be sup- ported by a pillar in the centre, fitting into a tube provided with a screw, so that the table may be raised or lowered, and the images exactly focused on its surface. A still more perfect optical arrange- ment for projecting brilliant images of distant objects on to a white surface for the purposes of the artist is shown in the figure annexed. (Fig. 68.) In this camera the rays of light, after falling on the convex sur- face, enter the prism, and, being totally reflected from the side A B, pass into the box through the concave surface, and fall upon a sheet of paper laid out on a pro- per framework. The picture thus obtained has not the fault of those produced by the ordinary arrangement of the mirror or con- vex lens, being free from spherical aberration, which is neutralized in this instance by the concave surface of the prism. As these prisms are difficult to make, the same result is attained by care- fully cementing with Canada bal- sam a piano - convex lens on one side of the prism, and a plano-concave on the other, whose focal lengths are equal to each other. (Fig. 69.) The magic lantern apparatus, the dissolving view and the phantasmagoria lantern apparatus, are all refracting optical instruments, very easily constructed. The magic lantern was contrived, about the year 1650, by the celebrated Kircher, and is described in his work entitled, "Ars Magna Lucis et Umbrae." FlG. 69. ABC, the prism, with plano- convex and plano-concave lens attached at A E and c E. THE REFRACTION OF LIGHT 59 There is, however, a curious account of phantom figures or demons, made to appear in the smoke of a fire and thrown upon walls, ascribed to Cellini, who lived nearly a century before Kircher. If the story be true, it would seem to show that phantasmagorial effects preceded the magic-lantern pictures, and that Cellini must have been acquainted with the construction of the instrument, or such effects as described could not have been produced. The magic lantern consists of a box provided with a chimney, containing a good lamp, or, still better, an oxy-hydrogen light ; when the former is used, a reflector is usually FlG. 70. Common Magic Lantern. B, the box ; c, the lamp and reflector ; A, the plano-convex lens ; c c, the tube sliding within the first tube, and containing a double-convex lens, A'. placed behind the flame, in order to increase the illumination of the pictures, The lime-light is placed behind the lenses called condensers (Fig. 71); these are usually composed of two plano-convex lenses, with the flat side placed towards the lamp, and the convex side touching, or nearly so, the convexity of the other lens, the flat side of which is towards the picture. The picture, painted or carefully photographed on glass, is placed in front of the condensers, and the whole projected and properly fo- cused on a white screen by means of two other plano-convex lenses ; the flat side of one lens being to- wards the picture, and the convex side towards the flat side of the second lens. The focusing lenses are contained in a tube which slides within the other, and is moved back- wards and forwards with a simple rack-work. The dissolving view arrangement, long kept a secret by Mr. Child, the inventor, is nothing more than two magic lanterns (Figs. 74, 75) placed side by side, and provided with slid- ing plates so arranged that, as one picture is gradually cut off, the second FIG. 71. Section of Superior Magic Lantern. diaphragm to reduce the aberration of light. 6o ON LIGHT. is disclosed; and by alternately throwing on one picture and cutting off the other, the most pleasing effects are obtained, provided the two lanterns are precisely similar. To save gas, it is sometimes usual to turn off the oxygen from one lantern and to supply it to the other, and thus by alternately raising and lowering the lights in the lanterns the same result is obtained. (Fig. 76.) The phantasmagorial effects first ascribed to Cellini are produced by painting in the figure-picture on glass, and then blackening out the whole of the ground, and either by carrying the lantern and moving backwards and forwards behind the sheet, or by a me- chanical arrangement in which the lan- tern runs on a tramway, and is focused as it approaches or recedes from the transparent disc the pictures are made to increase or diminish at pleasure. In practice it is better to allow the lantern and person showing it to be carried on the same carriage, as the lever arrange- ment shown in Fig. 72, and attached to the focusing lenses is very apt to get out of order. One of the most useful instruments for public exhibitions is that designed by Messrs. Chadburn, of 71 Lord Street, Liverpool, for the purpose of producing enlarged images upon a screen (similar to those of the magic lantern) from opaque objects, such as photographs, carte de visites, engravings, drawings, relievos, natural objects in all their colours, mechanical apparatus, or deli- cate mechanism in motion, such as the various parts of a watch or, still better, of a repeating watch. The instrument is simple in its construction, and con- sists of a lantern box, containing in the centre a pillar with adjusting screw, upon which the lime cylinder is placed ; behind it the metallic reflector, which must be so adjusted that the picture is evenly illuminated. The reflector can be raised or lowered, or moved back- wards and forwards ; it receives the light, and throws it upon the condensing lens, by which it is concentrated upon the picture placed in the sliding door in the angular box joined to the square compartment. The light thrown off from the highly illuminated picture is received by the achromatic objective lenses (the focus of which is adjusted by the rack upon them), and projected upon the screen. The angular compartment may be removed, and replaced by a part with lenses for direct light and transparent pictures, as in the ordi- nary magic lantern. An oyster directly after it is opened, the 'half of an orange, particularly if squeezed, as the effect is most ridiculous, the juice and pips appear to fall FIG. 72. THE REFRACTION OF LIGHT. 61 upwards all bodies being reversed in this instrument, the hand and orange are shown upside down the human hand, the face of a watch, a gold or FIG. 73. Part Section and Elevation of Chadburtfs Lantern, A, the light; B, reflector c, condensing lens; D, the picture; E, the achromatic focusing lenses. FIG. 74. Improved Dissolving View Apparatus by Highley, IOA Great Portland Street. silver coin, and photographs of distinguished persons, are all good objects for this instrument. In 1857 the writer introduced at the Polytechnic photographs- of original ON LIGHT. FIG. 75. Section of Highlefs Dissolving View Apparatus (Fig. 74). drawings made by Mr. George Hine, the distinguished artist. The whole of the pictures illustrating the amusing story of Blue Beard were done in this FlG. 76. Arrangement for saving oxygen gas, which is supplied alternately to one lime light and then to the other. way, and were most effective and successful, as every touch of the original artist is thus delineated in the photograph and subsequently thrown on the THE REFRACTIOM OF LIGHT. FlG. 77. Highley'' s complete Apparatus for Dissolving Views ; all packed in two boxes, screen. Messrs. Negretti and Zambra followed up the idea by using photo- graphs of statuary, which they displayed at Manchester with astonishing success, the Mechanics' Institution there realizing something like ^600 by the exhibition in a few months. Mr. Highley has continued in the same track, and deserves notice for the admirable photographs of natural objects which he prepared for the dissolving-view apparatus his arrangement of the latter contrivance, shown in Fig. 74 and in section Fig. 75, is good and convenient. The arrangement for saving oxygen gas (Fig. 76) is also extremely useful where the gas cannot be obtained easily. Portability and economy of space have all been carefully studied by Highley in Fig. 77, where the gases (oxygen or hydrogen) are condensed in separate strong copper cylinders which pack in one box, and the lantern, the slides, and the stand upon which they are placed, come out of and belong to the second box. 64 ON LIGHT. THE HUMAN EYE. This elaborate and wonderful work of the Creator, built up of the usual constituents of animal substances, viz., albumen, gelatine, fibrine, with a little fatty matter, all marvellously shaped and fitted to their purposes, repre- sents an optical instrument which transcends every contrivance made by the hand of man. The camera obscura is the nearest approach to an imita- tion of the eye. It is fitted with a double-convex lens ; the rays of light thrown off from any object placed before the apparatus are brought to a focus, and received upon a sheet of paper or piece of ground glass. In the eye the same result is brought about by the refraction of light in the crystalline lens and the other humours ; the rays are brought to a focus, and impinge upon a nerve, spread out as a delicate network to catch the beams, and to vibrate in sympathy with those exquisite undulations which cause the propagation of light, and thus to produce the sensation of vision. Anatomists have given this organ their most careful attention, and published elaborate drawings of the various parts of the eye. By the permission of Messrs. Chadburn, of Shef- field, a copy of their instructive diagrams of the eye is added (page 65). A. The Pupil, or circular opening in the iris, capable of being contracted or enlarged, according to the amount and intensity of light. B. The Iris, a flat circular membrane, of a grey, blue, or black colour, forming the anterior and posterior chambers of the eye. It performs the same functions as a diaphragm in an optical instrument. C. The Sclerotic Coat, a tough white membrane, to which the muscles for moving the eyeball are attached. D. The Eyelids, containing the tarsal fibro-cartilages. E. The Cornea, composed of tough transparent laminae, forming the front of the eye ; the first surface, where the rays of light are refracted. Some anatomists have considered the sclerotica and cornea as one and the same, and have termed the cornea the transparent, and the sclerotica the opaque cornea. F. The Aqueous Humour, contained in a delicate membrane filling the space from the cornea to the crystalline lens. The space occupied by this humour is divided into two parts by the iris, forming, as shown at B, the anterior and posterior chambers of the eye. G. The Crystalline Lens, contained in a transparent membrane called the Capsule, the principal refracting medium of the eye. The capsule adheres by its edge to the ring-shaped body called' the Ciliary Circle or ligament, N. H. The Vitreous Humour, contained in the hyaloid membrane a jelly- like substance, resembling. the.white of an egg, filling the body of the eye. I. The Retina, a membrane which receives the impression of light, and transmits it to the brain through the optic nerve, K. j. The Choroid Coat, a delicate membrane lining the sclerotica, covered on its inner surface with a black substance (pigmentum nigrum, resembling the colouring matter of the negro's skin) contiguous to the retina. The choroid, by its vascular tissue, serves to carry the blood into the interior of the eye. K. The Optic Nerve. L. Canal of Petit. M. Central Artery of the optic nerve. U. Ciliary Circle or ligament. THE HUMAN EYE. FIG. 74. 77^ ////;;m ^. FIG. ^.The Eyeball, showing the Coats,-&. of the Eye. FIG. 76. Longitudinal Section of the Eye and Orbit, through the dotted tines on Fig. 74. 5 ON LIGHT, 66 U. Ciliary Nerves. P. Vasa Vorticosa. M. The Ciliary processes. k. Tunica Conjunctiva. k s. Tunica Conjunctiva collapsed, as when the eye is closed. s. Elastic Muscle of the Eyelid. T. Elastic Muscle of the Eye. U. Superior Oblique Muscle. v. Depressive Muscle of the Eye. w. Section of Oblique inferior Muscle. X. Nerves and Arteries. Y. Tube conveying the optic nerve to the brain. z. Bone forming the socket of the eye. N.B ""The same letters apply to each figure. Brewster found the following to be the refractive powers of the different humours of the eye, the ray of light being incident upon them from air : Aqueous humour . . i'336 Crystalline lens, surface I '3767 M ,i centre 1-399 Crystalline lens, mean . 13839 Vitreous humour . . i'3394 Water .... 1*3358 But the rays of light are not all incident upon them from the air, and as the rays refracted by the aqueous humour pass into the crystalline, and those from the crystalline into the vitreous humour, the indices of refraction of the separating surfaces of their humours will be From aqueous humour to outer coat of the crystalline . 1*0466 From i: ii to crystalline, using the mean index 1*0353 From vitreous to crystalline, outer coat I '0445 From i, to \\ using the mean index . . 1.0332 The eye,' as already described, consists of four coats or membranes, which are disposed in the following order, viz., ist, the sclerotic; 2nd, the cornea, which fits into it like the glass of a watch; 3rd, the choroid; and 4th, the retina; of two fluids or humours, the aqueous and the vitreous, and of a lens called the crystalline. Over the cornea and sclerotic is expanded a delicate mucous membrane, called the conjunctiva. The iris is suspended across the eye, and in its centre is an opening, termed the pupil, which immediately opens when the light diminishes, and closes if the light is too strong. The posterior convexity of the lens is greater than the anterior. Sometimes, from a too great convexity of the lens or the cornea, the rays of light which enter the eye come to a focus before they impinge upon the retina, producing the defect called short-sighted vision. Optical science corrects this inconvenience by the use of a concave lens. If the crystalline lens is not sufficiently convex, the rays of light come to a focus behind the retina ; this defect is surmounted by the use of a convex lens, which diminishes the divergence of the rays. Such ingenious artificial additions to the eye' are common enough at the present day, but it may be asked, how did our forefathers bear these infirmities? Spectacles are supposed to have been unknown to the ancients, and it is stated by Francisco Redi that they were invented in the I3th century, between the years 1280 and 1311, probably about the year 1299 or 1300; he gave the honour of the discovery to THE HUMAN EYE. 67 Alexander de Spina, a monk of the order of Predicants of St. Catharine, at Pisa. Muschenbroek, the old electrician who discovered the Leyden jar, observes that it is inscribed on the tomb of Salvinus Armatus, a nobleman of Florence, who died in 1317, that he was the inventor of spectacles. This may have been the person who had the secret as well as the learned monk, because Redi states that the latter only disclosed the secret upon learning that another person had it as well as himself. Mr. Acland makes the following practical and valuable observations on defects of vision : " On the Symptoms indicating a Necessity for Spectacles. " The natural decay of vision occurs usually from thirty to fifty years of age, varying according to habits and employment of the individual. Sometime during this interval the refractive power of the crystalline humours of the eye slightly alters its condition, whilst the crystalline lens and cornea change their form, so that a difficulty of distinct vision is felt. The eye loses a portion of its power of seeing at varying distances, or its power of adjustment ; and near objects are no longer as easily seen as in youth. Reading small print by candle-light is difficult, as the book requires to be held at a greater distance from the eye than formerly, and a more powerful light is needed ; and even then the letters appear misty, and to run one into the other, or seem double. And still further, in order to see more easily, the light is often placed between the book and the eye, and fatigue is soon felt, even with moderate reading. " When these symptoms show the eye to have altered its primitive form, spectacles are absolutely needed. Nature is calling for aid, and must have assistance, and if such is longer withheld, the eye is needlessly taxed, and the change, which at first was slight, proceeds more rapidly, until a permanent injury is produced. " There is a common notion that the use of spectacles should be put off as long as possible, but such is a great mistake, leading often to impaired vision for life, and is even more injurious than a too early employment. " Timely assistance relieves the eye, and diminishes the tendency to flat- tening, whereas should the use of spectacles be longer postponed, the eye changes rapidly, and when the optician is at last consulted, it is found that a deeper focus spectacle must be used than usual for the first pair, and even these suit but a short time, and have to be again exchanged for those of still deeper power; and these frequent changes become a matter of necessity which, unless judiciously checked, continue during life. " It must not be forgotten that, when first using spectacles, they are not required during daylight, but only for reading, &c., by artificial light, and it may be from six months to two years from the time of first adopting them ere they will be required for day use. " Spectacles for the Short-sighted. Short sight is often present at birth, but is little noticed, nor its inconveniences felt, until study becomes imperative. When this occurs, the power employed should be always slightly under that needed to remedy the defect, otherwise the eye will gradually accommodate itself to the lenses, and require constantly an increase of power. In all cases leave some little for the adjustment of the eye to do, and then you may, after a time, diminish the power of the lenses needed. " The Optician's Knowledge. Having now shown when spectacles should be employed, let us for a moment consider what are the requirements that 5 2 68 ON LIGHT. should in all cases be possessed by the optician to whom the selection of spectacle lenses is entrusted. ' These requirements are ' i st. An intimate knowledge of the anatomical structure of the eye, and of the theory of vision. ' 2nd. An extensive acquaintance with the science of optics. ' 3rd. A sound mathematical knowledge. '4th. A practical acquaintance with the manufacture of lenses and spectacle frames. " Having for the last fourteen years made the adaptation of spectacles my especial study, at the establishment of Messrs. Home and Thornthwaite, 122, Newgate Street, I have frequently met with cases where great injury has been done to the weak-sighted by the ordinary optician's improper selection of spectacles ; and I could heartily wish more of my medical brethren would bring their knowledge to bear on this subject, which demands, and frequently calls forth, all the science and skill we possess, to meet the requirements of some abnormal cases that present themselves." The knowledge which the eye conveys to the mind is boundless ; the rela- tive condition of matter, large and small, of motion or rest, of colour, of solidity, of transparency, of brilliancy, of opacity, of space or distance, are only a few of the results attained by the exercise of the faculty of vision. THE STEREOSCOPE. This most valuable and instructive instrument, and now not only a " house- hold word," but a piece of domestic apparatus without which no drawing-room is thought complete, was invented by Professor Wheatstone, and subsequently modified by Sir D. Brewster. It demonstrates that man must have two eyes in order to enjoy the appreciation of distance, or, like the fabled Polyphemus, we might only have had one eye. Mr. Woodward gives the following excel- lent and familiar explanation of the phenomena produced by the stereoscope. FIG. 77. PROFESSOR WHEATSTONE'S REFLECTING STEREOSCOPE. A familiar explanation of the phenomena produced by the Stereoscope. " The name is derived from two Greek words, signifying to view solid things, and the instrument is so constructed that two flat pictures, taken under certain conditions, shall appear to form a single solid or projecting body. THE STEREOSCOPE. 69 "A picture of any object is formed on the retina of each eye ; but although there may be but one object presented to the two eyes, the pictures formed on the two retinas are not precisely alike, because the object is not observed from the same point of view. "If the right hand be held at right angles to, and a few inches from, the face, the back of the hand will be seen when viewed by the right eye only, and the palm of the hand when viewed by the left eye only ; hence the images formed on the retinae of the two eyes must differ, the one including more of the right side and the other more of the left side of the same solid or pro- jecting object. Again, if we bend a card so as to represent a triangular roof, place it on the table with the gable end towards the eyes, and loolj; at it, first with one eye and then with the other, quickly and alternately opening and closing one of the eyes, the card will appear to move from side to side, because it will be seen by each eye under a different angle of vision. If we look at .the card with the left eye only, the whole of the left side of the card will be plainly seen, while the right side will be thrown into shadow. If we next look at the same card with the right eye only, the whole of the right side of the card will be distinctly visible, while the left side will be thrown into shadow ; and thus two images of the same object, with differences of outline, light and shade, will be formed the one on the retina of the right eye, and the other on the retina of the left. These images falling on corresponding parts of the retina convey to the mind the impression of a single object j * while experience having taught us, however unconscious the mind may be of the existence of two different images, that the effect observed is always produced by a body which really stands out or projects, the judgment naturally determines the object to be a projecting body. " It is experience also that teaches us to judge of distances by the different angles of vision under which an object is observed by the two eyes ; for the inclination of the optic axes, when so adjusted that the images may fall on corresponding parts of the retina;, and thus convey to the mind the impression of a single object, must be greater or less, according to the distance of the object from the eyes. " Perfect vision cannot then be obtained without two eyes, as it is by the combined effect of the image produced on the retina of each eye, and the different angles under which objects are observed, that a judgment is formed respecting their solidity and distances. " A man restored to sight by couching cannot tell the form of a body without touching it, until his judgment has been matured by experience, although a perfect image may be formed on the retina of each eye. A man with only one eye cannot readily distinguish the form of a body which he had never previously seen, but quickly and unwittingly moves his head from side to side, so that his one eye may alternately occupy the different positions of a right and a left eye ; and, if we approach a candle with one eye shiit, and then attempt to snuff it, we shall experience more difficulty than we might have expected, because the usual mode of determining the correct distance is wanting. "In order, then, to deceive the judgment, so that flat surfaces may represent * That this is the correct theory of single vision with the two eyes is evident. For if, while looking at a single object with both eyes, we make a slight pressure with the finger on one of the eyeballs, \ve shall immediately perceive two objects j but, on removing the pressure, only one will be again seen. 70 ON LIGHT. solid or projecting figures, we must cause the different images of a body, as observed by the two eyes, to be depicted on the respective retinas, and yet to appear to have emanated from one and the same object. Two pictures are therefore taken from the really projecting or solid body, the one as observed by the right eye only, and the other as seen by the left. These pictures are then placed in the box of the stereoscope, which is furnished with two eye- pieces, containing lenses so constructed that the rays proceeding from the respective pictures to the corresponding eye-pieces shall be refracted or bent outwards, at such an angle as each set of rays would have formed had they proceeded from a single picture in the centre of the box to the respective eyes, without the intervention of the lenses ; and as it is an axiom in optics that the mind always refers the situation of an object to the direction from which the rays appear to have proceeded when they enter the eyes, both pictures will appear to have emanated from one central object ; but as one picture represents the real or projecting object as seen by the right eye, and the other as observed by the left, though appearing by refraction to have pro- ceeded from one and the same object, the effects conveyed to the mind, and the judgment formed thereon, will be precisely the same as if the images were both derived from one solid or projecting body, instead of from two pictures, because all the usual conditions are fulfilled ; and consequently the two pictures will appear to be converted into one solid body. "The necessary pictures for producing these effects, excepting those of geo- metrical figures, which may be laid down by certain rules, cannot, however, be drawn by the hands of man ; for, as Professor Wheatstone has observed, ' It is evidently impossible for the most accurate and accomplished artist to delineate, by the sole aid of his eye, the two projections necessary to form the stereoscopic relief of objects as they exist in nature, with their delicate dif- ferences of outline, light, and shade. But what the hand of the artist was unable to accomplish, the chemical action of light, directed by the camera, has enabled us to effect.' FIG. 78. Breivster's Refracting Stereoscope. " Daguerreotype portraits and Talbotype pictures are therefore taken, usually, by two cameras placed towards the object, with a difference of angle equal to the difference of the angle of vision of the two eyes, which is about 18 when the object is eight inches from the eyes ; hence, if these be carefully examined and compared with the original projecting objects, they will be found to be faithful representations of the object as seen by each eye respectively." PERSISTENCE OF VISION. 71 DIRECTIONS FOR USING THE STEREOSCOPE. " The objects must be so adjusted in the box, that only one picture may be seen in the centre, care being taken that the pictures are not reversed so as to be seen by the right eye instead of the left, and vice versa. " The proper position of portraits, buildings, and similar objects cannot be mistaken ; but where this is not readily perceived, it should be ascertained, when the object can be marked so as at once to be properly placed. "The eye-pieces, if allowed to turn, are marked with arrows, to indicate their proper position, these must be placed inwards, and in a right line with each other. " The eye-pieces in some instances are made to draw out to suit the foci of different persons. But those who use spectacles will generally see best with' them on, bringing them forward so as to lie flat on the eye-pieces, which in such cases should not be drawn out. " Persons, however, with a defective sight in either eye will not be able to perceive the astonishing effects of the arrangement, as two different images will not be perfectly formed on the retinae of the respective eyes." FlG. 79. Example of the zigzag path oj Lightning. PERSISTENCE OF VISION. There is a most interesting class of experiments that depend chiefly upon another property or faculty of vision, by which we retain for a certain limited period the images of objects presented before us. It may be premised that the term image refers to that picture which remains upon the eye as long as the object is present ; whereas the spectrum, which every one knows is the Latin for spectre, is that lingering impression left upon the eye after the real object has been removed. This property, like binocular vision, may be satisfactorily proved in various ways. Thus, if a broom-stick be thrust into the fire and burnt, so as to obtain a mass of ignited charcoal, and then whirled rapidly round in a circle, a complete circle of light is visible. Now, 72 ON LIGHT. it is evident that the hand or stick cannot be in every part of the circle at the same -instant of time ; the mind is therefore obliged to confess, in tracing the stick through the quarter, half, three-quarter, and whole circle, that of course the impression of the train of light must have remained upon the eyes, or else a single spot of light moving in a circle could only have been visible. A planet, if it moved fast enough, would leave a train of light, indicating, like the burning stick, its particular path or disc. The meteors move with such amazing velocity that their trains of light are extremely vivid, marked, and lengthened out, and show distinctly the direction or path they lake. A discharge of natural electricity or lightning would, if it moved slowly, be represented by a ball of fire travelling from one point to another ; it is, however, usually represented by a lengthened-out zigzag. (Fig. 79.) It is then called " forked lightning," and every part of its track remaining impressed on the vision, the whole appears as a series of continuous lines of fire, which, although diverted right or left, in a horizontal, perpendicular, or angular direction, pursue their path to the point where the discharge occurs, they are visible as a whole, and called a flash of lightning. The act of winking the eye is another familiar example of the same truth ; the eyelid closes and re-opens so rapidly, for the purpose of lubricating the eyeball, that the object we may be looking at does not become invisible, but remains impressed upon the eye. It has been ascertained that the impression lasts for about the seventh or eighth part of a second, and although some- times it may last for the third part of a second, it depends, no doubt, upon the amount of sensitiveness belonging to the organ of vision. There are very curious modifications of this property of vision, whereby colours and their complementary tints are impressed upon the eye. Thus, if a red wafer is placed on a sheet of black paper, and well illuminated by a sunbeam or any brilliant light, it will appear again to a spectator looking from the black 'to a white paper as a green one ; the red wafer being the real image, whilst the green one is the spectrum. The experiment may be varied with a yellow wafer on a black ground, which appears violet when the eyes are turned rapidly away to a white surface. On this principle a very entertaining book has been published. The reader, after staring at one of the illustrations, is directed to look up to the ceiling or wall, to observe the spectral effect. Sir D. Brewster explains these curious results, spoken of as accidental colours, by supposing that the eyes, after staring at any particular colour, say a bright red, become so fatigued or partially paralyzed that they cannot receive or appreciate the wave of red light, but as white light is made up of various waves of coloured light, the remaining sets of waves viz., blue or yellow-- can impress the vision by producing the complementary green colour. The late Dr. Golding Bird describes the following mode of demonstrating this fact, giving the merit of the experiment to the late Professor Cowper, who invented so many clever illustrations : " Cut in a piece of cardboard a series of holes, so that when folded to- gether they will exactly correspond, the whole resembling open lattice-work. Provide some sheets of thin tissue-paper of various colours, selecting those presenting strongly defined tints ; place one of them between the folds of the cardboard and hold it up to a vivid light, keeping the eye fixed on the lattice- work whilst the light penetrates the coloured paper ; in a few seconds the white colour of the pasteboard will vanish, and be replaced by a strongly marked tint complementary to that of the paper placed in it. Thus, with PERSISTENCE OF VISION. 73 yellow paper the framework will appear violet, with blue it will be orange, and with red it will be green. This illusion is so complete that it always excites surprise in those who see it for the first time." A little gunpowder placed on a block of wood, with iron filings sprinkled over it, throws up a shower of brilliant sparks of burning particles of iron when fire is applied ; and if the experiment is performed in a dark room, and the eyes of those standing near the experiment are closed directly after witnessing the real image of the burning particles of metal, they will see a volume of faint light, sometimes coloured, which remains upon the retinae, and forms a spectral image. If the colours of the solar spectrum are painted FIG. 80. The Polytechnic Phenakistiscope. on a glass disc, to which rapid motion may be imparted, after being fitted into the oxy-hydrogen lantern, a large disc can be thrown upon the screen, which changes to a greyish white directly it is set in motion. The change of the disc of many colours to a grey is very impressive, and is probably understood better by suggesting that the spectator should look through an aperture made in some opaque screen at the coloured disc ; the red, orange, yellow, green, blue, indigo, and violet pass before the aperture with such rapidity that they have not time to impress the retina as single colours, succeeding each other one by one, and they must therefore act collectively on the vision ; if collectively, then synthetically ; or, in plainer terms, the colours are caused to unite and reconstitute white light, or the nearest approach to it that can be produced by a mechanical contrivance of this nature. Many years ago the juveniles discovered that by twirling a halfpenny you could see both sides of it ; not only the portrait of the reigning monarch, but the usual figure of Britannia. This simple arrangement appears to have been succeeded by a more elegant contrivance, invented by the late Dr. 74 ON LIGHT. Paris, and called the Thaumatrope, or " Wonder-Turner," like many other clever things, a " nine days' wonder," and succeeded and surpassed by a very ingenious optical toy, invented by Plateau, called the Phenakistiscope. In connection with the name of Plateau, the Rev. Mr. Shaw, in a letter to the writer, says : " It may enhance the interest connected with the Phenakistiscope, if not known to you or your auditory, to learn that this gentleman, now re- siding in Ghent, Belgium, is and has been for years totally blind, carrying out his discoveries and observations entirely through the intervention of his wife. I mention this from personal experience, having assisted him some years ago to translate a treatise on capillary attraction for English publica- tion." Plateau's instrument, as arranged for the oxy-hydrogen light by Soleil Duboscq, is a very complicated affair, consisting of the usual condensing lenses, in front of which is the disc of glass with devices in regular order painted upon it. The latter, of course, rotates, and at the same time another wheel, containing four double-convex lenses set in the four quarters of the wheel, supplies that intermittent and separate light to each picture, which, when focused by the front lenses, produces all the effects of the popular Zoetrope (Fig. 81). FlG. 81. The Zoetrope at rest, showing tJie simple construction of the Instrument. In order to produce the best effect, it is absolutely necessary that each picture should be impressed separately but quickly upon the vision; and this is secured by the apertures followed by a certain opaque space, as employed in Plateau's original device so long exhibited at the Poly technic. This old-fashioned apparatus consists of a wheel perforated with apertures, on the back of which the figures are painted, and when the spectator looks through the slits into a plane mirror the figures appear to move. If the figures are painted in the usual manner on a disc, they all merge one into the other when the disc is set in motion, and a series of circles and eccentrics alone become apparent, which do not afford the slightest idea that they represent the figures; but Sir Charles Wheatstone has^shown that by constantly checking the motion, by a peculiar mechanism, so that each sepa- rate figure is impressed momentarily on the vision, then the same effects of motion are obtained without the intervention of the usual revolving slits or PERSISTENCE OF VISION. 75 FIG. 82. The Zoetrope in motion, simulating exactly the motions of a little girl playing with a skipping-rope. apertures. This important experiment establishes the basis of this class of illusions ; and the fact is further proved by the penny book now sold in the streets. The little pages have printed on them a series of devices representing any ordinary act of motion, such as a see-saw, and by rapidly passing the pages over the thumb with the first finger the effect of apparent movement is secured, as it would be with Plateau's apparatus, the Zoetrope, or Wheatstone's disc, with the checking and arresting mechanism. The best apparatus for showing to a large audience all the effects of per- sistence of vision, and the curious and elaborate movements obtainable from painted discs, is undoubtedly that devised by Mr. Thomas Rose, of Glasgow.* But before explaining this contrivance it will be advisable to study Faraday's paper. One of the first and most interesting papers written on the effects which are produced by persistence of vision is that of the late Dr. Faraday, and entitled, " On a Peculiar Class of Optical Deceptions ;" and, as the apparatus used chiefly consists of models constructed in cardboard, some copious quotations from that paper are here made.f " The preeminent importance of the eye as ah organ of perception confers an interest upon the various modes in which it performs its office, the circum- stances which modify its indications, and the deceptions to which it is liable, far beyond what they otherwise would possess. The following account of a * Fully described in article " Persistence " in a new edition of the " Popular Encyclopaedia." Blackie "Journal of the Royal Institution," vol. i , p 205. and Sons, London, Glasgow, and Edinburgh. 7 6 ON LIGHT. peculiar ocular deception, which, in a greater or smaller degree, is not uncommon, and which, if looked for, may be observed with the utmost facility, may therefore prove worthy of attention ; and I am the more inclined to hope so, because in some points it associates with an account and explana- tion of an ocular deception given by Dr. Roget in the 'Philosophical Transac- tions' for 1825, page 121. " The following are some cases of the appearance in question. Being at the magnificent lead-mills of Messrs. Maltby, two cog-wheels were shown me moving with such velocity that if the eye were retained immovable no distinct appearance of the cogs in either could be observed ; but, upon standing in such a position that one wheel appeared behind the other, there was imme- diately the distinct, though shadowy, resemblance of cogs moving slowly in one direction. " Mr. Brunei, junior, described to me two small similar wheels at the Thames Tunnel ; an endless rope, which passed over and was carried by one of them, immediately returned and passed in the opposite direction over the other, and consequently moved the two wheels in opposite directions with great but equal velocities. When looked at from a particular position, they presented the appearance of a wheel with immovable radii. " When the two wheels of a gig or carriage in motion are looked at from an oblique position, so that the line of sight crosses the axle, the space through which the wheels overlap appears to be divided into a number of fixed curved lines, passing from the axle of one wheel to the axle of the other, in form and arrangement resembling the lines described by iron filings between the oppo- site poles of a magnet. The effect may be obtained at pleasure by cutting two equal wheels out of white cardboard (Fig. 83 or 84), each having from twelve FIG. 83. FIG. 84. to twenty or thirty radii, sticking them on a large needle two or three inches apart, revolving them between the fingers, and looking at them in the right direction against a dark or black ground : the greater the velocity of the wheels, the more perfect will be the appear- ance. (Fig. 85.) "When the dark-coloured wheel of a carriage is moving on a good light-coloured road, so that the sun shines almost directly on its broadside, and the wheel and its shadow are looked at obliquely, so that the one overlaps the other in part, then in the overlapping part luminous or light lines will be perceived, curved more or less, and conjoining the axle and its shadow, if the wheel and shadow are FlG. 85. superposed sufficiently, or tending to do so if they PERSISTENCE OF VISION. 77 are superposed only in part. The more rapid the motion, the more perfect is the appearance. The effect may be easily observed (Fig. 86) by making a pasteboard wheel like one of those just described, blackening it, sticking it on a pin, and revolving it in the sunshine or candle-light before a sheet of white paper. " If the wheel be converted into a teetotum or top, by having a pin thrust through its centre and spun upon a sheet of white paper, the effect produced by the wheel and its shadow will be obtained with facility, and in form will resemble Fig. 85. In all these cases no rims are required; the spokes or radii will produce the effect. If a carriage wheel running rapidly before upright bars, as a palisade FlG. 86. or railing, be observed, the attention being fixed on the wheel, peculiar stationary lines' will appear ; those perpendicular to the nave or axis will be straight, but the others curved ; and the curve will be greatest in those which are furthest from the upper straight line. These curves are the same in form as those already described and explained by Dr. Roget,* and the appearance itself is produced in a similar manner ; but the phenomena are distinct, and the causes different. The effect at present re- ferred to is best observed when the velocities are great, whereas that explained by Dr. Roget takes place only when the velocities are moderate. It is pro- bable that the effects briefly mentioned by an anonymous writer in the 'Quarterly Journal of Science,' first series, vol. x., p. 282, and already referred to by Dr. Roget, were of the kind now to be explained ; for, though the de- scription is not accurate, either for the effects which form the object of this paper or that explained by Dr. Roget, and is, indeed, inconsistent with the observation or explanation of any of the phenomena, it probably had its origin in the occurrence of some of both kinds under the eyes of the writer. " The effect is easily obtained by revolving a white pasteboard wheel before a black or dark ground, and then, whilst regarding the wheel fixedly, traversing the space before it with a grate also cut out of white pasteboard. By altering the position of the grate and direction of its motion, it will be seen that the straight lines in the wheel are always parallel to the bars of the grate, and that the convexity of the curved lines is always towards that side of the grate where its motion coincides in direction with the motion of the radii of the wheel. By varying the velocity of the wheel and grate, the curves change in their appearance, and the whole or any part of the system, as described and figured by Dr. Roget, may be obtained at pleasure. " I have had a very simple apparatus constructed by which these and many other analogous appearances may be shown in great perfection and variety. One board was fixed upright upon the middle of another, serving as a base ; the upright board was cut into the shape represented in Fig. 87 ; the middle and two extreme projections, forming points of support, were supplied with little caps cut out of copper plate and bent into shape (Fig. 88), so that, when in their places, they offer four bearings for the support of two axes, one on each side the middle. The axes are small pieces of steel wire tapered at the extre- mities ; each has upon it a little roller or disc of soft wood, which, though it "Philosophical Transactions," 1825, p. ui. ON LIGHT. can be moved by force from one part of the axis to another, still has friction sufficient to carry the latter with it when turned round. These axes are made to revolve in the following manner: a circular copper plate, about 4 in. in diameter, has three pulleys of different dia- meters fixed upon its upper surface, whilst its lower surface is covered with a piece of fine sand-paper, attached by cement. A hole is made through the centre of the plates and pulleys, and guarded by a brass tube, so fitted as to move steadily but freely upon an up- right steel pin fixed in the middle of the cen- tre wooden support ; hence, when the plate is in its place, it rests upon the two rollers belonging to the horizontal axes, whilst it is rendered steady by the upright pin. It can be easily turned round in a horizontal plane, and it then causes the two axes with their rollers to revolve in opposite directions ; and the velocities of these can be made either equal to each other, or to differ in almost any ratio, by shifting the rollers upon the hori- zontal axes nearer to or farther from the centre of the stand. FIG. 87. FIG. "To produce motions of the axes in the same direction, an aperture was cut in the lower part of the upright board; a roller turned for it, which loosely fitted within the aperture ; a steel pin or rod passed as an axis through the roller. The roller hangs in its place by endless lines made of thread, passing under it and over little pulleys fixed on the horizontal axis. When, therefore, it is turned by the projecting pin, it causes the revolution of the axes. The variation in velocities is obtained by having the roller of different diameters in different parts, and by having pulleys of different dimensions. This description will be easily understood by reference to the figures 87 and 88. FIG. 89. " This apparatus had to carry wheels, either with cogs or spokes, which was contrived in the following manner: The wheels were cut out of cardboard, were about 7 in. in diameter, and were formed with cogs and sookes at pleasure. PERSISTENCE OF VISION. 79 A piece of cork, being the end of a phial cork, about the tenth of an inch in thickness, was then fastened by a little soft cement to the middle of the wheel, and a needle run through both and then withdrawn. These wheels could at any time be put upon the axes, and, being held sufficiently firm by the friction of the cork, turned with them. By these arrangements the axes could be changed, or the wheels shifted, or the velocities altered without the least delay. " The beauty of many of the effects obtained by this apparatus has induced me to describe it more particularly than I otherwise should have done. The appearance which I first had shown to me by Mr. Maltby was exhibited very perfectly : two equal cog-wheels were mounted (Fig. 89) so as to have equal opposite velocities ; when put into motion, which is easily done by the thumb and finger applied to the upper pulley and the horizontal copper plate, they presented each the appearance of an uniform tint at the part corresponding to the series of cogs or teeth, provided that the eye was so placed as to see the whole of both wheels ; but when a position was taken up so that the wheels were visually superposed, then, in place of an uniform tint, the appearance of teeth or cogs were seen, misty, but perfectly stationary, whatever the degree of velocity given to the wheel. By cutting the cogs or teeth in the wheel nearest to the eye deeper (Fig. 90), the eye could be brought into the prolongation of the axes of the wheels, and then the spectral cog- wheel appeared perfect (Fig. 91). The number of intervals thus occurring was exactly double the number of teeth in either wheel ; thus a wheel with twelve teeth produced twenty-four black and twenty-four white alternations. When one wheel was made to move a little faster than the other, by shifting the wooden roller on its axis, then the spectrum travelled in the direction of that wheel having the greatest velocity, and with more rapi- dity the greater the difference between the velocities of the two wheels. When the wheels were looked at so that they only partly visually superposed each FIG. 91. other, the effect took place only in those parts; and it was striking and extraordinary to observe, as it were, two uniform tints mingling and instantly breaking out into the alternations of light and shade which I have described. There are many variations in the curvature and other appearances obtained by altering the position of the eye, which will be imme- diately understood when observed, and which, for brevity's sake, I refrain from describing. "Wheels were then fixed on the machine, consisting of radii or spokes, each having twelve, equal in length and width (Fig. 84). When revolving alone, each wheel gave with a certain velocity a perfectly uniform tint ; but when visually superposed there appeared a fixed wheel, having twenty-four spokes, equal in dimensions to the original spokes. Variations of the position of the eye, or of the relative velocity of the two wheels, caused alternations similar to those I have referred to with the cog-wheels. " In observing these effects, either the wheels should be black or in shade, whilst the part beyond is illuminated ; or else the wheels should be white and enlightened, whilst the part beyond is-in deep shade. The cog-wheels present nearly a similar appearance in both cases, though in reality the parts of the spectrum which appear darkest by one method are lightest by the other. The' 8o ON LIGHT. spoke-wheels give a spectrum having white radii in the first method, and dark radii in the second. * Placing the wheels between the eye and the clouds, on a white wall, or a lunar lamp, answers very well for the first method ; and, for the second, merely reversing the position, and allowing the light to shine on the parts of the wheel towards the eye, whilst the background is black or in obscurity, is all that is required. Strictly, the phenomena should be viewed with one eye only, but it is not often that vision with two eyes disturbs the effects to any extent. " The cause of these appearances, when pointed out, is sufficiently obvious, and immediately indicates many other effects of a similar kind, and equally striking, which are dependent upon it. The eye has the power, as is well known, of retaining visual impressions for a sensible period of time ; and in this way recurring actions, made sufficiently near to each other, are percep- tibly connected and made to appear as a continued impression. The lumi- nous circle visible when a lighted coal or taper is whirled round, the beautiful appearance of the Kaleidophone, the uniform tint spread by the revolution of one of the spoke or cog wheels already described, are few of the many effects of this kind which are well known. " But during such impressions the eye, although to the mind occupied by an object, is still open, for a large proportion of time, to receive impressions from other sources ; for the original object looked at is not in the way to act as a screen, and shut out all else from sight. The result is that two or more objects may seem to exist before the eye at once, being visually superposed. The school-boy experiment of seeing both sides of a whirling halfpenny at the same moment, the appearance of the Thaumatrope, and the transparency of the revolving cog or spoke wheels referred to in consequence of which other objects are seen through the shaded parts are all effects of this kind ; two or more distinct impressions, or sets of impressions, being made upon the eye, but appearing to the perception" as one. " So it is in the appearances particularly referred to in this paper. They are the natural results of two or more impressions upon the eye, really, but not sensibly, distinct from each other. If, whilst the eye is stationary, a series of cogs, like those represented by the continuous outline (Fig. 92), pass rapidly before it, they produce a uniform tint to the eye ; and, for the purpose of following out the description, let it be supposed the cogs are in shade between the eye and a white background, the tint is then a hazy semi-transparent grey. If another series of cogs, represented by the dotted outline, and close to the first, so as to give no sensible angular difference to the dimensions of the cogs, pass with equal velocity in the same direction, it will produce its corresponding tint. If the two sets of cogs be visually superposed in part, as in the figure, there will be no alteration in the uniformity of the tint. If the cogs of one set be more or less to the right or left of the other, then the superposed part will approach more or less to the tint of the shaded and uncut part of the card- board wheel, and be less transparent. But if, instead of the motion being equal, the velocities are unequal, then total changes of the appearance super- vene ; the spectrum (if I may so call it) of the superposed parts becomes alter- PERSISTENCE OF VISION. 81 nately light and dark, and the alternations take place more or less rapidly as the velocities of the two sets of cogs differ more or less from each other. " When the cogs move in opposite directions, the uniform tint which each alone can produce is soon broken up in the superposed parts into lighter and darker portions ; and when the velocities of both are equal, the spectrum is resolved into a certain number of light and dark alternations, which are perfectly fixed, and which to the mind offer a singular contrast to the rapidly moving state of the wheels, and to the variations which their velocity may undergo without altering the visible result. " The effect, strange as it at first appears, will be easily understood by reference to Fig. 91. Suppose the eye directed to the part / beyond the cogs, and the sets of cogs to be moving with equal velocities in opposite directions, indicated by the arrow-heads, the part / will be eclipsed by the cogs a and b simultaneously, and for exactly the same time ; for they begin to cover it and leave it together. /, therefore, is alternately open to and shut from the eye for equal times ; for what these cogs have done will be performed by all the other cogs in turn, and the cogs are equal in area to the spaces between ; FIG. 92 A. FIG. 92 B. half the light, therefore, from that part of the background comes to the eye, and produces a corresponding impression. But with respect to the point d, although the cog b is just leaving it exposed, the cog a is just beginning to eclipse it ; and by the time the latter has passed over, the edge of the cog e will be upon the spot, and that cog will therefore hide it until f comes up, so that in fact the point d is always hidden ; no light comes from that part of the background, and it consequently appears dark. /' is circumstanced just as / was, for the cogs a and e cover it simultaneously, and so do all the other cogs in pairs ; it is, therefore, a light space in the spectrum, d 1 is a repetition in everything of d, and is a dark space. The parts intermediate between the maxima of light and darkness will, by examination, be found to be eclipsed for intermediate periods, and to appear more or less dark in consequence, so that the appearance of the spectrum belonging to the visually superposed parts of the two sets of cogs is as in Fig. 92 A. " In the case of equal wheels with radii, the fixed spectrum produced when the wheels superpose each other has twice the number of radii of either wheel, that being, of course, the number of times which the radii coincide with each other in one revolution. " Fig. 92 B represents the fixed spectrum produced by two equal wheels of eight radii each. When the radii or spokes are narrow, the difference in the intensity of tint between the middle and the edges of each image of a spoke is so slight as to be scarcely perceptible. But as this circumstance and many others will explain themselves immediately they are experimentally observed, 82 ON LIGHT. it is unnecessary to dwell minutely upon them here. A very simple experi- ment will render the whole of these effects perfectly intelligible. " If a little rod of white cardboard, five or six inches long, and one-thirtieth of an inch wide, be moved to and fro from right to left before the eye, an obscure or black background being beyond, it will spread a tint, as it were, over the space through which it moves. (Fig. 93, A.) A similar rod held and moved in the other hand will produce the same effect ; but if these be visually superposed, i.e., if one be moved to and fro behind the other, also moving, then in the quadrangular space included within the in- tersection of the two tints will be seen a black line, sometimes straight, and connecting the opposite angles A of the quadrangle, at other times oval or round, or even FIG. 93. square, according to the motions given to the two card- board rods (Fig. 94, B). "This appearance is visible even when the rods are several inches or a foot apart from each other, provided they are visually superposed. It is produced exactly as in the former case, and the black line is in fact the path of the intersecting point of the moving rods. As their motions vary, so does the course of this point change, and, wherever it occurs, there is less eclipse of the background beyond than in the other parts, and consequently less light from that spot to the eye than from the other portions of the compound spec- trum produced by the moving rods. "In this experiment the eye should be fixed, and the part looked at should be between the planes in which the rods are moved. The variation produced by using black rods, and looking at a white ground, will suggest itself. Those who find it difficult to observe the effect at first, will instantly be able to do so if the rod nearest the eye is black, or held so as to throw a deep shade the line is then much more distinct ; but the explanation is not quite the same, but nearly so it will suggest itself. Two bright pins or needles produce the effect very well in diffused daylight ; and the line produced by the shadow of one on the other, and that belonging to the intersection, are easily dis- tinguished and separated. "If whilst a single bar is moved in one hand several bars or a grate is moved in the other, then spectral lines, equal to the number of bars in the grate, are produced. If one grate is moved before another, then the lines are proportionably numerous ; or if the distances are equal, and the velocity the same, so that many spectral lines may coincide in one, that one is so much the more strongly marked. If the bars used be serpentine or curved, the lines may be either straight or curved at pleasure, according as the positions and motions are arranged so as to make the intersecting point travel in a straight, or a curve, or in any other line. " The cause of the curious appearance produced, when spoke or cog wheels revolve before each other, already described, will now be easily understood ; the spokes and cogs of the wheels produce precisely the same effect as the bars held in the hand, and the fixedness of the position of the spectrum depends upon the recurrence of the intersecting or hiding positions, exactly in the same place with equal wheels, provided the opposite motion of each be of equal velocity and the eye be fixed. " When the wheels were used in the little machine described (Fig. 87), having PERSISTENCE OF VISION. 83 equal but oblique teeth, and the obliquity in the same direction, the spectrum was also marked obliquely ; but when the obliquity was in opposite directions the spectrum was marked as with straight teeth. " When equal wheels were revolved with opposite motions, one rather faster than the other, the spectrum travels slowly in the direction of the fastest wheel ; when the difference in the velocity of the two wheels was made greater, the spectrum travels faster. These effects are the necessary consequence of the transference of the intersecting points already described, in the direction of the motion of the fastest wheel. When one wheel contains more cogs than the other, as, for instance, twenty-four and twenty-two, then with equal motions the spectrum was clear and distinct, but travelled in the direction of the wheel having the greatest number of teeth. " When the other wheel was made to move so much faster as to bring an equal number of cogs before the eye, or rather any one part of the eye, in the same time as the other, the spectrum became stationary again. The explana- tion of these variations will suggest themselves immediately the effects are witnessed. When the motion of the wheels upon the machine is in the same direction, the velocities equal, and the eye placed in the prolongation of the axis of the wheels, no particular effect takes place. If it so happens that the cogs of one coincide with those of the other, the uniform tint belonging to one wheel only is produced. If they project by the side of each other, it is as if the cogs were larger, and the tint is therefore stronger. But, when the velo- cities vary, the appearances are very curious ; the spectrum then becomes altogether alternately light and dark, and the alternations succeed each other more rapidly as the velocities differ more from each other. " When wheels with radii are put upon the machine, it is easy to observe, in perfection, the optical appearance already referred to, as exhibited by car- riage-wheels, &c. (Fig. 85). They should be looked at obliquely, so as to be visually superposed only in part ; and, provided the wheels are alike, and both revolving in the same direction with equal velocities, they immediately assume the form described, passing in curves from the axis of one wheel to the axis of the other, much resembling in disposition those curves formed by iron filings between two opposite poles of a magnet. "If the wheels revolve in opposite directions, then the spectral lines, origi- nating at each axis as a pole, have another disposition, and, instead of running the one set into the other, are disposed generally like the filings about two similar magnetic poles, as if a repulsion existed; not that the curves or the causes are the same, but the appearances are similar. A very little attention will show that all these lines are the necessary consequence of the travelling of successive intersecting points ; and any one of them may be followed out by experimenting with the two pasteboard rods already described, these being moved in the hand as if each were the spoke of a wheel. " All these effects may be simply exhibited by cutting out two equal paste- board wheels without rims, passing a pin as an axis through each, spinning one upon a mahogany or dark table, and then spinning the other between the fingers over it, so that the two may be visually superposed. If the appear- ances are observed by a lamp or candle, the wheels should be so held to the light that the shadow of the upper may not fall upon the lower ; otherwise the effects are complicated by similar sets of lines which appear upon the lower wheel, and are produced by the shadow of the upper. " These are the same in form and disposition as in the former, and are even 62 ON LIGHT. more distinct ; they should be viewed, not through the upper wheel, but directly upon the lower; their explanation has in part been given, and will be sufficiently evident." Returning to the consideration of Mr. Rose's Photodrome, or "Light- Runner," the construction is simple, and not likely to get out of order. It consists of two parts. The first part (b,. Fig. 94) consists of a wheel about four feet in diameter, provided with eight spokes, and wholly constructed of the the best seasoned mahogany. The wheel is driven by a gut band proceeding from a smaller flying- wheel, which is worked by .hand. This large wheel is so FIG. 94. The two portions of Rose's Photodrome, viz., the large and small wheels alluded to. arranged on a platform, or other convenient place, that a strong light, arranged in a lantern with a proper lens, casts its rays through one of two apertures in a second disc (a, Fig. 94), about two feet in diameter, placed below and in front of it, so that the shadow of the large wheel is distinctly thrown upon the white screen behind. When the large wheel is set in motion, and a certain velocity, from about 250 to 300 revolutions per minute, obtained, all the spokes and the shadows of them disappear, and then the curious effect of the rim or ring of the wheel is shown revolving without any apparent connection with the central axis. Whilst this large wheel is going round, if the spectator looks obliquely through the spokes of the real wheel to those of the shadow-wheel, he will see the curved lines described by Faraday, as obtained by him with his cardboard models in Fig. 86, p. 77. In a favourable position the whole distorted shadow- wheel, with curve lines on a grey ground, becomes visible on a scale not probably contemplated by Faraday with his small cardboard spokes. These effects being shown with the large wheel, attention is now directed to the second portion of the apparatus, consisting of a disc about two feet in PERSISTENCE OF VISION. FIG. 95. Exhibition of the Photodrome at the Polytechnic. diameter, and provided with two apertures. With an ingenious sliding arrangement six or eight apertures can be obtained, if required, but two are preferred for this experiment. The rays of light, as already described, pass through these apertures every time they come round, and the large wheel being still in motion and the spokes invisible, directly the small disc begins to move and attains a moderate velocity all the spokes and their shadows return. At first they are very hazy and indistinct, and almost semi-transparent ; but, as the velocity increases, they become distinctly apparent, and the large wheel appears to be going round slowly and nearly to stand still. The next change in the velocity of the small disc throwing the flashes causes the spokes to be multiplied, generally by five ; thus, forty spokes and forty shadows may be counted, the latter being grey, and not black, like the original eight shadows. The next and last increase of velocity in the small disc, which brings it up to about a thousand revolutions in a minute, causes the large wheel the eight spokes and eight shadows to appear quite distinct, and at that moment, although the large wheel is going round three hundred times in a minute, it appears to stand still. The flashes of light perform the same duty as the slits or apertures in Plateau's apparatus, and before the large wheel has time to move the light arrives and passes away. If the large wheel was moving at the rate of one thousand revolutions in a minute, no change would occur. It is the difference in the two velocities which determines this curious form of the illusion. Mr. Rose mentions a most amusing story in connection with the curious illusions produced by the Photodrome, viz., that of the large wheel apparently standing still when it is really moving very fast. It appears that whilst 86 ON LIGHT. showing the experiment to a number of working men, at a lecture-hall in Glasgow, one of them rose from his seat, and wanted to creep up quietly to the large wheel, for the purpose of convincing himself by touch that it really was moving. Fortunately, they stopped the man in time, or he would pro- bably have received a blow from the spokes of the wheel which might have broken some finger-bones. This incredulity was an interesting example of the effect of that teaching which grows up with us, viz., that " seeing is believing." Here was a man who had evidently never seen an optical illusion before, and, doubtless, by the time Mr. Rose had finished his beautiful experiments,, he discovered that the eye, like the ear, is easily deceived. LIGHT AND COLOUR. SPECTRUM ANALYSIS. ABERRATION AND ACHROMATISM. In the frontispiece of this book is shown the beautiful apparatus attached to the Duboscq lantern, containing the electric lamp. Such a contrivance, with its lenses and prisms, is a great contrast to the simple means employed by Sir Isaac Newton, nearly two hundred years ago, to effect the same object viz., the decomposition of light. The great discovery made by Newton, about the year 1672, that light is not of a simple but of a compound nature, was estab- lished by the help of a prism (an optical instrument already described at p. 50) through which a sunbeam was permitted to pass. No doubt, whilst moving the prism about in the light the production of colour might have been accidentally discovered, as it would have been by any other careful experimentalist ; but it fortunately happened that the discovery fell to the lot of a mind already well prepared to grapple with difficult phenomena, and Newton was soon able to convince himself and others that he had analysed light, and resolved it into seven colours viz., Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Here was light, not only refracted or bent from its natural course, but spread out a phenomenon to which the term dispersion is now given. Other lenses or optical instruments possess the same property in a more limited degree, and hence the edges of the pictures or images thrown by convex lenses from the magic lantern show colours. In what are called achromatic lenses, the disagreeable effect upon the eye produced by ordinary lenses is prevented, and the colours neutralized and destroyed. The value of science-teaching as a part of regular education is now fully recognized ; but schoolmasters have little time to spare to superintend the manufacture and collection of oxygen in bags, or to put together a voltaic battery, for the purpose of obtaining either the oxy-calcium or the electric light ; consequently, the phenomena of light are only taught theoretically instead of experimentally. If a master could teach the leading principles of optics by merely closing the shutters of his room, and allowing a sunbeam of a greater or lesser diameter, determined by different- sized diaphragms, to enter through an aperture into the darkened room, he would be more disposed to impart this kind of knowledge to his boys, because LIGHT AND COLOUR. FIG. 96. The Heliostat, placed on a shelf outside the window, reflecting the ray of light which passes through a hole in a sJmtter on to a prism, to show the decomposition of light. the sunbeam would cost nothing, and with the help of an instrument called the Heliostat* (77X10$, the sun, and SPECTR UM AN'AL $$&.+' IOI they cause groups of dark lines, and, group of dark lines produced by each vapour is iden- tical in the number of lines and in their position in the spectrum with the group of lines of which the light of the vapour consists when it is luminous. The reversal of the spectrum of coloured flame, and the mode in which he obtained the proof of the identity between the terrestrial sodium line and the dark lines similarily placed in the solar spectrum, is thus described by Kirchoff : " In order to test by direct experiment the truth of the frequently asserted fact of the coincidence of the sodium lines with the lines D (Frauenhofer), I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D change into bright ones. The flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame upon the slit, and, to my astonishment, I saw that the dark lines D appeared with an extraordinary degree of clearness." With respect to this important experiment, showing the reversal of the sodium lines, perhaps the most simple experiment is that of Roscoe, who seals up some of the metal sodium in a vacuum tube, and on volatilizing the metal the vapour is colourless by white light, but dark and opaque when the monochromatic or yellow light of sodium is shown behind it. It was by the exact reversal of the bright terrestrial lines, and the absolute identity in position of the bright terrestrial and dark solar lines, that Kirchoff discovered the elements that exist in the sun, viz., hydrogen, sodium, magne- sium, iron, calcium, nickel, chromium, copper, zinc, barium, and probably strontium, cobalt, cadmium. At p. 92, and in Fig. 101, are shown the lines B, c, D, E, F, G, and H, which are called Frauenhofer's principal fixed dark lines in the solar spectrum. The labours of Kirchoff have now almost interpreted the whole of these lines, which are read as follows : FIG. 1 08. The section of the Crucible to be used for showing the re- versal of the bright sodium lines, of which A is the cen- tral hole, and contains some chloride of sodium, and B B a ring or trench all round A, in which metallic sodium is placed; c, the upper char- coal pole. C, F, and G are Hydrogen. D is Sodium. E is Iron. H, Aluminium. C, Magnesium. The limits of this work do not permit the consideration of stellar che- mistry, and the extremely valuable researches of Mr. Huggins and Dr. Miller in this direction ; but the reader is referred to Mr. Huggins's discourse " On the Results of Spectrum Analysis applied to the Heavenly Bodies," published by Ladd; or to Mr. Watt's "Dictionary of Chemistry," for a complete resume of this subject This much may be said, that spectrum analysis proves that the fixed stars are suns like our own a fact which could only be assumed and taken for granted before the important experiments of Kirchoff, Huggins, and Miller. 102 ON LIGHT. FIG. 109. Star Spectroscope, with adjust ible Reflecting Prism and Mirror. With finest object-glass micrometric apparatus tor measuring the lines of the spectrum to i-io,oooth of an inch, extra eye-piece, and ivory tube to reader of vernier, as made for W. Huggins, Esq., F.R.S., and used during the observation of the red flames of the sun in India, August, 1868. Moreover, the spectroscope has discovered the real nature of the " red flames " or " prominences " of the sun, which are invisible under ordinary cir- cumstances, being overpowered by the dazzling brilliancy of the rays which proceed from the sun ; but visible during the few minutes that elapse during a total eclipse of the sun, as in the one which created so much interest in August of the present year, 1 868, visible only in the line or path of the shadow, which fell in India. Four xpeditions went to India to observe the red flames; they were all armed with the spectroscopic apparatus, and their united statements all agree that the red flames belong to the sun, and that, as they give bright lines which belong only to spectra of the second order, they must be enormous gas- heaps, intensely ignited or self-luminous. The bright lines chiefly observed appear to be those which belong to hydrogen gas and sodium, at least so far as we know at present (September, 1868) ; and this interesting statement was made through the telegrams from Major Tennant, Lieutenant Herschel, and M. Jannsen, which arrived in England, and were all sent independently of each other. As the red flames belong to the sun and show bright lines in the spectroscope, are they. great volumes of the photosphere thrust out (like the pips and juice 'of a squeezed gooseberry) beyond the last or gaseous atmosphere, which usually robs the light from the photosphere of its beautiful coloured bands r LIGHT AND COLOUR. 103 and changes them to dark lines ? for where light is not, there can only be darkness. These and other facts are discoverable by another modification of the specr troscopic arrangement (Fig. 109), as constructed by Mr. Browning. SPHERICAL ABERRATION. In using an ordinary concave mirror the experimentalist cannot fail to notice that the rays reflected from the part near the circumference do not come to the same meeting-point or focus as the rays reflected from parts near the centre. (Fig. 1 10.) It is evident that the rays A B, A c, come to a focus at G, which is further off than the focus F from the parallel rays D D, D D. The distance between F and G, the two foci, is called the longitudinal spherical FIG. 1 10. Concave Mirror, showing the Aberration of the Rays of Light. aberration. The natural consequence must be that an image projected by an ordinary concave mirror will be confused, because the eye has to look at a double image, the one superposed on the other. To get rid of the rays from the outer part of the mirror it is usual to employ a screen, so that the rays D D, D D, from the central part of the mirror only are used. FlG. in. Production of Caustic Curves. FIG. 112. Arising from this circumstance is the unequal illumination of a white ground on which rays are reflected to different foci, and the production of. symmetrical curves, termed caustic lines or caustic curves, in the study of which mathematicians have been most industrious. Brewster lays claim to the following method of exhibiting caustic curves. He recommends the use io 4 ON LIGHT. of a piece of steel spring highly polished, or, better still, polished silver, which is to be bent into a concave figure and placed vertically on its edge upon a piece of card or white paper, and when exposed either to the rays of the sun or any good artificial light, the curves shown in Fig. 1 1 1 are well defined. In the same way, passing from reflecting to refracting bodies, the spherical figure of a convex lens causes the rays which fall near the outer edge to come to a focus nearer the lens than the rays which are refracted from the centre. The result, as might be expected, is just the reverse of the concave mirror. The rays A B, A B, Fig. 1 1 2, falling on the margin of the double-convex lens are refracted to a focus at F, whilst those rays, D D, D D, which fall near the axis of the lens come together at a more remote point, viz., at C. Here again a screen or diaphragm cutting off the rays refracted from the outer edge of the lens gives a better image ; the picture produced by such a lens, provided with a screen, can be focused more distinctly; hence telescopes, microscopes, cameras, oxy-hydrogen lanterns, c., &c., are usually fitted with diaphragms, which reduce the light, but cause the images to become more distinct. The lens of the eye would, from this cause, project on to the retina a confused or double picture, which might render vision extremely imperfect ; this, however, is prevented by the iris, which acts as a diaphragm, thus the aberration of sphericity is corrected. THE DISPERSION OF LIGHT, OR CHROMATIC ABERRATION. If light consisted of a series of coloured rays, every one of which possessed the same index of refraction when they fall upon a glass lens, they would all come together in the same spot, and white light only would be obtained ; but this is not the case, and it is known in practice that lenses, and especially con- densing lenses, project coloured rings, and give images with coloured edges. And this is not remarkable when it is remembered that a double-convex lens may be regarded as a series of prisms united at their bases, and therefore capable of decomposing or dispersing light. It is a singular fact that Sir Isaac Newton considered, from the experiments he had tried with various prisms, that dispersion was proportioned to refraction, and he believed that all substances had the same chromatic aberrations when formed into lenses, and that any combination of a concave with a convex glass would produce colour with refraction. Newton reasoned only from the facts he had acquired on the dispersive powers of bodies, and pronounced the construction of achromatic telescopes which should not project images with coloured edges to be impossible. ' The fallibility even of his great mind is shown by the fact that, a few years after his death, Hall in 1733, and Dolland, the famous optician, in 1757, demonstrated that by using two media, viz., crown and flint glass, of different refractive and dispersive powers, a lens may be formed which is achromatic. The principle of the achromatic lens is not complicated or difficult to understand, provided the previous matter relating to compound and simple colours (p. 89) has been already studied. Given a lens made of a certain glass, and projecting, amongst other colours, a ring of red light, what colour, pro- jected from another lens, is required to neutralize it ? The answer is obvious : any colour which together with the red light would form white light. That colour must be green, because it contains yellow and blue ; and, as already shown, red, yellow, and blue form white light. In the adjustment of the two lenses LIGHT AND COLOUR. I0 5 forming the achromatic (Fig. 113), it is so arranged that the colours which would be separately produced by each lens shall, when combined, by their unequal dispersion fall together at the same spot and unite together. Any two colours which unite and form white light are said to be complementary, and there is a very conclusive experiment which may be performed with polarized light passed through a selenite slide placed behind a Nicol's prism composed of I. FIG. 113. No. I., Dolland's Achromatic Lens, consisting of one double-convex crown glass lens, a, and another concavo-convex lens of flint glass, b; No. II., Dr. Blair's Achromatic Lens, composed of two double- convex lenses of crown glass, enclosing a solution of chloride of antimony. FIG. n AT Complementary Colours overlapping and forming IVhite Light. double-refracting spar. The two discs of light projected on to the screen separately are green and red; but when caused to overlap each other by enlarging the aperture through which they pass, the two colours unite in the centre, forming white light, whilst red and green remain intact in those po- sitions which do not overlap. (Fig. 114.) Other complementary colours would be yellow and indigo, blue and orange. FIG. 115. Arrangement of the Composite Lenses in an Achromatic Telescope. Flint glass has a greater dispersive power than crown glass ; it will spread or disperse the spectrum over a larger space. The dispersive power of the prism used in decomposing light for showing the spectra of incandescent metal is increased by rilling them with carbonic disulphide (bisulphide of carbon), and the composition and dispersive powers of the three bodies is as follows: Crown glass . Flint glass Carbonic disulphide 0-039 0-048 io6 ON LIGHT. THE INTERFERENCE OF LIGHT. COLOURS OF THIN PLATES. About the year 1672, Dr. Hook, a very clever mechanician, and learned in all the science of his day, discovered that by splitting mica, which is free from colour, and sometimes used instead of glass, into very thin films, they exhibited the most beautiful colours. But as they were less than the twelve- thousandth part of an inch in thickness, Dr. Hook could not measure them, and was therefore unable to determine the law that regulated the production of any particular colour, according to the thickness of the film of mica. In due course of time the experiments engaged the attention of Sir Isaac Newton, and directly he touched the subject it was truly, so far as intellect was con- cerned, with the hand of a giant, and he soon discovered a method of measur- ing the films. He did not begin with mica, because it would have been very troublesome, if not impossible, to split it into a graduated series of films of the extreme thinness required to produce colour. Newton therefore commenced with air, and having once determined the law, it was easy, knowing the index of refraction of all other transparent bodies, to work out by calculation the respective thicknesses required to produce the same colours. He took a plano- convex lens, the radius of whose convexity was 14 ft., and placed it on a double-convex lens, the radius of whose convexity was Soft., and by means of proper clamps and screws the surfaces of the two lenses could be brought closely together. The convexity of the lower lens being so extremely slight, it might indeed be almost regarded as a flat surface, like any moderate area on the surface of the globe, because the sphere of glass (of which the lens would be a slice) had a theoretical diameter of 100 ft. (Fig. 116.) FIG. 1 1 6. Instrument used by Newton to obtain the Rings of Colour from Thin Plates of Air. L L, upper lens pressed on the lower one, / I, by the thumb-screws p p p. When the two lenses were pressed together, concentric rays of colour maae their appearance ; indeed the same kind of effect is often produced acci- dentally when a number of flat plates of window-glass are piled one above the other, the enclosed air being then pressed by the weight of the superin- cumbent'glass into a film sufficiently thin to show coloured rings. The Hon. Robert Boyle first discovered that thin bubbles of the essential oils, spirit of wine, turpentine, soap and water, produce the colours, and he THE INTERFERENCE OF LIGHT. 107 succeeded in blowing glass so thin that, like the mica, it displayed varieties of colour. Lord Brereton observed the colour of thin oxidized or decomposed films, such as are produced by the action of the weather during a prolonged period on the ancient glass in church windows, or on glass which has been buried in the earth. When steel is tempered, the regular gradations of colour produced by the oxidation of a very thin outer film are a guide to the skilled workman who tempers the metal. Mr. De la Rue, by floating a very thin film of a quick-drying varnish on the surface of hot water, and then receiving this on a sheet of paper, was enabled to secure in the most perfect manner those lovely tints, which are sometimes associated with the surface of ponds into which greasy matter or oil may pass, or in the puddles after rain in the yard of a gas-works where liquor containing coal oil has been spilt. The variety of colours which Newton describes in his important "Table of the Colours of thin Plates in Air, Water, and Glass," are given by him in the suc- cession of spectra or order of colours, where he enumerates seven spectra or orders of colours ; these are different from reflected and transmitted rays, and are produced by thicknesses of air, water, or .glass, estimated from a scale of 'an inch divided into one-million parts. FIG. 117. Woodward's Model of Waves, 'with movable Rods. Newton measured the diameter of every coloured ring ; he did not depend merely upon calculation, but tried a number of experiments with the colours of the spectra, allowing each to fall separately on his apparatus, and dis- covered that under these circumstances he no longer obtained a variety ol coloured rings, but observed that the central dark spot was surrounded by rings of the same colour as the light incident on the lenses alternating with dark rings. Thus, supposing Newton to have placed the apparatus for producing the rings into the yellow part of the spectrum, there would be a dark spot in the centre, then a yellow ring, now a dark, again a yellow ring, and so on ; he then squared the diameters of the reflected coloured rays, and obtained the odd numbers, I, 3, 5, 7, 9, &c., while the square of the diameters of the dark rings were as 2, 4, 6, 8, 10, &c. When the rings were observed by transmitted light, the order was reversed the coloured rings being at the even numbers, and the dark ones at odd integers. These effects Newton called fas of transmission andfas of reflection ; they could not be reconciled or explained by his own favourite theory, and, to the honour of this great philosopher, he did not attempt to press the corpuscular theory, and compel it to his own use, but simply left behind him a record of facts, only naming that which he had proved to exist, and giving the relative thicknesses of the plates of air by which each colour is reflected. io8 ON LIGHT. The undulatory theory of light alone is adopted to explain these phenomena, and by what is termed the interference of the waves the effects are supposed to be produced. Ingenious models have been made to explain the law of FIG. 1 1 8. A AT odd of Fixed Waves. interference ; but those of Mr. Charles Woodward, the President of the I sling' ton Scientific Society, are the most simple, and are thus described by him in his admirable little work on the " Polarization of Light : " A B (Fig. 1 17) represents a model with rods freely moving in a perpendicular direction through the frame A B, and furnished with pins resting upon the FIG. 119. Intensity of Waves doubled by the Superposition and Coincidence of two equal Systems. upper part of the frame, so that when at rest the whole may assume the appearance of waves, as in the diagram. CD (Fig. 1 1 8) represents a fixed model with waves of similar size and intensity, and numbered so as to distinguish each half-undulation. FlG. 1 20. Waves neutralised by the Superposition and Interference of two equal Systems. It will be seen that when the stars indicating the highest point of the waves, as A B, correspond with the odd numbers of half-undulations on c D, each system of waves will be in the same state of vibration; and, if so superposed, a series of waves of doubled intensity will be the result, as in Fig. 119. THE INTERFERENCE OF LIGHT. 109 If, on the other hand, the two systems be so superposed as that the stars on A B shall coincide with the even numbers on C D, as in Fig. 120, there will be a difference of half an undulation in the two systems ; the one will neutra- lize the other by interference, and darkness will be the result. If C D be continued so that A B may be moved forward indefinitely, it will be obvious that the waves will be equally increased in intensity by a difference in the two systems of any even number, and neutralized by a difference of any odd number, of half-undulations. These models are, therefore, well suited to teach matter of fact, viz., that two sets of waves of water may come together and obliterate each other, as in the tides of the port of Batsha, described by H alley and Newton, where the two waves arrive by channels of different fengths, and produce a smooth surface; or two waves of light may come together in such phases that in one case they exalt each other and produce a wave of double intensity, and in the other phase they may destroy one another and cause darkness. A wave of white light is, however, made up of other waves of coloured light ; so that when such a complicated series of differer.t- coloured waves interfere, it is easy to perceive that certain coloured waves may coincide and extinguish each other, whilst the remaining colours may unite and intensify each other. That waves of light do so interfere is placed beyond all doubt by the expe- riments of Dr. Young, and even more elaborately by the following beautiful experiment devised by Fresnel : FIG. i2i.--FresneFs arrangement to show the Interference of the Waves of Light. A sunbeam from the Haeliostat is passed through a narrow rectangular slit in the shutter (as described at p. 87), covered with red glass to secure a mono- chromatic light, or wave of simple light. The red light is brought, by a cylin- drical lens of very short focus, to a point at A. The rays cross each other and fall upon two mirrors of parallel glass B C, B D, placed at a very obtuse angle, having their line of intersection parallel to the line of light. After re- flection the rays proceed as if they came from the two points F F behind the two mirrors ; they interfere at G, and at other points not marked out in the no ON LIGHT. diagram, and produce light and dark fringes ; but, if one of the beams of light proceeding from the points E E be intercepted by a screen, the whole of the alternations of red and dark fringes disappear, and the only light left is that derived from the single ray of red light which remains after the other was removed by the screen. In this diagram two sets of waves only are used, but, of course, the same law applies to all. It is this principle of interference which produces coloured fringes by inflexion or diffraction, such as rays passing along the edge of a screen, or the fringes at the edge of a plane mirror, or fringes produced by narrow rectangular openings, fringes by two narrow slits very close together, and those obtained through gratings or networks. The word grating might deceive the reader, and lead him to suppose that the effect was caused by some rough arrangement ; but these beautiful experiments were carried out by Frauenhofer by tracing parallel lines on a film of gold leaf fixed on a plate of glass, and look- ing through them with transmitted light. Nature supplies us with striated bodies, which are in effect reflecting gratings. Brewster calculated that there were three thousand lines in an inch of a piece of iridescent mother-of-pearl. But this number has been surpassed by Barton, who ruled from two to ten thousand lines on steel, which he afterwards hardened and used as a die to stamp bright brass buttons. These, when illuminated by the various rays emanating from the numerous lighted wax candles in a ball-room, flashed with the splendid colours of the diamond. The colours of Newton's rings are due to the interference of the light reflected from the upper and under surface of the film of air ; for, however thin this may be, it must have an upper and an under surface, like a sheet of paper. Let Figs. 117 and 118, pages 107, 108, represent two equal systems of waves from red light reflected to the eye from the upper and under surface of Newton's thin plates of air. If they be superposed, as in Fig. 119, page 108, the waves will coincide, and there will be red light, as in the first coloured ring. On moving A B a distance equal to one half-undulation at Fig. 1 20, the waves will be neutralized by interference, and there will be darkness ; on moving A B a second half-undulation, there will be light, and so on ; for when the stars indi- cating the highest part of the waves of A B coincide with the odd numbers of half-undulations of C D, there will be light, as in Fig. 119; and when they coincide with the even numbers, darkness will be caused by interference, as in Fig. 120. Dr. Young proved that each of Newton's fits of transmission and reflection was equal to half a wave of each colour, and this is equal in length to the thickness of the plate of air at which that colour is first reflected, and there- fore a whole undulation would be equal to two of Newton's spaces or fits, or what he termed the length of an interval between the fits of easy reflection. Thus, the thickness of the plate of air required to produce red light being determined by Newton to be 133 ten-millionths of an inch, double that number, or the length of a wave of red light, would be 266 ten-millionths of an inch. For orange yellow green blue . ;, indigo violet 240 ten-millionths of an inch 227 211 196 185 167 THE INTERFERENCE OF LIGHT in Herschel's table is, perhaps, the most complete record of the invaluable work of Newton. The figures are Newton's, although the meanings of them have been altered to comply with the undulatory theory. Colours of the Spectrum. Lengths of an Un- dulation in parts of an inch. Number of Undu- lations in an inch. Number of Undulations in a second. Extreme red 0'0000266 37,640 45 8,000000,000000 Red ... 0*0000256 39,180 477,000000,000000 Intermediate o '0000246 40,720 49 5 ,000000,000000 Orange 0'0000240 , 4I,6lO 506,000000,000000 Intermediate 0'0000235 42,5*10 5 17,000000,000000 Yellow . 0*0000227 44,000 535 ,000000,000000 Intermediate 0'00002I9 45,600 5 5 5,000000,000000 Green . 0*00002 1 1 47,460 5 77,000000,000000 Intermediate O*OOOO2O3 49,320 600,000000,000000 Blue . 0*OOOOI96 51,110 622,000000,000000 Intermediate 0*0000l89 52,910 644,000000,000000 Indigo . O*OOOOl85 54,070 658 ,000000,000000 Intermediate ' 0*0000181 55,240 67 2,000000,000000 Violet . . 0*0000174 57,490 699,000000,000000 Extreme violet 0*0000167 59,750 727,000000,000000 A very good idea may be given of the effect of the law of interference by means of a simple contrivance proposed by Sir Charles Wheatstone, called the Eido- trope. It is made of two circular pieces of ordinary perforated zinc, one of which is made to turn round in front of the other by means of a band and pulley, the whole being arranged as an ordinary magic-lantern slide. Wire gauze or perforated cardboard may be substituted for the perforated zinc. If the two zinc plates were perforated exactly alike, little or no effect would be observed ; but as one set of perforations is always a little in advance of the other, certain shadows, which assume interesting forms, are perceptible when the instrument is used in the magic lantern, and the figures projected on to the disc. The dark shadows are caused by the mechanical interference of the zinc plates in the proportion to represent the half-undulation, and in some positions are very distinct. If wire gauze is employed, the shadows assume just the same appearance as the surface of watered silk. DOUBLE REFRACTION AND THE POLARIZATION OF LIGHT. When a ray of light falls upon the surface of Iceland spar, it is divided into two colourless rays, one of which is called the ordinary, and the other extra- ordinary, ray of light ; both rays possess physical properties different from those which belong to common light, and if reunited they would again form common light. In the year 1817, Dr. Young, the famous revivalist and supporter of the undulatory theory whilst considering the results of the speculations of 112 ON LIGHT. Huygens, Wollaston, and Brewster, and the cause of double refraction, was led to believe that the effect must arise from a difference of elasticity in the crystal of Iceland spar; and being aware that Newton had expressed the idea that a ray of light possesses sides, he first proposed the hypothesis of trans- versal vibrations of light. The theory is, that in the progress of a ray of light the forward motion is made up of two sets of vibrations, which are either longitudinal or transversal. The longitudinal vibrations represent the path or direction of the ray, whilst the transversal ones take place at right angles to the former. This peculiar motion may be compared to the particles of water which move up and down whilst the wave advances horizontally. Dr. Young illustrated these vibrations by the propagation of undulations along a stretched FIG. 122. A Rhomb of Iceland Spar, showing the double Refraction of Light. cord agitated at one end, which supposing a person to hold in his hand, and by moving first quickly up and down, a wave will be produced, that will run along the cord (see p. 6) to the other end, and then by a similar movement, but from the right side to the left, another wave will be produced, which will run along the cord as the former ; but the vibrations and undulations of each will be in planes at right angles to each other, and independent of each other, FIG. 123. A, Woodward's cardboard model representing a ray of common light; B, transverse section, shewing the figure of a cross. one being in a perpendicular plane and the other in a horizontal plane, so that, according to this theory, A (Fig. 123) may be considered to represent a ray of ordinary or unpolarized light, a cross section of which would give the simple figure B, it being understood that the vibrations take place in planes all round the direction of propagation. With the help of this hypothesis of transversal vibration, double refraction is easily explained, and is put into the most concise terms by the editor of THE POLARIZATION OF LIGHT. XI 3 the late Dr. Young's lectures : " A ray of light falls on the surface of a crystal the elasticity of which is. different in different directions. The motions conse- quently are not all transmitted with the same velocity, and, as the index of refraction depends on the velocity, one set of vibrations will, on emergence, be totally separated from another. Moreover, the light, on emerging, is quite different from common light. In each ray it consists only of vibrations in one direction. Suppose, therefore, one of these rays to fall on a second crystal placed in a similar position with the first ; it will not now be divided into two, but will emerge just as it entered. Light which consists of vibrations in one direction is called polarized light. In 1810 it was discovered by Malus, an officer in the French engineers, that light reflected from the same face of unsil- vered glass is more or less polarized, and Brewster ascertained that it is per- fectly so when the tangent of the angle of incidence is equal to the refractive index, and also that the transmitted ray is partially polarized." But why called polarized ? The term, perhaps, is not a very happy one, but was suggested by analogy to the poles of a magnet. Dr. Whewell thus defines polarity: " Opposite properties in opposite direc- tions, so exactly equal as to be capable of accurately neutralizing one another." FIG. 124. A, magnet made of watch-spring with north and south poles ; B, same magnet bent round, and polarity neutralized; c, common light; D D, polarized light. A piece of steel watch-spring, when magnetized, has a north and south pole (see A, Fig. 124) ; but when the same piece of steel is bent round in a circle, as at B, Fig. 124, the two forces neutralize each other, and the polarity is gone. Such a circular piece of steel might be compared to common light : it is like the section of a hoop-stick, c ; whilst polarized light may be compared to the straight steel magnet A, or to a lath. A hoop-stick is the same all round ; but a lath has a top and bottom and sides. The former may represent common light, and the latter polarized light ; and thus polarization is simply the separa- tion of the two sets of undulations or vibrations, D D, Fig. 124. When common light is passed through transparent refracting bodies per- fectly homogeneous in their structure, and of a uniform temperature throughout, such as gases, common air, pure water, annealed glass, jelly, and many kinds of crystallized bodies, the form of whose primitive crystal is the cube, the regular octahedron, and the rhomboidal dodecahedron, such as alum, common salt, or fluor spar, the beam of light is refracted singly ; but in nearly every other crystalline body the rays undergo double refraction, and, although this is not apparent at once, like it is with Iceland spar, the property of double refraction is soon discovered by using polarized light. ON LIGHT. Polarized light may be obtained in four different ways, viz. Firstly, by reflection ; Secondly, by simple refraction ; Thirdly, double refraction ; Fourthly, by transmission through a plate of tourmaline, slit parallel to the axis of the crystal. Thirty years ago, Mr. J. F. Goddard, then of the Polytechnic, London, received from the Society of Arts a silver medal for his apparatus for experiments on polarizing light. The description which accompanied the apparatus is so good and so little known, that the writer has quoted the most important part of it, in order to explain, with the assistance of the appa- ratus invented by Mr. Goddard, this most difficult branch of optical science. POLARIZATION BY REFLECTION AND SIMPLE REFRACTION. " Polarization may be effected with common crown glass, either by ordinary reflection or refraction, each of which will exhibit the same effects. In order to understand this, let b b (Fig. A 125) represent a bundle of plates of common FlGS. A and B 125. Explanation of Polarisation by Reflection and Simple Refraction. glass, placed so that a ray of ordinary light, a a, may form an angle of incidence of 56 45' with a line perpendicular to their surface; then the light reflected and represented as passing off at a will be polarized light ; and if a proper number of plates, which for the same angle of incidence is twenty-seven, be employed, the light transmitted at c will be polarized also, the two rays pos- sessing the same properties, but at right angles to each other. " Thus in the reflected ray d the vibrations are supposed to take place in a perpendicular plane, this being a bird's-eye view (Fig. B 125 being a horizontal view of the same thing), whilst in the refracted ray c the vibrations are per- formed in a horizontal plane. This will be easily understood on analyzing either of the rays, which may be done by the same means as that by which the original beam is polarized. Thus, supposing we experiment with, test, or THE POLARIZATION OF LIGHT. analyze the reflected ray \ so that it is evident, in these experiments, that there are two posi- tions, shown in Figs. 125 and 126, in which the same ray of polarized light d is wholly reflected, as at d 1 d\ and two other positions, A, D, Figs. 125 and 126, in 82 n6 ON LIGHT. COMMON LIGHT 1 . Is capable of reflection at oblique angles of incidence in every position of the reflector. 2. Will pass through a bundle of plates of glass in any position in Avhich they may be placed. 3. Passes through a plate of tour- maline, cut parallel to the axis of the crystal, in every position of the plate. which it is wholly transmitted by the analyzing bundle of glass, as at rV, all of which are easily understood by bearing in mind the description of the physical nature of common light according to the undulatory theory, and the action of the first or polarizing bundle of glass, or transversal vibrations. " Thus we obtain experimental data, which may be expressed as follows : POLARIZED LIGHT 1. Is capable of reflection at oblique angles of incidence in certain positions only of the reflector. 2. Will pass through a bundle of plates of glass only when they are placed in certain positions. 3. Does not pass through a plate of tourmaline cut parallel to the axis of the crystal, except in certain posi- tions; in others, the tourmaline, though quite transparent, stops the whole of the polarized light as if it was opaque. " A bundle of plates of glass or a slice of tourmaline is consequently to be regarded as a test of polarized light, and enables the physicist to distinguish between the latter and common light, which he is said to analyze, the bundle of glass or the tourmaline being called the analyser. POLARIZATION BY THE TOURMALINE. " Amongst crystallized minerals there .are many possessing the property of polarizing the light transmitted through them, the most remarkable of which, however, is the tourmaline. This mineral crystallizes in long prisms, whose primitive form is the obtuse rhomboid, having the axis parallel to the axis of the prism. " It must be remembered also that the axis of crystals is not, like the axis of the earth, a single line within the crystal, but a single direction through the crystal; for supposing Fig. 127 to represent a crystal of any kind, the axis of FIG. 128. A, single plate of tourmaline ; B, superposition of the second plate on the first. which is in the direction A X, if we divide such a crystal into four along the lines B B and C C, each separately will have its axis A O, O X, c B, and B c, which, when united in one crystal, are all parallel ; every line, then, within the crystal parallel to A X is an axis. THE POLARIZATION OF LIGHT. 117 "If \ve cut a crystal of tourmaline of a proper kind parallel to the axis into thin plates of an uniform thickness (about one-twentieth of an inch), and polish each side, it possesses the property of polarizing light transmitted through it in a remarkable manner. Fig. A 128 represents one of these plates, the lines across which we may suppose to be parallel to the axis. Now, if we hold such a plate before the eye, and look at the light of the sun, or flame of a candle, or any artificial light, a great portion will be transmitted through the plate, which will appear quite transparent, having only the accidental colour of the crystal, which in specimens suited for these experiments is generally brown or green; but the light so transmitted will be polarized light, and, on being analysed by a second plate, which may be done by looking through both at the same time, we shall find that when the axes of both plates coincide, i.e., are parallel with each other, the light which is passed through the first will also freely pass through the second, and they will together appear per- fectly transparent ; but when one is turned round, so that the axes of each plate are at right angles (across each other), as represented in B, Fig. 128, not a ray of light will pass through they will appear perfectly opaque, although we may be looking at the meridian sun. If we suppose the structure of the crystal to be represented by a grating, the bars of which are the axis, we may conceive that its action on ordinary light will be to transmit such vibrations only as are performed in a plane parallel with the axis, and to stop all others. Hence, the light transmitted through a single plate will be polar- ized, and possess exactly the same properties as the light polarized by any other means, as may be proved by analyzing it by any of the means which have been described. But let us suppose a second tourmaline to be used, and, as it is understood that in the light which makes its way through the first tour- maline the vibrations are parallel to the axis, all other vibrations being stopped when the axis of the second or analyzing plate is perpendicular to the first, as represented in B, Fig. 128, the vibrations which have passed through the first, being now perpendicular to the second, will also now be stopped by the second plate in such a position; and, as it is turned round, there will be found two positions in which it will not pass through, being wholly stopped, these posi- tions being at right angles to each other, as will be understood by B, Fig. 128, where a a is the first or polarizing plate, and c the second or analyzing plate, overlapping the first." Mr. Goddard then describes the instrument for which he received the silver medal the oxy-hydrogen polariscope. (Fig. 129.) " In this instrument A represents the hydro-oxygen blowpipe ; B, the lime cylinder and diverging rays of light refracted by the condensing lenses c c c and falling upon a mirror bb, composed of ten plates of thin flattened crown glass placed in the elbow of a tube bent to the polarizing angle of crown glass ; d, converging rays of polarized light reflected from the mirror ; h h, a bundle of sixteen plates of mica, for analyzing the light previously polarized by reflec- tion ; c, a double-refracting crystal (film of selenite) placed in the focus of the object-glass I, which forms an image of the crystal upon a disc or screen at r. As the analyzing bundle of mica, h h, 'is made to revolve (or turn round), the image of the selenite upon the disc undergoes all the changes, and exhibits alternately the primary and complementary colours at the same time, one being reflected in the direction s, and the other transmitted and seen at r. " The great advantage of polarizing the light from a number of plates is the obtaining a beam of any required dimensions, of much greater intensity than Tl8 ON LIGHT. 113 20 FIG. 129. Goddard's Oxy-hydrogen Polariscope. by any other means; for whatever single surface may be employed that polarizes light at the same angle as the glass used (which for crown glass is 56 45'), we obtain an additional quantity by laying on it a single plate of such glass, and a further quantity by the addition of a second, third, or any further number ; the quantity of light added by each succeeding plate being, how- ever, less in proportion to the number of plates through which it has pre*- viously to pass. In this respect the single-image (Nicol's) prism of Iceland spar is decidedly the best for analyzing, as by this a great variety of objects may be exhibited. Its application is shown in Fig. 130, where e, the selenite, THE POLARIZATION OF LIGHT. 119 is placed in the rays, ddd,o>i polarized light, an image of which is projected by the lenses ; h is the analyzing prism through which the rays of light r r are refracted. FlG. 130. Use of the Nicol's Prism as an analyzer. " But there is one class of phenomena, viz., the rings seen to encircle the optic axes of crystals, the number of which increases in some crystals (the topaz, for instance) with the divergence of the rays of polarized light passing through them. It will be evident, then, that the tourmalines enable us to exhibit more of these rings, and upon a larger scale, than the prism, which will be better understood by the arrangement shown in Fig. 131. FIG. 131. "ddd, converging rays of light polarized by reflection ; /, a lens of short focus transmitting a cone of light with an angle of divergence from its ray r r of 45; e, a crystal, say topaz; /*, the tourmalines for analyzing; so that, even for these purposes, the cost of the tourmalines is reduced one-half by Goddard's polariscope, as only one need be used." The writer frequently uses Goddard's instrument as made by Mr. Darker, jun., of Paradise Street, Lambeth, whose father before him earned so much credit in the practical parts of this branch of optics. Darker also makes the most elaborate and beautiful designs in selenite or sparry gypsum, being the native crystallized hydrated sulphate of lime, from which plaster of paris can .be made by driving off the water of crystallization. This mineral, split into thin films, and cut under water, or oil, or turpentine, is laid upon glass with Canada balsam. The greatest nicety is required in the manufacture of the selenite slides, or else all the edges of the figures would be rough. 120 ON LIGHT. A piece or film of selenite of unequal thicknesses exhibits the most varied and beautiful colours when placed in the polariscope, the colours transmitted by the analyzer being complementary to those reflected from the bundle of glass plates. Any transparent substance in which unequal elasticities occur will present phenomena of colour when placed in the polariscope. A piece of plate glass, if well annealed, shows no colour until it is bent or sq being placed in a strong frame provided with a screw. it is bent or squeezed by FIG. 132. Apparatus for compressing Glass. A A, the press; B, the piece of plate-glass. On the same principle, unannealed glass exhibits some of the most vivid colours and figures. (Fig. 133.) Or if a rod of plate glass is placed in the polariscope and heated with a FIG. 133. Unannealed Glass. red-hot copper bar, the unequal expansion of the particles causes that retar- dation in the path of the rays which results in interference, and the produc- tion of colours, and these disappear gradually when the hot copper bar is removed. A little jelly allowed to solidify in a proper frame, the sides of which are of glass, exhibits no double refracting power until it is subjected to pressure. A quill pen flattened out and arranged for exhibition in the polariscope will give some very pleasing tints. Water of an uniform temperature has no double refracting power, but when frozen and converted into ice the particles exhibit unequal elasticities, and colour is the result when it is placed in the polariscope. If plates of selenite or any doubly refracting crystal of considerable thick- ness be ground away on one edge, so as to give them a wedge-shape, they will present bands or fringes composed of all the colours of Newton's table, arising from the various thicknesses which such a shape possesses ; or by grind- ing a concavity in a similar plate a number of concentric rings (reminding THE POLARIZATION OF LIGHT. 121 the spectator of Newton's rings) are produced. Small crystals obtained by evaporating single drops of solutions of acetate of zinc, chlorate of potash, sulphate of soda, oxalic acid, oxalate of ammonia, sulphate of copper, borax, ferrocyanide of potassium, &c., may be exhibited in the polariscope. The lovely rings obtained by using uniaxial and biaxial crystals are well shown by Goddard's apparatus, with a large Nicol's prism or a good tourmaline as the analyzer. To exhibit these coloured rings a higher microscopic power FIG. 134. Appearance of the rings produced by Iceland spar cut perpendicularly to the principal or shortest axis, and alternate appearance of the black and white cross with complementary colours, as the analyzer is rotated. is used. This is always supplied with the instrument, and is put on before using the polariscope. For these experiments Iceland spar, rock crystal, emerald, sapphire, beryl, ice, furnish good examples of uniaxial crystals. A very large number of crystals are biaxial, and have two axes of double FIG. Double curves or sets of elliptical or oval-like rings produced by a plate of nitre 1-12 or 1-15 in. thick, cut perpendicular to the prismatic axis. refraction, which are more or less inclined to each other. These are termed biaxial crystals, or crystals with two optic axes. Nitrate of potash exhibits this optical property very perfectly, also Rochelle salt, selenite, sugar, borax, and many others. 12,2 ON LIGHT. The coloured bands obtained from biaxial crystals are not concentric, but somewhat oval, with two centres, which represent the two axes of the crystal. The splendid phenomena of colours produced by various substances in polarized light are the results of transversal vibrations. When a single wave or vibration in any one plane is divided into two, at right angles to each other, one will of necessity be half a wave behind the other, the two being opposite halves of the same wave ; and as each gf these again is divided or resolved into two others, there will be four waves or vibrations produced from the original one. Two of these in one plane coincide and strengthen each other, while the two in the other plane oppose and destroy each other. This difficult subject may be summed up and concluded with Woodward's very instructive diagrams, exhibiting at one view POLARIZATION, ANALYZATION, INTERFERENCE OF LIGHT. FIGS. 136, 137. FIG. 136. A, B, c, D, common light; E a plate of tourmaline, or a bundle of plates o.f glass, termed the polarizer; F, polarized light; G, a plate of selenite; H, dipolarized light; i, a plate of tourmaline, or a bundle of thin plates of glass, called the analyzer; K, coincidence of waves for red light; L, inter- ference of waves for yellow, and M. of those for blue light; N, the result red light. FIG. 137 i, the analyzer turned round 90; K, interference of waves for red light; L, coincidence of waves for yellow, and M, those for blue light; N, the result green light. HEAT. THERMOMETRIC HEAT. AMONGST the physical forces, the corellation of which has been so well discussed by various philosophers, that termed caloric (at one time, like Jight, considered to be a direct emanation of some rare and subtle form of matter) has received the most careful attention. Light is discoverable by two most sensitive inlets the eyes. The sensation termed heat is not more appreciable by the eyes than by any other part of the human body, and yet the mind may be easily deceived by sensations caused by heat or its absence, termed cold. The body may experience the greatest torture by an excess of heat or burning, and it may derive pleasure from the application of a moderate amount of the same power, as in the use of the Turkish or other baths. The nervous system distributed over the surface of the body cannot, how- ever, distinguish properly degrees of heat, and we seem to be able to discover only when heat is entering or leaving our bodies, and then the exclamations referring to extremes, such as "how hot .-" or. "how cold ! " escape us. And even this faculty is limited, because the sensations caused by touching a lump of frozen mercury and a hot iron are the same. The unfortunate person who does this will complain as if he were burnt with the intense cold of solid mercury. We cannot, as with the eye or the ear with light and sound, discern gradations of heat; hence artificial means have been invented to supply this want. It is not surprising that heat should have been considered to be a material body, entering into combination with solids, fluids, or gases, because it is so i2 4 HEAT. readily evoked from ponderable substances. A clever blacksmith, with his hammer, anvil, and a rod of good iron, will dexterously obtain, by hammering the metal, enough heat to light his forge fire, provided a little sulphur is used as the intermediate combustible body. Heat travels with light from the sun ; and as Newton succeeded in con- vincing his contemporaries that the latter was a material body, it came to pass by a natural sequence of reasoning that the former should also be regarded as a subtile rare form of matter opposed to cohesion. The material theory of caloric the hypothesis of "emission" has given way to the more rational theory of "undulation." If, as has been explained at p. I, an im- ponderable elastic ether pervades all space, a peculiar vibratory motion set up in the material particles of a body may be communicated to this ether; and then, on the same principle that a glass trembles whilst producing sound in air, so the minute particles or molecules of solid fluids or gases oscillate, and these oscillations or vibrations are communicated to and transmitted by the ether. Physicists, however, prefer to speak of their favourite hypothesis as " The Dynamical Theory" (Sai/ot/Ais, power). The title at once shows that heat, and not light, is intended to be expressed. Heat is in every sense of the word a " power ;" the terms are mutually convertible the one into the other. The combustion of coal produces heat, which generates steam, and the latter is the greatest modern representative of power. Power, as shown by the muscular force of the arm conveyed through a hammer, generates heat when metals are beaten on the anvil. This connection between heat and power is shown in the most perfect and masterly style by Dr. Tyndall,* the industrious and worthy successor of Faraday. He has enriched this branch of philosophy with a vast number of practical demon- strations and experiments, giving quite anew and fresh appearance to a science which seemed to have reached its limits in the stereotyped repetition of descrip- tions of thermometers, pyrometers, calorimeters, and eternal disquisitions on specific heat and latent caloric. Referring back to heat as the equivalent for power, there is a telling experiment of Tyndall's, in which a brass tube con- taining water is connected with a whirling table, and whilst it is going round with great velocity, it is rubbed with the wood of a lemon-squeezer; the friction soon generates enough heat to cause the water to boil, and to eject a cork with which .the tube is closed. Power generates heat, and vice versa. If a mode- rate-sized piece of lecture-table apparatus generating heat is to be regarded as a power, what must be the energy of the sun ? what kind of force is at work to produce so much heat ? Pouillet has carefully ascertained the total heating effect of the sun's rays upon the earth, and estimating the whole heating power of the sun as 2,300 millions of parts, he calculates that less than one of those parts only reaches our earthi, and yet it would melt a layer of ice thirty-five yards thick over the whole surface of our globe. This proportion of heat is not all available : some of it is at once converted into power by setting the air in motion, to create the winds; another portion raises the water of the ocean into vapour, which, descending in the form of rain on high levels, such as the mighty water-shed which supplies the great lakes (discovered by Speke and Grant and Sir Samuel Baker), the sources of the Nile, flows down to the lowlands, giving rise to water power, which is again the equivalent for * Heat Considered as a Mode of Motion. By John Tyndall, F.R.S., etc. Longman, Green, Longman, Roberts, and Green. THERMOMETRIC HEAT. 125 heat ; another part stimulates and increases the growth of plants ; and thus, in ages long since passed away, the heat of the sun's rays was not all lost, as the older Stephenson insisted, but stored up ready for man to use in another form, viz., coal, and therefore called potential heat. The plants, being the food of animals, again contribute to the production of animal heat and muscular force. The sources of heat are all connected with motion of some kind. No. i. Friction is a notable illustration, and it was by causing two pieces of ice to nib one against the other that Sjr Humphrey Davy generated heat, liquified the ice ; and like Dr. Young, who proved that light could turn a corner, and established by his experiments with inflection a sort of basis upon which the undulatory theory of light was again reconstructed, so this famous experiment of Davy supplied a great fact, and gave the first blow to the old theory which said that the ice melted because latent heat was made sensible heat, when it was well known that water at a temperature of 32 Fahrenheit contains much more heat than ice ; how, then, could the ice, already deficient in heat, supply enough to satisfy the condition of water? There are plenty of illustrations of the generation of heat by friction, The flint and steel ; the attrition of dried wood, as used by savage tribes ; the famous experiments of Count Rumford whilst boring cannon, when enough heat was generated in two hours and a half to cause two and a half gallons of water to boil ; the friction of railway- wheel axles, which have been known to become red hot and to set fire to the woodwork of the carriage. In North America, a case is quoted where heat was intentionally generated by waste water power and used for heating purposes, the generator being two flat plates of iron which rubbed against each other. No. 2. Percussion. It was said formerly that metals when struck with a hammer, or with a die in the coining-press, became hot because their density was increased, and 'therefore their capacity or containing power for heat was altered ; but it is clearly shown that this is not the true explanation. Lead, for instance, which becomes hot by percussion, does not increase in density and yet becomes hot so hot that when projected from the steam gun in the form of bullets against a wrought-iron target, a flash of light is apparent in a darkened room. The heavy shot used for battering iron plates always become very hot after they have struck the plate. No. 3. Chemical Action. The bringing together of a number of atoms, however small, the clashing together (as Tyndall calls it) of particles to pro- duce new compounds, as in the heating and combustion of finely powdered antimony when it is brought in contact with chlorine gas, or the heat gene- rated by combustion or from other chemical changes, are all to be regarded as the result of motion which the eye cannot detect, but which must occur before the elements come in contact, combine, and form new compounds. There are many chemical changes accelerated by motion, and hence the stirring-rod is an important mechanical means to secure the more rapid ^nion of particles. No. 4. Electrical Action. The very essence of the existence of electrical power is circulation or motion. The intense heat generated by the discharge of a powerful Leyden battery through a thin iron wire seems to be increased by the resistance offered to the passage of the current, and thus work is con- sumed. The ignition of a platinum wire by a current of voltaic electricity affords a further instance of resistance ; whilst another wire of the same size made of silver, offering less resistance an*d consuming less work, does not become red hot. We speak of a current of electricity : a current is something 126 HEAT. flowing; it is of course motion. Here again the two forces are similarly con- vertible. The heat generated by the passage of a current of electricity through a platinum wire will set up another current of electricity, if the heat is applied to a series of bars of bismuth and antimony arranged properly, and thus called a thermo-battery or multiplier a most delicate indicator of heat, which in connection with the galvanic needle is usefully and extensively employed in experiments where heat, inappreciable by a thermometer or other ordinary means, is generated. No. 5. Vital Power, impossible without food, appears to be the result of a kind of slow combustion, or change of carbon and hydrogen into carbonic acid and water, and furnishes another illustration of heat generated by chemical action. The muscular power of a horse, as sagaciously observed by Count Rumford, might certainly be used to produce by friction (as in the boring of iron) enough heat to cause water to boil for the purpose of cooking victuals, if a quicker -and more advantageous mode were not suggested by the direct combustion of the fodder which the horse must eat to maintain the animal heat, in order to be able to exert his muscular energy. To work out the relation between heat and mechanical power, it has been found necessary to establish a standard of comparison, or unit of work, which latter in England is defined to be " the force required to overcome the pres- sure of one pound through the space of one foot." By a very extensive series of experiments Dr. J. P. Joule determined that 772 foot-pounds, or units of work, have to be performed to raise a pound of water at about 50 Fahrenheit one degree; 772 units of work would, therefore, be called the mechanical equivalent of heat, and an equivalent to a force that would raise one pound 772 feet high; or,, if we reverse the statement, and imagine the same water falling through 772 feet, it would be raised one degree Fahrenheit. The power or force used was measured by the descent of weights, which caused the apparatus, viz., an iron paddle-wheel, to rotate in water or mercury, and, by the friction of the iron and mercury or water, to eliminate heat, which was estimated in the most careful manner. "Joule's equivalent" is, therefore, a standard of the most valuable and truthful kind, verified by another great man, Dr. Mayer, who, by different means and by calculation, makes out the equivalent to be 771*4 foot-pounds, instead of 772, and thus ? roved how correct had been the previous experiments and calculations of oule. Dr. Young says, " If heat is not a substance, it must be a quality; and this qua- lity can only be motion. It was Newton's opinion that heat consists in a minute vibratory motion of the particles of bodies, and that this motion is communi- cated through an apparent vacuum by the undulations of an elastic medium, which is also concerned in the phenomena of light. It is easy to imagine that such vibrations may be excited in the component parts of bodies by percussion, by friction, or by the destruction of the equilibrium of cohesion and repulsion, and by a change of the conditions on which it may be restored in consequence of combustion or of any other chemical change." Further on, he says, " The effect of radiant heat in raising the temperature of a body on which it falls re- sembles the sympathetic agitation of a string, when the sound of another string, which is in unison with it, is transmitted to it through the air. " All these analogies are certainly favourable to the opinion of the vibratory nature of heat, which has been sufficiently sanctioned by the authority of the greatest philosophers of past times and of the most sober reasoners of the THERMOMETRIC HEAT. 127 present. Those, however, who look up with unqualified reverence to the dogmas of the modern school of chemistry will probably long retain a par- tiality for the convenient, but superficial and inaccurate, modes of reasoning which have been founded on the favourite hypothesis of the existence of caloric as a separate substance ; but it may be presumed that in the end a careful and repeated examination of the facts which have been adduced in confutation of that system will make a sufficient impression on the minds of the cultivators of chemistry to induce them to listen to a less objectionable theory." These anticipations of Young have been fulfilled : the re-establishment of the undulatory theory of light, by his exertions, has been slowly followed by the reception of the dynamical theory of heat. THE COMMON EFFECTS OF HEAT. When a solid is raised in temperature, either by percussion or by the direct application of heat, the vibratory motion supposed to be set up in the mole- cules or atoms of the substance appears to overcome for a time their cohesive force, and they are separated : they occupy more space ; they expand, and, im- perceptible as that expansion must be to the eye, it may still be made apparent by a proper instrument. A miniature house, fitted with a number of movable metallic tiles, is so arranged that, when the outer walls are driven apart by any means, the roof and tiles fall in. Between the parts of the model representing the walls of the house is arranged a broad band of brass, which is nicely adjusted by a screw, so that it just touches the sides, which are held together with a spring. When a series of spirit-lamps are lighted under the bar, it FIG. 138. a, the spirit-lamp; b, the brass ring; c, the brass ball. soon expands with great force, and, overcoming the springs which hold the sides together, they are pushed out, and the rattle of the falling roof and tiles shows to the eyes and ears the catastrophe that might happen on a larger scale. The expansion of the brass rod is thus indicated in a simple and effective manner. When the contents of warehouses provided with great iron girders take fire, the latter expand, push out the walls, and ultimately bend themselves (when they become red hot) with the superincumbent weight above them. What is called Gavesande's ball (Fig. 138) is a simple and effective mode of showing cubical expansion. A brass ball is carefully turned and polished, so that it exactly fits, and will pass through a metal ring ; but, when heated, 128 HEAT. expansion takes place, and, instead of falling through the ring, it is held up as in a ring-sta.nd, and will no longer pass through the opening. The expansion of a fluid body is also shown by placing some coloured water in a flask ; to this is fitted a cork and tube with a small bore, which is bent round at the top, so that .any liquid ejected by expansion may fall into a shallow dish containing some bits of potassium ; the rise of the liquid in the tube may be watched by placing a piece of cardboard behind it, and directly the full expansion occurs the liquid is ejected and the potassium takes fire. The expansion of gases by heat is readily shown by various simple, experiments. The neck of an empty retort is placed under water, and directly the body is heated the air expands and passes in bubbles through the water; before removing the lamp a little ink should be stirred into the water, so that when the heat is withdrawn the amount of expansion may be shown by the rise of the coloured water to fill the space at first occupied with air, but now lost by expansion. The Montgolfier or fire-balloon has never ceased to please, because its inflation is so simple and rapid. The only difficulty seems to be to avoid setting the paper on fire ; this is easily prevented i, the flask with cork and tube, by using a metal funnel with coarse wire gauze at iied with water coloured with *the top, and inside of which the large piece of tow, P n omt r frffcuV conSming Etc w e"ed with spirits of wine, is allowed to burn ; the potassium ; d, the ring-stand and inverted funnel may be supported on three legs, spirit-lamp. an( j the mouth. should be at least 3 in. in diameter. in order to allow the heated air to pass rapidly into the paper bag. By attaching a thin string, the balloon may be let up and down any number of times. When the balloon is intended for the amusement of young people, at an out-door fete, the balloon can be sheltered from the wind by a blanket stretched between two poles ; and if the balloon, when nearly ready to start, is blown by a sudden gust of wind across the heating apparatus, it does not catch fire, because it is protected from the flame by the funnel and wire gauze. This mode of sending up a fire-balloon is the safest, because it does not carry any fire. The late accidental and total destruction by fire of the immense fire- balloon in the grounds of the Crystal Palace sufficiently indicates the danger of these aeronautical machines, and how soon they may ignite ; indeed, no Montgolfier balloon on the large scale should be used on this principle without first rendering the material of which it is composed incapable of combustion, by preparing it with a solution of phosphate of ammonia. Robertson, in his " Recreative Memoirs," gives a very interesting account of the construction and ascent of an enormous Montgolfier or fire balloon at Vienna, in the year 1781. It was the first experiment of the kind tried there, and was carried out in a most fearless, not to say reckless, manner by Gaspard Stuver. ' The length of the balloon (Fig. 141), which was constructed like a cylinder closed at both ends with cones, amounted to 60 ft. It was made of FIG. 139. THERMOMETRIC HEAT. 129 FIG. 140. The Paper M outg The sheet-iron funnel, with coarse wire gauze at the end, supported on three legs about i ft. from ground. An iron ladle containing tow moistened with spirits of wine, or a small fire fed with shavings, will do. canvas lined with sized paper. Three persons ascended with Stuver in a Danube boat arranged as a car, and attached to the balloon by proper cords. They entered the car with the greater courage because they did not intend to allow the balloon to travel where the winds migh't direct it, but to retain it as a " captive balloon " (like the one at the Crystal Palace) by a strong cord ; but, unfortunately, the rope was not strong enough : it broke, and away went the balloon with immense rapidity, not without considerable peril to the unfortunate passengers. The shock was so violent that the boat tilted on 9 L'30 HEAT. one side, and the fire was thrown out on the canvas. By a happy forethought, the men were provided with water and long rods to which were attached large sponges, and with these they courageously stopped the further progress of the flames. The voyage was not very long : the un wieldly machine descended a little, and knocked down a large wooden framework prepared for some pyrotechnic display : it then reascended, grazed the tops of the trees in the Prater or park, and fell on the grass on the other side of the Danube. AMOUNT OF EXPANSION IN SOLIDS, LIQUIDS, AND GASES. MEASURES OF HEAT THERMOMETRY. The fact that the particles or molecules of a solid body are pushed away from each other by heat, and suffer a certain increase in dimension, called expansion, has been already mentioned, and in every-day life examples are not wanting. The moulds for casting in metal are always made larger than the size required, in order to allow for the expansion of the metal when in the liquid state. The iron hoops of carriage or cart wheels are put on red hot, and being cooled suddenly by the application of cold water, they contract with great force and draw all parts of the wheel firmly together. In bottles the stopper is often fixed tight, and cannot be removed by any force that is applied ; when this is the case, the outer part of the neck should be carefully heated over the flame of a spirit-lamp ; expansion takes place, and then the neck is tightly grasped with the hand, protected by a duster in case of FIG. 142. A B, cast-iron frame; c c, red-hot iron bar, passed through the holes in A B, and fitted tight by the screw D. E represents the same iron frame broken by the contraction of the bar. accidental breakage; a slight effort, and particularly moving the stopper backwards and forwards in one direction only, and carefully avoiding a motion which would cause the stopper to turn round, will soon be rewarded by the extrication of the stopper from the tight embrace of the neck of the bottle. A piece of iron, cast with two elbow-pieces, each bored so as to allow an iron bar to be placed through them when red hot, and then screwed up with a thumb-screw as tight as possible, instantly breaks off one or both the elbow- pieces when the red-hot bar is cooled by being suddenly plunged into water. The amount of expansion, or coefficient of expansion, of solids is, however, exceedingly small, and requires the utmost nicety of experiment to discover its amount. The difference between linear expansion, or the increase of length, and superficial expansion, or the increase in the area of a surface, must of course be remembered. THE EXPANSION OF SOLIDS. The unequal expansion of metals is shown by riveting together two flat plates of iron and brass ; the latter, expanding in a greater degree than the former, causes the compound bar to take the form of an arch or curve when heated ; the brass side being uppermost, the arch ascends, and a convex figure is observed ; but if downwards, the reverse or concave form is produced. The rise or arching of the riveted bars is easily shown if united with an electrical battery ringing a bell with which contact only is made when the curve is produced. Attention is further directed to the result of unequal expansion when the spirit-lamp is withdrawn and the bar cooled with ice or cold water ; the bell ceases to ring, and the bar again becomes straight. FIG. 143. A A, The compound bar of brass and iron, heated by a spirit-lamp, and rising in a curve towards the wire B, connected with the bell c and battery D, of which the otheu wire is attached to the compound bar at E. When the spirit-lamp is removed, the bar contracts, and the contact is broken at B. It is on this principle that Breguet constructs his most delicate metallic thermometers. The solid affected by heat consists of a thin metallic ribbon, composed of three strips of platinum, gold,, and silver, passed through a rolling-mill together. The ribbon is then coiled in a spiral form, the platinum being outside and the silver inside ; one end is fixed to a proper support, and the other is attached to a copper needle. The spiral unwinds when the heat increases, and the contrary result occurs if heat is withdrawn, and it cools. The needle moves round a scale which is graduated by direct comparison with a standard mercurial thermometer. The following table shows the comparative increase of length or linear expansion in bars or rods of various substances when they are heated, from the freezing to the boiling point of water viz., from 32 to 212 Fahrenheit, according to the authority of Graham : Zinc (cast) . Zinc (sheet) . Lead . Tin . Silver . Copper Brass . on 323 340 35i 5*6 524 ,,58i 584 Pure gold . Iron wire . Palladium . Glass without lead Platinum . Flint glass Black marble . on 682 812 ,,i,ooo 1,142 " 1^248 2,833 92 i,32 HEAT. In this table it will be noticed that glass and platinum elongate nearly the same, and this fact explains why platinum wire can be melted into glass tubes made of glass not containing lead, such as the hard German glass, without causing the tubes to crack, either by expansion or contraction, at the points where the wires are inserted. The minute linear expansion of black marble, only one on 2,833, is of course the reason why it is used as the pendulum-rod in the clock of the Royal Society of Edinburgh. Even a bar or rod of ice elongates by an increase of temperature, and is found even to surpass zinc ; the ice will elongate I part on 267, the zinc I on 323 parts. Ice will also contract when exposed to a temperature lower than its freezing-point, and the amount of contraction has been carefully observed up to 30 or 40 below the freezing-point of water. A solid may expand at a uniform rate up to a certain point, and then, if the heat is increased, the elongation becomes more rapid. Amongst metals, platinum is found to expand with the greatest uniformity, most probably in consequence of its great infusibility. Certain crystals of the same nature throughout expand very unequally in their several dimensions of length, and breadth, and height, and they are found to elongate in a greater degree in the direction of one axis than in another. Iceland spar possesses this property ; and was found, by Professor Mitscherlich, to expand in one direction, the crystallographic axis, and to contract in the other at right angles to the former, so that the anomaly of expansion and contraction in one body was apparent. Another remarkable anomaly of the same kind will be noticed presently in connection with a fluid viz., water when reduced to a certain temperature. The difference between linear, surface, and volume expansion is determined by the geometrical prin- ciple, that when a solid increases in magnitude without undergoing a change in figure, taking the linear expansion as the unit, or say 100, the superficial expansion will be twice the linear, 100x2 = 200, and cubic expansion three times the linear, 100X3 = 300. Thus the linear coefficient of expansion of glass being 0-000,008, the cubic expansion of the same will be 0-000,024 ; the dilatation of volume and surface of solids being calculated from linear expansion. THE EXPANSION OF LIQUIDS. It has already been noticed that the expansion of solids by heat is so con- trolled by the antagonistic force of cohesion, that it is not perceptible to the vision without the use of secondary means, such as those described at page 127. With liquids the amount of expansion is very perceptible when they are inclosed in narrow glass tubes, and in fact is much greater than that of solids, because the force of cohesion is diminished so much that every particle is free to move upon its neighbouring particles. Some fluids expand more than others. Alcohol is more expansible than water, and water more than mercury ; in fact, alcohol expands six times more than mercury. Messrs. Dulong and Petit employed the most refined means to ascertain the rate of, and absolute expansion of, mercury, and they found that the coefficient of the latter was 1-5550 between the freezing and boiling point of water, the rate being as follows : THE EXPANSION OF LIQUIDS. 133 From o to 100 Centigrade, mercury expands i measure, or 55^ 100 to 200 i 54^ 200 to 300 i 53 Liquid carbonic acid expands four times more than air, and, when heated from 32 to 86 Fahrenheit, 100 measures expand to 140. There are other fluids, such as liquid sulphurous acid, hyponitrous acid, cyanogen, and the chloride of ethyle, which also expand very considerably when heated. Alcohol and bisulphide of carbon expand uniformly, which is another curious fact, because their boiling-points are so different, the former alcohol being 173, the latter 116. Liquids, as a rule, expand by heat, and contract by cold. There is, however, a remarkable exception, probably more apparent than real when the theory of the expansion of liquid is better understood, that water which becomes solid in all parts of the globe at the level of the sea at 32 Fahrenheit, or of o Centigrade, expands instead of contracting when the water reaches a temperature of 40 Fahrenheit, and falls to 32 : the amount of expan- sion is not very great, being one part in ten thousand at 32. But the fact, which at first was thought illusory, is indisputable, as proved by the experiments of Dr. Hope. He placed two thermometers in a large vessel of water, the one being at the top, and the other at the bottom. Up to a temperature of 40, the cold water contracted, and, being the heavier, sank to the bottom, and the lower thermometer registered the greatest cold. After 40 was passed, the water evidently expanded ; the coldest water was found to be at the top, and duly recorded by the thermometer sinking to 32, whilst the warmer water, which ought, according to the law of expansion, to have been uppermost, remains at the bottom, and therefore was heavier, bulk for bulk, than the water about to crystallize. It is this remarkable exception that preserves the fish in the lakes and rivers. During the severe winters of Siberia the water is frozen many feet thick; but it is related by one of the exiles in this roomy but severe prison, that part of their amusement in certain seasons consisted in fishing in great holes in the ice, and all they caught they partially but imme- diately ate raw and living, biting out a piece of the back, which was declared to be a most agreeable tit- bit. It is evident that the fish, if frozen, could have no power of locomotion they must die ; so that on the arrival of winter the Siberian waters would throw up their dead fish, as all would be killed if the water, which is a very bad con- ductor of heat, did not remain at 40 at the bottom of the lakes, rivers, and seas. Bismuth is said to possess the same curious property of expanding whilst it is being cooled, and thus iron bottles filled with melted bismuth, and plugged with a screw, burst at the moment the metal assumes the solid state. A bomb-shell or cast-iron bottle filled with water, and screwed up, bursts in the FIG. 144. s experiment. i 3 4 HEAT. same manner if surrounded with a freezing mixture of pounded ice, or snow and salt. With respect to bismuth it is right to state that Professor TyndalPs conclusion on this similarity, stated in his work on " Heat as a Mode of Motion," in which it is asserted "that the anomalous expansion of water in the act of cooling below 40 Fahrenheit is by no means an isolated instance of the kind, but that other bodies, and particularly melted bismuth, participates in this extraordinary property of expanding near the point of solidification," is opposed by Mr. Alfred Tribe, who, after making experiments upon this FIG. 145. Siberian Exiles fishing. subject, considers that the analogy between water and bismuth is imperfect, since in the case of the molten metal there is no perceptible range of tem- perature through which it expands on cooling. The act of solidification is itself accompanied by an increase in bulk ; but there is no evidence of this expansion taking place prior to the act of crystallization. When the crystal- lization of any salt is exhibited on the disc with the oxy-hydrogen microscope, the visible illustration of the motion of the particles is very decided, as the crystals shoot out and interlace with each other. The act of solidification is, therefore, one of motion, and heat is produced, and very decidedly so in the case of sulphate of soda or glauber salt. A large flask filled with a satu- rated solution of sulphate of soda, and carefully closed with a cork and- bladder, so that the air is excluded, does not solidify when cold. Crystalliza- tion only begins when the air is admitted ; the solution of a minute volume of air liberates a tiny crystal, and, the nucleus once formed, cohesion sets in with rapidity ; the molecules are set in motion, and sufficient heat is produced to be felt by the hand, and becomes still more apparent if a delicate air-ther- mometer is used. India-rubber caoutchouc, when stretched and apparently expanded, becomes warm instead of cold ; is it possible to suppose that the expansion in the direction of the length may cause contraction in the trans- verse direction in the breadth, and the sum of this violent motion is in favour of the contraction, and thus heat is the result? The contraction of stretched or expanded caoutchouc by heat is another THE THERMOMETER. 135 remarkable anomaly. It was suggested by Mr. Thompson that it might shorten if heated, and the fact was proved by Tyndall, who first stretched a vulcanized india-rubber tube by a ten-pound weight, and surrounding it with hot air, the caoutchouc tube contracted and lifted the weight. In this case, motion, the stretching of the caoutchouc, eliminates heat, which again pro- duces motion when the stretched caoutchouc is warmed and lifts the weight by the contraction of its substance. The expansion of fluids by heat, and the reverse, is taken advantage of in the construction of those useful instruments called thermometers, or heat- measurers. A glass tube with a small bore, sufficiently so to be capillary, is selected with care, in order to secure the same diameter throughout. The bores of some tubes are like an elongated cone, and, if they were used, the mercury would expand much more in some parts of the tube than in others, and hence the indications of such a thermometer would be incorrect. A little mercury, amounting to an inch in length, is allowed to enter the tube, and being moved from one end of the tube to the other, it is soon discovered whether the mercury increases or decreases in length, or remains, as is usually the case, of the same linear dimensions in all parts. The proper length having been cut off, one end is melted and blown out into a bulb, the other being formed into a cup or funnel- shape form, to hold the mercury, which is forced in; the tube is now inclined slightly, and the air in the bulb expanded by heat; it is afterwards allowed to cool, and, as the air cools and contracts, the mercury from the upper funnel is forced in by the pressure of the air, and enters to supply the place of the air driven out by expansion. To get rid of the rest of the air, the mercury is alternately boiled and cooled until the bulb and part of the tube are full of mercury. Having thus filled the bulb and one-third of the tube, the next step is to seal it hermetically, which is done by heating the mercury to the boiling-point, and at the moment the mercury is overflowing at the summit the glass is fused with a flame, urged by a blow- pipe (Fig. 147), before the mercury has had time to contract ; and if this operation has been skilfully per- formed, a perfectly void space, or vacuum, free from air, is obtained as the mercury sinks or contracts in the bulb and tube. The instrument in its present state will show an increase or decrease of heat by the rising or falling of the mercury ; but such indications would be useless, and it would be impossible to compare the observations made by any two ungraduated bulbs or tubes filled with mercury in the manner already described. The graduation of the instrument is, therefore, of paramount importance, and standard-points must be obtained such, that they shall 'be the same in every thermometer, whatever may be the scale. Sir Isaac Newton enjoys the merit of having selected the temperature of FIG. 146. 136 HEAT. ice, which is dissolving and liquifying, for one point, and boiling water, emit- ting steam freely and without pressure, for the other. Ice always melts at the same temperature, and pure water invariably boils at the same temperature, when the barometer stands at 29 8 in. It is only necessary to immerse the thermometer alternately in melting ice and in boiling water, with certain precautions, and to mark the point at which the mercurial column stands one being called the freezing-point, and the other the boiling-point. The instrument must be immersed in the melting ice until the mercury becomes perfectly stationary. The immersion in boiling water requires the greatest care, and a time should be selected when the barometer stands at 29'8 or 30 in. The depth of the water in the vessel should not exceed 2 in. FIG. 147. Blow-pipe work in Negretti and Zambra's Thermometer Room. The vessel must not be a shallow one, but sufficiently deep to contain the bulb and nearly all that part of the tube up which the mercury will rise when placed in boiling water. Distilled water should be used, and brisk ebullition maintained, and the steam allowed to escape freely, as any confinement of it would raise the temperature above that of boiling water. The space or interval between the two points is now divided into any number of equal parts, which vary according to the scale used (Fig. 148.) In England, the interval according to Fahrenheit is divided into 180 parts, the zero being 32 below the freezing-point. On the Continent, the interval is divided by Celsius into 100 parts, and is called the Centigrade scale, the zero commences with the freezing-point ; sometimes into 80 parts, called Reau- mer's scale, the zero, as before, being the freezing-point of water. Of the v .hree, that of Celsius is the most simple, and will be gradually adopted throughout the civilized world. THE THERMOMETER. FlG. 148. Graduation, by a Machine, of the Tubes after the freezing and boiling points have been determined. These scales are easily reduced from one to another by ascertaining their numerical relation. Thus 1 80 is to 100 as 9 to 5 ; 1 80 80 9 4. Fahrenheit's is, therefore, reduced to the Centigrade scale by multiplying by 5 and dividing by 9 ; or to that of Reaumer by multiplying by 4 and dividing by 9. The Celsius or Centigrade and Reaumer's scales are reduced to Fahrenheit's scale by reversing the process : the multiplier in both cases being 9, the divisor will be 5 with Centigrade and 4 with Reaumer. The reduction is, however, a little complicated when it is remembered that Fahrenheit's zero is 32 below the freezing-point of water, so that in all these calculations 32 must be first subtracted when Fahrenheit is reduced to Centi- grade or Reaumer, and added when the contrary is required. ist Example. To reduce 212 Fahrenheit to Centigrade: 212 32=i8oX 5 =900-^9= 1 00 C. 2nd Example. To reduce 100 Centigrade to Fahrenheit: iooX9'=9oo-^ 5 = I 8o +32 =2i2F. The freezing-point of water is therefore designated and known in all books by the following expressions : o C, o R., 32 F. The boiling-point of water, by 100 C., 80 R., 112 F. ; C. being Centigrade, R. Reaumer, and F. Fahrenheit. The limits to the use of the mercurial thermometer are the points at which the metal solidifies, or is frozen, viz., at 39 below zero F., or at which the metal boils* 662 F., or 450 above the boiling-point of water; hence in the one case degrees of extreme cold are registered by thermometers filled with 138 HEAT. alcohol, which has never been known to freeze at the greatest known cold ; and, in the other case, all temperatures above 662 may be registered, to a certain point, by the air-thermometer ; but all temperatures which soften glass and go beyond that point can be estimated only by the pyrometer. The air- thermometer will be explained in treating of the expansion of gases ; and in ending the description of the ordinary mercurial thermometer, it may be stated that the bulbs are liable to a permanent change of capacity, which displaces the zero ; hence it is usual to keep standard thermometers three or four years before they are graduated. Thermometers are constructed for a variety of purposes, and have, there- fore, different names given to them. In illustration of this statement, we give a drawing of Negretti and Zambra's maximum thermometer for registering the highest daily temperature of the air, or degree of heat at any particular hour of the day FIG. 149. The Maximum Thermometer , the construction of which is as follows : A small piece of glass is inserted in the bend near the bulb, and within the tube, which it nearly fills : at an increase of temperature, the mercury passes this piece of glass ; but on a decrease of heat, not being able to recede, it remains in the tube, and thus indicates the maximum temperature. After reading, it is easily adjusted. Hitherto every series of meteorological observations has been more or less broken by the frequent plunging of the steel index into the mercury, or be- coming otherwise deranged. Messrs. Negretti and Zambra have, in their maximum thermometer, supplied a want long felt. FIG. 150. The Alcohol or Minimum Thermometer consists of a glass tube, the bulb and part of the bore of which is filled with perfectly pure spirits of wine, in which floats freely a black glass index. A slight elevation of the thermometer, bulb uppermost, will cause the glass index to flow to the surface of the liquid, where it will remain, unless violently shaken. On a decrease of temperature, the alcohol recedes, taking with it the glass index ; on an increase of temperature, the alcohol alone ascends in the tube, leaving the end of the index furthest from the bulb indicating the minimum temperature. THE PYROMETER. 139 Directions for using the Minimum Thermometer for determination of the Minimum Temperature of the Air. Having caused the glass index to flow to the end of the column of spirits, by slightly tilting the thermometer bulb upper- most, suspend the instrument in the shade, with the air passing freely to it on all sides, by the two brass plates attached for that purpose, in such manner that the bulb is about half an inch lower than- the upper, or the end of the thermometer furthest from the bulb, then, on a decrease of temperature, the spirits of wine will descend, carrying with it the glass index ; on an increase of temperature, however, the spirits of wine will ascend in the tube, leaving that end of the small glass index furthest from the bulb indicating the minimum temperature. To re-set the instrument, simply raise the bulb end of the ther- mometer a little, as before observed, and the index will again descend to the end of the column, ready for future observation. The same instrument may be used as a terrestrial radiation thermometer, and when in use is to be placed with its bulb fully exposed to the sky, resting on grass, with its stem supported by little forks of wood. By no means jerk or shake an alcohol minimum thermometer when re-setting it, for by so doing it is liable to disarrange the instrument, either by causing the index to leave the spirit, or by separating a portion of the spirit from the main column. As alcohol thermometers have a tendency to read lower by age, owing to the volatile nature of the alcohol allowing particles in the form of vapour to rise and lodge in the tube, it becomes necessary to compare them occasionally with a mercurial thermometer whose index error is known ; and if the differ- ence be more than a few tenths of a degree, examine well the upper part of the tube to see if any alcohol is hanging in the bore thereof ; if so, the detached portion of it can be joined to the main column by swinging the thermometer with a pendulous motion, bulb downwards. THE PYROMETER. One of the most celebrated contrivances for estimating high temperatures was that of Mr. Wedgwood; but, as the indications depended on the con- traction of clay cylinders, which will contract as much by the long continuance of a comparatively low heat as by a short continuance of a high one, they were enormously exaggerated, and could not be correct. The late Professor Daniell improved greatly upon Wedgwood's instrument, and, by using the linear expansion of bars of metal, arrived much nearer to a correct estimate of temperatures above a dull red heat. Daniell calls his instrument the register pyrometer, and describes it as follows : " It consists of two parts, which may be distinguished as the register and the scale. The register is a solid bar of blacklead earthenware, highly baked. In this a hole is drilled, into which a bar of any metal, six inches long, may be dropped, and which will then rest upon its solid end. A cylindrical piece of porcelain, called the index, is then placed upon the top of the bar, and confined in its place by a ring or strap of platinum passing round the top of the register, which is partly cut away at the top, and tightened by a wedge of porcelain. When such an arrangement is exposed to a high temperature, it is obvious that the expansion of the metallic bar will force the index forward to the amount of the excess of its expansion over that of the blacklead, and that, when cooled, it will be left at the point of greatest elongation. What is now required is the measurement of the HEAT. distance which the index has been thrust forward from its first position ; and this, though in any case but small, may be effected with great precision by means of the scale. " This is independent of the register, and consists of two rules of brass accurately joined together at a right angle by their edges, and fitting square upon two sides of the blacklead bar. At one end of this double rule a small plate of brass projects at a right angle, which may be brought down upon the shoulder of the register, formed by a notch cut away for the reception of the index. ** A movable arm is attached upon this frame, turning upon its fixed extremity upon a centre, and at its other carrying an arc of a circle, whose radius is FIGS. 151, 152. exactly 5 in., accurately divided into degrees and thirds of a degree. Upon this arm at the centre of the circle another lighter arm is made to turn, one end of which carries a nonius with it, which moves upon the face of the arc, and subdivides the former graduation into minutes of a degree ; the other end crosses the centre, and terminates in an obtuse steel point, turned inwards at a right angle. " When an observation is to be made, a bar of platinum or malleable iron is placed in the cavity of the register ; the index is to be pressed down upon it, and firmly fixed in its place by the platinum strap and porcelain wedge. The scale is then to be applied by carefully adjusting the brass rule to the sides of the register, and fixing it by pressing the cross piece upon the shoulder, and placing the movable arm so that the steel point of the radius may drop into a small cavity made for its reception, and coinciding with the axis of the metallic bar. " The minutes of the degree must then be noted, which the nonius indicates upon the arc. A similar observation must be made after the register has been THE PYROMETER. 141 exposed to the increased temperature which it is designed to measure, and again cooled, and it will be found that the nonius has been moved forward a certain number of degrees or minutes, as shown at Figs. 151 and 152." Fig. 151 represents the register; A is the bar of black lead; a the cavity for the reception of the rnetallic bar; cc 1 is the index, or cylindrical piece of porcelain ; d, the platinum band, with its wedge, e. "Fig. 152 is the scale by which the expansion is measured: f is the greater rule, upon which the smaller, g, is fixed square. The projecting arc h is also fitted square to the ledge under the platinum band d. D is the arm which carries the graduated arc of the circle E, fixed to the rule f, and movable upon the centre i. C is the lighter bar fixed to the first, and moving upon the centre k. H is the nonius at one of its extremities, and m the steel point at the other. The rule^ admits of adjustment ony^ so that the arm h may be adjusted to the centre z, in order that at the commencement of an experiment the nonius may rest at the beginning of the scale. The term " nonius," used by Daniell, is only another name for vernier, a contrivance for measuring intervals between the divisions of graduated scales on circular instruments. The scale of this pyrometer is readily connected with that of the thermo- meter by immersing the register in boiling mercury, whose temperature is as constant as that of boiling water, and has been accurately determined by the thermometer. The amount of expansion for a known number of degrees is thus deter- mined, and the volume of all other expansions may be considered as propor- tional. The melting-point of cast iron has been thus ascertained to be 2786, and the highest temperature of a good wind-furnace about 3300 points which were estimated by Mr. Wedgwood at 20,577 and 32,277" respectively. Mr. Wedgwood, indeed, makes an observation which is calculated to throw suspicion upon the accuracy of his results ; for he says, " We see at once how small a portion (of the rays of heat) is concerned in animal and vegetable life, and in the ordinary operations of nature. From freezing to vital heat is barely i -5ooth part of the scale a quantity so inconsiderable relatively to the whole that in the higher stages of ignition ten times as much might be added or taken away without the least difference being discoverable in any of the appearances from which the intensity of fire has hitherto been judged of.". Now this, remarks Daniell, "is utterly unlike the gradual progression by which the operations of nature are generally carried on ; and the fact is, that a regular transition may be traced from one remarkable point of temperature to another." Thus from the freezing of water, 32, to vital heat in man is 60. 60 X 3= 1 80 Boiling water. 60 X 7= 420 Melted tin. 60X10= 600 Boiling mercury. 60 X 15= 900 Red heat. 60x31 = 1860 Melting silver. . 60x45=2700 Melting cast iron. 60 x 5 5=13300 Highest heat_ of wind-furnace. Before the invention of the register pyrometer, the expansion of solids had never been ascertained beyond the temperature of 527 : the following I 4 2 HEAT. table exhibits the progressive amount of several metals to their point of fusion, as determined by Daniell's pyrometer: PROGRESSIVE DILATATION OF SOLIDS. One millon parts at 62 . At 212. At 662. At Fusing-point. Blacklead ware . 1,000,244 I,OOO,7O3 Wedgwood ware . 1,000,735 1,002,995 Platinum 1,000,735 1,002,995 1,009,926 (maximum, but not fused). Iron, wrought 1,000,984 1,004,483 1,018,378 to the fusing-point of cast iron. Iron, cast . 1,000,893 1,003,943 1,016,389 Gold . 1,001,025 1,004,238 Copper 1,001,430 1,006,347 1,024,976 Silver . 1,001,626 1,006,886 1,020,640 Zinc . 1,002,480 1,008,527 1,012,621 Lead . 1,002,323 1,009,072 Tin ... 1,001,472 ... 1,003,798 Professor Daniell concludes his dissertation by the following passage, which is quite in accordance with those notions which Tyndall has so ably contended for viz., that heat is a mode of motion: "The amount of the force which produces these expansions and contractions, measured by any oppo- sing force, that of cohesion, for instance, is enormous. " Some idea may be formed of it, when it is understood that it is equal to the mechanical force which would be necessary to produce similar effects in stretching or compressing the solids in which they take place. Thus, a bar of iron heated so as to increase its length a quarter of an inch, by this slow and quiet process exerts a power against any obstacle by which it may be attempted to confine it, equal to that which would be required to reduce its length by compression to an equal amount. On withdrawing the heat, it would exert an equal power in returning to its former dimensions." M. Molard used this great moving force to restore the walls of a building to the perpendicular which had been bulged, and the same principle was used at the Cathedral of Armagh. THE EXPANSION OF GASES. We now come to the most expansible bodies viz., the gases; and, although at first there was considerable doubt whether they all expanded alike, because the experimentalists had neglected to remove the moisture the aqueous vapour from them, it was finally discovered, not only by Gay-Lussac in Paris, but by our own countryman, the illustrious Dr. Dalton, that all gases expand alike with the same amount of heat, and that the rate of dilatation continues uniform for all temperatures. In discovering the expansibility THE EXPANSION OF GASES. of liquids it was found that cohesion was not quite overcome, and that there was still a considerable amount of that force which tended to keep the par- ticles in contact. This, however, is not the case with gases ; the cohesive power is for the time completely overcome by the motion of heat. Sir H. Davy speaks emphatically upon this motion in his " Chemical Philosophy." " It seems possible to account for all the phenomena of heat, if it be supposed that in solids the particles are in a constant state of vibratory motion, the particles of the hottest bodies moving with the greatest velocity and through the greatest space; that in fluids and elastic fluids, besides the vibratory motion, which must be conceived greatest in the last, the particles have a motion round their own axes with different velocity, the particles of elastic fluids (gases) moving with the greatest quickness ; and that in ethereal sub- stances the particles move round their own axes, and separate from each other, penetrating in right lines through space. Temperature may be conceived to depend upon the velocity of the vibration, increase of capacity in the motion being performed in greater space ; and the diminution of temperature during the conversion of solids into fluids or gases may be explained on the idea of the loss of vibratory motion in consequence of the revolution of particles round their axes at the moment when the body becomes fluid or aeriform, or from the loss of rapidity of vibration in consequence of the motion of the particles through space." It has been proved that gases expand by i-49oth of their own volume lor every degree o Fahrenheit's scale between the freezing-point, 32, and the boiling-point of water, 212, and so on at higher or lower temperature, pro- vided the pressure of the air remains the same. If the Centigrade scale is used, the ratio of expansion of any gas will be 1-27 3rd of its volume for every degree. 490 cubic inches of air at 32 become 491 at 33 491 33 492 34 492 34 493 35 From a most careful series of experiments it has been determined that " the coefficient of expansion " of all gases, expressed in decimals, is 0-00,366. These figures are near enough for all ordinary calculations, although it must be observed that, speaking rigidly, this is not exactly the 'case, except probably with the three permanent gases, oxygen, hydrogen, and nitrogen, in all the other gases and vapours the expansion being greatest for those which are most readily condensible. M. Regnault has made the most elaborate and careful experiments, and determined that one thousand volumes of certain gases at o C. or 32 F. (the pressure of the air remaining unchanged) become expanded in the fol- lowing proportions when heated to 100 C., or 212 F. : Air . Carbonic acid Carbonic oxide Cyanogen . 1,367-06 1,370-99 1,366-88 1,387-67 Hydrogen . . . 1,366-13 Hydrochlorine acid . 1,368-12 Nitrogen . . . 1,366-82 Nitric oxide . . i,37i'95 It will be apparent that hydrogen expands the least, and, as might be expected, cyanogen, which is liquified with comparative ease, is much higher viz., 1,387*67. It is, therefore, apparent that if the coefficient of expansion remains the same with all gases, that cyanogen should have been represented 144 HEAT. by the same figures as those which belong to air instead of being 0*00,387 to 0-00,367 atmospheric air. The conversion of this property of expansion into power or motion is well described by Tyndall : " Suppose I have a quantity of air contained in a very tall cyliYider (A B, Fig. 153), the transverse section of which is one square inch in area. Let the top, A, of the cylinder be open to the air, and let P be a piston, which, for reasons to be explained immediately, I will suppose to weigh two pounds one ounce, and which moves air-tight and without friction up or down in the cylinder. At the commencement of 'the experiment let the piston be at the point P of the cylinder, and let the height of the cylinder from its bottom B to the point P be 273 inches, the air underneath the piston being at a tem- perature of o C. Then, on heating the air from o to i C, the piston will rise one inch; it will now stand at 274 inches above the bottom. If the temperature be raised two degrees, the pis- ton will stand at 275 ; if raised three degrees, it will stand at 276; if raised ten degrees, it will stand at 283; if 100 degrees, it will stand at 373 inches above the bottom; finally, if the tem- perature were raised to 273 C, it is 'quite manifest that 273 inches would be added to the height of the column ; or, in other words, that by heating the air to 273 C. its vohime would be doubled. -The gas in this experiment executes work. In expand- ing from P upwards, it has to overcome the downward pressure of the* atmosphere, which amounts to 1 5 Ibs. on every square inch, and also the weight of the piston itself, which is 2 Ibs. I oz. Hence, the section of the cylinder being one square inch in area, in expanding from P to P' the work done by the gas is equivalent to the raising a weight of 17 Ibs. I oz., or 273 ounces, to a height of 273 inches. It is just the same as what it would accomplish if the air above P were entirely abolished, and a piston weighing 17 Ibs. i oz. were placed at P. " Let us now alter our mode of experiment, and, instead of allowing our gas to expand when heated, let us oppose its ex- pansion by augmenting the pressure upon it ; in other words, let us keep its volume constant while it is being heated. " Suppose, as before, the initial temperature of the gas to be o C., the pressure upon it, including the weight of the piston P, being as formerly 273 ounces. Let us warm the gas from o C. to i C. ; what weight must we add at P in order to keep its volume constant ? Exactly one ounce. " But we have supposed the gas at the commencement to be under a pres- sure of 273 ounces, and the pressure it sustains is the measure of its elastic force ; hence, by being heated i, the elastic force of the gas has augmented by i-273rd of what it possessed at o. If we warm it 2, two ounces must be added to keep its volume constant ; if 3, three ounces must be added ; and if we raise its temperature 273, we should have to add 273 ounces, that is, we should have to double the original pressure to keep its volume constant. "In the first case marked out, it is shown that by heating the air to 273 C. its volume would be doubled. In the second, that by compressing the air with 273 ounces we may heat it to 273 C., and have, consequently, double the FIG. 153. THE EXPANSION OF GASES. original pressure to keep the air confined to the same volume. In fact, trie- volume being kept constant, the elastic force is doubled. ^ " But are the absolute quantities of heat imparted in both cases the same ? By'no means. Supposing that to raise the temperature of the gas, whose volume is kept constant, 273, ten grains of combustible matter are necessary; then to raise the temperature of the gas, whose pressure is kept constant, an equal number of degrees would require the combustion of 14^ grains of the same combustible matter. The heat prodttced by the combustion of the addi- tional 4^- grains in the latter case is entirely consumed in lifting the weight. Using the accurate numbers, the quantity of heat applied when the volume is constant is, to the quantity applied when the pressure is constant, in the pro- portion of i to 1*421. " This extremely important fact constituted the basis from which the mechanical equivalent of heat was first calculated." Various methods have been contrived to determine the amount of expansion of gases when subjected to a uniform pressure, and one of the most simple is that of Monsieur Pouillet (Fig. 154), described by Lardner. "An iron syphon tube, D c, is formed with short legs, from the bottom of which proceeds a pipe with a stop-cock F, under which is placed a cistern or reservoir G. In the legs of the syphon D c are inserted two glass tubes, D E and C B, of more than thirty inches in height. The tube D E is open at the top ; the tube C D is closed at the top, ' but has a horizontal branch united to it, at B, which is connected with a tube, A B, made of platinum, which terminates in a hollow globe or ball, A, also made of pla- tinum. In the tube B A is fixed a stop- cock in order to communicate at pleasure with the atmospheric air. " The stop-cock F being closed, and the stop-cock in the tube B A being open, mer- cury is poured into the tube D E, so as to fill the glass tubes D E and C B nearly to the top. Since the tubes D E and C B both communicate with the external air, the columns of mercury in them will stand at the same level. " To determine the expansion which air FlG - J 54. Pouillet s Apparatus. suffers when raised from the freezing-point to the boiling-point under uniform pressure, let the ball A be immersed in a bath of melting ice, so as to reduce the air included in it to the freezing- point. Let the stop-cock in the tube B A be then closed, and let the bulb A be removed to a bath of boiling water. The air in the bulb, expanding, will press down the column of mercury in B c, and will cause the column in D E to rise ; so that the levels of the two columns will no longer coincide. But they may be equalized by opening the stop-cock F, and allowing mercury to flow into the reservoir G from the syphon until the levels in the two legs come to the same point. When that is accomplished, the pressure upon the expanded 10 146 HEAT. air included in the bulb A and the tube communicating with it will be equal to that of the atmosphere, and equal to that which the same air has when at the freezing-point. " The capacity of the tube C B being known, the volume which corresponds to any length of it will be also known ; also the increment of volume which the air has suffered by expansion will be indicated by the height through which the mercury has fallen in the tube C D. This increment, therefore, will be the dilatation of the air included in the bulb A and the communicating tube between the freezing and the boiling points. In the same manner, by this apparatus, the dilatation corresponding to any change whatever of tem- perature under a given pressure can be ascertained." The expansion of air by heat, and the uniformity with which it takes place, suggested at a very early period of science the use of air-thermometers, which are the most delicate and, with certain precautions, the most reliable in certain cases where high temperatures have to be determined. The first is supposed to have been constructed by a learned Italian physician, named Sanctorius, about the year 1590. It is sometimes attributed to Cornelius Drebel, who in- troduced it in the year 1610; but this is a mistake. Drebel followed Sanctorius, and therefore cannot be the first inventor, although there is every reason to suppose that he made his air-thermometer in perfect ignorance of what Sanctorius had already done. The construction is very simple : it consists of a glass tube at the end of which a bulb or ball is blown; this tube, with its ball, is then fitted into some conve- nient glass vessel or bottle, containing a little coloured water. On the application of heat, either from the palm of the hand or the flame of a spirit-lamp, a por- tion of the air in the tube is expelled, and, when cold, the water ascends to fill its place ; the rise or fall of this column of coloured water by the expansion or contraction of the air in the bulb is supposed to indi- cate the difference of temperature. It was soon discovered that this air-thermometer was not correct in its indications, and was, in fact, affected by the pressure of air : when the barometer fell, the air expanded in the bulb, and the coloured fluid was driven downwards; or, on the contrary, if the barometer rose, the air, contracted by the increased pressure on the liquid, was pushed higher up the tube. Sir John Leslie greatly improved upon the rude appa- ratus already described, and invented a very elegant in- FlG. 155. strument, called the Differential A ir Thermometer (Fig. The Air-Thermometer 156), which has been of the greatest use in the refined of Sanctorius. experimental researches made for the elucidation of the more obscure properties of the force called heat. It consists of two glass bulbs or balls connected together by a tube bent twice at right angels. The balls contain air, and, just before they are her- metically closed, a little sulphuric acid, coloured with carmine, is introduced, so that it rises to about half the height of the two tubes bent at right angles. CONDUCTION. 147 The ball left open for the introduction of the coloured fluid is now finally closed, and as both bulbs must be equally affected by changes of tem- perature in the surrounding air, the liquids in the tubes remain in equilibrium. If, however, one of the balls is grasped by the hand, the air expands, and the fluid is driven up the other tube, which is provided with a proper scale ; thus at any moment, by placing one ball in a particular spot where heat is to be discovered, the expansion of the air becomes a most sensitive and delicate means of appreciating any small amount of heat. TIG. 156.- Leslies Differ- ential Thermometer. FlG. 157. Differential Tfiermometer used.to discover Focus of Heat Rays. CONDUCTION. Our ideas of this property of heat, of travelling along and through material substances, are quickly tormed and put in practice. If a bar of iron and a rod of glass are thrust between the bars of a grate containing burning fuel, we soon learn which we may first touch or take out with impunity. The iron rapidly becomes so hot throughout its length and breadth, that we cannot lay hold of it ; the glass rod may be quite softened within a few inches of the hand, and yet the heat is not sensibly felt or becomes so great as to prevent the rod of glass being held in the hand : in the one case there appear to be regular stepping-stones across which the heat may, as it were, take its way ; in the other there is no regular path provided, and the travelling power of the heat is interfered with, and so greatly impeded that a considerable time must elapse before any sensible progress or travelling of the heat can be recorded. Thus in early days the wise men of the period rudely divided all substances into conductors and non-conductors of heat. Such a division, however, is not in accordance with nature ; there are intermediate conditions of conductivity, and thus we come to speak of good and bad conductors of heat. 102 148 HEAT. In regarding heat by the dynamical theory, the student can have no diffi- culty in understanding that the position of the solid substance under exami- nation in the list of good or bad conductors must depend greatly upon its physical structure. The metals are good conductors ; there is uniformity of internal structure, and the vibratory movement necessary to set the heat- waves in motion is regular and not interfered with ; moreover, the particles are in close contact. Glass is a bad conductor, because those conditions which are necessary for the setting up of molecular motion are not fulfilled ; the vibrations are not communicated steadily from molecule to molecule, but broken up and thrown into confusion ; the glass has no regular molecular homo- geneity it is too heterogeneous. Any substance which can transmit molecular motion is a good conductor of heat, and those bodies which do not transmit this motion readily are bad conductors. FIGS. 158 and 159. Griffith? experiment. The difference oetween the conducting power of a metal, an earth, and art earthy compound may be illustrated by the following simple and instructive experiment : * Provide solid cylinders of these three materials, viz., iron, sandstone, and chalk ; let these be i in. in diameter and 6 in. long, and perfectly flat at each of their ends. Place a cup, containing an ounce of tallow, upon the warm hob of the grate; and when the tallow is perfectlymelted, dip into it for about half an inch one end of the iron cylinder, and then lift it out ; a portion of tallow will adhere,, and quickly become solid, because the iron, by good conducting power, deprives it of the heat of fluidity. Dip one end of each of the other cylinders in the same way; they will attract or absorb a considerable portion of the melted tallow, and some time will be required before it will become equally solid with that on the iron cylinder, because sandstone and chalk have not sufficient conducting power to deprive it of heat in a similar degree. Dip the end of all three cylinders again, and lift them out, and, when the tallow becomes solid, dip them again, and lift them out until they have all obtained an equal coating of tallow ; then allow them to cool. Pour boiling water into a " hot- water plate," and place the three cylinders to stand upon it at equal distances, with their coated ends uppermost, as shown in Fig. 158. * " Chemistry of the Four Seasons," Griffiths. CONDUCTION. 1 49 In the course of a few minutes, the iron will again prove its good conducting power by melting the tallow ; but the sandstone and chalk will prove their bad conducting power by the tallow remaining solid during the whole time that the water is cooling down to common temperature. By reversing the arrangement of the last experiment, namely, by applying heat above, instead of beneath, the cylinders, it can be proved that neither the conducting power of the iron nor the non-conducting power of the sand- stone and chalk are in the least degree affected or modified. Let the iron cylinder be again coated with tallow, but pare away all from its circular extremity, that it may now stand firmly upon this, and have only a ring of tallow, about half an inch wide, around its circumference ; do the same with the cylinders of sandstone and chalk; then set the three at equal distances within a circle similar in diameter to the bottom of the hot-water plate, that they may form a tripod for its support (this arrangement must be made upon a steady table) ; then remove the plate, without disturbing the cylinder, fill it with boiling water, and carefully replace it to stand upon them, .as represented in Fig. 159. The three cylinders will now be subjected to heat applied from above, instead of from below, as in the last experiment (Fig. 158); but this arrangement will -cause no difference in their conducting power, or non-conducting power, as will be proved in the course of a few minutes by the ring of tallow melting from the iron cylinder, whilst that upon each of the other cylinders remains solid as before. Starting with gold, and taking it as the type of a good conductor, and .giving it the first place in a scale amounting to 100, we have the following tabulated results obtained by Franklin and Igenhausz, by watching the rate at which wax was melted at the end of bars of Gold 100*00 Platinum .... 98*10 Silver . . . . . 97*30 Copper 89*82 Iron 37-41 Tin . . . . . 30*38 Lead 17*96 Marble ..... 2*34 Porcelain .... 1*22 Brick-earth . . . . 1*13 Zinc 36*37 The metals are evidently the best conductors ; but even these differ remark- ably, gold being 100, whilst lead has not one-fifth of the conducting property and power of transmitting molecular motion possessed by the first-named metal. Brick-earth is constituted of a number of distinct bodies; it is a mechanical mixture of a variety of compounds, each of which has an exact chemical composition. The particles are not only different from each other, but are widely apart ; the substance is of a porous nature. Asbestos, pumice- stone, charcoal and especially animal charcoal sand, are all porous, and well-known bad conductors, so much so that a red-hot ball of iron can be held in the hand for a certain time, provided a layer of either of the above- named substances intervene between the skin of the hand and the heated metal. By a more careful mode of experimenting, the conductivity of the various metals has been determined by Despretz, Wiedemann, and Franz. In this table it will be seen that silver occupies the first place, instead of gold, which is third. Platinum, again, which stands second in the first table, is very low down in the scale of conductivity ; and bismuth is the lowest of all. Silver . ICO Iron Copper Gold . 74 53 Lead Platinum 24 German silver . Tin . K Bismuth . HEAT. 9 8 6 2 Franklin and Igenhausz must therefore have committed some gross errors in their experiments, or the second table quoted here is wrong. Dr. Tyndall explains the cause of the difference with a very pretty experi- ment. He takes a short prism of bismuth, and another of iron, of the same size, and having coated the extremities with wax, they are both placed on the lid of a vessel filled with boiling water. Strange to say, the wax on the bismuth melts first, although it has six times less conductivity than iron. Here is a para*dox which requires explanation, and shows why the experiments conducted by Franklin and Igenhausz cannot agree with those of more modern physicists. In the first place, the test of conductivity employed by the earlier experimenters was the rapidity with which the wax and tallow coating a bar of any given substance melted in comparison with another just as Tyndall used the prisms of bismuth and iron. In the second place, the mode of experimenting employed by Despretz was not simply a determination of the rapidity with which the thermometer inserted in the bar was affected, as shown in a 5 c d e f illli FlG. 1 60. Desprettfs Mode of determining the Conductivity of Metals; A, the bar containing the thermometers, a, b, c, d, e,f; B, glass supporting A; D, the spirit-lamp. but he waited until the bar showed a stationary condition of heat, and the thermometers no longer continued to rise, and, by estimating the difference between each thermometer, he soon discovered that the best conductors produced the least amount of difference between the thermometers, and that the worst conductor gave the contrary result. Why did he wait until the heat of the bar became stationary ? To avoid the error caused by the difference of " specific heat," which varies with every substance. This difference is readily explained by the following experiments : A pint of water at 50 F. mixed with a pint at 100 F. will amount to a quart, which will have a mean temperature of 75 F. CONDUCTION. 151 50 F. 100 F. 2)150 Here the molecules are exactly the same ; it is water mixed with water, and the particular heat required to raise any given bulk to a certain temperature cannot alter. If, however, a pint of water at 100 F. is mixed with a pint of mercury at 40 F., the resulting temperature is not the mean, 70, but 80; the water has only fallen 20, whilst the mercury has risen 40. The 20 of heat from the water has been sufficient to heat the mercury 40. Hence it is apparent that mercury has a less " capacity for heat" (keeping to old expres- sions) than water, and it requires a smaller amount of heat to raise it to a given temperature, viz., 80. For the term, " capacity of heat," or " specific heat," substitute, according to the dynamical theory, the term, "power to get into molecular motion," or " capacity for molecular motion." We may once more return to Tyndall's paradox with the bismuth and iron. The "capacity for heat," or "specific heat," of iron is o'lisS; that of bismuth is only 0*0308 : like the mercury and the water experiment, it takes less heat to warm any given mass of bismuth than it does to heat an equal bulk of iron. The molecular motion which can be set up in bismuth occurs much quicker than it does in iron : one might almost say that the " inertia of heat " in iron was greater than that of bismuth. But this inertia once overcome, and each metal transmitting all the molecular motion which can be conferred from the vessel containing the boiling water, it will soon be found, according to the table quoted by Tyndall, that iron transmits six times more vibratory power, or motion of heat, than bismuth; it has less power to get into molecular motion than bismuth, but, once in motion, it sends vibration after vibration from molecule to molecule, and soon outstrips the bismuth in the race of conductivity. In this place it is desirable to speak of certain terms which have arisen and are used in conformity with the dynamical theory of heat. i. "POTENTIAL" FORCE. Potential force may be defined as a power waiting and ready to be used ; " the sword of Damocles suspended by a hair ;" the giant standing motion- less, but capable, at the word of command, of exerting great physical power. It is, in short, stored-up energy the gold in the bank cellars, potential, but not in circulation or use. Substitute for the word " force " heat, and you have potential heat. 2. "ACTUAL" FORCE, OR "ENERGY." As the first was dormant or passive, the second is " actual " or real, and makes itself apparent the hair broken, the sword in the act of descending. They are mutually convertible : as actual heat appears, potential heat is used up and disappears. You cannot store gold in a cellar and use it at the same time. The stored gold would represent potential heat ; the gold in use or circula- 152 HEAT. tion, actual heat. A country in a state of peace would have gold stored, and ready to pay an army ; but the latter, once formed and in actual service, must be paid ; and as the army becomes active, the potential energy the gold- disappears. One pound of hydrogen and eight pounds of oxygen contain potential energy which is enormous ; when they unite, they form nine pounds of water, and the mechanical value of the heat, or actual energy, set free is equivalent to a force that would raise forty-seven millions of pounds weight one foot high. The change of one pound of hydrogen, by combination with eight pounds of oxygen, into nine pounds of water would be an example of "chemical action." Action and reaction are equal, but contrary; and therefore Dr. Odling's admirable lecture " On Reverse Chemical Action," delivered before the last FIG. 161. A, the Hask of water boiled by spirit-lamp, and delivering steam to the platinum tube B, coiled round and placed in a hollow made in a firebrick, and subjected to the intense heat of the oxy-hydrogen blow, pipe c. D, small pneumatic trough and tube for collection of the two gases, oxygen and hydrogen, meeting of the British Association, held at Norwich, is most welcome, because it supplies the reasoning for the opposite effect viz., the conversion of "actual energy, or heat," into potential energy. By passing the vapour of water through a spiral platinum tube, made white- hot by the oxy-hydrogen flame, the vapour is divided again into its elements, oxygen and hydrogen. This beautiful experiment, so worthy of the author of the " Correlation of the Physical Forces," Professor Groves, is shown at Fig. 161. The platinum tube has no power to unite with the oxygen or the hydrogen ; it is simply the vehicle for the application of the intense heat of the oxy- hydrogen blowpipe. The potential energy of the mixed gases produces actual energy or heat, and the latter again stores up potential energy by the repro- duction of hydrogen and oxygen. Nothing can be more perfect as a train of experimental reasoning, or more decidedly illustrate the conversion of poten- tial into actual energy, and vice versa. It is a true illustration of " conserva- tion of energy," and enables the student to realise the magnificent principle which destroys nothing, nor admits the destruction of anything, because through- out the universe the sum of these two energies, called "potential" and CONDUCTION. 153 " actual," is equal. The conclusion of Dr. Odling's brilliant address, " On Reverse Chemical Action," admirably expresses these grand truths : " Reverse chemicaliActions are those which do not take place of themselves, but only by the appfication of some external force or agency, which force becomes as it were stored up in the product of the reaction ; in other words, it is attended by a conversion of potential into actual energy. It is an instance of winding up, and not of running down. Direct chemical action takes place of itself by virtue not of an innate tendency of the bodies, which acts, but of an energy which has been put into the bodies at some time or other ; it takes place of itself, and is attended by the liberation of pent-up forces contained in the reacting bodies, in other words, it is attended by a conversion of potential into actual energy. Every direct chemical combination has been preceded by some reverse chemical action, just as the falling down of a weight has been preceded by the winding of it up. When we consume wood and coal in our fires, or bread and wine in our bodies, we merely effect a combination whereby their potential is converted into actual energy, this potential energy having been stored up in them at the period of their formation ; this energy being, in fact, the robbing of the sun's rays, and the storing up the heat of these rays in these articles of fire and fuel. Under the action of the sun's rays the de- composition is effected of the carbonic acid and water into oxygen gas, restored to the atmosphere, and carbon-hydrogen, which is accumulated in the vegetable tissue. When we burn these tissues in our fires or bodies, we are simply restoring in the form of actual energy the potential heat of the sun's rays or its mechanical equivalent. We have all read of the Bourgeois Gentilhomme who had been talking prose all his life without knowing it. We have all our lives, and some of us without knowing it, been realising that celebrated problem of extracting sunbeams from cucumbers." It should be mentioned that Wiedemann and Franz did not employ ther- mometers ; they used a more refined arrangement with the thermo-electric pile and galvanometer needle a most delicate measurer of heat, which will be more fully explained presently. Wool, chalk, stone, fire-clay, ivory, are all bad conductors of heat." Asbestos, powdered pumice-stone, charcoal, saw- dust, and snow are still worse conductors of heat. The subdivision and pulverization of the substance increase porosity, and decrease conductivity. The wool and fur of animals, the plumage of birds, and especially the down (made into eider-down quilts), are all good examples of the wondrous care with which a superintending Creator has foreseen the various wants of the animal kingdom, and protected them even against the vicissitudes of tem- perature. The kettle-holder made of wool, the pieces of ivory which break the metallic communication between the good-conducting silver teapot and its handle and the soot charcoal covering the bottom of a kettle, which allows the vessel to be taken direct from the fire and, though full of boiling water, held upon the palm of the hand, are good and familiar examples of the application of bad conductors. One of the most interesting novelties displayed in the department devoted to Norway, in the French Exhibition of 1867, was the Self-acting Norwegian Cooking Apparatus,, constructed in the most simple manner, of a wooden box lined with four inches of felt, in which the saucepans containing the food, previously boiled and maintained at the boiling-point for five or ten minutes, according to the nature of the food to be cooked, are placed. The heated 154 HEAT. saucepans are covered with a thick felt cover, and, the lid of the box being fastened down, the rest of the cooking is done by slow digestion, no more heat being added. The heated vessels containing the food will retain a high temperature for several hours, so that a dinner put into the apparatus at 8 in the morning would be quite hot and ready by 5 in the afternoon, and would keep hot up to 10 or 12 at night, because the felt clothing so completely prevents the escape of the heat ; and as the whole is enclosed in a box, there are no currents of air to carry off any other heat by convection. FlG. 162. The Norwegian Self -Acting Cooking Apparatus. A, the box, lined with felt; B B, saucepans fitting into box; c, the felt cover to be placed on the top of the saucepans. The principle on which this cooking apparatus acts is that of retaining the heat ; and it consists of a heat-retainer or isolating apparatus shaped somewhat like a refrigerator, and of one or more saucepans or other cooking-vessels made to fit into it. Whereas in the ordinary way of cooking the fire is neces- sarily kept up during the whole of the time required for completing the cooking process, the same result is obtained, in using this apparatus, by simply giving the food a start of a few minutes' boiling, the rest of the cooking being com- pleted by itself in the heat-retainer away from the fire altogether. Directions for use. Put the food intended for cooking, with the water or other fluid cold, into the saucepan, and place it on the fire. Make it boil, and when on the point of boiling skim if required. This done, replace the lid of the saucepan firmly, and let it continue boiling for a few minutes. After the expiration of these few minutes, take the saucepan off the fire, and place it immediately into the isolating apparatus, cover it carefully with the cushion, and fasten the lid of the apparatus firmly down. In this state the cooking process will complete itself without fail. By no means let the apparatus be opened during the time required for cooking the food. The length of time which the different dishes should remain in the isolating apparatus varies according to their nature. It may, however,, be taken as a general rule that the same time is required to complete the cooking in the apparatus as in the ordinary way on a slow fire. The advantages of this apparatus are thus detailed by Herr Sorensen, the patentee, whose attention was first directed to the subject by the Norwegian CONDUCTION. 155 peasants, who heat their food in the morning, and whilst away in the fields keep the saucepan hot by surrounding it with chopped hay : 1. Economy of fuel varies according to the length of time required for cooking the different sorts of food. For those requiring, in the ordinary way, only one hour's cooking, the saving is about 40 per cent. ; two hours, 60 per cent. ; three hours, 65 per cent. ; six hours, 70 per cent. In the case of gas being used, the saving would be greater still. 2. Economy of Labour. A few minutes' boiling is sufficient. No fire is necessary afterwards. The cooking-pot once in the apparatus, the cooking will complete itself. Over-cooking is simply impossible, and the process of cooking is infallible in its result. The food will be cooked in about the same time as if fire had been continuously used. But the food need not be eaten for many hours after the cooking process is complete; so that half-an-hour's use of a fire on a Saturday night, for example, will give a smoking hot dinner on Sunday. 3. Portability. The weight of the apparatus complete varies from 18 to 5olbs. The apparatus can, in proportion to its dimensions, be carried about with great facility, without interfering with the cooking process. By means of a large apparatus for instance, following on a cart a detachment of soldiers on the march it is possible to provide them with a hot meal at any moment it might be found convenient (as may be proved by official reports from the officers of the Royal Guard at Stockholm, in the possession of the patentee). Again, fishermen, pilots, and others whose small vessels are not generally so constructed as to enable them to procure hot food while at sea, may easily do so, by taking out with them in the morning an apparatus prepared before their departure. It is, in short, a thing for the million, for rich and poor ; for the domestic kitchen, as well as for persons away from their homes. It cooks, and keeps food hot, just as well when carried about on a pack-saddle, on a cart, or in a fisherman's boat, as in a coal-pit or under the kitchen table. 4. Quality and quantity of the food prepared. Where other plans of cook- ing waste one pound of meat, this apparatus, properly used, wastes about one ounce. The unanimous testimony of those who have used it pronounces the flavour of food cooked in this manner incomparably superior to that which is ordinarily produced. 5. Simplicity of use. One of the greatest advantages of this invention is, no doubt, its simplicity and practical application. There is no complication of hot-water or air pipes to retain the heat, no mechanical combination what- ever for producing a high degree of heat by steam pressure ; consequently there is no necessity for steam-valves or other combinations which would render the use of the apparatus difficult and dangerous. Any person will, without difficulty, be able to use the apparatus to advantage after once having witnessed it in operation. No special arrangement is required in the kitchen for using the apparatus. Any fuel will do for starting the cooking. 6. In addition to all these advantages, the complete apparatus constitutes the * Simple Refrigerator' for the preservation of ice, which has attracted so much notice (see Letters in Times, July 30, 31, August 4, 1868), and had such warm approval from medical men. It will keep ice in small quantities for many days. In the organization of our bodies there are chemical changes going on which maintain a certain temperature. It matters not whether the living being, man, is a resident of tropical or polar regions ; the temperature required 156 HEAT. to promote and carry on vitality remains the same, or nearly so. If the cold or absence of heat is likely to be dangerous, man uses the skins and furs of animals for his clothing, and takes care to lose little or no heat. On the other hand, if the heat is excessive, increased action of certain powers throws out perspiration, which carries off the heat that might accumulate and prove dangerous. Solid bodies convey their heat rapidly to the human body, and the reverse. Somebody said that a frog could not be killed by any extreme of cold ; but when the animal was carefully dressed in tinfoil and then sub- jected to the cold produced by a freezing mixture, the conducting power of the metal was too much for the animal powers of the frog to resist, and he was killed. The air during the summer months is often very hot upwards of 1 00 F. in the glare of the sunlight ; but the heat from air is very slowly com- municated to the body, and the latter has time to neutralize the otherwise burning heat by consuming it in work, i. e., by forcing water through the pores of the skin, and converting it in part into vapour. The very low conductivity of the gases is shown by some very interesting experiments, performed by Tillet in France, and by Dr. Fordyce and Sir Charles Blagden and others in England, and thus related by Sir David Brewster in his charming little book called " Letters on Natural Magic :" " Sir Charles Blagden, Dr. Solander, and Sir Joseph Banks entered a room in which the air had a temperature of 198 F., and remained ten minutes ; but, as the thermometer sank very rapidly, they resolved to enter the room singly. Dr. Solander went in alone, and found the heat 210 F., and Sir Joseph entered when the heat was 21 1 F. Though exposed to such an elevated temperature, their bodies preserved their natural degree of heat. Whenever they breathed upon a thermometer, it sank several degrees : every expiration, particularly if strongly made, gave a pleasant impression of coolness to their nostrils, and their cold breath cooled their fingers whenever it reached them. " On touching his side, Sir Charles Blagden found it cold like a corpse; and yet the heat of his body, under his tongue, was 98 F. " Hence they concluded that the human body possesses the power of destroy- ing a certain degree of heat when communicated with a certain degree of quickness. This power, however, they concluded, varied in various media. " The same person who experienced no inconvenience from air heated to 211 could just bear rectified spirits of wine at 130, cooling oil at 129, cooling water at 123, and cooling quicksilver at 117. A familiar instance of this occurred in the heated room. All the pieces of metal there, even their watch- chains, felt so hot that they could scarcely bear to touch them for a moment, while the air from which the metal had derived all its heat was only unpleasant. " Messrs. Duhamel and Tillet observed, in France, that the girls who were accustomed to attend ovens in a bakehouse were capable of enduring for ten minutes a temperature of 270. "The same gentlemen who performed the experiments above described ventured to expose themselves to a still higher temperature. " Sir Charles Blagden went into a room where the heat was i or 2 above 260 F., and remained eight minutes in this situation, frequently walking about to all the different parts of the room, but standing still most of the time in the coolest spot, where the heat was above 240 F. " The air, though very hot, gave no pain, and Sir Charles and all the other gentlemen were of opinion that they could have supported a much greater heat. CONDUCTION. 157 " During seven minutes Sir C. Blagden's breathing remained perfectly good; but after that time he felt an oppression in his lungs, with a sense of anxiety, which induced him to leave the room. His pulse was then 144 double its ordinary quickness. "In order to prove that there was no mistake respecting the degree of heat indicated by the thermometer, and that the air which they breathed was ca- pable of producing all the well-known effects of such a heat on inanimate matter, they placed some eggs and a beef-steak upon a tin frame, near the thermometer, but more distant from the furnace than from the wall of the room. In the space of twenty minutes the eggs were roasted hard ; and in forty-seven minutes the steak was not only dressed, but almost dry. Another beef-steak similarly placed was rather over-done in thirty-three minutes. In the evening, when the heat was still more elevated, a third beef-steak was laid in the same place, and, as they had noticed that the effect of the hot air was greatly increased by putting it in motion, they blew upon the steak with a pair of bellows, and thus hastened the cooking of it to such a degree that the greatest portion of it was found to be pretty well done in thirteen minutes. " Sir Francis Chantrey, the late eminent sculptor, exposed himself to a tem- perature still higher than any yet mentioned. " The furnace he employed for drying his moulds was about 14 ft. long, 12 ft. high, and 12 ft. broad. When raised to its highest temperature with the doors closed, the thermometer stood at 350 F., and the iron floor was red hot. The workmen entered it at a temperature of 340, walking over the iron floor with wooden clogs which had become charred on the surface. On one occasion Sir Francis, accompanied by five or six of his friends, entered the furnace, and, after remaining two minutes, they brought out a thermometer which stood at 320. Some of the party experienced sharp pains in the tips of their ears and in the septum of the nose, while others felt a pain in their eyes." In this very interesting account we see it was assumed by the observers that the power of resisting the high temperature was due to some natural power or vitality, and yet it is stated that the tips of the ears and the septa of the nose were painfully affected. Certainly a live body resists a heat that would cook a dead one; therefore, in the abstract, vitality or the maintenance of the various processes inseparable from the living being, must not be wholly dis- regarded, as without vitality none of those changes of matter could occur which enable the living- tissues to resist the great heat ; but, after all, the " actual " heat is converted into " potential " heat, perspiration is secreted and escapes from the natural outlets of the body, the pores of the skin, and the lungs. Time, of course* is an important element in these experiments, and even the living body must succumb to any lengthened application of the great heat already described. Heated gases impart their heat very slowly to surrounding objects, because the gases are bad conductors of heat. If, for the sake of discussion, we could imagine an atmosphere composed of minute and rare atoms of silver, such an atmosphere, if it could be breathed, would impart its heat with dangerous rapidity to the body. Liquids, like gases, conduct heat very slowly. The hand may be placed within a short distance of a quantity of boiling water, and is wholly unaffected by its dangerous neighbour. The experiment is easily tried by first placing round a cylindrical glass, that will easily admit the hand, a large tube of caoutchouc. 158 HEAT. The large tube can be made in the usual manner, by cutting the edges of the sheet of caoutchouc first, and then winding it twice or thrice round some cylindrical vessel ; the whole, being kept together with tape, is then boiled and allowed to cool; a large india-rubber tube is then obtained, which can be stretched over one end of the glass cylinder and properly fixed with string ; the hand is then inserted, and the india-rubber tube tied round the wrist. The glass, containing the hand, is now held upright, and cold water poured in, so that the clenched hand is covered with one inch of water. Some boiling FIG. 163. The Hand placed in Water 'which is boiling above if. A, section of glass cylinder, made a little funnel-shaped at the top, with the caoutchouc tube B B attached by string to the lower part; c c, apparatus attached to the arm, and tied round tightly, so that the water cannot escape : this must be carefully attended to, because if the cold water runs away the boiling water will come down upon and scald the hand ; D, the red hot iron (the half of a dumb'- bell with a hole bored through it) held by a hook. water, coloured with a solution of indigo, is now carefully poured in down the sides of the glass, or, better still, on a thin disc of cork, floating on the cold water above the hand. The line of demarcation is readily seen by the differ- ence between the colourless cold and the coloured hot water. A red-hot ball, held by a hooked iron, is now applied to the top of the coloured water, which will soon enter into a violent state of ebullition : the water boils at the top, but does not communicate its heat by conduction downwards to the hand. After the experiment has been tried, of course the arm must not be reversed to pour out the water, or else the hand may be scalded. A syphon, protected by a fold of flannel or paper, may be filled with cold water in the usual way, CONDUCTION. 159 and the boiling water run off quickly. If the syphon was not covered with some bad conducting substance, the person helping to run off the boiling water might be inclined to leave go, when the hand inside would run a great risk of feeling the temperature of boiling water. It is, of course, one of those experi- ments which succeed thoroughly if all the manipulations are properly carried out from the beginning to the end. Another and very delicate proof of the bad conductivity of water can be shown by fixing a differential thermometer in a cork placed in the mouth of an inverted gas jar, and then heating the water at the top with a red-hot iron. FIG. 164. A A, inverted gas jar with neck c stopped with a good cork, through which the stem attached to the differential thermometer B B passes. The jar is rilled with cold water, and heated from the top by a common urn-heater, D. Although the thermometer is unaffected, it does not follow that water will not conduct heat. M. Despretz has ascertained that water will conduct heat very slowly. The motion of the particles which is immediately set up when the water is heated from the top must tend to destroy that similarity of molecules which seems so desirable to secure good conductivity. Directly any portion of the water is heated, its gravity is -altered, and it becomes lighter; this perpetual motion of the individual particles must inter- fere with the steady propagation of dynamical force, which has been shown to be essential to good conductivity. It appears to be doubtful whether gases do conduct heat : the molecules are too wide apart, and have greater mobility than liquids. Both with liquids and gases, circulation is a necessary condition if either are to be warmed, and hence, in speaking of the application of heat to these forms of material substances, another term is employed, viz., "convection," or carrying power. To heat a vessel of water to the boiling-point, the fire must be applied at the bottom ; a circulation of particles immediately commences ; the expanded or lighter particles rise by reduced specific gravity to the top, and, as they travel upwards, convey the heat " by convection " through the other and colder 160 HEAT. particles, which descend to take their place, and thus a constant circulation is set up until the whole is brought to one temperature, viz., the boiling- point, 212 F. When Sir Joseph Banks and others experimented with the atmosphere heated to 260, they found that if the heated air was set in motion and caused to travel rapidly, with the aid of the bellows, over the skin, the heat soon became disagreeable, and with dead matter (the beef-steak), at a higher tem- perature, it was distinctly shown that the process of cooking was more rapidly carried on when the hot air was kept in motion and its carrying power made use of. The same fact was observed by the Arctic discoverers, who could bear the most intense cold, viz., minus 55, or 14 below the freezing-point of mercury, when the air was still ; but, if set in motion, the wind, the current of air, or the cold blast dangerously affected the extremities, which were rapidly deprived of heat by this power of convection, and frozen or " frost-bitten." In all schemes for ventilating and supplying heated air, circulation must, of course, be maintained, either to impart or carry off heat. It is said that, if the hand is kept perfectly still in water heated to a tempera- ture of 150 F., the nerves are not disagreeably affected ; but directly the hand is moved, then the heat becomes painful, and cannot be borne. As an illustration of convection, or the carrying of heat, on the grand scale, there are the trade winds and the Gulf Stream. In the tropics the heated earth imparts some of its force to great volumes of air, which ascend and flow towards the poles; upper currents from the equator to the poles must be succeeded by under currents from the poles to the equator. The constantly ascending warm air is thus a carrier of heat to colder cli- mates, and vice versa. These currents are modified by the various physical conditions of the earth's surface. In like manner, a great current of warm water, which leaves the Straits of Florida at a temperature of 83 F., passes across the Atlantic in a north- easterly direction. It washes the north-western shores of Europe, and makes itself, or rather its heat-giving power, apparent by flowing round the coast of Ireland. In mild winters in England it is the diffusion of heat by certain winds, and the good offices of the Gulf Stream, which mitigate the severity of the season ; and these carriers of heat are only neutralized when similarly, but contrarily, enormous masses of ice, icebergs, are detached from the polar regions, and rob the water of its heat on its journey to our shores. LATENT HEAT. CAPACITY FOR HEAT SPECIFIC HEAT HEAT OF ATOMS ATOMIC HEAT. These somewhat difficult terms or titles, referring to truths that the young student does not, perhaps, fully appreciate at first, nay, to speak plainer, which he never will comprehend without industrious application to study, are set forth in the following chapters. In all the old standard works upon natural philosophy it is usual to state that there are two kinds of heat that may be resident in a body, viz., one kind LATENT HEAT. 161 called " sensible heat," which is designated as temperature, and is capable of measurement by the thermometer and other kindred instruments ; another and more subtle condition, not apparent to our nervous system, called " latent heat," and incapable, whilst in that condition, of affecting any measurer or test of " sensible heat." The dynamical theory substitutes the terms " actual energy," or force, for that of " sensible heat," and "potential energy" for that of " latent heat." The one, actual heat or energy, is in use ; the qther, potential heat or energy, is in store. A horse-shoe nail may be warmed by any convenient source of heat, and as long as it remains above the temperature of the air we have evidence of " actual heat." When cold it may be hammered on an anvil, by an expert blacksmith, and then becomes so hot it will set fire to sulphur or phosphorus. The heat thus evoked was formerly called " latent heat," and was supposed to be combined with the material substance of the iron ; the dynamical theory rejects the idea of its being a distinct subtle fluid, but ascribes the heat to the motion of the particles of the iron. It may be useful here to tabulate the new terms used by Clausius, Rankin, Tyndall, and others, in their exposition of the dynamical theory of heat. ENERGY OR HEAT. Defined to be the power of performing work. It may be latent or sensible. Latent. Possible energy, or work to be done. Potential energy is energy in store. Sensible. Actual energy, or work is being done. Dynamic energy is energy in action. One column of terms is the exact antithesis of the other. There is no mechanical machine by which we can tear asunder or separate the ultimate molecules of bodies. Cohesion, or molecular force, is too potent to be over- come by mechanical energy. Heat, another kind of energy, will, however, act where the former fails ; therefore heat is the equivalent for mechanical energy. When a metal is expanded by heat, every molecule is separated or forced asunder; the energy of heat must be enormous to overcome the force of cohesion. When a mass of metal is heated, there is not only the motion imparted the vibratory power set up to produce sensible or actual energy (heat) but the molecules or atoms of the metal are pushed asunder, as shown by their expansion. This work, which goes on inside and throughout the mass of the metal, is not visible, and therefore may be called " interior work." Tyndall compares this interior work to the raising of a weight from the earth the overcoming of the force of gravity, which attracts all things, and keeps all terrestrial bodies in their places. The raising of a weight by a cord from the earth, it is clear, confers " a motion-producing power." The weight can fall, and in its descent can perform work. Whilst hanging in the air, it represents possible energy, or " potential " energy. The pull, or attraction of gravity, causes this possible or "potential" energy. If there were no attraction between the substance and the earth, there would be no " possible " energy. Substitute the ultimate atoms of bodies for the weight and the earth : 11 102 HEAT. remember that the atoms of solid bodies are held together with molecular force (cohesion), and it must be evident that whenever they are separated, although the distance to which they are separated cannot be measured it is too minute still the fact remains, and when the atoms come together it is like the fall of the weight to the earth, and the result must be the production of actual energy, or heat. This is what Tyndall means when he speaks of the clashing together of the atoms. The heating of the cold horse-shoe nail by hammering, or the heating of cold bars by rolling, is simply the conversion of mechanical energy into molecular motion ; if the approach of the molecules of a body will produce actual energy, a still nearer approach must increase that energy, or heat. Indeed, the experiment already quoted, of heat produced by hammering and bringing the atoms nearer together, is a good illustration of the above argument. The " specific heat" (a term that must be carefully considered presently) of a metal like copper is altered when a nice, soft, well-annealed piece is ham- mered: heat is produced, and the specific heat changes from 0*09501, 0*09455, to 0*09360, 0*09330; and its specific gravity or density becomes higher.. When again heated red hot and allowed to cool slowly, as is done in the process of annealing, its specific heat returned to 0*09493, 0*09479, or very nearly the same that it was at first. Thus by alternately hammering and then heating or annealing a metal, the atoms are brought more closely together or pushed further apart. When the atoms are pushed further apart, the heat becomes potential or latent ; when advanced nearer to each other, the heat is actual or sensible. Nearly every philosopher selects a particular subject to which he devotes his special attention. Let us read what Dr. Tyndall says of latent heat in his standard work, " Heat a Mode of Motion." " We shall now direct our attention to the phenomena wjtiich accompany changes of the state of aggregation. When sufficiently heated, a solid melts ; and when sufficiently heated, a liquid assumes the form of gas. Let us take the case of ice, and trace it through the entire cycle. This block of ice has now a temperature of 10 C. below zero. I warm it ; a thermometer fixed in it rises to o, and at this point the ice begins to melt ; the thermometric column, which rose previously, is now arrested in its march, and becomes perfectly stationary. I continue to apply warmth, but there is no augmenta- tion of temperature ; and not until the last film of ice has been removed from the bulb of the thermometer, does the mercury resume its motion. It is now again ascending ; it reaches 30, 60, 100 C. ; here steam-bubbles appear in the liquid ; it boils, and, from this point upwards, the thermometer remains stationary at 100. But during the melting of the ice, and during the evapo- ration of the water, heat is incessantly communicated. To simply liquefy the ice, as much heat is imparted as would raise the same weight of water 79*4 C, or as would raise 79*4 times the weight one degree in temperature ; and to con- vert a pound of water at 100 C. into a pound of steam at the same temperature, 537*2 times as much heat is required as would raise a pound of water one degree in temperature. The former number, 79*4 C. (or 143 F.), represents what has been hitherto called the latent heat of water ; and the latter number, 537*2 C. (or 967 F.), represents the latent heat of steam. " It was manifest to those who first used these terms, that throughout the entire time of melting, and throughout the entire time of boiling, heat was LATENT HEAT. 163 communicated ; but inasmuch as this heat was not revealed by the ther- mometer, the fiction was invented that it was rendered latent. The fluid of heat was supposed to hide itself in some unknown way in the interstitial spaces of the water and the steam. " According to our present theory (the dynamical), the heat expended in melting is consumed in conferring potential energy upon the atoms : it is virtually the lifting of a weight. So likewise as regards steam, the heat is consumed in pulling the liquid molecules asunder conferring upon them a still greater amount of potential energy. " When the heat is withdrawn, the vapour condenses, the molecules again clash with a dynamic energy equal to that which was employed to separate them, and the precise quantity of heat then consumed now re-appears. " The act of liquefaction consists of interior work expended in moving the atoms into new positions. The act of vaporization is also, for the most part, interior work; to which, however, must be added the exterior work of forcing back the atmosphere, when the liquid becomes vapour Let us then fix our attention upon this wonderful substance, water, and trace it through the various stages of its existence. First, we have its constituents as free atoms of oxygen and hydrogen, which attract each other, fall or clash together. The mechanical value of this atomic act is easily determined. The heating of i Ib. of water i C. is equivalent to 1,390 foot-pounds ; hence the heating of 34,000 Ibs. of water i C. is equivalent to 34,000X1,390 foot- pounds. " We thus find that the concussion of our i Ib. of hydrogen with 8 Ibs. of oxygen is equal, in mechanical value, to the raising of forty-seven million pounds one foot high. " I think I did not overstate matters when I stated that the force of gravity, as exerted near the earth, is almost a vanishing quality, in comparison with these molecular forces. " The distances which separate the atoms before combination are so small as to be utterly immeasurable ; still it is in passing over these spaces that the atoms acquire a velocity sufficient to cause them to clash with the tre- mendous energy indicated by the above numbers. After combination, it is in a state of a vapour, which sinks to 100 C, and afterwards condenses into water. In the first instance the atoms fall together to form the compound ; in the next instance the molecules of the compound fall together to form a liquid. The mechanical value of this act is also easily calculated. 9 Ibs. of steam, in falling to water, generate an amount of heat sufficient to raise 537*2X9 4,835 Ibs. of water iC, or 967X9=8,703 Ibs. iF. Multiplying the former number by 1,390, or the latter by 772, we have in round numbers a product of 6,720,000 Ibs. as the mechanical value of the mere act of con- densation. " The next great fall is from the state of liquid to that of ice, and the mechanical value of this act is equal to 993,564 foot-pounds. Thus our 9 Ibs. of water, at its origin and during its progress, falls down three great pre- cipices ; the first fall is equivalent in energy to the descent of a ton weight down a precipice 22,320 feet high ; the second fall is equal to that of a ton down a precipice 22,900 feet high ; and the third is equal to the fall of a ton down a precipice 433 feet high. " I have seen the wild stone-avalanches of the Alps, which smoke and thunder down the declivities with a vehemence almost sufficient to stun the 11 2 1 64 HEAT. observer. I have also seen snow-flakes descending so softly as not to hurt the fragile spangles of which they were composed ; yet to produce from aqueous vapour a quantity, which a child could carry, of that tender material, de- mands an exertion of energy competent to gather up the shattered blocks of the largest stone-avalanche I have ever seen, and pitch them to twice the height from which they fell." CAPACITY FOR HEAT. This term, which is most simple and useful, expresses a fact that has been forced upon observers by numerous experiments made with the thermometer. The thermometer is usefully applied to determine the temperature of any solid, fluid, or gaseous matter ; but it will not tell the observer how much heat or actual energy is contained in different measures of the same fluid. A gallon of water in one vessel, and a pint of water in another, may be shown by the thermometer to have a temperature of 212 F. ; but the quantity of energy or heat must be much greater in the larger measure the one gallon than in the single pint. The thermometer fails to show the quantity of energy, whilst it gives relatively the "relative actual heat" the "temperature." A photo- meter, or measurer of light, will demonstrate the relative illuminating power of any given source of light ; but it cannot give the number of vibrations per second producing the light. A thermometer can tell us truthfully how m^.ch hotter or colder than 32 or 212 F. a substance may be ; but it cannot inform us what may be the amount of vibratory power given, and the molecular force detached, which', according to the dynamical theory, must be the equivalent for the expression or quantity of heat. There are certain facts, explain them how we will, which are indisputable. If 10 Ibs. of water (one gallon) at 100 F. are mixed with the same weight of oil at 50 F., the resulting temperature will not be the mean, 75 F., but 83-^ F. The water, therefore, has lost "actual energy" equal to i6|; but the same energy has caused the oil to rise 33f . If the experiment is reversed, and 10 Ibs. of oil at 100 are mixed with 10 Ibs. of water at 50, the mean will be 66|: the 33^ actual heat or energy given out from the oil is only able to raise the temperature of the water i6f. The actual energy which will raise the temperature of oil 2 will raise an equal quantity of water only i. The heat that will raise any given substance from o C. to i C., compared with the amount of " energy" required to heat an equal weight of water to the same point, is called its "specific heat." Therefore the specific or potential heat of oil will be a half, '5, as compared with the unit or one viz., water. As the oil has been quickly heated, so it will rapidly cool ; it has only half the "energy of heat" possessed by water to give up. If the water require one hour to cool to any given temperature, the oil would reach the same point in half-an-hour. Hence "time" is the test used sometimes to determine the specific heat of bodies the time required by a substance to cool. Or the process may be reversed by ascertaining the quantity of ice which exactly equal weights of other bodies can melt in falling from one temperature to another, say from the boiling-point to the freezing-point of water. As the process of mixture already described with the oil and water may be employed, there are there- fore three methods by which the specific heat of bodies may be determined : LATENT HEAT. 165 1. The direct method by mixture. 2. Time required to cool, and rate of cooling. 3. Heating of ice, and quantity liquefied by a given weight of the substance heated to 212 whilst falling to 32. By the first method viz., mixture or immersion the distinguished phy- sicist, Regnault, arrived at the following results : "SPECIFIC HEATS OF EQUAL WEIGHTS BETWEEN O C AND IOO C. Water .... rooooo Oil of turpentine . . 0*42593 Charcoal. . . . 0*24150 Brass . . . . 0*09391 Silver .... 0*05701 Tin . . . 0*05623 Glass .... 0*19768 I Mercury .... 0*03332 Iron , . . 0*11379 Zinc .... 0*09555 Copper .... 0*09515 Aluminium . . . 0*21430 Platinum . . . 0*03243 Gold . . . . 0*03244 Lead . . . . 0*03140 Bismuth .... 0*03080 For a lecture-table experiment there are none better than that devised by Tyndall, to show the time required by equal spheres of various solids, heated to the same temperature, to melt their way through a cake of beeswax. The metals used are iron, lead, bismuth, tin, copper : these are shaped as balls or spheres, and each furnished with a hook for conveniently removing them from the oil, in which they are heated to a temperature of i8oC. A framework of wood, shaped like the spokes of a wheel, with five strings, to which the balls are attached, may be used in order to remove the whole of the balls at once from the heated oil. When they are laid upon a cake of beeswax, 6 in. in diameter and half an inch thick, supported on the ring of a tripod or other convenient means of support, the iron and the copper balls go through first, the tin next, while the lead and bismuth are retained. If they contained the same amount of heat, or had the same " actual energy," they would all go through the wax in the same time : the difference in their specific heats determines the rate at which they perforate the wax. Messrs. Dulong and Petit have shown that the specific heat of bodies increases as their temperature rises. Any given substance will require more heat to raise it a certain number of degrees when at a high than at a low temperature. The variations of specific heat according to temperature are well shown in the case of iron. SPECIFIC HEAT OF IRON (DULONG AND PETIT). From 32 to 212 . . . 0*1098 392 . . . 0*1150 572 ... 0*1218 666 ... 0*1255 In a similar manner the specific heat of the gases has been carefully deter- mined, the methods employed involving one of the three modes already described. De la Roche and Berard caused a measured volume of the gas under examination, when heated to a fixed temperature and kept at a uniform heat, to pass through a spiral glass tube surrounded with water (this plan would be equivalent to the " mixture " of oil and water), and, by observing the increase of the temperature of the water surrounding the spiral tube, and other data, they determined the specific heat of certain gases. i66 HEAT. Dr. Apjohn devised another method, viz., that of vaporizing water by a current of the heated gases, and, by inverse proportion, viz., the greater the specific heat of the gas, the less time required to cool it, and vice versa, he has given the specific heats of gases already examined by De la Roche ; but unfortunately the figures of the two experimentalists did not agree, and there- fore a more careful investigation was made by Regnault, who, taking the specific heat of an equal weight of water as the unit of comparison, commences with air, and gives the following table of the specific heats of a number of gases and vapours with which he experimented ; and, what is still more valu- able, the table gives the specific heat of equal volumes and weights of the bodies examined : SPECIFIC HEAT OF GASES AND VAPOURS. GAS OR VAPOUR. Equal GAS OR VAPOUR. Eq Vols. ual Weight. Vols. Weight. Air ... 0-2375 0-2375 Sulphurous an- Oxygen 0'2405 0-2175 hydride . 0-341 1 0*1540 Nitrogen . 0-2368 0*2438 Hydrochloric Hydrogen . 0-2359 3-4090 acid 0-2352 0-1842 Chlorine 0-2964 0"I2IO Sulphuretted hy- Bromine . 0-3040 0-0555 drogen . 0-2857 0-2432 Nitrous oxide . 0-3447 0-2262 Water 0*2989 0-4805 Nitric oxide 0-2406 0-23I7 Alcohol 0-7171 0-4534 Carbonic oxide . 0-2370 0-2450 Wood spirit 0-5063 0-4580 Carbonic anhy- Ether . I-2260 0-4796 dride 0-3307 0-2163 Ethyl chloride . 0-6096 0-2738 Carbonic disul- Ethyl bromide . 0-7026 0-1896 phide 0'4I22 0-1569 Ethyl di sulphide I '2466 0-4008 Ammonia . 0*2996 0*5084 Ethyl cyanide 0*8293 0-4261 Marsh gas . 0-3277 0-5929 Chloroform 0-6461 0-1566 Olefiant gas 0-4106 0-4040 Dutch liquid 079II 0-2293 Arsenious chlo- Acetic ether 1*2184 0-4008 ride 07013 0'II22 Benzol I'OII4 o*3754 Silicic chloride . 07778 0'1322 Acetone 0*8341 0-4125 Titanic chloride 0*8564 0*1290 Oil of turpentine 2.3776 0-5061 Stannic chloride 0-8639 0*0939 Phosphorous chloride . 0*6386 0*1347 Regnault's experiments confute those of De la Roche and Berard, and deny that the specific heat of air and all gases rises with the temperature. Regnault's experiments were carried on with air between the limits of tem- perature expressed by 30 C. and 200 C. The same result was obtained with gases like hydrogen, which cannot be easily liquefied ; and the specific heat was not found to increase with the temperature, at least between 30 C. and 200 C. A gas which can be easily condensed, such as carbonic acid, shows, in accordance with the statement of De la Roche and Berard, an increased specific heat with an increased temperature. LATENT HEAT. 167 SPECIFIC HEAT OF CARBONIC ACID AT DIFFERENT TEMPERATURES. Between 30 and 8 C. . .' specific heat 0-18427 - 8 100 . . . 0-20248 - 8 210 ... 0-21692 Regnault also discovered that the specific heat of a given volume of a gas increases directly as its density is increased; and his valuable experiments show that the specific heat of the same liquid varies with the temperature. There exists a remarkable connection between specific heat and atomic weight, which has given rise to another term "atomic heat." This expression means the product obtained by multiplying the specific heat of a body by its atomic weight. The specific heat of an elementary body is inversely as its combining pro- portion. Regnault discovered in upwards of twenty bodies chemically pure, that the atomic heat ranged between 3-31 and 2*93, giving a mean of 3-13. Hence, if the above number 3*13 is divided by the number expressing the specific heat of iron, lead, mercury, tin, &c., the quotient gives very nearly the atomic weight of the metal. The term " atomic weight" must not be confounded with the term " chemical equivalent:" the latter is obtained by direct experiment, and means the com- bining proportion of the various elements, as, for instance, i being taken as the combining proportion or equivalent for hydrogen, 16 will be that of oxygen; or I of hydrogen may displace 65 of zinc: hence the former is equiva- lent to the latter. Atomic weight is a product arrived at by calculations carried out in various ways, as, for instance, when the number 3-13 is divided by the specific heat of a metal. Atomic weight is also arrived at by other methods ; it may sometimes coin- cide with the combining proportion, or equivalent number, or it may be a multiple of it. , "Actual energy" (heat) disappears during liquefaction. When matter passes from the solid to the liquid state, " actual " is converted into " poten- tial energy;" and the heat is said to disappear, and cold is produced. It is the enormous amount of actual heat, so slowly converted into potential heat, that prevents the sudden liquefaction of ice or snow, and the great damage which would occur to property if the snow could be quickly melted. Con- versely, when a liquid is changed to the solid state, the closer proximity of the molecules, the merging together of the particles by cohesion, converts the " potential " into " actual " heat ; and thus the very change of water into snow or ice produces actual energy, or heat, and helps to mitigate the effect of a sudden frost. Taking the fact (irrespective of theory) that liquefaction will produce cold, there are various solids and mixtures of solids which will produce a sufficiently low temperature, when quickly dissolved in water, to freeze water contained in a vessel surrounded with the mixture. The mere solution of nitre alone will lower the temperature of water from 50 to 35 F. Four ounces of nitre and four ounces of common sal ammoniac dissolved in four ounces of water reduce the temperature from 50 F. to 10 F. A mixture of equal parts of snow, or powdered ice, and salt will sink the thermometer from 32 F. to o, or 32 degrees below the freezing-point of water ; and two of snow and one of salt reduce the temperature to 4 F. A mixture of three parts by weight of i68 HEAT. chloride of calcium and two of snow will reduce the temperature from 32 F. to 50 F. ; and by powdering and carefully cooling the chloride to 32 F., and N using very thin vessels, mercury can be frozen. The liquefaction of a met-.llic alloy, composed of '207 parts by weight of lead, 118 ' tin, 284 bismuth, in 1,617 parts of mercury, will sink the thermometer from 63 to 14; and, of course, water can be frozen by this process. One of the most interesting experiments is that of Mousson, who contrived an apparatus by which ice was subject to a pressure equal to thirteen thousand atmospheres, and by which its bulk was reduced by thirteen-hundredths of that which it occupied at o C. (32 F.). The temperature of the ice was first reduced 20 C. ( 4 F.), and then subjected to the pressure of a copper rod, worked by a very powerful screw. FIG. 165. A Still, with " Still Head? and the Worm surrounded by Cold Water. Instead of increasing the solidity of the ice, the mechanical compression and motion of the molecules liberated the equivalent in actual energy or heat ; the ice liquefied, and the copper rod was found to have fallen to the bottom of the water, which again solidified directly the pressure was removed. The freezing-point of water is lowered to a minute extent by pressure. IV. liquid alloy of sodium and potassium is easily obtained by pressing pieces of the two metals together : if this liquid be brought into contact with mercury, the amalgam instantly solidifies and becomes hard ; at the same time so much heat is liberated that incandescence is apparent at the point where the metals come in contact, and any combustible fluid, such as naphtha, may be set on fire. Liquefaction produces cold ; congelation or solidification, heat. If liquefaction is pressed further by the addition of more heat, the water is converted into vapour, the molecules are thrust wider apart, and " actual heat" disappears. EBULLITION. -169 This is demonstrated very conclusively in the distillation of water. The heat is applied to the bottom of the vessel containing the water, and when it has once reached the boiling-point, 212, the steam the vapour (also at 212) carries off all the heat of the burning coals ; the heat disappears ; the ther- mometer, inserted in the still, remains stationery. When the steam is passed through the condensing apparatus the coil of pipe, called the worm, sur- rounded by cold water, and contained in what is called the worm-tub the heat or energy which it carries off from the fire becomes apparent ; the stored heat is so large in quantity that it soon raises the temperature of the water in the worm-tub, and the quantity of water in the tub, which may be raised to 212 F., is much larger than the water condensed. The stored "heat" (already so often spoken of as " potential heat ") in the steam becomes " actual " energy when the vapour passes to the liquid condition of matter ; and this heat, as already described, is so great, that it may be conveniently applied in the warming of buildings. The conversion of water into vapour by the method already described is progressive, and unattended with danger. If the water could be suddenly converted into steam, and the specific heat of steam was not so high, the attempt to boil water must always end disastrously, because it would be generated suddenly and explosively ; the steady " ebullition," or escape of bubbles of steam, as the cohesion of the molecules is gradually overcome, would not be maintained. The escape of air from water, heated to 212 F., is very apparent when it is boiled in a flask. Tyndall says the air acts as a kind of elastic spring, pushing the atoms of the water apart, and thus helping them to take a gaseous form. The cohesion of the particles of water appears to be greatly increased when the foreign matter viz., atmospheric air is removed. Thus, water allowed to fall through a tube from which the air has been ejected by boiling the water, and melting the glass and hermetically sealing the end, falls col- lectively, making a noise, and would break through the end of the glass tube like a solid substance. The vacuum-tube containing the water is called " the water-hammer," and if altered in shape by bending it into a V-shaped figure, nicely rounded off at the bend, some very amusing illustrations of the modification of the cohesion of the water and adhesion to the glass can be displayed. The mechanical nature of the interior of a vessel in which steady "ebulli- tion " is to be maintained greatly affects the escape of the vapour or steam. If the interior surface is too smooth, like that of a flask, and distilled water boiled therein, the flask is said to bump, i.e., the temperature of the boiling water rises a degree or so above the boiling-point, and every time steam is formed it escapes with a sudden jerk, as if it were a slight explosion, and the temperature falls to 212, again rising and falling with each rush of vapour. When this occurs, it may be instantly corrected by dropping in any metallic filings, zinc or copper, or by placing in the flask a bit of crumpled platinum- foil. The rough edges break up the continuity of the smooth surface of the glass, and serve to conduct the heat of the lamp into the particles of the water, and thus to hasten the disruption of their cohesive power. It is easy to follow out the idea further by lining a copper vessel with shellac. Water placed in a vessel prepared in this manner will not boil until it attains a temperature of 219 F.," /.., the steam can be made to press the piston up, as well as down. This adds considerably to the first expense and the continued expense of fire." This Cornish engine, made by Watt, is a singular contrast to the engines of the present day, in which the consumption of fuel, and consequently the work performed, is carried to the most refined and absolute degree of perfection. Engines have been made to perform the duty of raising one hundred million pounds of water one foot high by the consumption of a single bushel of coals. The essential portions of the steam engine are better studied in Watt's " Double-action Engine." In this, as in the single-acting engine, the " cylinder " holds the first place. This consists of a cylinder of metal, A D, provided with a piston, B, the end of which passes through a stuffing-box, c, and is connected with the beam by a beautiful arrangement called the parallel mo- tion (Fig. 177). The steam is passed into this cylinder both above and below the piston with the utmost regularity, by means of a sliding valve, E. This valve opens a communication between the interior of the boiler and the cylinder, and the condenser and the cylinder, in such a manner that, whilst the steam is using its power on one side of the piston, it is at the same time creating a vacuum on the other side, by passing into a box called the condenser, F the famous "separate con- denser" of Watt to which an air-pump is attached to remove any air that may collect, the condensed water, and also that used for injection. The sliding of the valve upward and down- ward is effected by means of another admi- rable mechanical arrangement, called the " eccentric." In nearly every kind of engine there is attached to the beam and piston-rod a " pa- rallel motion," in order that the piston-rod may always move in a straight line. This simple mechanical arrangement is one of the happiest of the inventions which seem to have come, as it were, intuitively to the well- educated mind of Watt. To render the working of the double-acting engine as perfect as possible, and to prevent the bad effects of sudden and violent work- ing by excess of steam, Watt caused his engine to regulate its own motion by FIG. ij$.The Cylinder, Valve., and Condenser . THE STEAM ENGINE. 187 FIG. 176. The Eccentric. This was not wholly the invention of Watt, as the same principle had been previously used in the regulation of sluices of water-mills, under the name of the " lift-tenter ;" but the merit due *o Watt is FIG. 177. j. ne Parallel Motion. a a, the beam ; ft, the piston-rod ; c, the air-pump rod ; d d Seekings, Great Exhibition, 1 862. The governor in the above engine acts upon an equilibrium or double- beat throttle valve, through the intervention of only a single lever; and the comparative absence of resistance renders its action peculiarly sensitive. The most important features of the " vacuum " or " condensing engine " of Watt having been discussed, the high-pressure steam engine, such as that delineated at Fig. 178, may next be considered. Their form is legion ; they may be beam engines or horizontal or vertical engines. The machinery comprised in their construction can be fitted up in a much smaller space ; and they differ from the " vacuum or condensing engine " by the absence of those parts which give the name to Watt's engine. The air-pump and condenser are removed, and the steam, after performing its work, is allowed to escape directly into the atmosphere. An illustration of a small engine is given, because the high-pressure principle is well adapted for nearly all small THE STEAM ENGINE. 189 engines, and it is especially to be noted in the locomotive. Portable engines, which can be removed from place to place, but are stationary when at work, are all worked on the high-pressure principle. At the great French Exhibi- tion of 1867, our manufacturers of portable steam engines for agricultural purposes, such as for working thrashing-machines, ploughing, &c., &c., received many more gold medals than those of other nations ; and the excel- lence of the machinery used by advanced and intelligent farmers in England has created a trade with foreign countries which, in spite of the low wages of the engineers of the Continent, is still most thriving and lucrative. FIG. 179. Howard's Patent Steam Ploughing and Cultivating Apparatus. The apparatus delineated in the above engraving includes the engine, the windlass, the wire rope, the cultivator, the anchors, and pulleys. The young people for whom this book is intended have so many opportunities of studying the locomotive engine at the various railway stations, that it is presumed the general outline of this most important class of engines must be sufficiently known to all. The interior of a locomotive can hardly be thoroughly understood without one of those valuable sectional models made by Messrs. Elliott Brothers, of Charing Cross. The models have sectional working gear, and accurately define the various parts and their respective uses;, and all good schools should possess sectional models of the Watt condensing engine and of the locomo- tive or high-pressure engine. At the Exhibition of 1862 was exhibited a locomotive engine, built for the London and North-Western Railway Company by Mr. Ramsbottom, their locomotive superintendent, at Crewe, being a good specimen of a first-class passenger engine. It was fitted with patent pistons, duplex safety-valves, and lubricators, and adapted for burning coals with great economy. An engine of this class ran the American express, on the 7th January, r862. a distance of 130^ miles without stopping, at an average speed of 54 FIG. 1 80. Apparatus for Supplying Water to Tenders whilst in motion. miles per hour. The tender attached (Fig. 180) was fitted with Mr. Rams- bottom's most ingenious apparatus for taking up water whilst running. The plan has been in daily operation on the Chester and Holyhead Railway since it was first adopted in the winter of 1859-60. By it various quantities of water, from 1,200 gallons downwards, can be picked up, at speeds ranging from 22 miles to 50 miles and upwards per hour. In the running of the Irish mails, the arrangement has the effect of reducing the dead weight of the tender about six tons, equal to the weight of a loaded carriage. These engines are added to the enormous screw engines manufactured by Messrs. James Watt & Co. The latter consist of four cylinders, each of 84 inches diameter. The paddle-wheels are driven by four engines, each of 72 inches diameter of cylinder and 14 feet stroke, and rated collectively at 1000 nominal horse- power. In the Exhibition of 1862 some good examples of high and low pressure marine condensing engines, with surface condensers, were shown by George Rennie & Son. The advantage of two cylinders in direct-acting marine screw engines is that of working steam expansively, whereby economy of steam and fuel is obtained, depending on the pressure of the steam and the relative volumes of the high and low pressure cylinders. These engines are fitted with surface condensers, with copper tubes and improved centrifugal pumps for circulating the water in the condensers, these pumps being made on a double-curvature principle of least resistance to the flow of water occasioned by the centrifugal force generated by the angular velocity of the pump. Engines on this principle are fitted with boilers in proportion. Apparatus for superheating steam and feed- water heaters may be made to consume not more than two pounds of coal per actual horse-power. The important principle of working steam expansively has been applied THE STEAM ENGINE. 19* FIG. 1 8 1. The P addle-Wheel Engines of the Great Eastern. with the greatest success in large engines, made like the Cornish ones, for pumping enormous quantities of water for the use of great cities like London. Steam of high pressure is used, and when admitted to the piston it is cut off at one-eighth or one-tenth of the stroke. At the Kent Waterworks the Cornish engines used are two with cylinders of 70 inches and 10 feet stroke, two with cylinders of 60 inches, and two smaller ones ; in these engines the expansion was not more than one-fifth. The high-pressure, condensing, double-cylinder engines erected at Ditton for the Lambeth Waterworks, and at Kingston for the Chelsea Waterworks Company, can accomplish, according to Mr. Simpson, in ordinary work, 90 million pounds raised I foot high per one hundredweight of coals consumed. The returns of the work performed by the Cornish pumping engines have been given from an early date, and are very interesting. 1769, John Taylor gave the return at only 5^ millions. In 1800 . . 20 1815 50 l8 35 .125 The latter 125 millions was at Fowey's Consols Mine, where Austen's engine was used. It might almost be disputed whether such an amount of duty was ever done ; but it was well authenticated by the report of the committee ap- pointed, and they reported that the work was done with a bushel of coals, weighing 94 pounds. HEAT. The principle of expansion was used by Watt, but without any very good result ; but Woolf and Trevithick applied the system with high-pressure steam, and realised the economical results already referred to. With respect to the double-cylinder engine, this was invented by Jonathan Hornblower, and not by Watt. It was patented by the former in 1781. The first and second patents of Hornblower contain the following : " First, I use two vessels in which the steam is to act, and which in other steam engines are generally called cylinders. " Secondly, I employ the steam, after it has acted on the first vessel, to operate a second time on the other, by permitting it to expand itself, which I do by connecting the vessels together, and forming proper channels and apertures, whereby the steam shall occasionally go in and out of the same vessel." The third invention was for " surface condensation," a term already used, and meaning the application of cold water on the other side of a plate forming the side of the box containing the steam. The more perfectly the circulation of the cold water can be maintained, the better is the condensation. A surface- condenser represents the worm attached to a common still, and this invention evades the " separate condenser " in the patent of Watt. FIG. 182. A Cornish Boiler. Professional men have discussed the respective merits of the single and double cylinders, and Mr. Hawksley stated, as the result of his experience, " that when raising water from a pit, the Cornish engine (single cylinder) would work well ; but it would perform best when pumping out of a deep pit, and when it had a large amount of heavy rods to continue its action and diminish its initial velocity. In the case of dear coal he would employ the double cylinder ; in the case of cheap coal he would employ the single cylinder ; and either not cut off the steam at all, or not much before it gets to the end of the stroke. With the double cylinder he would use eight expansions ; beyond that, so little was gained by the system of expanding steam, that it was not worth carrying it further. In order to economise coal, and, of course, to increase the stowage qualities E VAPOR A TION. 193 of a vessel, the " Combined Vapour Engine" was invented by M. du Trembley. This ingenious arrangement provided that, after the steam had done its work in the cylinder, it passed to the surface-condenser, which was surrounded with ether, and, causing this fluid to boil, the vapour passed to another cylinder, where it exerted its elastic force ; and after the vapour of the ether had done its work, it was finally condensed and pumped back again to the box surrounding the external condenser of the steam engine, the condensa- tion of the vapour of water causing another fluid (ether) to boil. This clever arrangement met with considerable approval, and has been tried on an exten- sive scale in the propulsion of vessels. Superheated steam, or steam passed through a coil of iron pipe placed in the furnace, has been proposed and used successfully in the working of marine engines in order to economise fuel. Rankin calls this superheated steam ^' steam-gas ;" and the Hon. John Wethered, of the United States, modified this superheated steam by mixing it with ordinary steam from the boiler, because he found that when the steam was heated sufficiently high to develop the full power, it destroyed the cylinder and slides. He considers the differ- ence between superheated steam and combined steam consists in this that the former, being of a gaseous nature, was a bad conductor of heat, and parted with it with difficulty ; whereas combined steam, being pure vapour and a better conductor of heat, parted with it more readily, and left more in the cylinder of the engine to be converted into mechanical power. Wethered claimed an economy of combined steam over ordinary steam of 52*5 per cent., and over superheated steam of 25 per cent. According to more recent experiments on the large scale, made by the eminent firm of Messrs. Penn, of Greenwich, with superheated steam, it is conclusively determined that an economy of 20 per cent, of fuel was realized in the working of marine engines, when the steam at a pressure of 20 pounds per square inch was raised by the superheating apparatus 100 Fahrenheit. The Cornish boiler, to which allusion in connection with the Cornish engine has already been made, is shown at Fig. 182. It consists of a double cylinder, the fire being placed on bars inside it, and is one of the most useful forms that can be employed, and is the kind of boiler used for working the steam engine at the Polytechnic. EVAPORATION. It is so common an act to boil water and convert it into steam, that non- scientific minds are sometimes puzzled when the more learned talk of steam, or the vapour of water, being always present in the air we breathe; they begin to ask themselves mentally for the visible presence of great cauldrons of boiling water to supply the vapour ; and, failing these proofs, subside into a sort of wondering doubt. The great evaporating surfaces of the oceans, rivers, lakes, &c., are always silently at work ; and Faraday, in one of his popular discourses, said that sixty sacks of coal must be burnt to produce an amount of steam such as would pass away gradually from the surface of an acre of ground during an ordinary summer's day. The oroof that the atmospheric air does contain invisible steam is shown 13 t94 HEAT. by the water deposited outside a tumbler containing iced water, or water drawn from a deep well a few degrees below the temperature of the air. Evaporation is confined to the surface of the liquid exposed to the air ; and that may be stopped, as in the case of water when oil is poured upon it. If, during evaporation, vapour forms under the ordinary pressure of the air, it is necessarily increased when produced in a vacuum, because there is no resistance to be overcome ; as the first is the slow production of vapour at the surface of a liquid, so the second is the quick production of vapour. If a number of barometer-tubes are filled with mercury, and placed in a proper vessel or trough, also containing mercury, they all exhibit a height cor- responding to the existing pressure of the air ; when, however, a few drops of water, alcohol, or ether, or a small lump of ice, are introduced respectively into the separate tubes, the mercury is depressed immediately, showing the evaporation which instantaneously takes place in the Toricellian vacuum, or space above the level of the mercury in the barometer. FIG. 183. The Catgut Hygrometer. The amount of depression, showing the elastic force of the vapour, varies with each liquid. At the same time, the above experiment shows that all vola- tile liquids are instantaneously converted into vapour in a void space, or vacuum. Faraday found that there was a limit even to evaporation, and, experi- menting with mercury, he noticed that a slip of gold leaf, suspended in the neck of a bottle containing mercury, was whitened by the evaporation and condensation of the quicksilver upon the gold. This effect did not, however, take place at a temperature of about 39'2 F. With sulphuric acid, which is a very permanent fluid, the temperature of the limit of evaporation was found to be much higher, viz., about 86 F. Various instruments have been devised, from the earliest times of scientific investigation, to determine the quantity of invisible steam or moisture in the air. All cords, and especially catgut (a string made from the peritoneal linings of the intestines of the sheep;, lengthen or shorten according to the state of the moisture in the air. If a piece of catgut, made fast at one extremity, .be conveyed, as in Fig. HYGROMETRY. '95 183, over a series of pulleys, A, B, c, D, E, F, G, so as to make several turns backwards and forwards, and if a weight, P, be suspended from the other extremity, the latter will fall as the string lengthens in damp weather, and rise as the air becomes drier. This is shown better by attaching an index or pointer, H K, turning on a pivot I, in such a manner that the length I K shall be greater than I H, and pointing to a graduated arc, L L. Saussure employed a human hair for the same purpose; but all such arrange- ments infallibly become deteriorated by time. M. Le Roi was the first to suggest that the temperature at which dew begins to be deposited should be employed as the measure of the moisture of the air. De Luc also proved that the quantity and force of vapour in vacua are FIG. 1 84. Regnaulfs Condensing Hydrometer ; by Negretti and Zambra. the same as in an equal volume of air of the same temperature, or that these two elements of vapour depend upon the temperature. The determination of the exact temperature at which dew is formed, and at which, in the open air, the dew disappears or ceases to be formed on the sides of the vessel producing it, is of the utmost importance, and was carefully investigated by the late Dr. Dalton. The observation is rendered more exact with a bright metallic vessel, as in Regnault's elegant apparatus. Regnault's Condenser Hygrometer consists of two highly polished silver cylinders, into the upper part of which are cemented thin glass tubes ; these have brass covers, arranged to receive and support two delicate standard thermometers, the bulbs of which descend nearly to the bottom of the silver portion of these chambers. Each chamber has a small internal tube carried 13 2 i 9 6 HEAT. down from the brass to within a short distance of the bottom, to admit the passage of the air, which is drawn through both chambers by an aspirator, connected to the base of the hollow upright and arms supporting the cylinders. To use this hygrometer, ether is poured into one chamber sufficient to cover the bulb of the thermometer, and then the thermometers being inserted into both cylinders, the instrument is now connected to the aspirator, and by it the air is drawn through both cylinders down the internal tubes, passing in one chamber in bubbles through the ether, and in the other chamber simply around the thermometer. The tube in this empty cylinder is of such a diameter as to ensure similar quantities of air passing through each chamber. After a short time the passage of the air through the ether will cool it down to the dew-point temperature, and the external portion of the silver chamber containing the ether will become covered with moisture. The degree 'shown by the thermometer in the ether at that instant will be the temperature of tho dew-point ; the second thermometer showing the temperature of the air at the time of observation. The late Professor Daniell, who paid much attention to the construction of hygrometers, and, indeed, constructed one of the best and most simple, says : " The more accurate mode of expressing the moisture of the air from an observation of the temperature and dew-point is by the quotient of the divi- sion of the elasticity of vapour at the real atmospheric temperature by the elasticity at the temperature of the dew-point ; for, calling the term of satura- tion 1000, as the elasticity of vapour at the temperature of the air is to the elasticity of vapour at the temperature of the dew-point, so is the term of saturation to the observed degree of moisture. Thus, with regard to the obser- vation in the Deccan, where, with a temperature of 90 F., the dew-point has been seen as low as 29, making the degree of dry ness 61. Force at 90. Force at 29. 1-430 0194 looo : 135 The fourth term is the degree of moisture on the hygrometric scale." RADIATION. It is not at aH surprising that the philosophers who first commenced experi- ments with heat, or caloric, should have regarded it as an imponderable and highly elastic fluid, which clothed, as it were, the material particles of solids, fluids, and gases ; the latter attracting the former, and sometimes emitting or throwing out their caloric, which was also supposed to be repulsive of its own particles. The material theory of heat is, however, not tenable : when we consider it as radiant matter, we are reminded at once of its analogy to light, and we understand that the undulations of the same ethereal medium may propagate heat as well as light: ,it is not necessary to suppose that the ether which gives us light is interpenetrated by another kind of ether that may give us heat. The examination of the invisible heat rays in the solar spectrum assist us greatly in taking a correct view of the phenomena. Whilst enjoying the social pleasures of the fireside, we are always reminded that heat can travel, like light, through space. At night, if travelling in a steamboat across the Channel, we approach the funnel, from which invisible heat is constantly radiating, we see no fire, and RADIATION. 197 yet we can understand by the pleasant warmth experienced that waves of heat may be impinging upon us, just as the waves of water dart against the sides of the vessel. We shall find presently that light-waves may be sepa- rated from heat-undulations ; and even when they travel together, and the light only is apparent, the heat may be rendered evident in various ways, as in the use of the burning-glass, or, by permitting the rays of the sun to pass through a glass containing some ether, the rays are freely transmitted, but if a piece of charcoal is placed in the ether, the heat rays are arrested, and vibratory power is soon conferred upon the charcoal, which in its turn com- municates motion to the ether, and raises it to the boiling-point. The intensity of the heat rays decreases or increases according to the same law which affects light, viz., as the square' of the distance inversely. The in- tensity of heat is less, the greater the obliquity of the rays with respect to the radiating surface. Avoiding a source of heat which may be accompanied with light, and using a canister filled with boiling water, and placing it in the focus of a polished concave metallic reflector, the rays are collected, and can be thrown off to another reflector, when they are again brought to a focus, discoverable by an air-thermometer (p. 147.) At the Polytechnic a small bit of meat can be cooked when placed in the focus of a large concave reflector, and opposite to another standing 100 ft. away, and containing in its focus a large wire cage full of burning charcoal. FlG. 185. The large Polytechnic Metallic Reflectors. A fire in the focus of one, and the meat in the foeus of the other. The power of reflecting heat rays is influenced by the condition of the sur- face. Polished metals possess the property in the highest degree ; and if the bit of meat were covered with gold leaf it would not be warmed through, whilst the opposite effect of first blackening the meat, by dusting finely powdered charcoal over it, assists the absorption of the heat rays very greatly. Melloni discovered that out of 100 rays Silver reflects 90 Bright lead '. 60 Glass 10 Hence, if a glass concave mirror is used to reflect the rays of an ordinary fire towards the face, little or no warmth is experienced ; on the other hand, a concave tin plate will reflect the heat very sensibly. I 9 8 HEAT. It was formerly supposed that the power of a body to absorb heat was in die inverse proportion of its power to throw off or reflect heat that the two properties exactly accounted for the heat originally falling upon any given surface. This, however, is not found to be the case. The heat waves which are incident upon any given surface are disposed of in three ways : I. Some portion is absorbed. II. Another portion is reflected according to the ordinary laws which govern the reflection of light. /I I. A third portion is scattered, and is then called diffused heat. The thinnest film of gold leaf will protect the parts of a sheet of paper exposed to radiation from some red-hot surface, whilst the blackening of any portion of the same sheet of paper will hasten its destruction. Radiation and absorption, according to the experiments of Leslie, are directly proportioned to each other ; a blackened tin vessel full of hot water, that will radiate heat freely, and soon fall to the temperature of the air, will, on the other hand, as rapidly increase in temperature if held near any good source of heat. The relation between radiation, absorption, and reflection, and the manner in which the two first may balance each other, was elegantly shown by the late Dr. Ritchie. He used for his experiments a metallic vessel filled with hot water, and a differential thermometer, one bulb of which was shielded by a bright metallic disc, and the other with a blackened one ; one surface of the metallic box containing the water was also polished, and the other blackened. When the blackened side of the box was placed opposite to the bright metallic screen, no effect was produced on the thermometer, because the radiating power of the black surface was neutralized by the non-absorp- tive and good reflecting power of the bright metallic disc. If, however, the same side was opposed to the blackened disc, then the thermometer was affected. Similarly, but reversely, when the polished side of the box was opposed to the blackened disc, little or no effect was perceptible, because the highly polished surface did not radiate heat easily. If boiling water be poured into two tea-pots, one of which is of bright block tin, and the other of black japanned tin, the latter cools more quickly than the former. The air exercises a retarding power on the waves of heat which are absorbed, and, as proved by Sir H. Davy, they travel much easier through a vacuum. Davy ignited charcoal points by a current of electricity, and, placing them in the focus- of a concave mirror, discovered that, when the receiver was exhausted to i- 1 20th, the effect upon a thermometer placed in the focus of another reflector was nearly three times as great as when the air was at its ordinary pressure. The absorptive power of bodies was supposed to depend greatly upon the particular colour used. Franklin placed pieces of coloured cloth in the sun's rays on the snow, and found they sank into the snow or melted it in the following order : black, blue, green, purple, red, yellow, white. Tyndall, how- ever, has explained the cause more correctly, and has discovered that the colour has not so much to do with the effect produced as the nature of the material used for the colouring agent. Although it has been stated by Leslie that white surfaces generally reflect heat well, and absorb it indifferently, there is the curious fact, ascertained by Melloni, that white lead has quite as great an absorbent power as lampblack ; and if the heat comes from boiling water (column i), it will absorb twice as much as it would do if it came from an RADIATION. 199 incandescent platinum wire. Melloni (p. 201) filled a copper canister with water, and kept it at the boiling-point, and by means of a very delicate instru- ment, called the thermo-multiplier, obtained the following relative absorptive powers, as shown in column I. If, however, the heat is derived from an incan- descent platinum wire, as in column 2, the figures are different ; and white lead is found to absorb a less quantity of the rays of heat when they are luminous, and Indian ink more. No. t. No. a. Lampblack . . . . 100 . . . 100 White lead .... 100 ... 56 Isinglass .... 91 ... 54 Indian ink .... 85 ... 95 Shellac .... 72 ... 47 Metals 13 ... 13*5 Leslie's principle does apply to clothing, and it appears that if we imitate nature, and, like the Polar bear, wear white, we shall be warmer in winter and cooler in summer. In running streams, and even in the Rhine, what is called "ground ice" is frequently found. This is no contradiction of the laws already explained with reference to the cooling of water. The ice is formed at the bottom of the stream, because the stones and other earthy matters forming the bed of the river emit or radiate heat when the sky is very clear ; and as the water of the stream is mixed by the current, and the temperature of the bed of the river is lowered by radiation, the ice forms in spongy masses, which may rise to the surface, carrying stones and even the anchors of ships with them. The rays of heat are more readily absorbed when they fall upon bodies at angles near the perpendicular ; hence the rays of the sun are hotter in summer than in winter, when they are more oblique. If the bulb of an air-thermometer be brought near a burning hydrogen flame, its radiating power is found to be very low, although, as is well known, the heat of the flame is so great that it will quickly ignite a spiral of platinum wire ; when the heat waves are set in motion, emission or radiation takes place, which will promptly affect the thermometer. Tyndall has investigated the radiating and absorbing powers of gases and vapours, and, although they are feeble, he has been able to discover that vapours and compound gases have a much greater absorbing and emitting power than any simple or elementary gas, such as oxygen or nitrogen, or when they are mechanically mixed, as in atmospheric air. Had our globe been surrounded with a gas like olefiant gas, the absorbent power would have been 240 times greater than that of oxygen. Amongst gases, those which absorb heat the most also radiate it freely. As might be expected from the analogy between light and heat waves, the latter may be reflected, refracted, may undergo double refraction, be absorbed, and even polarized ; the latter fact being proved by the use of tourmaline plates or bundles of plates of mica. 200 HEAT. TRANSMISSION OF HEAT. Melloni's name will ever be associated with all the more important experi- ments in which the course of heat-waves is traced through various media. As with light there are bodies called transparent, diaphanous, translucent or transparent, opalescent, and opaque, so with reference to the power of trans- mitting heat, bodies generally are divided into two classes : I. Diathermanous or diathermic bodies (Sia, through, and tfepyuos, heat), permitting heat-waves to travel through their substance. Examples rock salt and certain elementary gases. II. Athermanous or adiathermic bodies, which arrest or stop the progress of the heat-undulations. Examples all liquids in variable proportions ; alum in crystal and solution. Mr. B. Stewart has shown that bodies of the first class are bad radiators of beat, but that those of the second or adiathermic class are eood radiators. It does not follow, because substances like the diamond, glass, ice, &c., ermit light-rays to pass through them, that they will also allow the heat- rays to travel through in the same proportion. Glass permits the light to pass freely through its substance, but stops a considerable number of the heat-undulations ; and alum, nearly all. Rock salt is the only substance which is entitled to be placed in the first or true diathennanous class, and although it does, according to Krupland and Stewart, absorb certain of the heat-rays more than others, still at present it stands first, and is therefore used in the form of plates, prisms, and lenses for these delicate experiments. Melloni found that certain solids, cut into plates one-tenth of an inch in thickness, allowed the following percentage of heat waves from an Argand lamp to pass : Rock salt ..... 92, transparent Plate glass and Iceland spar . . 62, Smoky quartz 57, nearly opaque Transparent carbonate of lead . 52, transparent Selenite ,20, Alum 12, Sulphate of copper. ... o, deep blue With liquids, when the source of heat was an Argand oil lamp, and the fluids enclosed in a glass cell, the results given in Table I. were obtained. Table II. shows the results obtained by Tyndall from liquids enclosed in a rock-salt box, the source of heat being an ignited platinum wire : Table I. Table II. Bisulphide of carbon 63 83 transparent Olive oil 30 Chloride of sulphur ... 63 red Ether 21 41 Sulphuric acid .... 17 41 Alcohol 15 30 Solution of alum or sugar . .12 30 Water (distilled) 11 30 Water saturated with salt . 26 Rock salt stands in the same relation to heat, so far as transparency to TRANSMISSION OF HEAT. 201 FlG. 1 86. Mellon? s Apparatus. Argand oil lamp without a glass; spirit-1 imp and platinum wire , the copper box, blackened, to con- tain water at 212 F. ; stand, to place the ob)ec's upon, screen, with apertures of various sizes j the thermo-multiplier current, with the galvanometer needle. heat-rays is concerned, as colourless glass does to the light-rays. When a hot metallic ball is placed between the bulbs of a differential thermometer, the liquid remains stationary, because both are equally heated ; if, however, a plate of rock salt is interposed as a screen on one side of the ball, and a plate of glass on the other, the thermometer is immediately affected, as more rays pass through the rock salt than through the glass. Melloni's apparatus for these investigations may be regarded as the model of perfection. It includes the various sources of heat, such as a naked flame, an ignited platinum wire, a blackened copper vessej containing water at 100 C. (212 F.), or a copper plate heated 10400 C. (752 F.), and is plainly shown in Fig. 1 86. The delicacy of the thermo-multiplier as an indicator or measurer of heat is most remarkable, and it will be fully explained in another part of this work. The minute electrical currents set up in the thermo-multiplier are recorded by the galvanometer needle. It has already been shown that in bodies which arrest partially or wholly the heat-waves, the nature of the heat, or rather the particular source from which it is obtained, has a great influence upon the result. Thus fluor-spar permits 33 per cent, of the heat-waves derived from boiling water to pass through its substance, whilst the power rises to 78 per cent, when the source of heat is a burning lamp. Heat-waves which have passed through one plate of glass will also pierce another, with a small amount of loss ; the same waves are nearly all stopped b) alum. 202 HEAT. Tyndall's discovery, that the vapour of water absorbs thirteen times more obscure heat than air, is a most important fact, and shows why the air con- taining vapour nearer the earth is warmer than that which is dry and found on the summit of lofty mountains. The dry air allows the obscure heat- waves to travel through, and is too diathermanous, whilst air charged with moisture has considerable athermaneity for obscure rays, which are produced when the rays of the sun have passed through our atmosphere and fallen upon the earth. When the rays of the sun fall upon the earth to warm it, they are radiated and then diffused ; a change in their quality takes place, and they become obscure rays of heat. It is these obscure rays which melt snow, and perform other useful offices. THE CONVERSION OF LIGHT RAYS INTO HEAT RAYS, AND VICE VERSA, BY CHANGE OF REFRANGIBILITY. At the meeting of the British Association, held at Newcastle, in 1863, Dr. Akin proposed three experiments for the conversion of rays of light into heat- rays ; of these one is deserving of notice, viz., the proposal to collect the rays of the sun in a concave mirror, and then to cut off the light with " proper absorbents," and to bring platinum foil into the focus of invisible rays. Although Dr. Akin was the first to propose definitively to change the refran- gibility of the ultra-red rays of the spectrum by causing them to raise platinum foil to incandescence, yet the chief merit, in connection with this branch of heat, is due to Dr. Tyndall, because, in the spirit of Lord Bacon, he was not content with a theory which merely suggested that a certain result might be obtained, but industriously worked out the crude idea, and proved that it was substantially true, by devising a number of clever and original experiments, which had never been shown before. In the article on Light (p. 92), the change of refrangibility of certain rays "at the violet end of the spectrum,, and the beautiful experiments with " fluorescence," by Professor Stokes, have already been specially considered. And just as he obtained a large proportion of these r?ys, existing in and beyond the violet, by using prisms of quartz, so Melloni, by using a prism of rock-salt, was en- abled to prove that the ultra-red rays discovered by Sir W. Herschel formed an invisible heat spectrum as long as the visible one. Other experimentalists continued the -investigation, especially Professor M tiller, of Freiberg, who worked out a curve expressing the heating power of the whole spectrum ; but it was left for Tyndall to complete the investigation, and directly isolate the invisible or obscure rays of heat ; and as Stokes, by lowering the refrangibility of the invisible ultra-violet rays, rendered them visible, so Tyndall, by raising the refrangibility of the ultra-red rays, rendered them also visible. The instru- ments he used, to quote his own words,* " consisted of the electric lamp of "Duboscq and .the linear thermo-electric pile of Melloni. " The spectrum was formed by means of lenses and prisms of rock-salt ; it was equal in width to the length of the row of elements forming the pile; and the latter being caused to pass through its various colours in succession, and also to search the space right and left of the visible spectrum, the heat falling upon it at every portion of its march was determined by the deflection of an extremely sensitive galvanometer. * "Proceedings of the Royal Institution of Great Britain," vol. iv., partj. Professor Tyndall, "On Combustion by Invisible Rays." INVISIBLE HEAT RAYS. 203 D REO.OMNCE. YEUOW. GREEN ^BLVE FIG. 187. Dr. TyndalVs Diagram. " As in the case of the solar spectrum, the heat was found to augment from the violet to the red, while in the dark space beyond the red it rose to a maxi- mum. The position of the maximum was about as distant from the extreme red in the one direction as the green of the spectrum in the opposite one. " The augmentation of temperature beyond the red in the spectrum of the electric light is sudden and enormous. Representing the thermal intensities by lines of proportional lengths, and erecting these lines as perpendiculars at the places to which they correspond, when we pass beyond the red these perpendiculars suddenly and greatly increase in length, reach a maximum, and then fall somewhat more suddenly on the opposite side of the maximum. When the ends of the perpendiculars are united, the curve beyond the red, representing the obscure radiation, rises in a steep and massive peak, which quite dwarfs by its magnitude the radiation of the luminous portion of the spectrum. " Interposing suitable substances in the path of the beam, this peak may be in part cut away. Water, in certain thicknesses, does this very effectually. " The vapour of water would do the same; and this fact enables us to account for the difference between the distribution of heat in the solar and in the electric spectrum. The comparative height and steepness of the ultra-red peak in the case of the electric light are much greater than in the case of the sun, as shown by the diagram of Professor Miiller. No doubt the reason is, that the eminence corresponding to the position of maximum heat in the solar spectrum has been cut down by the aqueous vapour of our atmosphere.. Could a solar spectrum be produced beyond the limits of the atmosphere, it would probably show as steep a mountain of invisible rays as that exhibited by the electric light, which is practically uninfluenced by atmospheric absorp- tion. " Having thus demonstrated that a powerful flux of dark rays accompanies the bright ones of the electric light, the question arises, ' Can we not detach the former, and experiment on them alone?' " In the author's first experiments on the invisible radiation of the electric light, blaok glass was the substance made use of. The specimens, however, 204 HEAT. which he was able to obtain destroyed, along with the visible, a considerable portion of the invisible radiation.* But the discovery of the deportment of elementary gases directed his attention to other simple substances. He exa- mined sulphur dissolved in bisulphide of carbon, and found it almost perfectly transparent to the invisible rays. He also examined the element bromine, and found that, notwithstanding its dark colour, it was eminently transparent to the ultra-red rays. Layers of this substance, for example, which entirely cut off all the light of a brilliant gas-flame, transmitted its invisible radiant heat with freedom. Finally, he tried a solution of iodine in bisulphide of carbon, and arrived at the extraordinary result, that a quantity of dissolved iodine sufficiently opaque to cut off the light of the mid-day sun was, within the limits of experiment, absolutely transparent to invisible radiant heat. " This, then, is the substance by which the invisible rays of the electric light may be almost perfectly detached from the visible ones. Concentrating by a small glass mirror, silvered in front, the rays emitted by the carbon points of the electric lamp, we obtain a convergent cone of light. Interposing in the path of this concentrated beam a cell containing the opaque solution of iodine, the light of the cone is utterly destroyed, while its 'invisible rays are scarcely, if at all, meddled with. These converge to a focus, at which, though nothing can be seen even in the darkest room, the following series of effects may be produced : " When a piece of black paper is placed in the focus, it is pierced by the invisible rays, as if a white-hot spear had been suddenly driven through it. The paper instantly blazes, without apparent contact with anything hot. u A piece of brown paper placed at the focus soon shows a red-hot burning surface, extending over a considerable space of the paper, which finally bursts into flame. " The wood of a hat-box similarly placed is rapidly burnt through. A pile of wood and shavings, on which the focus falls, is quickly ignited, and thus a fire may be set burning by the invisible rays. " A cigar or a pipe is immediately lighted when placed at the focus of invi- sible rays. " Discs of charred paper placed at the focus are raised to brilliant incan- descence ; charcoal is also ignited there. " A piece of charcoal, suspended in a glass receiver full of oxygen, is set on fire at the focus, burning with the splendour exhibited by this substance in an atmosphere of oxygen. The invisible rays, though they have passed through the receiver, still retain sufficient power to render the charcoal within it red hot " A mixture of oxygen and hydrogen is exploded in the dark focus, through the ignition of its envelope. " A strip of blackened zinc-foil placed at the focus is pierced and inflamed by the invisible rays. By gradually drawing the strip through the focus, it may be kept blazing with its characteristic purple light for a considerable time. This experiment is particularly beautiful. " Magnesium wire, presented suitably to the focus, burns with almost into- lerable brilliancy. "The effects thus far described are, in part, due to chemical action. The substances placed at the dark focus are oxidizable ones, which, when heated sufficiently, are attacked by the atmospheric oxygen, ordinary combustion 1 "The glass in thin layers had a greenish hue: 1 have since found black glass far more diathermic." INVISIBLE HEAT RAYS. 205 being the results. But the experiments may be freed from this impurity. A thin plate of charcoal, placed in vacuo, is raised to incandescence at the focus of invisible rays. Chemical action is here entirely excluded. A thin plate of silver or copper, with its surface slightly tarnished by the sulphide of the metal, so as to diminish its reflective power, is raised to incandescence either in -vacua or in air. With sufficient battery-power and proper concentration, a plate of platinized platinum is rendered white hot at the focus of invisible rays ; and when the incandescent platinum is looked at through a prism, its light yields a complete and brilliant spectrum. In all these cases we have, in the first FIG. 1 88. TyndalVs Apparatus for showing the heating-power of the Invisible Rays. A, the lantern containing the electric lamp and silvered mirror; B, the plate-glass trough, having an outer jacket, through which cold water circulates, to prevent the solution of iodine in bisulphide of carbon boiling ; c, the cistern of water and pipe passing to jacket, B, and flowing away to D -, E, stand to cairy zincfoil. place, a perfectly invisible image of the coal-points formed by the mirror ; and no experiment hitherto made illustrates the identity of light and heat more forcibly than this one. When the plate of metal or of charcoal is placed at the focus, the invisible image raises it to incandescence, and thus prints itself visibly upon the plate. On drawing the coal-points apart, or on causing them to approach each other, the thermograph of the points follows their motion. By cutting the plate of carbon along 'the boundary of the thermograph, we might obtain a second pair of coal-points, of the same shape as the original ones, but turned upside down ; and thus by the rays of one pair of coal-points, ao6 HEAT. which are incompetent to excite vision, we may cause a second pair to emit all the rays of the spectrum. " The ultra-red radiation of the electric light is known to consist of ethereal undulations of greater length, and slower periods of recurrence, than those which excite vision. When, therefore, those long waves impinge upon a plate of platinum, and raise it to incandescence, their period of vibration is changed. The waves emitted by the platinum are shorter and of more rapid recurrence than those falling upon it; the refrangibility being thereby raised, and the invisible rays rendered visible." ELECTRICITY, FRICTIONAL OR STATICAL. '"THERE is no branch of science more fascinating to the youthful mind than . this most curious form or mode of motion. By motion it is evoked. There is nothing more to do than to rub some body, such as glass or sealing-wax, with silk or flannel, or to lay a warm sheet of brown paper on a tea-tray, and rub it well with india rubber ; and the electric force becomes apparent, either by creating motion again, causing light substances, such as feathers or the down of feathers, to move towards the surface on which the force has been set free, or if observed in a darkened room, the sheet of brown paper is found to give light, a crackling sound is heard, and small sparks are visible as the sheet of paper is drawn up from the tea-tray. This can be done over and over again. It is only necessary to dry the paper by holding it before the fire, and the same attractive power, the same curious fire, is apparent. The sheet of paper itself, after being well rubbed, will move towards the body of the person who holds it up by one corner, and is said to be attracted because it is electrified or electrized. One of the " seven wise men of Greece," named Thales, from whose school at Miletus, in Ionia, came Socrates and his disciples, has always been con- sidered as the first who introduced a scientific method of philosophising among the Greeks, 600 years before the Christian era. To this philosopher is ascribed the following : " That God is the most ancient being, who has neither beginning nor end ; that all things are full of God ; and that the world is the beautiful work of God." A principle of motion, wherever it exists, is, according to Thales, mind. 208 ELECTRICITY. Hence he taught that the magnet and amber (r/AeKrpov) are endued with a soul, which is the cause of their attracting powers.* It is from the Greek name of amber, a fossil resin, that the science derives its name " Elec- tricity." There are many substances which are electrized by friction gutta-percha, the skin of a cat, sulphur, the different resins, and especially shellac, the chief constituent of good sealing-wax, glass, and the greater number of crystals, &c. On the other hand, there are many bodies, such as the metals, in which, apparently, the power cannot be developed. The earlier experimentalists divided all bodies into electrics and non-electrics: the former they considered could be electrized by friction; the latter, apparently, not so. It was then discovered that this classification was not a correct one, and that the reason the so-called non-electrics did not show any electrical energy when rubbed was because of their " conductivity ; " as fast as the electricity was produced, it was conducted away to the earth and lost. Finally, they discovered, by cutting off the conducting communication with the earth by attaching the so-called non-electrics, such as a rod of metal to one of glass and then rubbing it, that now the metal could attract light particles down, pith of the elder, gold leaf, c. and was then said to be " insulated." An instrument had now to be invented to indicate the disturbance of electrical equilibrium : this instrument was appropriately called an " electroscope," or instrument for showing electrical excitation. Commencing with the more simple forms, we may trace them up to the most refined and delicate instru- ments. FlG. 1 89. A simple form of Electroscope. A, the needle and cork; B, the cup attached to the feather. I, The mouth of a clean, dry, empty wine-bottle is closed with a cork, through which a short needle has been passed, the point being up- * " Enfield's History of Philosophy," p. 82. ELECTRICITY. 209 wards. On this point is balanced an eagle's feather, to which a little cup made of glass, or any other convenient hard substance, has been fixed. The glass cup, or cap, with the feather attached, resting on the point of the needle, offers little or no resistance or friction, and hence the feather moves freely like a suspended magnet in any direction. When a stick of sealing-wax is rubbed and advanced towards the feather, the latter is immediately attracted, and will follow the sealing- wax round with great rapidity. After the feather has been touched several times by the electrized wax, it is now found, on approaching the electrified sealing-wax, that the feather is repelled not so energetically as it was attracted, but quite sufficiently so as to be distinctly apparent. " Attraction " and " repulsion " are thus illustrated : II. A glass tube or rod is bent at right angles, and the end fixed to some convenient support, viz., a round or square piece of wood. A pith- ball suspended from it by a silk filament becomes a sensitive and simple electroscope or electric pendulum. FIG. 190. An Electroscope. A A, the glass support j B, the pith-ball suspended; c, the electrized glass. If two balls are suspended side by side, and the electrified wax or glass brought towards them, they are found, after being attracted to and touching the electrized glass, to repel each other. "Attraction" and "repulsion" are again demonstrated: Another modification of the above may be arranged by making two similar supports, like that in Fig. 190, and suspending a pith-ball from each. If the two balls placed close together are electrized, they repel each other; but if the two stands are moved a little way from each other, 14 110 ELECTRICITY. and one electrized with the rubbed glass and the other with the rubbed wax, the two balls attract each other. FIG. 191. The two Stands and Pith-balls. G is electrified with the rubbed glass ; w, with the rubbed wax. N.B. A little tinfoil neatly pasted round the joints where the threads are suspended assists the accumulation of electricity ; and if the pith- balls are gilt and suspended by very fine hair-like wires of silver or gold, the effects are more decided the pith-balls do not cling together. In this experiment it would appear that the electricity from glass attracts that from the wax; whilst separately (Fig. 190) they are mutu- ally repulsive of their own particles, and hence one electricity was called vitreous and the other resinous. III. A very delicate electroscope is that in whicn the material to be moved by the electrical force is itself remarkably light, and must be screened from the air to prevent it being agitated or blown off by any current of wind suddenly impinging upon it. The material is gold leaf, which can now be purchased in books cut ready for use. It is usual to attach two gold leaves to the opposite sides of a thin plate of brass, or card covered with gold paper ; this is held by a pair of pincers, at the end of a brass rod passing through a glass tube cemented in a brass cap, at- tached to a bell-glass. By this mode of suspension the brass wire, which terminates with a circular brass plate or table, is supported on the glass tube (a bad conductor of electricity), and the tube and cup are again supported by the bell-glass, so that good insulation is secured. When great refinement is required, it is usual to place a glass shade over the whole ; the latter is perforated at the top with a hole, about one inch in diameter, through which the brass rod and table are passed, and lumps of lime being placed in both glasses, the air is kept dry, and, the aqueous vapour being absorbed, there is no deposit of dew-like moisture under either of the glasses. (Fig. 192.) A, brass table or disc, with wire attached, and pincers P, to hold the . gilt card to which the gold leaves are attached ; C, the inner bell-glass, ELECTRICITY 211 upon which the cap carrying the glass tube D, through which the brass rod passes, is cemented ; E E, the outer glass shade, perforated with a hole in the top, about I in. in diameter, to allow the brass rod to pass through. N.B. The table or round plate unscrews from the wire, in order to allow this to be done : both the inner bell-glass and the outer glass shade fit nicely into grooves made in a square mahogany stand, G G, neatly fitted with a drawer to hold quicklime. The part of the stand covered with the two glasses is perforated with holes, in order that the desiccating power of the lime may take full effect on the air enclosed by the two glasses. It is sometimes usual in this electroscope, called Bennet's, to place two rods and balls in the stand; so that, if the gold leaves are too highly charged, they may not be torn off, but, by touching the brass rods, the excess of electricity, which might damage FIG. 192. A more delicate Electroscope. the instrument, is carried off to the earth. For other reasons, the brass rods connected with the earth exalt the power of the electricity applied, however feeble it may be. An electrized glass rod, brought towards the cap of the instrument, causes the gold leaves to diverge or repel each other ; when left diver- gent with the electricity from glass, they instantly fall on the approach of an electrized piece of wax. The little table is convenient for stand- ing any object on, or else a plain ball would perhaps be a better ter- minal, as the edges of the table, unless nicely rounded, are apt to dis- sipate the electricity. It is not necessary to touch the cap of the electroscope with the 14 2 212 ELECTRICITY. electrized bodies, in order to pass into or on the rod connected with the gold leaves the electricity we wish to examine. By an influence called " induction," to be more fully explained hereafter, the gold leaves are found to possess the same kind of electricity as that enjoyed by the electrized body. Another electroscope invented by Dr. Robert Hare, of the University of Pennsylvania, in which one gold leaf only is used, is worthy of par- ticular notice here, and is described in Noad's " Manual of Electricity:" " The leaf, about 3 in. long and 3-ioths of an inch wide, is suspended, according to Singer's method, in the centre of a globular or other shaped glass vessel from a brass wire surmounted with a brass cap. A similar rod of brass, carrying at each end a small disc of brass or gilt wood, about half an inch in diameter, passes through the side of the vessel, so that the internal disc shall be immediately opposite the lower end of the suspended leaf. This wire slides freely through a socket, so that the internal disc may be adjusted at any required distance from the leaf. " When it is employed to detect electricity, the lateral wire is uninsulated by hanging a wire from it to the earth, and the body to be tested is brought into contact with the cap. If the distance between the gold leaf and the disc B is very small, the most minute force of attraction is rendered apparent. When it is required to determine the kind of elec- tricity with which a body is charged, the insulated disc B is brought as near as possible to the leaf, and electrified vAhzv positively (with excited glass) or negatively (with excited wax) ; the gold leaf is first attracted, and then repelled. Under these circumstances the body to be tested is brought into contact with the cap or with D : if its elec- tricity be of the same nature as that with which the leaf is charged, the latter will diverge more freely; if of the contrary nature, it will collapse towards B. " By placing a gilt disc on each side of the gold leaf, Mr. Gassiot obtained signs of electrical exci- tation from a single cell of the voltaic battery." From the preceding experiments the following conclusions may be arrived at: I. That an electrified body has the power to attract another which is not electrical. II. That two bodies similarly electrified repel each other. III. That the electricity derived from glass is different from that obtained from wax ; and that, being dissimilar, they attract each other. IV. The two electricities have names to distinguish them from each other: one is called vitreous, because obtained from glass ; and the other resinous, because usually obtained from sealing-wax. The whole is summed up in the two simple statements : Similar electricities repel each other ; dissimilar electricities attract each other. V- The electricity a substance gives out by friction is not always the same, but depends on the nature of the rubber used, and other circumstances. FIG. 193. Dr. Robert Hare's single- leaf Electroscope' THEORIES OF ELECTRICITY. 213 Glass, when rubbed with a cat's skin, gives resinous electricity, and vitreous if rubbed with silk. Polish and temperature, as shown by De la Rive, exercise a remarkable influence. When bodies are highly polished, they have a greater tendency to give by friction vitreous elec- tricity, or to acquire it ; by elevating the temperature of bodies, they have a greater tendency to acquire resinous electricity. A piece of roughened or ground glass, rubbed against a smooth and highly polished piece of glass, becomes resinous, whilst the smooth glass is negative. VI. No single electricity can be evolved without an equal excitation of the other or opposite electrical force ; the rubber and the substance rubbed are always in opposite states the silk handkerchief being resinous, the glass vitreous. Electricity being, as it were, a resident in all substances, it is said to be quiescent when the two opposite forces have neutralized each other. It is then called the static state of electricity; and this state is supposed to be the normal condition of all bodies before they become electrical. When the two electricities travel towards each other, or pass in sparks through intervals of air, or move insensibly along a wire or other conductor, it is said to be in a dynamic state, or condition of motion or circulation, which becomes very evident in watching the motion of an electrical machine, or the single voltaic circle of zinc and copper placed in acid and water. The dynamic state is sometimes spoken of as electric tension, and an electric current as a continuous dynamic state. THEORIES OF ELECTRICITY. By the theory of Du Fay, as altered by Symmer, it is supposed that two forces, called fluids, exist in every substance, whatever may be its nature solid, liquid, or gaseous. Each of the two fluids is supposed to be very subtile and rare, quite impon- derable, and consisting of particles that repel each other. When the two fluids are separated, electrical effects are obtained ; and when they unite, the electrical power ceases, for they have now combined to form neutral fluid, or natural electricity. As before stated, one electricity is called vitreous, and the other negative. The repellent nature of the electrical particles is supposed to cause them to arrange themselves on the surface of conducting bodies, where they remain, because they are checked in their movement by the non or badly conducting air with which they are surrounded. Non or bad conductors are supposed to retain the fluids, and to interfere with their movements. This theory of Symmer is a most convenient and simple one for the young student, and will help him to fix the main experimental truths of electricity in his mind. The second theory, devised by Benjamin Franklin, supposes that one fluid only exists, the particles of which mutually repel each other. The electrical fluid is supposed to be combined with all matter : matter without electricity is supposed to be repulsive of its own particles. When a body is in a quiescent 2i4 ELECTRICITY. electrical state, then the matter is exactly saturated with electricity, and it is in a natural condition. If the substance is rubbed, it either gains or loses the electrical fluid. The acquisition of more electricity is said to confer a plus or positive state of elec- tricity : the loss of the electricity places the substance in a minority with regard to electricity; it is now said to be indued with minus or negative electricity. What Symmer terms vitreous electricity Franklin calls positive electricity; what Symmer styles resinous electricity is called by Franklin negative electri- city. It is of little consequence which theory we adopt, for one or the other must be wrong; most likely, both are untrue. We have seen that a certain vibra- tion of particles will produce invisible heat rays, and, when they are quickened in their pulsation, light rays ; as in TyndalFs experiments, the concentrated invisible rays of heat, falling on a piece of platinum-foil, are converted into visible or light rays. The same wave theory will doubtless be ultimately applied to electricity, which may only be some remarkable vibratory state of the ether pervading all matter and space. And this opinion was held, forty years before Galvani, by Sultzer, who first experimented with pieces of silver and lead. By placing them on opposite sides of the tongue, and then bringing the two in contact, he noticed a peculiar metallic taste, like vitriol. Here again it will be understood why so much space was devoted to the consideration of the "universal ether," at the commencement of the article on Light. EXPERIMENTS WITH THE ELECTROSCOPE. An electroscope is easily made with a wide, clean lamp-glass. A cork is fitted into it, and through the cork is passed a wire, one end of which is beaten out, so as to give a sufficiently large and flat surface; a pair of small gold leaves are attached to this end of the wire, and to the other is fixed a round piece of cardboard, covered with tinfoil or gold paper. When the wire is passed through the cork, the gold leaves may be attached by moistening the flattened end of the wire with a little gum, and bringing it carefully down upon the cut gold leaves in the book. The second gold leaf is the most diffi- cult to get on. When both leaves are in their places, the cork, wire, and leaves may be placed in the lamp-glass, and the cardboard table fixed on the wire. I. A little coffee, quickly ground in a mill, received in a warm dry beaker glass, and then sprinkled upon the table or plate of the electroscope, causes the leaves to diverge. II. Some whiting or chalk, dried and put into the valve of a pair of bellows, and then forced out upon the electroscope with the wind, very soon causes the leaves to be deflected. III. A large lump of sugar held over the electroscope, and sawed in various places with a saw, affects the instrument as the sugar-dust falls upon it. IV. After playing a tune on a violin with a dry and well rosined bow, if the latter is passed lightly over the electroscope, electrical excitation is apparent. V. A roll of dry warm flannel rubbed against a stick of sealing-wax EXPERIMENTS WITH THE ELECTROSCOPE. 215 causes the leaves of the electroscope to stand out, and repel each other ; but they fall directly the sealing-wax is applied, because the two electrical and opposite forces vitreous from the flannel and resinous from the wax neutralize each other, the rubber and the substance rubbed giving always the opposite states. VI. While the leaves are divergent with the rubbed wax, bring an excited glass rod or tube towards the electroscope, as before ; the leaves fall immediately. VII. Mr. Symmer, whose name has already been mentioned in connection with one of the theories of electricity, tried some very amusing ex- periments with silk stockings. He put upon the same leg a worsted stocking, and over that a silk one, and rubbing the outer stocking before a fire, he slipped the silk one suddenly off, and, the sides re- pelling each other, the stocking appeared to be inflated, and to retain the same shape as if the leg were in it ; and of course, if the silk stocking had been carefully approached towards the electroscope, the leaves would have been rendered powerfully divergent. VIII. A crystal of Iceland spar cemented to an insulating glass rod, then pressed in the hand, and placed immediately on a very delicate elec- troscope, will cause a slight divergence. IX. A disc of insulated cork, gently warmed and simply pressed against another one of the same material, will show a certain minute amount of electrical energy when applied to the electroscope, the warm disc being usually resinous, and the cold one vitreous. X. A stick of sealing-wax broken, and the fractured portion applied to the electroscope, gives abundant evidence of electrical excitation. XI. On a sheet of mica place the end of a stick of sealing-wax whilst in the melted state, and as hot as possible ; allow the stick of wax to cool and to adhere to the mica. If now the wax is suddenly pulled so as to tear away a film, the fracture will disturb the electrical quiescence of the mica, and it affects the leaves of the electroscope. XII. A roll of sulphur broken across, and the bits powdered up in a mortar, produce a very lively effect upon the gold leaves when brought in contact with the cap or table of the electroscope. XIII. The crystals of tartaric acid, boracite, and the tourmaline all become electrically excited when heated, and affect the electroscope. Choco- late fresh from the mill, as it curls in the tin pans in which it is received, becomes strongly electrical. When turned out of the pans, it retains this property for some time, but soon loses it by handling. Melting it again in an iron ladle, and pouring it into the tin pans as at first, will, for once or twice, renew the power ; but when the mass becomes very dry, and powdery in the ladle, the electricity is revived no more by simple melting ; but if then a little olive oil be added, and mixed well with the chocolate in the ladle, on pouring it into the tin pans, as at first, it will be found to have completely recovered its electric power. M. Becquerel's experiment with heating the tourmaline is performed as follows : The crystal of tourmaline is supported in a stirrup of paper, attached to a few filaments of silk, hung on to an insulating rod of glass, attached to an upright pillar, so that it can be moved up or down. The crystal is lowered so as nearly to touch a plate of copper, 2l6 ELECTRICITY. heated below with a spirit-lamp ; and resting on the plate is a cylin- drical glass, open top and bottom, like a wide but short lamp-glass. Two pieces of covered bent wire, each carrying a little disc of gilt paper, are placed over the top edge of the cylinder, and so arranged that each disc shall nearly touch the end of the crystal ; or, better still, the cylinder is perforated with two holes, opposite each other, and the wires cemented in with their discs, and made to face the poles or ends of the tourmaline. If each wire is separately con' FIG. 194. BecquereVs experiment with the heated Tourmaline. A, the suspended and heated tourmaline; B+, the wire conveying the + or vitreous electricity to the electroscope c + ; B , conveying the or negative electricity to the electroscope c ; D, the spirit-lamp heating the copper plate E. nected with a delicate electroscope having very small gold leaves, and the crystal warmed and then raised so as to be opposite to and just touching the little gilt discs, one end of the crystal will give vitreous or -j- electricity, the other resinous or electricity. The effect is most powerful whilst the temperature is rising; when tr? temperature becomes fixed, the electrical effect ceases. On reversing the experiment and allowing the tourmaline to cool, the electricity again becomes apparent ; but the electrical poles of the crystal are reversed, the end that was -f- whilst being heated becoming in EXPERIMENTS WITH THE ELECTROSCOPE. 217 the act of cooling. If the crystal is broken, the fragments, like the parts of a broken magnet, each exhibits the opposite electricities at their extremities. M. Gaugain states that the crystal should not be heated beyond about 302 F. If raised to 752 F., the tourmaline becomes a conductor of electricity ; it recovers its insulating powei on cooling, but is then rendered hygroscopic ; this property it again loses on being washed and dried at 302 F. XIV. In the article on Electrical Induction, a still more delicate electroscope called Volta's condenser electroscope, and another termed Peclet's Multiplying Condenser, will be described. With the first of these instruments the electricity derived from " chemical action " is dis- tinctly shown. A clean platinum capsule, containing some distilled water, is placed upon the Volta electroscope ; into this is immersed a plate of zinc connected by a wire with the earth. The liquid acquires a very feeble charge of + or positive electricity, and the metal is found to be or negative : the very slight oxidizing power of the water upon the zinc is supposed to produce this result. There is no advantage gained by the addition of a little sulphuric acid, because the conducting power of the water is increased, and the two electricities have a tendency to re-unite directly they are liberated : hence pure water is the best for this experiment. XV. With the same electroscope (Volta's) the electricity eliminated by combustion may be rendered apparent. The carbonic acid is allowed to impinge upon a metallic plate placed in conducting communica- tion with the instrument, the charcoal being burnt in connection with the earth. The electricity is extremely feeble, but is found to be definite, the carbon being --or negative, whilst the carbonic acid is + or positive. The combustion of hydrogen gas produces water ; and in this combination of the former with oxygen, the hydrogen is found to be or negative, and the steam + or positive. XVI. It was contended by Pouillet to whom we are indebted for a large number of these delicate experiments that when water is evaporated electricity is always liberated ; if the water was alkaline, it charged the electroscope with positive, if acid, with negative electricity; hence it was easy, and seemed feasible, to propose a theory which should account for the accumulation of electricity in the clouds, the enormous amount of evaporation going on from the surface of rivers, lakes, seas, being supposed to be a constant source of electric power. Peltier has shown that the electrical effects are most likely due to friction of the evaporating fluid against the sides of the vessel, as the electricity is only liberated at the last moment, when the alka- line matter is crackling against the vessel in the act of becoming solid. Moreover Faraday demonstrated that the steady evaporation of water from a platinum dish did not produce electricity ; if, however, the dish was made very hot, and a large drop of water allowed to fall into it, the latter assumed the spheroidal state, and no electricity was apparent until the temperature of the platinum dish was allowed to fall, and the drop of water to boil violently and to rub against the sides of the vessel. It will be seen presently that the further develop- ment of this idea led to the construction of the powerful steam hydro- electric machine at the Polytechnic Institution. 218 ELECTRICITY. XVII. The slow oxidation of zinc by the air has been used by De Luc, who contrived the dry pile. The dry pile is, however, useless if allowed really to become dry; it has been found that, when the moisture naturally present in all paper is thoroughly removed, the action of the dry pile diminishes and almost ceases, but is easily restored by the admission of damp air, which gives back to the paper its natural amount of moisture. The dry pile is usually made by arranging, in a tube capped at both ends with brass, discs of thin sheet-zinc paper or silver-foil, and the following are Mr. Singer's directions for the construction of a dry pile : " The materials I prefer for these piles are thin plates of flatted zinc, alternating with writing or smooth cartridge paper and silver leaf. " The silver leaf is first laid on paper, so as to form silvered paper, which is afterwards cut into small round plates by means of a hollow punch. "In the same way an equal number of plates are cut from thin flatted zinc and from common writing-paper. " These plates are then arranged in the order of zinc paper, silvered paper with the silver side upwards, zinc 'upon the silver, the paper, and again silvered paper with the silvered side upwards, and so on ; the silver being in contact with zinc throughout, and each pair of zinc and silvered plates separated from the next pair by two discs of paper. " An extensive arrangement of this kind may be placed between three thin glass rods, covered with sealing-wax, and secured in a tri- angle by being cemented at each end into three equidistant holes in a round piece of wood ; or the plates may be introduced into a glass tube, previously well dried, and having its end covered with sealing- wax and capped with brass ; one of the brass caps may be cemented on before the plates are introduced into the tube, and the other after- wards. Each cap should have a screw pass through its centre, which terminates in a hook outside. This screw serves to press the plates closer together, and to secure a perfect metallic contact with the extremities of the column." FlG. 195. De Luc's "Dry Pile" connected with two Electroscopes. If a tube containing one thousand alternations is laid upon two electroscopes, as in Fig. 195, the zinc end is found to be positive, and the silver negative. Mr. Singer continues : u I found a series of from twelve to sixteen hundred groups,- which EXPERIMENTS WITH THE ELECTROSCOPE. 219 are arranged in two columns of equal length, which are separately insulated in a vertical position : the positive end of one column is placed lowest, and the negative end of the other, their upper extremi- ties being connected by a wire, they may be considered as one continuous column. A small ball is situated between each extremity of the column and its insulating support ; a brass ball is suspended by a thin thread of raw silk, so as to hang midway between the balls ; and at a very small distance from them. " For this purpose the balls are connected during the adjustment of the pendulum by a wire, that their attraction may not interfere with it ; and when this wire is removed, the motion of the pendulum commences. The whole appararus is placed upon a circular mahogany base, in which a groove is turned to receive the lower edge of a glass shade, with which the whole is covered." Mr. Singer directs that, in order to preserve the power of the columns, the two ends should never be connected by a conducting sub- stance for any length of time. It is there- fore necessary, when laid by, that it should be placed upon two sticks of sealing-wax, and that the terminal balls be half an inch or so from the table. If a column which appears to have lost its power be thus insulated for a few days, it will recover. There is another cause of deteriora- tion, which is more fatal: this is too much moisture. The paper discs therefore should be made as hot as possible before they are ~, , , . put together ; or even subjected to a con- The per P etlial Chune,, tinued but gentle heat for some time before they are inclosed in the glass tube, and, that being heated also, the plates may be inclosed without the presence of any appreciable moisture. The size of the plates may be f ths of an inch in diameter, or less. With a column of 20,000 alternations a Leyden jar may be charged, and minute sparks are visible when contact is made with the fine points of wire connecting the two extremities. When the dry pile is attached to the electroscope of Hare by sub- stituting the poles of two of De Luc's columns for the gilt disc (Fig. 193, p. 212), the instrument is made wonderfully delicate, so much so that Mr. Sturgeon describes an arrangement of this kind, the delicacy of which he states to be such that, the cap being of zinc, and of the size of a sixpence, the pendent leaf is caused to lean towards the negative pole by merely pressing a plate of copper, also the size of a sixpence, upon it, and when the copper is suddenly lifted up the leaf strikes. The different electrical states of the inside and outside of various articles of clothing were readily ascertained by this deli- cate electroscope. Bohnenberger has the credit of making the first of these instruments. FIG. 196. constructed with De Luc's cohtmns. 220 ELECTRICITY. FIG. 197. Bohnenberger's Electroscope. Tfye gold leaf, being in equilibrium, and neither attracted or repelled, is instantly moved to one side or the other when the very smallest amount of electricity is evolved on the cap of the instru- ment. From these various experiments with electro- scopes it maybe learned that friction under every circumstance, and even when disguised, as in the rapid evaporation of water from a hot surface, is an important source of electricity ; That there are two kinds or conditions of elec- tricity which exactly neutralize each other, and they are always evolved together ; That pressure, or any modification of mecha- nical motion, such as fracture, rending, or tear- ing, all cause electrical quiescence to be dis- turbed ; That heat, as applied to various crystals, sets their particles in motion, and causes the evolu- tion of electric force ; That chemical action is a source of electricity, of a tension similar, though not equal, to that of ordinary friction, as shown in De Luc's column. A, the gold-leaf suspended between the two poles, B B, of the dry pile. ELECTRICAL MACHINES. In the year 1777, Tiberius Cavallo, a thoroughly practical and learned elec- trician, describes, in his " Complete Treatise on Electricity," the construction of the cylinder electrical machine of his day. It will not be found to differ materially from that made in 1868. He remarks " The principal parts of the electric machine are the electric, the moving engine, the rubber, and the prime conductor, i.e., an insulated conductor which immediately receives the electricity from the excited electric." The electric formerly used was made of different substances, as glass, resin, sulphur, sealing-wax, &c.; and in different forms, as cylinders, globes, sphe- roids, &c. (Fig. 198.) The three glass globes are made to rotate and rub against three cushions. The conductor, a piece of metallic pipe or a gun-barrel, was suspended from the ceiling by silken cords, and connected with the globes by unravelled gold lace hanging down, the latter being used for the same purpose as the points now attached to all conductors of electrical machines. "This diversity," continues Cavallo, speaking of the various shapes and nature of the electric used, " then obtained on two accounts : first, because it was not ascertained which substance or form would answer best; and, secondly, on account of producing a negative or positive electricity at the pleasure of the operator ; for, before the electricity of the insulated rubber was discovered, sulphur, rough glass, or sealing-wax was generally used for the negative elec- ELECTRICAL MACHINES. 221 FlG. 198. Dr. Watson's Electrical Machine, Showing the first application of the cushion as a rubber, instead of the hand. tricity." The reader will perceive that Cavallo adopts the Franklinian theory. " At present smooth glass only is used ; for, when the machine has an insu- lated rubber, the operator may produce positive or negative electricity at his pleasure, without changing the electric. " In regard to the form of the glass, those commonly used at present are globes and cylinders. " The cylinders are made with two necks ; they are used to the greatest advantage without any axis (or rod passed through from neck to neck) ; and their common size is from 4 in. diameter and 8 in. long to 12 in. diameter and 2 ft. long, which are perhaps as large as the workmen can conveniently make them. " The glass generally used is the best flint, though it is not yet absolutely determined which kind of metal is the best for electrical globes and cylinders. The thickness of the glass seems immaterial, but perhaps the thinnest is preferable. " It has often happened that glass globes and cylinders in the act of whirling have burst in innumerable pieces with great violence, and with some danger to the bystanders. Those accidents are supposed to happen when the globes and cylinders, after being blown, are suddenly cooled. " It will, therefore," prudently remarks Cavallo, " be necessary to enjoin the workmen to let them pass gradually from the heat of the glass-house to the atmospherical temperature." The author prefers a single handle, instead of the multiplying gear, which is very apt to get out of order, and the cord to stretch or break when most 222 ELECTRICITY. wanted. The various parts of the machine just described were gradually in- vented and applied by various clever electricians, Otto Guericke, Hawkesbee, Abbe* Nollet, Dr. Watson, Wilson, Nairne, Dr. Priestley; and many years elapsed before the machine attained anything like the perfection of that em- FlG. 199. Cylinder Electrical Machine, used by Cavallo in 1777, Showing the glass cylinder A A, with a pulley attached to one neck, B, round which an endless cord passes to a large or multiplying wheel, c; the cushion E, and silk flap F: the cushion, placed on a glass pillar let into a piece of wood, moves backwards and forwards in a groove, c, and is se- cured by a screw ; before use, is covered with amalgam. The machine is clamped to the table at H. The prime conductor i i, with collecting points K, is supported on glass legs, L L, let into a maho- gany stand. The amalgam used by Cavallo consisted of two parts mercury and one of tinfoil, with a little powdered chalk, all rubbed up with grease. ployed by Cavallo in his experiments. A more elegant and compact form is now given to the cylinder machine by Messrs. Elliott, of the Strand. The most convenient form is undoubtedly the plate electrical machine. Of this Cavallo says "Next to Dr. Priestley's machine, I shall describe another, which was invented by Dr. Igenhouz, and which, from its simplicity and conciseness, makes a fine contrast with the former. " This machine consists of a circular glass plate, about i ft. in diameter, which is turned vertically by a winch fixed to the iron axis that passes through its middle ; and it is rubbed with four cushions, each about 2 in. long, situated at the opposite ends of the vertical diameter." Fig. 200 is a drawing of the- large plate electrical machine in use at the Polytechnic. The plate glass is 7 ft. in diameter, and rather more than fths of an inch thick ; it has two large rubbers, and, when these are well amalga- mated, and the weather is propitious at least dry very long sparks -of great intensity may be obtained; when the atmosphere is damp, in spite of the rapidity and power with which it is turned round by a four-horse power steam engine, it will hardly give a spark an inch in length. The prime conductor is a large globe, about 3 ft. in diameter; and inserted into this is a large ring of wood, 4 ft. in diameter, and raised 6 ft. from the globe being an arrangement first proposed in connection with the Austrian electrical machines exhibited in the Great Exhibition of 1862. The ring, no doubt, theoretically speaking, should act as a condenser, and assist, by induc- tion, to increase the tension of the electricity ; but whether it be due to the ELECTRICAL MACHINES. 223 FlG. 200. The great Plate Electrical Machine at the Royal Polytechnic. height of the building in which the ring is placed, or from other causes, the effect produced did not appear to be increased by this addition to the apparatus. The power of an electrical machine is greatly influenced by the nature of the glass. There is a very fme-looking machine at the Polytechnic, constructed on the plan of the late Sir William Green Harris : the plate is 3 ft. in diameter; but, in consequence of the alkali of the plate glass, its power is very slight, and not half so good as that of many small cylindrical machines. _ The best -amalgam for an electrical machine is made of I part of tin, 2 of zinc, and 6 of mercury. Melt the zinc and tin together, and, when approaching solidification, add the mercury, and stir till the whole is solid : if the latter is added when the alloy of zinc and tin is too hot, much of it may be dissipated in vapour ; and the amalgam should be made under a chimney, to avoid the fumes of mercury. Sometimes the above are rapidly melted together, and then placed in a wooden box and shaken until quite cold. The shaking reduces the greater part to a fine powder, which may be sifted out and used 224 ELECTRICITY. with grease. The author always lays a coating of tallow-grease on the cushion first, and then carefully sifts the amalgam upon it, laying all smooth with a clean broad knife or spatula. Very cheap machines can be made frc-m common window-glass, to the centre of which, and on the opposite sides, two wooden caps, turned convex, may be cemented without any perforation of the plate, the axle being made of glass rod fitting into holes in the wooden caps. Mr. Goodman recom- mends that the cement used should be made of equal parts of black resin and beeswax ; but the writer recommends the use of less beeswax, because it renders the cement liable to melt easily if the electrical machine is placed before a fire to be warmed ; the quantity may be I Ib. of black resin to 3 oz: or 4 oz., at the most, of beeswax. A plate of this kind would cost half-a-crown. Sometimes two circles of common window-glass are cemented together to increase the thickness, and prevent the chance of breakage. The common window-glass, from its hardness, gives a large quantity of electricity when the friction is properly and equally applied. Very excellent machines are now made of plates of vulcanite, and, in fact, are used for mining purposes. The plate is enclosed in a box of vulcanite, and turned by a handle on the outside. It contains one or more Leyden plates ; and after these are charged by a few turns of the machine, the discharge may be sent through covered wire into one of Professor Abel's fuses at a distance of many hundred yards. The writer has used such a machine on a damp night in November, in the grounds of Haileybury College, Herts, with the greatest success, exploding three separate charges one of gun- powder, directed against a heavy gate; another, a mine, blowing many tons of earth into the air ; and a third, a keg of 9 Ibs. of gun-cotton, made by Messrs. Prentice, of Stowmarket, which nearly emptied a pond in which it was exploded, and, sad to relate, broke a great many windows in the chapel by the terrific concussion of the air, although the building was at least three hundred yards from the pond where the explosion took place. Mr. Hart, of Edinburgh, describes a very compact and well-arranged ma- chine, called Winter's Electrical Ma- chine. Winter's Electrical Machine is one of the most perfect forms of the plate friction machine that has hitherto been made. It distinguishes itself from other machines by the extraordinary length of the spark that it gives, by sim- plicity of construction, and by the uniformly good results that are obtained from it, even when the state of the air is not favourable to the display of electrical phenomena. The annexed drawing represents one of these instruments. It FIG. 201. Winter's Electrical Machine. ELECTRICAL MACHINES. 225 will be seen from it that the glass plate is fixed into an axle, which revolves in two upright supports. One of these, in which the shorter wooden end of the axle revolves, is made of glass, and the other, in which the longer glass end of the axle revolves, is made of wood. By this means the electricity formed upon the plate cannot on either side reach the ground, for on the one side the insulating glass pillar, and on the other the insulating glass axle, prevents it, and thus complete insulation of the plate forms one of the elements of the ex- cellence of Winter's machine. The friction in this, as well as in all friction- machines, is caused by pressing on the plate of glass a flat surface of leather, covered with an amalgam of mercury, zinc, and tin, which is put on with the aid of a little grease. The frame standing on the low glass support to the right of the figure is the wooden rubber frame, into the notches of which fit two flat pieces of wood covered in front or on the s4de next the plate with leather and a very little stuffing, and provided on the other side with springs, which, acting against the frame, keep the front surface uniformly pressing against the plate. There is only one pair of rubbers, not two, as in ordinary machines, and this enables them to be placed at a greater distance from the prime conductor of the machine.* The brass ball standing on the tall glass support to the left is the prime conductor. For the sake of more perfect insulation, this ball is fitted on to the support by means of a. trumpet- shaped opening made in it, thereby preventing the dispersion of electricity that would arise from the sharp edge of a hole exactly large enough for the rod. There are three other openings in this ball, one on each side and one at the top. The two small rings which are seen projecting upon the plate fit into one of these by means of a T-shaped piece of brass. They are made of wood, and have a groove cut in them on the side turned towards the plate, into which a row of fine pin-points is fixed for collecting the electricity formed upon it. These points are connected with the prime conductor by means of a strip of tinfoil which lines the bottom of the groove. Two wings of oiled silk attached to the rubbers stretch between them and these rings, so as to prevent the electricity from dissipating itself before reaching them. The opening on the top of the ball is made to receive the stalk of the large wooden ring, which is seen surmounting it, and which forms the most peculiar feature of the instru- ment. An iron wire forms the core of this ring, and is in metallic connection with the prime conductor. The function performed by this remarkable appen- dage is to lengthen the sparks given by the machine. In a 24 in. plate, for instance, with the aid of the ring, the sparks are 14 in. in length, and without it scarcely two. The remaining opening in the prime conductor is for the stalk of the small brass ball from which the sparks are obtained. To the left of the figure is the spark-drawer for receiving the sparks from the machine. The length of the spark given by an electrical machine is by far the most severe test of the excellence of its construction, and, in this respect, Winter's machine is entitled to hold the first rank among friction-machines. A machine 12 in. in diameter costs ^5. Another and most interesting electrical machine, by which the apparent anomaly of frictional electricity without friction is realised, was exhibited in the Great French Exhibition of 1 867, and described by a careful observer, in the Mining Journal.' " In appearance, Holtz's machine resembles the ordinary plate machine; in fact, the most prominent part is a glass disc, which is mounted and .revolved in the usual manner. But the plate is thinner the thinner the 15 22 6 ELECTRICITY. flG. 202. The Holtz Electrical Machine, giving "frictional electricity" without friction. better and as it is desirable to revolve it very rapidly, a multiplying wheel is connected with the plate, so that the speed may be increased to the extent desired. The machine, however, has really but little resemblance to the plate machine, for it has no rubbers ; it produces torrents of frictional elec- tricity, but the electricity is not generated by friction ; there is no friction about the machine, except at the axle bearings. The plate revolves in free air, and nothing should touch it. In the place of rubbers are what are called inductors, which are strips of paper 3 or 4 in. long, and about i in. wide. They are supported and insulated on pieces of glass, which are of spear-head form. The inductor is made complete by Dasting on to the paper pointed pieces of cardboard, which project beyond the glass spear-heads an inch or two. The spear-heads are attached to the framework of the machine, so that they shall be parallel, and as near as possible, to the plate on its crank side. Opposite the inductors, at the front of the plate, are the comb points, which serve to collect the electricity, and convey it to the conductors for use. Each inductor is furnished with its set of points. The combs are attached to brass rods, terminated at their other ends by brass balls. The rods are fastened to the framework of the machine, and are insulated from it. The balls at the ends of the rods may be connected to each other in any desired order by means of bent wires. " To obtain the electricity, one of the inductors is slightly charged, by means of an excited rod of hard rubber, glass tube, or otherwise, and turning the crank. Its power progressively increases for about a minute, and until it reaches the maximum, when it furnishes a steady supply of electricity as long ELECTRICAL ATTRACTION. 227 as the disc is revolved. The amount of electricity which a disc of only 2 ft. in diameter will yield is enormous. " To explain the action of the machine three elements must be considered the inductor, the plate, and the comb points. If a pointed wire be brought opposite an electrified body as, for example, a prime conductor the positive electricity of the prime conductor attracts the negative of the wire, and repels its positive, and a stream of negative electricity flows out of the wire at its point, while the positive flows to the opposite direction. Now, suppose a sheet of glass be interposed between the point and the conductor. The attraction of the positive electricity of the conductor for the negative of the wire is by no means lessened ; the negative is accumulated towards the point, and, by reason of its higher tension, flows out on to the glass. But the glass is impervious to the electricity, and it remains on its surface; the glass becomes electrified. In Professor Holtz's machine we have the electrified body in the inductor, the wire point opposite, and the glass plate interposed. Suppose inductor No. i electrified positively, this positive electricity attracts negative electricity out of the comb points on to the interposed plate. The plate moving on the part electrified negatively comes opposite card points of inductor No. 2. Here the negative electricity of the plate draws out of the card points positive electricity on to the glass, and inductor No. 2 becomes charged negatively, while the glass is negatively charged on the further side, and positively charged on the near side. Inductor No. 2, being charged negatively, draws positive electricity out of comb points No. 2, and neutra- lizes the negative drawn from comb points No. i. Card points No. 3 dis- charge negative electricity on the plate, and inductor No. 3 becomes positive, and, like No. i, draws negative electricity out of the corresponding comb point. It will be seen that the alternate inductors are oppositely electrified, and that their corresponding comb points give out or receive accordingly. By varying the manner of connecting the balls at the extremities of the comb points a considerable variety of changes in the relation of the quantity and intensity may be obtained. These variations are somewhat similar to those which are secured by varying the order of connecting the elements of the galvanic battery. The greatest intensity is obtained by connecting the inductors as they stand in numerical order round the disc. By connecting one of the poles with the ground, the other may be used as a prime conductor for charging Leyden jars, &c. It is found advisable, in order to secure more perfect insulation, to varnish the plate and the inductors with shellac varnish." ELECTRICAL ATTRACTION AND REPULSION GOVERNED BY CERTAIN LAWS. The electroscopes already described are merely intended to indicate the development of electricity ; their construction does not permit any calculation to be made as to the quantity of the force present in or upon any given surface. In using a common magnet to attract a needle, it is evident that distance regulates the intensity of the power, which increases rapidly as the two are brought in closer proximity, or decreases as quickly by increasing the distance between them. 15 2 228 ELECTRICITY. The influence of distance is particularly shown in experiments with static electricity, and the phenomena were carefully examined by Coulomb, who determined the laws which bear his name. First Law of Coulomb. Two electrified bodies attract or repel each other with a force which is inversely proportional to the square of the distance that separates them. Example : An electrified body at a certain distance exerts a force which may be called unity or one ; at half that distance the power is four times greater; at one-third, nine times; one-fourth, sixteen times greater, and so on. Second Law. The distance remaining the same, the attractions or repul- sions are in the compound ratio to the quantities of electricity which the two bodies possess. Example : A fixed electrified ball, which will repel another and movable one to a certain distance, called unity or one, will have only half the power if con- nected with another ball of the same size, the charge distributed over one surface is now spread over double the surface ; and if this again is connected with another ball, the force is halved again, and possesses only a quarter of its original power. FIG. 203. A, an insulated ball electrized with a force to be called unity, i ; A B, the same ball touching another ball of the same size, B. The charge is now spread over twice the surface, and the force is reduced one-half. B, the ball with one-half the charge ; B c, the same ball touching another, c. The original charge is again spread over twice the surface, and the force reduced at A B to one-half is now reduced at B c to a quarter. On the same principle, by reversing the previous experiments and increasing the charge, if a series of balls, gradually decreasing in size, are attached to any given-sized ball, they must end in a very small ball, or that to which it is equivalent, viz., a point ; consequently the charge increases in intensity, instead of diminishing : and hence the use of points, which discharge electricity very rapidly ; or receive it, as in the points attached to the prime conductor of an ordinary electrical machine. The electric force tends to escape from the sur- face of conductors by virtue of the repulsion of its own particles. The force it exerts is considered proportional to the square of the quantity ; hence, if the accumulation of electricity in eight balls decreasing in size be taken as I, 2, 3, 4, 5, 6, 7, 8, the force will be the square, as shown in Fig. 204. The last ball, which is eight times less in area than the first, is charged eight times more than the first, and the force, or desire to escape or polarize the surrounding particles of air, is increased by the square, viz., sixty-four times. ELECTRICAL ATTRACTION. 229 64 4;9 36 25 16 FIG. 204. The rationale of a Point, and why it gives off Electricity. The lines show how a point is arrived at from a series of spheres gradually decreasing in size. These laws, and the applications which flow from them, were discovered by Coulomb'with the very delicate instrument called the Torsion Electrometer, or Torsion Balance (Fig. 205). It consists of a cylindrical or cubical glass box, carrying upon, but communicating through, the upper pane of glass a vertical tube 15 or 20 in. high. The box may be 12 in. high. FIG. 205. At the top of the tube is a graduated circle and pointer, and inside this, and exactly in the centre, is attached a fine silver or platinum wire, stretched by a little weight. The wire suspended from the top of the tube is long enough to reach to the centre of the box ; and through the weight that stretches it is passed a hori- zontal needle of gum lac or glass. The needle is not suspended like a balance ; but one arm is longer than the other, and carries a little disc of gold paper or a small gilt pith-ball. In order to measure the space traversed by the needle, a proper scale is 230 ELECTRICITY. placed in the centre of the front glass pane ; and, before commencing experi- ments, the zero of the circle carried by the tube is made to correspond with the zero of the scale in the box ; and this can be done by carefully moving round the top scale, to the inside of which is attached the metallic wire carry- ing the little weight and needle. The needle is affected by the electricity from a ball or disc of exactly the same size as that attached to the needle, which is supported by an insulating stem. It is introduced vertically in another hole made through the top pane of glass, and the whole is so arranged that, when the ball of the needle is in contact with the other, the needle is in the direction corresponding to the zero or o of the two scales. In the cylindrical glass Coulomb balance the vertical ball is suspended by a metallic rod, which goes through the top, and is attached to another ball. It is not then removed as in the square-box balance ; but a " proof plane " on an insulating handle is applied to the electrified body under examination, and this is caused to touch the outer ball of the balance. In the square-box balance the ball is removed by its insulating handle, and the electrified body under examination is touched with it, and the ball placed inside the box. According to the first law, it immediately divides its electri- city with the ball of the needle, which latter, being repelled, describes a larger or smaller arc, according to the intensity of the charge. Directly the needle is repelled, the wire must be twisted ; and this is called the force of torsion. Coulomb ascertained that the forces of torsion are propor- tional to the angles of torsion ; or, in other words, the force that causes the torsion or twisting of the wire is exactly proportional to the arc described by the needle. Supposing the needle to be repelled to the distance of 36, in order to compel the needle to come to 18, the top circle on the tube must be moved round 1 26 degrees ; from which it follows that the wire, if twisted 1 8 below and 126 above, makes up a torsion equal to 144. Under the same circum- stances, to reduce the arc to 9, an angle of 576 of torsion must be used. The relation of the 36, 1 8, and 9 are to each other as the angles of torsion, 36, 144, 576, or these angles are to each other as I 14: 16; hence, if the distances are to each other as I : \ : , the repulsive forces are to each other as i 14: 1 6, and the first law of Coulomb is proved. The late Sir William Snow Harris employed a delicate brass scale-beam, suspended from a curved brass rod fixed to an insulating support ; the beam carried a circular gilded plane from one arm, and the scale from the other ; the gilded plane is suspended over another plane of the same size, which can be raised or depressed at pleasure. The attraction between the two planes was estimated by the weights raised, and the instrument is known as Harris's Balance Electrometer (Fig. 206). It is with these apparatus (the bifilar balance and balance electrometer), and by greatly varying his experiments, says De la Rive, " that Sir W. Harris found that the law of the inverse of the square of the distance is not exactly sustained, except when the balls or the discs are charged with an equal quan- tity of electricity, when this quantity is not too feeble, and, finally, when the angular distance that separates them is greater than 9. " Otherwise, and especially if the electric charges of the two bodies are very different, the force becomes the inverse of the simple distance, within certain limits. The same causes equally modify the second law, which establishes ELECTRICAL ATTRACTION. 231 the relation existing between the quantities of electricity and the attractive or repulsive forces. Thus in one experiment, the respective quantities of elec- tricity being successively on each of two discs in turn i and 2, the corre- sponding repulsive forces, instead of being i and 4, were i and 5. This devia- tion from the law was due to the absolute intensity of the electricity being too feeble. But it is much more sensible when there is inequality in the electric charges of two bodies, and when this -inequality is very great. " These numerous exceptions to Coulomb's laws are in a great part due to there occurring to electrised bodies, when in presence of each other, impor- tant modifications in their electric state, by the effect of influences whose action we shall study further on influences which are the more sensible as the elec- tric charges are more different. "They depend also upon its being very probable that the laws in question are general only for points almost ma- thematical, and not for bodies of any forms or dimensions. " Now we conceive that they must be so when we employ, as Coulomb did, small equal spheres for electrised bodies ; for, as is demonstrated in me- chanics, the action of a sphere is always the same as that which would be exer- cised by its centre, supposing all the forces with which the sphere is en- dowed were concentrated in this centre. We see, therefore, that Coulomb's laws may be regarded as general by restrict- ing them to the cases of electrised molecules or points ; and that in other cases they maybe regarded as deviating less from the truth as the bodies are of smaller dimensions, and as the forms approach more or less the spherical form." When a sponge, or any other porous matter, is dipped in water, the latter is taken up by the whole mass, and dif- fused through it. Similar ideas might be formed with respect to a charge of electricity that it spreads itself through the whole body of the conductor on which it was rendered evident ; this, how- ever, is not the case ; the electricity arranges itself on the surface of electrified bodies. Hence balls and cylinders used in the construction of electrical apparatus, such as conductors, are always made hollow : solidity is not necessary. The most conclusive experiment to prove the fact already stated was that made by Coulomb. Having insulated a sphere of metal, it was charged with electricity, and on the application of two hemispheres, supported, of course, by glass rods, FIG. 206. Harris's Balance Electrometer. 2 3 2 ELECTRICITY. the whole of the charge was removed when they were taken away and applied to an electroscope ; whereas the original ball first charged did not exhibit the slightest charge when tested with the same instrument. FIG. 207. Coulomb's Experiments, showing the distribution of the Electricity on the surface of insulated Conductors. The ball being first electrified, and the hemispheres applied, which remove the charge. Faraday, whose name is so completely identified with the subject of electri- city, devised many clever experiments to show the fact. One of the best is where he uses a conical muslin bag attached to an insu- lated ring. At the apex of the cone, both outside and inside the net, is a silken thread for the purpose of turning it inside out. When the bag is charged, a "proof plane," i.e., a metallic plate attached to a glass or wax rod, or a small disc of gilt paper fixed to a thin rod of shellac or glass, is placed in the interior, and then applied to a delicate electroscope, which remains unaffected. If, however, the proof plane touches the outside of the bag, a charge is obtained, which is rendered evident directly the elec- troscope is used. By turning the bag inside out (whilst insulated and charged) with the dry silk string, silk being a non-conductor, the condition is reversed ; that which was FIG. 208. the outside is now the inside, and gives no Faraday's Experiment with the evidence of electrical excitation, whilst that conical Muslin Bag. which was inside is now outside, and, of ELECTRICAL ATTRACTION. 233 course, will deliver a charge to the proof plane. A bird, a white mouse, some gunpowder, and a delicate electroscope, placed inside a wire-gauze cover, such as might be used for protecting meat, standing on a stool with glass legs, and connected with the electrical machine when in full action, will give an abun- dance of sparks from the outside, which do not affect the living things, the gunpowder, or the electroscope in the slightest degree. FIG. 209. The Mouse, the Bird, Gunpowder, and Electroscope under a Wire Gauze Cover. When two gilt pith-balls, hanging side by side, and suspended from an in- sulating stand, are electrised, they stand out and repel each other, with a force indicated by the distance at which they are separated. The distance is a rough measurer of the intensity or energy of the charge. By attaching the pith-balls to an insulated cylinder (Fig. 210), round which a riband is wound up close, and conveying a charge from the electrical machine, they repel each other for some time, and remain in that state in dry and moder- ately warm air. If, however, the flap or riband of silk is unwound by the glass handle, the electricity is spread over a larger surface, the intensity of the ori- ginal charge is diminished, and this is shown by the pith-balls falling together, and again returning to their original distance, or nearly so, when the glass is again wound up. This experiment is quite in accordance with the laws of Coulomb, already explained at page 228. Instead of the proof plane, a " carrier ball," as Fara- day termed it, may be used ; this is made of some nicely turned light wood, covered with gold paper, and supported by a silk thread, well dried, and satu- ' rated with shellac. The latter is easily dissolved in methylated spirit, by digestion in the cold for a day or so. Of course, warming the spirit by putting the bottle on a piece of wood standing on the hob of a grate will accelerate the solution ; but care must be taken to avoid the chance of its taking fire. Most of the above experiments were devised by Faraday ; but it is easily shown that the principle of distribution and the proof that electricity resides 234 ELECTRICITY. FlG. 210. The Cylinder charged, and the flap unwound. on the surfaces of metallic electrified bodies was well known and shown by Cavallo in his book published in 1777. The experiment quoted is called THE ELECTRIC WELL, " Place upon an electric stool a metal quart mug, or some other conduct- ing body nearly of the same form and dimension ; then tie a short cork-ball electrometer at the end of a silk thread proceeding from the ceiling of the room, or from any other proper support, so that the electroscope may be sus- pended within the mug, and no part of it may be above the mouth; this done, electrify the mug by giving it a spark with an excited electric, or otherwise, and you will see that the electroscope, whilst it remains in that insulated situation, even if it be made to touch the sides of the mug, is not attracted by it, nor does it acquire any electricity ; but if, whilst it stands suspended within the mug, a conductor, standing out of the mug, be made to communi- cate with or only presented to it, then the electroscope acquires an electricity contrary to that of the mug, and a quantity of it which is proportionable to the body with which it has been made to communicate ; and it is then imme- diately attracted by the mug. Cavallo explains the cause in his own quaint language, and his theory is in accordance with that taught in these days, only the technical names are changed ; thus, in modern style, the fact would be explained by stating that " polarity cannot be set up when opposing actions are at work in different directions, as in the inside of an insulated metallic vessel." Cavallo says, " The reason why, in this experiment, the electroscope contracts no electricity whilst suspended entirely within the cavity of the mug is because the electricity of the mug acts upon the electroscope on all sides, and this has no opportunity of parting with its fluid when the mug is electri- fied positively, nor of receiving' any when the mug is electrified negatively. But, as soon as any conductor communicates with it, the electroscope becomes immediately possessed of the electricity contrary to that of the mug ; for, if the mug be electrified positively, the fluid belonging to the electroscope will be repelled to that body which communicates with it, and which, being out THE ELECTRIC WELL. 235 of the mug, cannot be affected by its electricity ; and if the mug is electrified negatively, it will attract the fluid of the electroscope, which actually receives an additional quantity of it from that conducting body with which it com- municates. FIG. -211. C avail Js Electric Well A, quart mug insulated, and containing the electroscope inside; B, the threads raised above the edge of the vessel, or, still better, touched with an insulated brass rod extending into the air. In A, opposing forces, + and +, oppose in different directions In B, polarity can be set up j because the inside is 4-, the electroscope , and the extremity of the rod in the air +. " The electroscope, therefore, becoming always possessed of a contrary elec- tricity, must necessarily be attracted. " If, by raising the silk thread a little, part of the electroscope, i.e., of its linen threads, are lifted just above the mouth of the mug, the balls will be immedi- ately attracted ; for then, by the action of the electricity of the mug, it will acquire a contrary electricity by Diving to or receiving the electric fluid from the air above the cavity of the mug. " It has been supposed by some that the electroscope in the above experi- ment (or any other small insulated body), hanging in the cavity of an electrified vessel, or the like, is not attracted by the sides of the vessel because the attrac- tion of electricity, being as the squares of the distances inversely, cannot affect the electroscope one way more than another ; it being demonstrable that if to every point of a spherical concave surface equal centripetal forces are directed, decreasing as the squares of the distances from those points, a small body- situated anywhere within that surface would remain there without being at- tracted one way more than another.* But to this it may be replied that the demonstration of the above-mentioned proposition, if it is applicable to sphe- rical or cylindrical concave surfaces, cannot, however, be applied to every kind of irregular cavities, with which, if they exceed not a certain size, the above experiment succeeds as well as with the cylindrical cavity of the mug." Cavallo proceeds to give what he considers to be the proper theory, which in the main is right ; but, as before observed, the explanation is simplified by stating that, as polarity cannot be set up inside a vessel, so a charge cannot be maintained. * Newton's " Principia," Book I., prop. Ixx. 236 ELECTRICITY. ELECTRICAL INDUCTION. In studying the phenomena of light and heat, it will be necessarily observed that these forces have a radiant power. A heated body may be brought towards another which is not heated, and impart Jo it a certain amount of its warmth ; the latter gains what the former loses : the vibratory power set up in the heated body is supposed to be conveyed by the undulations of the ether to the body which is not heated, and setting up therein similar vibrations ; the result is that heat is produced in a cold substance by the approach of a heated body, which loses its vibrating energy in warming the other. Loss of power, independent of any conducting power of damp air, curious to say, is not observed when an electrified body is gradually brought towards another which is not electrified ; and yet the electrical quiescence of the latter is disturbed, and may give rise to large quantities of electricity, as in Holtz's electrical machine (Fig. 202) ; the effect thus obtained is called " induced elec- tricity." The fact is well shown by using a cylindrical conductor, the two halves of which can be separated with their respective insulating glass columns. On the underside of the conductor small rings or hooks may be inserted for the convenience of attaching pairs of gilt pith-balls, which should be as light as possible. FIG. 212. A, the electrified ball approached to the conductor, B c, made in two halves to separate at D; each half to have one suspended pith-ball at r3, so that, when joined together, they form a pair of balls as in the ordinary electroscope ; also each to have a pair at the extremities B and c. Directly the charged ball A has approached sufficiently near to the conductor B c, the pith-balls show by their mutual repulsion that its electrical quiescence is disturbed, and that, in fact, if the ball has been charged with positive or + electricity, it will cause negative or electricity to become apparent at B, whilst positive or + electricity will be found at C. The pith-balls hanging at D will hardly be disturbed, if at all, showing that there is a neutral point, like the centre of a bar magnet, where the forces are balanced. When the disturbing cause A is removed, the separated electricities rush together again, the electrical equilibrium of the cylinder is restored, and the pith-balls no. longer repel each other. ELECTRICAL INDUCTION. 237 No advantage, therefore, so far as the production of a permanent charge of electricity, has been obtained in the above experiment, which, it must be re- membered, is performed with a conductor of electricity. If, however, the expe- riment is repeated, and, whilst the conductor is under induction from the ball A, the two halves are separated, then it will be found that each half is per- manently electrified. FIG. 213. A, the electrified ball ; B, the half of the cylinder, separated from the other, and showing a charge of negative or electricity ; C, the other half, showing positive or + electricity ; p, the single balls, sus- pended from B and c, attract each other, as they represent the opposite electricities, + and . The separation of the halves of the conductor whilst under induction has prevented the opposite forces reuniting; the pith-balls remain deflected on each half, and the single balls, suspended at the place where the two halves are separated, incline towards each other, because dissimilar electricities attract. The equality of the electrical disturbance is again beautifully shown by bring- ing the halves together, when the electrical excitation set up entirely ceases, as the two opposite forces exactly neutralize each other. The experiment may be once more repeated, and the two halves separated whilst under induction. If a stick of excited wax is approached to the half of the cylinder marked B, minus, the pith-balls are deflected still further from each other ; but when the same stick of excited sealing-wax is brought towards C, or plus electricity, the balls drop down. In the first case, the increased deflection shows that the electricity on B is negative, because the wax is negative, and exalts the previous charge. In the second, the diminished deflection and falling down of the balls show that the electricity on c is positive, as it is neutralized for the time being by the in- fluence of the negatively electrified wax. Two electroscopes, one placed in connexion with each half of the conductor, may be substituted for the pith-balls, and are, perhaps, more certain and truthful in their indications ; moreover, they are more delicate, and would show a smaller amount of electrical disturbance. These experiments demonstrate that, in conductors, polarity, i.e., the sepa- ration of the electricities, the production of opposite properties in opposite di- rection, may be set up by induction, but is not maintained; and this is, in fact, as contended by Faraday, the essential difference between conductors and non- conductors : in the former polarity is not maintained ; in the latter, as we shall 2 3 8 ELECTRICITY. now see, polarity, being set up, is maintained, or it would be impossible to charge a Leyden jar. When a plate of glass is held against the ball attached to the prime con- ductor of an electrical machine, and a pith-ball, suspended on a glass sup- port, is approached towards it, the ball is energetically attracted towards the glass ; and yet the latter, being called a non-conductor, ought not to have per- mitted the electricity to have apparently travelled, like heat, through its sub- stance. FIG. 214. A, one side of the glass plate, which may be one foot square, and is held against the ball of the elec- trified conductor ; B, the ball suspended on the glass stand, and attracted to the other side of the glass plate. The electricity does not travel through the glass plate, but, like the brass conductor (Fig. 212), is thrown into an electro-polar state, the one side touch- ing the conductor being positive, and the other side, to which the pith-ball is attracted, being negative ; a very slight charge is thus conferred upon the glass plate, which will not rise higher until one side is put in conducting communi- cation with the ground. The small charge, however, is retained when the glass is removed, and thus the polarity is shown to be maintained by non-con- ductors^ constituting the essential difference between them and conductors of electricity. The sheet of glass cannot be charged properly unless both surfaces are ELECTRICAL INDUCTION. 239 coated with tinfoil, within, say, 2 in. of the outer edge. On a sheet of glass i ft. square, the tinfoil will be 8 in. square. If this plate is supported on an insulating stand, by being placed in the cleft or groove of a piece of mahogany fitted on the top of a glass rod, fixed in a proper foot, the charge, as before stated, is very slight, because the force called positive electricity applied to one side of the plate, which may polarize the particles of the glass, is opposed by the positive electricity resident on the other side of the glass, and a balance is arrived at a dead lock ; the particles cannot increase their charge, because the order is broken, and instead of the continuity being represented by Fig. 215, where + is at one end and at the other, the regu- FIG. 215. larity is destroyed by the last particle being -f- instead of , as shown at Fig. 216; and the molecules are now + at one end and + at the other, and must therefore oppose (and thwart, as it were) each other. FIG. 216. The difficulty is, however, overcome by connecting one side of the plate with the earth, when the order shown in Fig. 215 is restored, and the + elec- tricity is said to escape to the ground, which latter, in its turn, represents a vast series of particles all polarized ad infinitum, but decreasing in intensity as the distance from the disturbing source is increased, according to the law already explained at page 228. Faraday insisted that electrical induction was an action of contiguous par- ticles, whether it took place through a metal, or glass, or air ; he opposed the " emission " theory of electricity, as others had done before with respect to the emissive theories of light and heat. Formerly .it was supposed that electricity travelled through air without affecting the particles of the air ; it was imagined to be a subtle form of matter of its own kind. Faraday laboured to prove that every particle of air becomes polar, and takes part in the propagation of the force, just as the particles of the glass become polar when charged with electricity. A thin leaf of gold may be -f- on one side and on the other, so long as it is subjected to the inductive action of an electrified body brought near it ; 40 ELECTRICITY. that which occurs in a large conductor, as shown in Fig. 212, may occur, microscopically, as it were, in a gold leaf* If once the student grasps the idea of the polarity of each minute and con- tiguous particle,, the difficulties of Faraday's inductive theory vanish. It is well, here, to dwell on the condition of the surfaces of a glass plate whilst under induction, and receiving a charge of electricity. The late Professor Darnell's diagrams are very excellent* FlG. 217. Danulfs L Explaining the condition of the surfaces of an insulated catted wife tuftfoO, d plate of glass, ** Upon the molecular hypothesis of induction, No. I may represent a plate of glass with its metallic Coatings, a b and c d^ in its neutral state. In No. 2, we suppose the same plate, with its metallic coating, a , in contact with the charged conductor of an electrical machine. Its other coatings we also suppose to be insulated; and, as we know, the plate cannot be charged. The coating a , however, being positive or +, not only will the particles of the glass be thrown into a polar state, but the coating c d win also be polar, H , by induction to surrounding objects ; but the charge will not rise to any degree of intensity, because the + electricity of the latter cannot be carried off ? or diffuse itself upon the earth, but will react upon the glass. But if we uninsulate this coating, then will No. 3 represent the high state of tension (charge) which the forces will assume under the inductive process, when a high charge of + electricity upon a b will sustain an equal charge of elec- tricity upon d c by the polar arrangement of the particles of the interposed dielectric (glass). "In the above diagrams the unshaded circle represents the particles of glass in a state of electrical quiescence ; the shaded circles represent polarity, the shaded half being supposed to be + (plus), the unshaded half (minus) electricity.* When explaining the cause of electricity residing on the surface of an insu- lated conductor, it was stated that the interior of the vessel (Fig. 21 1) was not found to give the slightest charge to the proof plane, because polarity could not be set up properly in consequence of opposing forces in different directions : we may trace out the latter in the next diagram (Fig. 218). Suppose a set of molecules in a polar state starting from A are met by another column of particles in the opposite direction B, which virtually undo all that might '. Darnell's "Xnttodnctkm to Cb ELECTRICAL INDUCTION. 241 been done by A. The polar state cannot be set up in the carrier-ball or. in fact, in the particles of air contained in the vessel under examination by the carrier-ball or proof plane. The same reasoning applies to all sets of molecules coming in the direction c as opposed to c, Das opposed to D, E as opposed to E. The nomenclature of the phenomena of induced electricity is thus expressed by Fa- raday : - 1 . The excited body, glass or wax, is catted the inductric or inductive body. 2. The effect of the inductric on a distant body, and where no loss of electricity is sus- tained, as by contact, is called induction. 3. The electricity thus obtained is called inducd electricity. 4. The body subjected to the action of the inductric is caned the inducteout body. 5. The medium, such as air, through which the electric may act upon the induc- teous body is termed the dielectric {&<*, through, and rjX&cirpov, electricity). A di- electric may be solid, fluid, or gaseous. When the above principles are once com- prehended, it is easy to conceive that every kind of electrical attraction must be pre- l ceded by induction; to demonstrate this fact, Harris attached a gold leaf to a disc of FIG. 219. >$"/> William Sassr gilt paper, neatly fixed on a filament of shel- Harri/s Experiment dtmw- lac, and suspended by a silk thread. The disc may be attached to one end of a. wen- dried straw suspended by a thread ; a little bit of tinfoil on the other end will balance strating that Attraction is prec&led by Inductum. ' 242 ELECTRICITY. the gilt disc. Directly another disc electrified is brought towards the sus- pended disc, the little gold leaf on the other side stands out and is repelled, showing distinctly that the opposite kind of electricity to that which is the disturbing cause must be eliminated, or the gold leaf would not move until the suspended disc touched the electrified disc. At page 211, whilst explaining the construction of the electroscope, it was stated that the instrument could be made more delicate by the introduction of a simple arrangement, through which inductive action is brought to bear upon the cap, and through that to the gold leaves. The part attached to the electroscope-stand is called the condenser, and assists in increasing any minute evolution of electricity that would otherwise be insufficient to overcome the weight of the gold leaves, and cause them to repel each other. FIG. 220. The Electrical Condenser. A, plate supported on glass stem ; B, plate on a conducting stem, jointed at bottom so as to move to any position c. It (Fig. 220) consists of two circular brass plates, one supported by a glass insulating stem, and the other resting upon a conducting stem jointed at the bottom. When the plate on the insulated stem is connected by means of a wire with the cap of the electroscope, which may be very feebly excited, as with the pressure of Iceland spar, on the removal of the uninsulated plate, the gold leaves of the electroscope indicate the minute electrical disturbance. It is evident that between the two plates there must be a dielectric air, the particles of which we have already seen are capable of assuming the electro- polar state. The electricity from the tourmaline on the cap of the electroscope has charged the insulated plate A, Fig. 220; this throws the intervening air into a polar state, so that the air is in the same condition as the glass plate with its coatings of tinfoil, the latter being represented in this apparatus by the two brass plates. If both plates were insulated, there would be opposing forces, as shown at p. 239; but one, plate B, is connected with the earth. At first, and whilst the plates are near each other, the electricity is said to be disguised. All this time, if the electricity on the cap of the electroscope is positive (+), it has, by induction through the film of air, thrown the second plate into the opposite condition, negative or . The two electricities on the two plates are, as it were, engaged to each other ; the desire to unite, or their tendency towards one another, is simply arrested by the intervening air, and this for the time disguises the electrical energy which really exists ; but when the second plate is removed, and the two elec- tricities are separated, then it is found that the feeble original charge has been ELECTRICAL INDUCTION. 243 exalted ; for, as the feeble charge from the cap, connected with the insulated brass plate, acted on the other uninsulated brass plate, the latter, by con- nection with the earth, like the outside of a Leyden jar, reacts upon the insulated plate ; so that, when the two are separated, a greater elec- trical effect is perceptible. By the repeated application of the pressed Iceland spar, and the withdrawal and return of the plate B to A, the charge is virtually in- creased or condensed on A. The closer the two plates can be brought together, the better the effect ; but, as the particles of air are soon broken through by a disruptive discharge in the shape of a spark, and particularly so if the air is at all humid, it is found better, as in Volta's condenser, to use a thin plate of some non-conducting material, such as shellac, instead of air. The disguise of the two electricities is the more complete when the metallic discs are brought very close to each other, because the attraction of the two electricities becomes stronger as the distance is diminished. The inductive power of the electrified plate must be increased, and the reactionary force of the second plate, con- nected with the earth, also rises to a more exalted state. FiG. 221. The Gold- Leaf Condenser Is so called because it is adapted to a gold-leaf electroscope. The nicety of manipulation required in order to use the instrument properly is described by M. de la Rive, in his "Treatise on "Electricity," translated by Mr. Charles V. Walker : " It is composed of two metal plates, nicely adjusted, of not less than 6 in. nor more than i ft. in diameter. On of these plates is screwed on the exterior extension of the metal stem of the electroscope by which the gold leaves are supported, and has a wire and ball attached to it, A ; the other, B, is provided with an insulating handle, c, fixed vertically at its centre, and is placed upon the former so as exactly to cover it. 162 244 ELECTRICITY. " The two plates have been coated on their surfaces in contact with several layers, successively applied, of a very liquid varnish, formed of a solution of shellac in alcohol. This varnish, in drying, forms a pellicle whose thickness does not exceed i-25oth part of an inch, but which is sufficient to prevent the recomposition of the electricities when they are not very strong. " The plates are thus almost in contact, and the disguise of the electricity is as complete as possible ; and the condensing power of this apparatus is very considerable ; but it can only support very feeble charges, which, indeed, are all it is intended to receive. It is important that the two plates be fitted to each other as accurately as possible, and, consequently, that their surfaces be very even. For this reason there is a limit to the size of these surfaces that cannot possibly be exceeded, because their construction would become too difficult, in consequence of the conditions we have pointed out. The manipulation also would be very troublesome ; for it is essential that we should be able to raise the upper plate easily, and should take care to raise it perpendicularly, with- out exercising 2^ friction against the other, which of itself would be a source of electricity, and would consequently interfere with the results. " This reservation being once made, it is advantageous to have the largest possible surface, because the quantity of electricity accumulated is propor- tional to this surface. " Experiment has demonstrated that we cannot exceed a foot in diameter, without falling into the inconveniences that we have just pointed out. The plates are generally of brass, and, if possible, of gilt brass, so as to be pro- tected against the chemical action of the moist air, and of the vapours and liquids with which they may have occasion to come in contact. " Electrical signs are sometimes found on separating the two plates, even although there may be no electrical source in communication with either of them. This error is due to a small quantity of electricity arising from preced- ing experiments, which has penetrated into the layers of varnish, and which is not got .rid of without some difficulty. "In order to remove it, we must place a very thin sheet of tinfoil between the two discs, and leave it there until we have satisfied ourselves that, after having been placed in immediate contact with each other, the plates liberate no trace of electricity by the mere fact of their separation. It is essential always to determine this absence of spontaneous electrical signs before making an experiment. " For greater convenience, the source of electricity is generally placed in com- munication with the upper plate of the condenser B, which is termed the col- lector; and the lower plate, or its connected brass ball, is touched with the finger. " When the two plates are separated, it is the electricity of the lower plate, now become free, that affects the electroscope ; but we must not lose sight of the fact of its being of a contrary nature to that of the upper one, and, con- sequently, to that of the source subjected to experiment. " Before beginning a second experiment, we must not forget to discharge both the plates by touching them with the fingers ; and generally we must never leave them charged, especially when they are in contact, because the electri- city that they retain penetrates into the layers of varnish, from which, as we have seen, it is a very difficult matter to expel it. " By the assistance of this instrument Volta succeeded in showing that a plate of zinc, when held in the hand and put into contact with the upper plate, THE ELECTROPHORUS. 245 charged it with negative electricity an experiment that was the origin of the voltaic pile. When this experiment is made, care must be taken that the zinc plate be well cleansed, especially in the points where it touches the disc. " In like manner, we can charge the plate with positive electricity by inter- posing between the plate and the zinc plate, which is still held in the hand, a disc of cloth or paper slightly moistened with salt water. In each case we must not neglect to touch the lower plate with one of the hands, whilst the zinc plate is held by the other in contact with the upper plate. The experiments that we have just quoted, and the other delicate experiments in which the con- denser is used, require the air of the room in which the operation is carried on to .be as dry as possible, or at least the electroscope and all the pieces of which it is constructed to be well protected from moisture. With this view, the whole is covered with a glass cage, in the interior of which chloride of calcium is placed, in order to produce the dryness." Space does not permit us to describe Peclet's instrument, which is still more sensitive, but requires precautions to be taken in its use that almost negative its other valuable qualities. If the condenser cannot be understood, the youthful student is supplied with fresh ideas, which will help him to do so, in the old-fashioned and most useful instrument, called the ELECTROPHORUS , electricity; os, carrying). FIG. 222. The Electrophorus. A B, the tin dish, with the sides sloping inwards, so that the composition cannot fall out; c c, the upper metallic plate and glass handle, D ; E E, two spots of sealing-wax, dropped and mtlted on to the lower side of the metallic plate, to keep it opposed to, but not touching, the resinous plate. This instrument is spoken of by Cavallo as "a machine for exhibiting per- petual electricity ;" though he explains afterwards that, being only an excited electric, it must gradually lose its power like all other excited electrics, but being flat it is not exposed to currents of air which may circulate around a stick of sealing-wax, and carry off the charge more quickly. To make an electrophorus, a circular tin, with a rim f in. deep, may be pro- vided, about I ft. in diameter, and into this, whilst warm, should be poured a mixture of two parts of shellac and one part of Venice turpentine, after they are carefully melted and well incorporated together. When cold, the surface 246 ELECTRICITY. has a bright polish, and is, of course, remarkably smooth ; indeed, care should be taken not to scratch it. The second part of the apparatus for it consists only of two parts is the circular flat plate, 10 in. in diameter, made of tin, or gilt copper, or cardboard covered with tinfoil, in the centre of which is a glass rod so fixed that it will lift the metallic plate. The instrument is charged by gently rubbing or striking the resinous plate with a cat's skin or a warm piece of flannel, and, like the charged pane of glass, described at page 238, the thinner the resinous plate can be cast, the better, as after being rubbed, and always supposing the tin dish is in conducting com- munication with the earth, it acquires a charge like the Leyden jar, to be described presently. The electro-polar plate having been set up in the resinous plate, the metallic plate with the glass handle (which, in common with all the glass supports of electrical apparatus, should be varnished with shellac var- nish) is brought down upon the excited resinous plate ; no direct transfer of electricity takes place except when the plate happens to touch the excited wax, and this is prevented, in a great measure, by the two little studs of sealing- wax, E E, already spoken of in Fig. 222. When the plate is in position, and held by the glass handle, the two elec- tricities, positive and negative, naturally resident in the metal, separate, as already described in the explanation of the phenomena of induction, at page 237 ; because induction may not only take place in a long conductor, but on the opposite sides of a tin, copper, or other metallic plate. If the plate is now removed and examined, it is not found to have acquired any charge of electricity ; conductors do not retain polarity; and the two forces, separated whilst the metallic plate was in the neighbourhood of the excited resinous plate, come together again, as already described fully at page 236. The metal plate is again laid upon the lower excited resinous plate, and now, if touched by the finger just the moment before it is raised by the glass handle for the act of touching and raising should be almost simultaneous, and is soon learnt with a little practice then, on applying the knuckle to the edge of the metallic plate or to the brass ball, a spark immediately passes ; and thus, by continually touching, raising, and applying the top metallic plate by its glass handle to a small Leyden jar, the latter is speedily charged. The rationale of the necessity for touching is easily explained. When the plate is under induction, the lower side facing the negatively electrified resinous plate is positive, and the upper side negative ; on touching the plate, positive electricity passes to the negative, and the upper surface receives a charge in excess of its natural quantity, and, instead of the two sides being represented by +, plus, and , minus, the plate, when removed, is found to be H K Here is an excess of electricity, which passes to the knuckle in the form of a spark, and again restores the equilibrium to + and It is in this way that the metallic plate can be charged any number of times by alternately touching and raising, and the resinous plate loses no electrical power whatever. Holtz's electrical machine, described at p. 226, is another and very notable instance of the same kind. If the resinous plate in its tin dish, before being rubbed, is placed on an insulating stand, so as to be well insulated, and is then rubbed, care being taken not to touch the metallic dish, it acquires little or no charge. The under side of the resinous plate must be in conducting communication with the ground, like the glass plate with the tinfoil coatings, described at p. 239. THE ELECTROPHORUS, 247 When the whole apparatus, previously excited and ready for use, is placed on the insulating stand, and the metallic plate raised, it acquires so slight a charge that it will not give a spark, and would only affect an electroscope, which, Cavallo says, " shows that the electricity of this resinous plate will not be con- spicuous on one side of it, if the opposite side is not at liberty to part with or acquire more of the electric fluid." The original electrophorus invented by Volta was a circular glass plate, covered with a composition made of equal parts of shellac, rosin, and sulphur; and these plates, no doubt, from their thinness, would answer the purpose re- markably well. Cavallo, who is always so thoroughly practical in his electrical experiments, FIG. 223. Electrophorus^ made of Glass, and covered with Sealing-wax. ' says that he made one of a glass plate, and no more than 6 in. in diameter: when once excited, it could charge a coated Leyden phial several times suc- cessively, so strong as to pierce a hole through a card with the discharge. Sometimes the metal plate, when separated from it, was so strongly electrified that it darted strong flashes to the table upon which the electric plate was laid, and even into the air, besides causing the sensation of the spider's web upon the face brought near it, like an electric strongly excited. " The power of some of my plates " (which he covered with sealing-wax, second quality), he says, " is so strong, that sometimes the electric plate adheres to the metal, when this is lifted up ; nor will they separate, even if the metal plate is touched with the finger or othe^ conductor." Thus, with a circular piece of window-glass, covered with sealing-wax melted on to it, a circular piece of wood or card covered on both sides with tinfoil, and fixed by a pasteboard tube to a glass rod, a very serviceable and cheap electrical machine can be made by young people. The Leyden jar is nothing more than the coated glass pane (p. 238) rolled up or made into a cylinder. It was discovered by three philosophers, who were working together at Leyden, viz., Muschenbroeck, Allaman, and Cuneus. They were attempting to collect and store electricity in a bottle, containing some water, through the cork of which was thrust a nail, touching the water ; the first shock was re- ceived when Muschenbroeck, holding the bottle in one hand, touched the nail with the other accidentally. One smiles, thinking of personal experience in 248 ELECTRICITY. these matters, to imagine the half-frightened wonderment of the worthy sage, who might have supposed that he had invoked the demon or " genius," good or evil, of the bottle. Of course, everybody throughout Europe was made acquainted with the electric shock by travelling electricians, who, like the travelling "ghost" show- men of the present day, relieved Muschenbroeck of any trouble in communi- cating his discovery to the world in general. As the water was found to be inconvenient, in consequence of the vapour condensing in the upper part of the bottle, and thus reducing the distance between the outer and inner surface of glass, so that a small charge only could be obtained, brass filings, fixed on with some varnish, were next tried ; and Cavallo devotes more than a page of his " Complete Treatise on Electricity " FIG. 224. The Leyden Jar and Discharger. to the narrative of a grand explosion and smoke arising from the interior of his Leyden bottle, prepared with varnish and brass filings, in consequence of the latter taking fire with the electric spark, which, darting from point to point of the filings, set the inflammable mixture of air and spirit vapour from the varnish on fire ; and he adds, regretfully, that, after it had burnt out, all the brass filings fell to the bottom of the bottle, because the adhesive quality of the varnish was destroyed by fire. The older electricians sometimes used mercury instead of water; but this was soon found to be very expensive, and not applicable to large jars, in con- sequence of the great weight of the metal. The principle of the Leyden jar being once understood, viz., that the water accidently used by Muschenbroeck in his bottle was the inner conducting coating that conveyed the electricity to all parts of the interior surface of the glass, and that the undesigned application of the hands on the outside served for the outer coating, a little more consideration brought electricians to the use of tinfoil, no doubt suggested by the use of this metal in the art of silvering looking-glasses. There are no better directions for coating and preparing Leyden jars and batteries than those given by Cavallo, who says, " When glass plates or jars, THE LEYDEN JAR. 249 having a sufficiently large opening, are to be coated, the best method is to coat them with tinfoil on both sides, which may be fixed upon the glass with varnish, gum-water, paste, beeswax, &c. ; but in case the jars have not an aperture large enough to admit the tinfoil, or an instrument to adapt it to the surface of the glass, then brass filings, such as are sold by the pin-makers, may be advantageously used, and they may be stuck with gum-water, bees- wax, &c. ; but not with varnish, for this is apt to be set on fire by the discharge. Care must be taken that the coatings do not come very near the mouth of the jar, for that will cause the jar to discharge itself (now called a spontaneous discharge. " If the coating is about two inches below the top, it will in general do very well ; but there are some kinds of glass, especially tinged glass, that, when coated and charged, have the property of discharging themselves more easily than others, even when the coating is five or six inches below the edge. " There is another sort of glass, like that of which Florence flasks are made, which, on account of some unverified particles in its substance, is not capable of holding the least charge. On these accounts, therefore, whenever a great number of jars are to be chosen for a large battery, it is advisable to try some of them first, so that their quality and power may be ascertained. " If a battery is required of no very great power, as containing about eight or nine square feet of coated glass, I should recom- mend to make use of common pint or half-pint phials, such as apothecaries use. They may be easily coated with tinfoil, sheet lead", or gilt paper on the outside, and brass filings on the inside. They occupy a small space, and, on account of their thinness, hold a very good charge; but when a large battery is required, then these phials cannot be used, for they break very easily, and for that purpose cylindrical glass jars of about fifteen inches high, and four or five inches in diameter, are the most convenient." It is easily shown, by charging a Leyden jar fitted with, shifting coatings, made of light tin-work or of wire gauze, that they have nothing to do with the maintenance of the charge ; they simply act as chan- _ nels for the conveyance of the electricity to all parts * .\ ^ of the glass. It is the polarity of the particles of the with s]n J tm glass, which is kept up as long as the jar is charged, and is only destroyed when the interior of the jar is brought in conducting communication with the exterior by means of the useful instrument called the discharger, already shown in Fig. 224. ^The Leyden jar with shifting coatings, having been charged, is discharged with a loud snapping noise, by bringing one ball of the discharger to the outside, and the other to the ball coming from the inside. The jar is again charged, and the arm of the discharger is used to take out the interior coating. Directly that is removed, the jar may be lifted out of its outer coating, and, if the air of the room is dry, may be left some time without fear of its losing the charge. The charged Leyden jar would keep its elec- trical polarity still longer, if put under a dry glass shade, as the air around the Leyden jar would then remain still, and would thus retard the slow discharging of a charged glass surface, when the air of the room is in constant motion, by 250 ELECTRICITY. reason of the warmth of the fire, or the movements of persons about the room who are engaged in making the experiments. After waiting a reasonable time, the jar may be lifted into its outer coating, and the inner one can be quickly and dexterously returned, by the assistance of the discharger, to the interior; and now, on applying the discharger as before, a loud cracking and brilliant spark prove that the charge was confined to the particles of the glass. B FIG. 226. A, the conical glass jar; B, the outer coating ; c, the inner coating; D, the discharger. An insulated Leyden jar, like the coated pane described (page 238), cannot sustain a charge. Franklin soon discovered this fact, and hence the experi- ment is usually called " Franklin's experiment with the Leyden jar." The jar may be supported on a stand with a long glass support, which of course must be dry, and insulate perfectly. It should always be remembered that a steady gentle warmth is far better than roasting the apparatus before a large fire ; indeed, a great deal of damage THE LEYDEN JAR. 251 is done to electrical apparatus by foolishly exposing to a strong heat instruments which are partly put together with cement : the latter melts, and the symmetry and perfection of a piece of apparatus is often entirely spoilt ; because it requires some experience to cement a brass cap on to a glass vessel, and the young electrician can do little or nothing with his apparatus when the cement is melted and running down the inside or outside of it. FIG. 227. Franklin's Experiment with the Insulated Jar. The interior of the jar is now connected with the ball of the prime con- ductor of the electrical machine, and, after receiving some sparks, it will be noticed that they cease to pass, and that the conductor is showing, by its electrical brushes and dischaYges through the air, that there is no charge passing into the Leyden jar. When removed from the conductor, by pushing the insulating stand on one side, and tested with the discharger, little or no spark is perceptible. If, however, the wire and ball on which the Leyden jar stands usually inserted into and made movable on the top of the insulating stand is now connected by a chain with the ground, the jar is very quickly charged, when sparks are received from the prime conductor. The rationale has already been explained at page 240, but may be repeated here. When insulated, the positive electricity naturally resident on the outside of the glass opposes any accumulation of positive electricity in the interior; the chain of particles is not continuously charged in the order of plus and minus, but is interrupted by plus coming in the wrong place ; the order, however, is restored when the outside qf the jar is connected with the ground, as the 252 ELECTRICITY. natural positive electricity finds a channel through which it can escape, and no longer opposes the accumulation of the positive charge inside the jar. When a number of jars are insulated on glass stands and placed in regular order, the knob of the first being connected with the prime conductor, the knob of the second to the outside of the first, the knob of the third in con- <*-~imrm 4*=aSWM ^BB^^e - FlG. 228. Charging t)ic j^eyaen Jar Uy Cascade. tact with the outside of the second, and the outside of this last connected with the ground, the whole series is charged by the first, because the first loses exactly the proportion of positive electricity which enters its interior ; this passes to the second, which in its turn loses the equivalent from the out- side, and finally passes or flows, as it were, into the third jar, the outside of which is connected with the ground. Thus the positive or plus electricity of the first jar, like a continuous cascade, flows from one jar to the other, and, all being charged, they cannot be discharged together ; to effect this, the interior of all the jars must be connected together, and the same must be done with the exteriors. FlG. 229. The Jars turned round by their Insulating Glass Supports. A A, brass rod, laid on the wires and knobs connected with the interior of the jnrs, not by the hand, but with a silk thread; B B, brass rod, laid on outside of jars with hand; c, discharger, bringing the end* of two rods in conducting communication, and spark discharged. Each jar can be turned round at right angles, and a brass rod, with balls at each end, suspended by a silk thread, can be laid across all the wires and knobs of the jars, and another wire laid along the exterior of the jars ; then, THE LEYDEN JAR. 253 if the two extremities of the rods in conducting communication with the out- sides and insides of the jars are brought in contact with the discharger, a brilliant spark and louder noise announces the discharge of the series of three jars which had been charged with electricity according to the original method discovered by Franklin. Mr. I sham Baggs displayed some very brilliant experiments at the Poly- technic with Leyden jars, charged in the manner already described ; and, by a particular mode of arranging them in positive and negative series, a very long and brilliant spark was obtained.* It has been shown by the Franklin experiment that a jar cannot be charged unless the outside is placed in communication with the ground ; it has also been pointed out that Leyden jars are usually charged by passing the electri- city to the interior. A Leyden jar can, however, be charged from the exterior ; and the arrangement for this purpose is shown at Fig. 230. FIG. 230. The Leyden Jar .charged from the exterior. A brass disc, c, is screwed on the top of the ball of the large jar A, in order to carry the smaller one B. When A is charged, B becomes polarized, but cannot accumulate a charge until the positive electricity from the inside is allowed to escape ; this is done by touching the knob of B and the outside of A with the two balls of the ordinary discharger. A flash takes place when this is done, and no\v both A and B are charged. The inner surface of B is nega- * " Journal of the Royal Society," Jan. 13, 1848. 2 54 ELECTRICITY. live, the inside of A is positive ; the outside of A is negative, the outside of B is positive. Both jars may be discharged by using two dischargers : one connects the outside of A with the inside of B, thus bringing together the two negative sur- faces ; and the other discharger by touching the first one and then being advanced to the stage C, which represents the positive electricity, the usual flash and discharge follow directly the discharger comes within the striking distance. A collection of Leyden jars, fitted up with wires and balls communicating with each other, and placed on a sheet of tinfoil, so that the exterior of the jars, like the interior, may be in conducting communication, constitutes what is termed a Leyden battery. (Fig. 223.) FIG. 231, The Leyden Battery. The five large jars are coated with tinfoil, and the brass balls belonging to each jar are supported by a method proposed by the Rev. F. Lockey, and recommended because it sometimes occurs that a jar will break during the discharge of the battery, although the electricity may pursue the path intended for it. Jars are more likely to break if the wire to which the ball is attached is carried down to the tinfoil inside. Direct metallic communication with one point of the interior of the jar is not so safe as having four contacts, and this is secured by the bar of wood, covered with tinfoil, and connected with two cross-pieces of thinner wood laths, also covered with tinfoil, and shown at A B, Fig. 231. It is evident that contact is made at two places, A, B, at the top, and two at the bottom, C, D. The writer has in his possession two very large jars, which he coated with EXPERIMENTS. 2 55 \v tinfoil, after first pasting a coating of paper, such as paper-hangers use, on the jars, and allowing the paper to rise one inch above the tinfoil coatings. The jars expose a surface of six square feet of glass, and have been in use, without fracture, for the last twenty-five years, although frequently very highly charged, to break square pieces of maho- gany, to demonstrate the mechanical power of the electric discharge. Young experimentalists would do well to avoid these trying experiments, as the electricity may prefer to break through the glass, instead of travelling only through the fibres of the wood. Henley's electrometer, shown at H, Fig. 231, should always be inserted in one of the balls of the battery whilst it is being charged, as it indi- cates, by the rise of the arm carrying a light pith- ball, the amount of charge, and when it reaches 90 the jars are fully charged. It is sometimes convenient to keep a jar charged for a considerable time, and particularly if the electricity is required for medical purposes; this is done by passing a glass tube through the wooden cover of the Leyden jar: the tube is lined half-way up from the bottom with tinfoil, and terminates at the top with a brass cap ; to con- nect this with the interior of the jar, a wire with a loop at the top passes through the brass cap, and, after the jar is charged, may be removed by turning the jar upside down, when it tumbles out ; or, better still, it may be taken away with a curved wire and ball, supported on a glass handle. FIG. 232. The ordinary Leyden Jar, coated with Tinfoil, And containing the glass tube A B, capped with brass at A, and passing through the wooden top, which is usually cemented in and well varm.shed. c shows the height to which the tube is lined with tinfoil ; D is the wire, with ring at the top, removable by the insulated curved wire w. EXPERIMENTS WITH THE ELECTRICAL MACHINE, THE LEYDEN JAR, AND LEYDEN BATTERY. I. The charging and discharging of a Leyden jar is beautifully shown by coating the inside and outside with diamond and spotted coatings, or little bits of tinfoil cut in the form of diamonds or spots, and pasted on so that an interval of, glass surface may occur between each of them. When connected with the prime conductor, the jar presents a brilliant and most pleasing appearance during the time it is being charged, and also at the moment when the discharger is used ; and the jar so coated is usually called a spangled jar. II. Similar spots or small circles of tinfoil pasted round a glass tube show a brilliant spark between each interval or space left between the spots, when held to the prime conductor, or at the moment that the charge of a Leyden jar is sent through them. The tube is usually capped with brass at each end. 2 5 6 ELECTRICITY. FIG. 233. A Spangled Jar. FIG. 234. A Spangled Tube. FIG. 235. 1 1 1. Narrow strips of tinfoil are arranged in parallel lines on a plate of glass, so that a continuous conducting strip, commencing with a ball at the top of the glass and ending with one at the bottom, is obtained. The strips are then neatly cut out, so as to leave a small interval sufficiently wide to show the spark, and delineate in a succession of sparks any word, such as the name of FIG. 236. EXPERIMENTS. 257 IV. A glass bottle may be coated inside and outside with weak glue and rather large brass filings shaken inside and sifted over the glue out- side. Of course, one side must be done first viz., the inside ; and the coating should be carried about as high as the usual coating of tinfoil. It should terminate top and bottom with a band of tinfoil, and it exhibits a very pretty effect when hung on to the conductor of the electrical machine, the outside being connected with a wire or chain with the ground. The intervals between the filings give rise to the most varied and beautiful appearances of lines and forked electric sparks ; and as the jar discharges itself when the accumulation reaches a certain point, measured by the distance between the wire from the inside and the outside coatings, the effect is continuous as long as the elec- trical machine is turned. V. A little tow wrapped round one of the balls of the discharger, and dipped in alcohol or ether, is set on fire directly the spark of the Leyden jar passes through it. VI. A person standing on a stool with glass legs, and holding in one hand the chain from the prime conductor of an electrical machine in motion, may set on fire spirit or ether (held to him by some one else) in a metallic spoon, by merely allowing a spark to pass from his finger to the inside of the edge of the spoon. The hair of the person standing on the stool, and connected with the electrical machine, stands out in a very fantastic manner, if the hair is fine, silky, and well combed out previously. VII. When a blunt wire, say -f in. thick, and nicely rounded off at the end, is fixed into the conductor of an electrical machine (there are holes drilled expressly for putting in wires), as the handle is turned, a feeling like a gentle current of air is felt, when the face is approached to it, and, if the room be darkened, very pretty brush-like discharges are seen. FIG. 237. The brush discharge from a positively electrified wire. The reverse : the concentration of the same brush into a glow or star when positive electricity is drawn towards the negative conductor. The one is the reverse of the other. If the same blunt wire is placed in the negative conductor and the electrical machine put into rapid motion, a sort of glow or star is seen on the end of the blunt wire. In the first case, the positive electricity is escaping from the wire ; in the second, it is going into and towards the wire. VIII. An egg-shaped glass vessel, provided with a ground-glass plate, a collar 17 258 ELECTRICITY. of leather at the top, and through which a brass rod and ball move so as to approach to or recede from another ball fixed into the lower cap, cemented on to the glass and provided with a stop-cock, is first, FlG. 238. exhausted of air with the air-pump. Directly it (Fig. 238) is con- nected with the electrical machine, a beautiful glow of delicate violet- coloured light is seen to pass between the balls. IX, FIG. 239. The electrical inclined plane (Fig. 239) is formed by two inclined wires stretched between four glass pillars. When a very light rod of wood, covered with burnished gilt paper, having fine wires inserted at right EXPERIMENTS. 2 59 angles, with their ends all bent exactly alike, is placed on the wires which are connected with the conductor of the electrical machine, the rod revolves by reason of the reaction of the dispersed particles of electrified air upon those which are still, and it rolls up the inclined plane. If the experiment is tried in a darkened room, all the points exhibit pretty brushes of electric light. X. If the inside of a clean dry tumbler or, better still, a German beaker glass, is held over the brass rod and ball of the conductor, and, after being well electrified, is put down over a number of light pith-balls FIG. 240. The Electrical Dance of Puppets. placed on a metallic plate ; the latter are attracted and repelled in the most amusing manner, and, if the glass will take a good charge, the effect lasts some time, and, when apparently stopped, may be often renewed by drawing the finger over various parts of the outer surface. XI. Light pith figures, if well made and balanced, perform a sort of dance, by jumping up and down between a flat brass plate connected with the conductor and suspended opposite another plate connected with the ground (Fig. 240). When the shadow of the figure is cast on a disc, everybody can see the experiment, which then assumes gigantic proportions. XII. A bell (Fig. 241) may be constantly struck with clappers, so arranged that, whilst the bell is insulated and electrified, the clapper is alternately attracted and repelled. Or, if the bell is placed in connection with the inside of a Leyden jar (Fig. 242), and the outside with another bell, the two being opposite to each other, and having between them a suspended clapper, the bells will continue to ring until the jar is discharged. XIII. A very elegant experiment devised by Lichtenberg, and called after him Lichtenberg figures, is thus described by De la Rive: "Lichtenberg figures make manifest without an electroscope, and in 17 2 260 ELECTRICITY. FlG. 241. The Electric BelL FlG. 242. The Ley den Jar and Bells. a directly visible form, the nature of the electricity with which the inner coating of a jar is charged. This experiment consists in slowly passing over a cake of resin (or flat plate of vulcanite) the knob of a Leyden jar, while the outer coating is held in the hand : we may even trace figures with the knob. " The free electricity of the inner coating, which is constantly renewed in proportion as it escapes, because the other coating is held in the hand, remains adhering to all the points of the cake which the knob has touched. " If, after having thus traced out lines with the knob of a jar charged interiorly \vith positive electricity, we trace others beside them with the knob of another jar.charged with positive electricity, we may render each of them visible and distinct by powdering the cake with a powder formed of a mixture of sulphur and red lead that have been rubbed together. We perceive that all the particles of sulphur place them- selves on the positive lines, and all those of red lead upon the negative ; and they remain adhering there, even when we blow them or shake the cake strongly, so as to make the portion cf the powder disappear which is not upon the parts of the surface that had been touched by the knob. " The effect that we have just described arises from the particles of sulphur, during their mutual trituration, having acquired negative electricity, and those of red lead positive, which causes the former to pass upon the positive traces, and the latter upon the negative. We also remark that the sulphur forms a small tuft round each of the positively electrified points, whilst on each of the negative points the red lead leaves only a circular spot. This phenomenon, establishing, as it does, a very remarkable difference between the two electricities, is due to a more general cause. " The property that we have thus recognised in resin, of retaining both ELECTRICAL DISCHARGERS. 261 FIG. electricities adhering to its surface, is not peculiar to this substance alone : all bodies that are insulators possess it in a more or less marked degree. We have already seen that it exists in glass, when we elec- trized the interior of a glass jar, to produce the dance of pith balls. A Leyden jar, the coatings of which are movable (see Fig. 226), fur- nishes a further proof of this. " The jar is charged as usual ; then with an insulating handle the inner coating is lifted away, and afterwards the glass itself is lifted out: the two coatings, being thus detached, manifest no electrical signs. The two electricities have, in fact, remained adhering to the glass, the positive on the interior surface, and the negative on the ex- terior. . Leyden Jar, with Lane's Discharger. FIG. 244. Harris's Improved Lane's Electrometer. " These two electricities are recovered again by replacing the jar within its outer coating, and placing within it its inner coating ; the discharge takes place between the two coatings as if they had not been deranged. The fact just pointed out explains why a Leyden jar always retains electricity after a first discharge, even when the latter has given rise to a strong spark. We can obtain a second discharge, much weaker, it is true, than the former, but yet very sensible, and sometimes, indeed, exceedingly violent, if the jar is large, and has been strongly charged. "This second discharge arises from a portion of the two electricities having remained adhering to the glass after the first discharge, not- withstanding the contact of all the points of the two surfaces of the 262 ELECTRICITY. jar with the metal surfaces ; but the second discharge is generally sufficient to make all the remaining traces disappear." XIV. A very portable and simple apparatus for obtaining electricity and charging a Leyden phial was arranged by Mr. Adams, an optician of the same date as Cavallo. It consists of a half-pint phial, coated inside with brass filings, and outside with tinfoil, and is charged by a var- nished silk ribbon, which is rubbed by being passed through hare-skin rubbers placed, like finger-stalls, on the first and middle fingers of the left hand. The following directions are given for the proper manipu- lation of the silk rubbers : Place the two finger-caps of hare-skin on the proper fingers ; hold the phial at the same time at the edge of the coating, on the outside, between the thumb and first finger of the left hand ; then take the ribbon in your right hand, and steadily and gently draw it between the two ribbons, over the two fingers, taking care at the same time that the brass ball of the jar is kept nearly close to the ribbon while it is passing through the fingers. By repeating this operation thirteen or fourteen times, the electrical fire will pass into the jar, which will become charged, and, by placing the discharger against it, you will see a sensible spark pass from the ball of the jar to that of the discharger. If the apparatus is dry and in good order, you will hear the crackling of the sparks when the ribbon is passing through the fingers, and the phial will discharge at about the distance of half an inch from the balls. XV. In order to regulate the proper discharge of single Leyden jars and batteries, very useful contrivances have been invented. The arrangement (Fig. 243) consist of a bent glass arm, which is fixed to the rod and ball passing to the inside of the jar; the arm carries a tube through which a rod, with balls at both ends, slides. The dis- tance between the two balls, one of which represents the interior and the other the exterior of the jar, is regulated according to the scale graduated on the sliding rod, so that a discharging spark of any re- quired length (confined within the limits of the charged surface of the jar) may be obtained. Sir W. Snow Harris improved the arrange- ment of Lane's discharging electrometer, by making it an indepen- dent piece of apparatus, that might be adapted to one or more jars. The exploding balls of this instrument (Fig. 244) are supported be- tween a bent glass arm and a vertical tube of brass, and may be set at any given distance by means of a graduated slide. The bent arm of glass is attached, and is movable on a stout glass cylindrical rod, so as to insulate the whole, if required, and adjust the ball to be con- nected with the inside of the jar or battery to any given height. These and other pieces of electrical apparatus are made most correctly and elegantly by Messrs. Elliott Brothers, of 5 Charing Cross, the worthy successors of the old firm of Watkins and Hill, so long celebrated for their electrical apparatus. XVI. Cuthbertson's Balance Electrometer is an extremely useful contri- vance, where large Leyden batteries are required to be rapidly and uniformly discharged, as at the Polvtechnic, where the deflagration of metallic wires* is displayed. The apparatus consists of a wooden stand, in which two glass rods or supports are fixed : one of the insu- lating rods or pedestals supports a arass ball, which has a little hook ELECTRICAL DISCHARGERS. 263 below it, for the convenience of attaching the chain passing from the outside of the Leyden battery ; the other and higher glass pedestal supports a large brass ball, in which is arranged a long brass rod, supported on knife-edges, and acting like a balance ; above this, and proceeding from the same large brass ball, is another rod and ball, placed so that the ball of the latter is exactly over, and almost touching, the other and lower one, that works on knife-edges. FlG. 245. Cuthbertson's Balance Electrometer. A, B, glass supports. The hook of A is connected by a chain with the outside of the battery. B carries the large ball through which the balance-rod, D, works. The sliding weight, E, like that of a steel rod, enables the experimenter to adjust the balance perfectly. H, the upper and fixed wire and ball, which, when sufficiently electrified by contact with the inside of the battery, by the hook and chain at K, repels the movable balance D, and, making the circuit complete (as shown by the dotted lines) by touching the brass ball on A, the whole discharge of the battery is sent through any substance. With Cuthbertson's compound universal discharging electrometer, the experimenter may always have notice when the battery is nearly charged and ready, by inserting in the upper ball a Henley's quadrant electrometer, with graduated arc. The oscillation of the balance, when the battery is almost ready, will likewise serve to warn the person using it that he may expect the discharge to occur. XVII. In connection with the Leyden battery, a Cuthbertson balance elec- trometer and Henley's universal discharger and press are always employed when a variety of substances are to be subjected to the powerful effects of a large charged surface of glass. The mechani- cal arrangements are such that the direction of the charge is certain and precise. The annexed figure (246) hardly requires any explanation, as the parts are so simple. It consists of two glass legs, which support, by 264 ELECTRICITY. hinged joints, two brass rods and balls with glass handles attached. The latter slides through tubes, and may be caused to advance or recede from each other, or they move right or left, as the hinged joints work in sockets. The balls meet either on the little table, in which a piece of ivory is inserted, or the little table can be removed and the press substituted for it ; as, for instance, when it is required to show the immense FlG. 246. Henley's Universal Discharger and Press. mechanical force of the electrical discharge by putting gold leaf between glass plates, and passing a charge through them, which shatters the glass to fragments, and frequently forces the gold leaf into the body of the glass. In this experiment, it is usual to put the glass plates in the press, and, to prevent accident from the pieces of glass flying about, it is better to cover the whole with a dry clean duster. XVIII. Unscrew by a turn or two the balls attached to the arms of the Hen- ley discharger ; take some very fine iron wire, such as is used by silversmiths for making scratch-brushes, and having twisted a little in the crack or opening left by unscrewing the balls, screw them up again, when the thin wire will be held tightly, and, the length having been adjusted to the power of the Leyden battery employed, the whole is dispersed in minute white-hot globules when the electric charge is sent through the wire. XIX. Place the balls of the Henley discharger on the little table, about one inch apart ; put some gunpowder between them. When the dis- charge of the Leyden battery is sent across and through the gun- powder, it is not ignited, but every grain is dispersed and thrown away by the mechanical violence of the discharge, which occurs so rapidly, that the heat of the electric discharge does not appear to have time to affect the gunpowder. When the great steam hydro-electric machine was in use at the Polytechnic, it was possible, by directing from a point the whole dis- charge of the mammoth machine for some minutes into a heap of gun- powder, to accumulate heat and set it on fire ; but it was always very troublesome to do, and a great deal of steam had to be used to effect this object. If, however, a damp string formed part of the conduct- ing arrangement, then the powder fired almost instantaneously, as EXPERIMENTS. 265 the damp string exercises a retarding action on the velocity of the current of electricity, and it then appears to have time to give its heat to the powder. To fire gunpowder by the Leyden jar, a little cardboard tray may be placed on the table of the Henley discharger ; and in order not to spoil the polish of the balls, two copper wires are thrust through the sides of the tray containing the powder, and the brass balls of the Henley discharger connected with them. A wet string may be tied to one rod and also to an ordinary discharger, the other rod being con- nected by a chain with the exterior of the Leyden jar; the ordinary discharger with the wet string is made to touch the knob, and, although it sometimes fails, the powder is very generally ignited directly con- tact is made. To fire gunpowder, a wet string must form part of the circuit. The powder may be placed in a closed case or cartridge, so that it cannot be scattered by the mechanical violence of the dis- charge. Sturgeon retards the velocity of the discharge by placing the gun- powder in a boxwood cup which is insulated and connected with the outside of the jar. An insulated brass wire and ball is placed directly over the cup, and, directly contact is made with this and the interior of the Leyden jar by the ordinary discharger, the powder is usually fired. XX. A piece of mahogany, about two inches long and in. square, may be split by passing the discharge into and through it by two copper wires inserted about half an inch, one at each end. The softer the wood, the safer the experiment so far as the jars are concerned, and, as already observed at page 255, this experiment must not be pushed too far by using larger and thicker pieces of wood. XXI. When a lighted composite candle is blown out carefully, there rises from it a column of gas and smoke, which is inflammable. If such a candle is placed on the table of the Henley discharger, and the balls adjusted so that the spark will go through a point just above the burning wick, and the whole connected with a charged Leyden jar, the spark will relight the candle, if, simultaneously with the blowing out of the flame, contact is dexterously made with the Leyden jar. FIG. 247. The Electric Bomb. XXII. The expansion which air undergoes during the passage of an electric discharge through it is shown by a very nicely constructed mortar, to the mouth of which is accurately fitted a ball of some light wood. 266 ELECTRICITY. When the discharge passes, the ball is forced out ; and if the whole is made of ivory(Fig. 246), the effect is very certain. The expansion of the air in this experiment will help the student to understand why so much noise (thunder) is heard, when the electrical discharge takes place from hundreds of acres of charged clouds. , XXIII. The experiment called the " Shooting Star" is extremely beautiful, but, like many other illustrations, requires considerable pains to be taken in order to obtain a good result. In the first place, a long tube must be pro- vided at least four feet in length ; this is properly capped, and provided with a stopcock at one end and a plain cap on the other, which should be nicely rounded off, and inside the cap a small ball may be screwed. The electrical machine being in good order, and the Leyden battery, of six square feet of glass, warm and dry, one assistant may proceed to charge it gradually, whilst another may be pumping the air out of the long tube. When the electrometer shows that the battery is nearly charged, one end of a chain is attached to one of the balls of the discharger, and the other end to the top of the long tube. The air- pump or stopcock end of the tube is, of course, in conducting com- munication with the outside of the battery jars. The circuit is now suddenly completed, and sometimes a continuous flash through the whole length of the tube marks the discharge of the battery ; but it may occur that it discharges itself in a brush, and that the battery must be recharged, and the experiment tried again. To insure per- fect success, the experiment should be tried with a barometer attached to a pump, and then it- will soon be ascertained what vacuum is the best for the experiment. Success greatly depends on the right management of the vacuum, which must not be a perfect one. XXIV. The velocity of electricity, and the consequent amazing rapidity of the spark-discharge, and appearance or disappearance of the light, is admirably shown by Mr. Rose's photodrome apparatus described at p. 85, Fig. 95. The writer uses the disc four feet in diameter, having a series of black balls painted on a white ground ; when this is rotating three hundred times in a minute, and the black balls have all merged one into the other, according to the law of persistence of vision, already explained at p. 84, they produce (instead of twelve distinct black balls) three continuous rings, dark in the centre, and lighter towards the edges, because there the greatest surface of the white disc is exposed. The disc should be illuminated with a lime light and lens, and, directly this is cut off, a Leyden jar, provided with a Lane's discharger, is per- mitted to discharge itself regularly, by keeping the electrical machine in motion ; all the black balls now return, and the disc, though going round three hundred times in a minute, appears frequently to stand still. The same fact is observed during a storm at night, accompanied with thunder and lightning : all objects seen by the light from the elec- tric flash appear to stand still, although they may be in rapid motion at the time. Captains of ships have frequent opportunities of noticing this : a storm comes on suddenly, and some, if not all, the sails of the ship require to be furled; the command is given, up. fly the sailors, and the deck and rigging swarm with men who are actively engaged ; EXPERIMENTS. 267 but if at this moment the ship is illuminated with a flash of lightning, every officer, every man, the ship tossing about, and the waves of the sea, all appear at rest, as if they were parts of a magnificent stereo- scopic picture. The fact is, that the light from a flash of lightning, as proved by Sir Charles Wheatstone, comes and goes in the millionth part of a second ; so that before the wheel, going round three hundred times in a minute, has time to move, the electric light has arrived and passed away. The same thing occurs with all other movements viewed with the electric flash, and the fleetest racehorse even, under these circum- stances, would actually appear to be standing still. XXV. Many years ago, Sir Charles Wheatstone invented a most ingenious arrangement for measuring the velocity of electricity through a copper wire, and it was from these experiments he deduced the almost instantaneity of the light from the electric spark. His apparatus consisted of a Leyden jar, which was charged in every experiment to the same amount, and the discharge sent through a copper wire about half a mile long. FIG. 248. The copper wire was insulated and interrupted at three points, viz. one, A A, within a few inches of the inner coating, one at the middle of the circuit, B B, and one at the same number of inches of the outer coating, c C, of the Leyden jar as the first which was in con- tact with the inner coating. A very cleverly arranged insulated disc (Fig. 248) contained the three breaks in the circuit, where the spark discharges took place ; so that when the Leyden jar was discharged, all the sparks could be seen at once, and were reflected in a small 268 ELECTRICITY. revolving mirror. If observed without the mirror, the three sparks appeared to occur simultaneously ; but when looked at in a small revolving steel mirror through a plate of glass, the sparks, accord- ing to the law of persistence of vision, become lines of light, of which two are equal, whilst the third, representing the middle of the circuit, is sufficiently delayed to give a shorter line, and, as the velocity of the steel mirror is known, by a proper register, the exact angular deviation of the image of the central spark is easily ob- tained ; and from these data the retardation of the current by the long copper wire is correctly calculated. FlG, 249. Apparatus, made by Messrs. Elliott, to show the time occupied by the transmission of an Electric Current by reflection. B, the revolving mirror. The three sparks, when seen in the revolving mirror, appear as three straight bright lines; and, if the motion is very fast, the lines assume the appearance A, Fig. 250, when the mirror is rotated to the :B FlG. 250. Lines of Light reflected from Revolving Mirror. right ; but, if reversed, then they appear as in B, Fig. 250 : but the lines were nevei seen as at c or D, Fig. 250, which should have been the case according to the Franklinian theory of a single fluid. Thus EXPERIMENTS. 269 Wheatstone's ingenious and beautiful experiment supports most powerfully the theory of the two fluids, which seem to meet in the centre of the wire, as if they rushed with equal speed to unite with and neutralize each other. The spark disc (Fig. 249) was placed 10 ft. away from the re- volving mirror, and the summing-up of the experiments gave the following conclusions : 1. That electricity travels, through a copper wire arranged as in the experiment described, faster than light in its passage from the sun. 2. That the electricities of the two kinds, viz., that from the interior of the jar and the other from the exterior, travel at the same velo- city, and meet in the middle of the wire. 3. That the light from the electric flash or spark-discharger does not last longer than the millionth part of a second. 4. That the delicate optic nerve is capable of appreciating an interval of that duration, or, in other words, can see objects which are only illuminated for the millionth part of a second. FIG. 25 1. Appearance of the Card after sending the discharge through Silver Wire \-yx>th of an inch thick. XXVI. Very fine .^old, silver, copper, brass, and iron wires can be obtained of Messrs. Johnson and Matthey, at their assay office in Hatton Garden. About three inches of either metallic wire is stretched across a plain white card by making a small cut in the card at the opposite ends, and then placing the wire in the cuts, which may be neatly closed with little slips of tinfoil. The card with the wire is then covered with another card, and placed between the boards of the little press attached to Henley's universal discharger (Fig. 245, p. 264). When tightly screwed up and the brass balls of the discharger brought in contact with the ends where the tinfoil marks the termination of the two ends of the wire, the discharge from the Leyden battery can then be sent through it. The result is that the wire is completely disintegrated, 270 ELECTRICITY. and so perfectly divided that nothing remains upon the two cards but certain curious marks (Fig. 251), which are no doubt caused by the finely divided metal being driven bodily into the card, although it is usually ascribed to oxidation, and this may be the case with metals which unite easily with that element. When a very thin iron wire is deflagrated alone by passing the battery discharge through a length of nine or twelve inches, the effect is very beau- tiful, as it is dispersed in a shower of red-hot globules, which are well displayed in a darkened room. The dissipation of gold by a powerful electrical discharge can also be shown in a similar manner. The metal is vaporized, and disappears in the form of a red vapour. By receiving the vapour from gold on a piece of silk, a portrait or other figure may be printed upon it. To obtain these portraits a likeness of any known personage is cut out in a small piece of cardboard, so that, if held against the wall with a candle behind it, the shadow cast indicates that the portraiture is successful ; the portrait-card is now laid upon a sheet of gold leaf pasted to another card ; and, as the electrical discharge would act unequally upon the gold if merely conveyed through the brass balls of the dis- charger, it is usual to paste a slip of tinfoil on the opposite edge of the gold leaf, thus bringing all the gold at once in conducting communicating with the brass balls. FIG. 252. A, card covered with gold leaf, and edges prepared with tinfoil ; B, portrait- card ; c, the two cards, A and B, in press, and in contact with the brass balls of the discharger. XXVII. With the powerful hydro-electric machine at the Polytechnic (p. 273) (to be hereafter described) a most beautiful effect was produced by sending the discharge through a long chain composed of beads of glass and copper strung on a stout silk cord ; and as the latter was at least forty feet in length, the effect was very imposing. On the smaller scale a piece of brass chain, hung in festoons on a plate of glass blackened at the back, affords a very pretty experiment, being illuminated throughout its entire length when the electrical discharge is sent through it. XXVIII. To imitate and demonstrate the effects of discharges of natural electricity, or lightning, on buildings, &c., many ingenious models, such as the gable end of a house, a pyramid, a powder-magazine, EXPERIMENTS. 271 a ship, or mast of a ship, are made by Messrs. Elliott, of the Strand, London. FIG. 253. All these models act upon one principle, viz., that as long as the conductor is continuous throughout and unbroken, no harm or damage occurs to the model ; but directly the conducting chain is broken, by removing or altering the position of some part of the conductor, then tha following results occur. In the first place, the charged cloud is represented by Sir William Snow Harris's thunder- cloud needle (Fig 254), formed by a brass horizontal rod or needle balanced and movable upon the point of a vertical metallic rod connected with the interior coating of a large Leyden jar. FlG. 254. Harries Thunder-cloud Needle. One end is covered with the finest cotton wool : a little good gun-cotton increases the effect, as it may be so arranged that every time the flash occurs the cotton shall ignite, and the sudden flash with the crack and light of the spark is remarkably telling. The cotton is intended to represent a cloud hovering over the chimney or highest part of a house or church-steeple ; and, when the jar has been sufficiently charged, it is attracted, according to the law of induction, to the nearest object, and the simulated cloud descends upon the top of the model, at the same time discharging the jar 272 ELECTRICITY. through the parts of the models. As before stated, when the lower portion of the conductor attached to either of the models is connected with the outside of the jar by a chain, the Harris's thunder-cloud needle being in connection with the interior of the jar, the discharge causes no change in the disposition of the parts of the toy model; but if, as in A, Fig. 253, the little piece of square wood at B is turned round at right angles, the continuity of the wire is broken, and it is blown out when the discharge takes place. The model B maintains its erect position if the conductor is undisturbed ; but when a little bit of tinfoil is removed from D, it topples over in the most natural fashion when the miniature thunder-cloud is discharged upon it. The model E affords a good bang, and the roof is blown off when the powder in the tube F is ignited ; but care must be taken not to use too much gunpowder. The writer well remembers helping poor young Mr. John Cooper, many, many years ago, at a lecture delivered at the Southwark Institute, and, being directed sotto -vocc to give them a " good one," he attended too implicitly to his in- structions. Luckily, this was the concluding experiment : the powder-house blew up with astounding effect ; but, unfortunately, the roof descended into the middle of a large cylindrical electrical machine, and the result, of course, was total annihilation. The audience, it is believed, thought it was all part of the experiment, and applauded in the most cheering manner ; but the glances ex- changed between the lecturer and his assistant were of the most desponding kind, considering that the large electrical machine had only been borrowed for the occasion. G, Fig. 253, is called the "fire-house," and exhibits the heat of the electrical discharge, and its power to set fire to gun-cotton or tow dipped in ether or alcohol ; and, as it is made of tin and glazed with glass windows, the conflagration inside betrays the lamentable effects that might and do occur when houses are struck and set on fire by lightning. XXIX. A lightning conductor, if intended to last, should be made of copper rod, at least half an inch better three-quarters in diameter. It should be carried above the highest chimney-top, and be well pointed and doubly gilt ; the lower end must be carried down to the clay, and must enter the first stratum of earth known to be always damp. If the building is a long one, it is better to have a light- ning conductor at each end, as a cloud, in coming up to a lightning- conductor, is always discharged through the shortest road ; and if a chimney-pot at the other end of the building rises as high as the lightning conductor at the other end, it may divide the honours and dangers of the discharge with the conductor, provided the cloud arrives at the side opposite to that where the metallic safety-rod is fixed. XXX. The hydro-electric machine affords a magnificent example of electricity derived from friction, and it continued for a lengthened period to be one of the greatest attractions at the Polytechnic. In the " Philosophical Magazine," vol. vii., appeared a letter from Mr. (now Sir William) Armstrong, giving a curious and most inte- EXPERIMENTS. 273 FIG. 255. The Hydro-Electric Machine at the Polytechnic. resting description of the accidental production of the electric spark by high-pressure steam escaping through a fissure or crack in the cement by which the safety-valve ought to have been fitted in steam-tight to the boiler of a locomotive standing at Sedgehill, six miles from Newcastle. Every time the engine-man passed his hand through the steam he received an intense electric spark, which he spoke of as " fire." Mr. Armstrong investigated the phenomena, and, continuing a very laborious and clever series of experiments, arrived by gradual steps to the production of a per- fect steam machine, in which the particles of water, impelled by steam, rubbing against the interior of a series of jets lined with partridge-wood, produced effects which have never been surpassed in England. At that time Dr. Bachoffner was the very popular lecturer on Natural Philosophy at the Polytechnic, and he assisted at and conducted most patiently the vast number of experiments which had to be carried out before the ponderous machine was considered ready to be exhibited to the public. With Dr. Bach- offner were of course associated the contriver, Mr. Armstrong, and Mr. Walker ; and fearful that our readers may think the writer too prone to talk of Polytechnic doings, he has preferred to take Dr. Noad's account of the machine as exhibited fifteen years ago at that Institution: 18 274 ELECTRICITY. " Shortly after these experiments were made, the directors of the Polytechnic Institution determined on constructing a machine, on a large scale, for the purpose of producing electricity by the escape of steam ; and under the supcr- intendance of Mr. Armstrong, assisted by Dr. Bachoffner, the ' Hydro- Electric Machine' was finished, and placed in the theatre of the Institution, where by its extraordinary power it soon excited the astonishment of all who beheld it. The machine consists of a cylindrical-shaped boiler, similar in form to a steam-engine boiler, constructed of iron plate f in. thick ; its extreme length is 7 ft. 6 in., one foot of which being occupied by the smoke-chamber makes the actual length of the boiler only 6 ft. 6 in. ; its diameter is 3 ft. 6 in. The furnace and ash-hole are both within the boiler. When it is required entirely to exclude the light, a metal screen is readily placed over these. By the side of the door is the water-gauge and feed-valve. On the top of the boiler, and running nearly its entire length, are forty-six bent iron tubes, terminating in jets having peculiar-shaped apertures, and formed of partridge- wood, which experience has shown Mr. Armstrong to be the best for the pur- pose ; from these the steam issues. The tubes spring from one common pipe, which is divided in the middle, and communicates with the boiler by two elbows. By this contrivance the steam is admitted either to the whole or part of the tubes, the steam being shut off or admitted by raising or lowering the two lever Handles placed in the front of the boiler. Between the two elbows is placed the safety-valve for regulating the pressure, and outside them, on one side, is a cap covering a jet employed for illustrating a certain mechanical action of a jet of steam, and on the other a loaded valve for liberating the steam when approaching its maximum degree of pressure. At the further extremity of the boiler is the funnel-pipe or chimney, so contrived that, by the aid of pulleys and a balance-weight, the upper part can be raised and made to slide into itself (similar to a telescope), so as to leave the boiler entirely insulated. To prevent as much as possible the radiation of heat, the boiler is cased in wood, and the whole is supported on six stout glass legs, 3! in. diameter and 3 ft. long. In front of the jets, and covering the flue for con- veying away the steam, is placed a long zinc box, in which are fixed four rows of metallic points, for the purpose of collecting the electricity from the ejected vapour, and thus preventing its returning to restore the equilibrium of the boiler. The box is so contrived, that it can be drawn out or in, so as to bring the points nearer or further from the jets of steam : the mouth or opening can also be rendered wider or narrower. By these contrivances the power and intensity of the spark is greatly modified. A ball-and-socket joint, furnished with a long conducting-rod, has been added to the machine, so that by its aid the electricity can be readily conveyed to the different pieces of apparatus used to exhibit various phenomena. The pressure at which the machine is usually worked is 60 Ibs. on the square inch. " As it is now fully established that the electricity of the hydro-electric machine is occasioned by the friction of the particles of water, the latter may be regarded as the glass plate of the common electrical machine, the partridge- wood as the rubber, and the steam as the rubbing power. The electricity produced by this engine is not so remarkable for its high intensity as for its enormous quantity. The maximum spark obtained by Mr. Armstrong in the open air was 22 in., the extreme length under present circumstances has been 12 or 14 in.; but the large battery belonging to the Polytechnic Institution, exposing nearly 80 ft. of coated glass, which under favourable circumstances THE HYDRO-ELECTRIC MACHINE. 275 was charged by the large plate machine, 7 ft. in diameter, in about 50 seconds, is commonly charged by the hydro-electric engine in 6 or 8 seconds. The sparks which pass between the boiler and a conductor are exceedingly dense in appearance, and, especially when short, more resemble the discharge from a coated surface than from a prime conductor. They not only ignite gun- powder, but even inflame paper and wood shavings when placed in their course between two points. In the I5ist number of the 'Philosophical Magazine,' a series of electrolytic experiments made with this machine are described by Mr. Armstrong. True polar decomposition of water was effected in the clearest and most decisive manner, not only in one tube, but in ten different vessels, arranged in series, and filled respectively with distilled water, acidified with sulphuric acid, solution of sulphate of soda tinged blue, and red solution of sulphate of magnesia, c., c., and the gases were obtained in sufficient quantities for examination. " The following curious experiments are likewise described : " Two glass vessels containing water were connected together by means of wet cotton. On causing the electric current to pass through the glasses, the water rose above its original level in the vessel containing the negative pole, and subsided below it in that which contained the positive pole, indicating the transmission of water in the direction of a current flowing from the positive to the negative wire. Two wine-glasses were then filled nearly to the edge with distilled water, and placed about 4-ioths of an inch from each other, being connected together by a wet silk thread of sufficient length to allow a portion of it to be coiled up in each glass. The negative wire, or that which com- municated with the boiler, was inserted in one glass, and the positive wire, or that which communicated with the ground, was placed in the other. The machine being then se-t in action, the following singular effects presented themselves : " i. A slender column of water, inclosing the silk thread in its centre, was instantly formed between the two glasses, and the silk thread began to move from the negative towards the positive pole, and was quickly all drawn over and deposited in the positive glass. " 2. The column of water, after this, continued for a few seconds suspended between the glasses as before, but without the support of the thread; and when it broke, the electricity passed in sparks. " 3. When one end of the silk thread was made fast in the negative glass, the water diminished inthe positive glass, and increased in the negative glass, showing, apparently, that the motion of the thread, when free to move, was in the reverse direction of the current of water. " 4. By scattering some particles of dust upon the surface of the water, it was soon perceived by their motions that there were two opposite currents passing between the glasses, which, judging from the action upon the silk thread in the centre of the column, as well as from other less striking indica- tions, were concluded to be concentric, the inner one flowing from negative to positive, and the outer one from positive to negative. Sometimes that which was assumed to be the outer current was not carried over into the negative glass, but trickled down outside of the positive one, and then the water, instead of accumulating, as before, in the negative glass, diminished both in it and in the positive glass. ' 5. After many unsuccessful attempts, Mr. Armstrong succeeded in causing the water to pass between the glasses without the intervention of a thread for 18 2 276 ELECTRICITY. several minutes, at the end of which time he could not perceive that any material variation had taken place in the quantity of water contained in either glass. It appeared that the two currents were nearly, if not exactly, equal, while the inner one was not retarded by the friction of the thread. Mr. Arm- strong likewise succeeded in coating a silver coin with copper, in deflecting the needle of a galvanometer between 20 and 30, and in making an electro- magnet by means of the electricity from this novel machine. " Extraordinary as is the power of the Polytechnic machine, it was after- wards entirely eclipsed by a similar apparatus constructed at Newcastle under the direction of Mr. Armstrong, and sent out to the United States of America. In the arrangement of this machine, the boiler of which is not larger than that at the Polytechnic Institution, Mr. Armstrong introduced certain improve- ments, suggested by the working of the latter, and which had reference to those parts of the apparatus more immediately concerned in the production of the electricity, viz., the escape apertures and the condensing pipes. It was found to be a matter of extreme nicety to adjust the quantity of water depo- sited in the condensing pipes, so as to obtain the maximum excitation of elec- tricity. If, on the one hand, there be an excess of water, then two results will ensue, each tending to lessen the electricity produced: ist, the mean density of the issuing current of steam and water is increased, which causes the velocity of efflux, and consequent energy of the friction, to be diminished ; and, 2iidly, the ejected steam-cloud is rendered so good a conductor by the excess of moisture, that a large proportion of the electricity manifested in the cloud retrocedes to the boiler, and neutralizes a corresponding proportion of the opposite element. On the other hand, if the quantity of water be too small, then, although each particle of water may be excited to the fullest extent, the effect is rendered deficient, in consequence of the insufficient number of aqueous particles which undergo excitation. " In the Polytechnic, arrangement for condensation of the steam in the tubes is effected by contact with the external air ; and when the density of the steam in the boiler is diminished rapidly, they do not cool down with sufficient rapidity to condense the' requisite quantity of water. To remedy this defect in the American machine, Mr. Armstrong adopted a method of condensing by the application of cold water. A number of cotton threads were suspended from each condensing pipe into a trough of water, from which, by capillary attraction, just as much water was lifted as was required for the cooling of the pipe, since it was easy, by increasing or diminishing the quantity of cotton, to increase or diminish the supply of cold water ; and this method of keeping down the temperature proved so effective, that two or three times the number of jets that were before used could now be employed. The number in the American machine was 140, ranged in two horizontal rows, one above the other, on the same side of the machine. The sparks obtained, though not longer than those upon the London machine when it stood in the open air, succeeded each other with three or four times the rapidity, and, even under unfavourable circumstances, charged a Leyden battery, consisting of thirty-six jars, containing 33 ft. of coated surface, to the utmost degree that the battery could bear, upwards of sixty times in a minute, being equivalent to charging nearly 2000 ft. of coated surface per minute, which is at least twenty times greater than the utmost effect that could be obtained from the largest glass electrical machine ever constructed." The Polytechnic apparatus, itself unique, enormous, and powerful, was well THE HYDRO-ELECTRIC MACHINE. 277 adapted for the purposes of the Institution, but could not be carried about or fitted up in another lecture-room. The writer had a portable apparatus fitted up, which gave safely, on the small scale, all that could be witnessed with the great hydro-electric steam machine. It consisted of a cylindrical furnace and strong copper boiler, supported on a stool with stout glass legs, each of which rested on a disc of shellac. The boiler was provided with a safety-valve and all necessary taps, and proceeding from it, and fitted with a ball-and-socket joint, was a copper tube, I in. in diameter, curved round, and having a hollow copper ball at the end, to which three stop-cocks were fitted. Whilst steam was getting up, the copper tube was left off the boiler, and only screwed on just before the experiments were shown. The chimney of the furnace was so arranged that the portion connected directly with the furnace could be removed, disclosing a square iron box, into which a few pieces of burning charcoal were placed, so that, when the copper tube and ball were screwed on, the first stop-cock exactly faced the iron box containing the charcoal; and, of course, when the steam was turned on, it blew out of the latter into the charcoal, and, causing the charcoal to burn with greater rapidity, created a good draught, which carried off the steam, and prevented it doing harm to the other electrical apparatus, which had to be kept dry and warm. FlG. 256. Portable Apparatus for showing the Electricity of Watery Steam. A is the copper boiler, safety-valve, copper curved tube, with hollow ball and three stop-cocks ; the lower one enables the operator to remove condensed water, the upper one to introduce any different fluid ; the third contains the jet made of hard partridge-wood (Fig. 257), from which the watery steam escapes into the charcoal-box and chimney, D D. The dotted lines, C c, show the portion of the chimney removable before the experiments commence, in order to insulate the furnace B, which stands on a stool with strong glass legs, resting on plates of shellac. The operator must remember to keep a sufficient quantity of damp sand in the bottom of the ash-pit, which should be regularly wetted by the assistant, or the stool may catch fire, and great confusion caused by this untoward result. The chimney D D is rendered independent of all extraneous support by being attached to a strong iron pillar with claw feet, screwed to the floor with (E E) 278 ELECTRICITY. stage-screws, i.e., spiral screws with handles, much used for theatrical purposes, to support small bits of scenery on a stage. The boiler being insulated, and the steam up to a pressure of at least 30 Ibs. on the square inch, a number of interesting experiments may be performed. FlG. 257. Section of the Jet used for the Hydro-Electric Machine, Being a conical plug of hard wood (partridge-wood is preferred), terminated by a brass mouth-piece. The shaded parts are brass. I. Mere emission of dry steam produces no electricity, and will hardly affect the gold leaves of an electroscope. II. The copper ball is now purposely cooled a little by pouring cold water and applying a wet flannel to it, so as to obtain some condensed water; and now, when the steam is turned on, the usual signs of electrical excitement become apparent, and sparks are easily procur- able. The handles of the stop-cock must be covered with flannel, or the operator will be unable to manipulate the opening and shutting of them. The watery steam, rushing through the tube, evolves elec- tricity, because the particles of water forced through by the jet of steam rub against the inside of the jet, thus proving in a satisfactory manner that friction is the exciting cause, and not the mere change of form of water into steam. The copper boiler, whilst the steam is issuing, is negatively electrified; the issuing steam, positively. III. The steam being raised to 50 Ibs. on the square inch, the electric spark, the inflammation of combustible matter, and the charging of the Leyden jar, can be displayed, the boiler and steam remaining in the same state of electricity. IV. Altering the rubbing fluid, by substituting oil for the water in the copper globe (easily done by pouring in a few drops of oil of turpentine through the upper stop-cock), changes the state of the electricity of the boiler from negative to positive, and the steam from positive to negative, because the globules of water become coated with oil, and thus expose a different surface against the rubber, viz., the inside of the hard partridge-wood jet. V. The electrical exaltation is destroyed for a time by putting a solution of common salt into the copper globe, because the particles of water are then made good conductors, and as fast as the electricity is ob- tained it is neutralized (returned again to the boiler), just like rubbing a piece of sealing-wax with a damp flannel. The gradual rise and return of the electrical force is shown, as the conducting matter, the salt, is blown out of the copper globe, as if the damp flannel had been dried, and thus lost its conducting power. VI. Dry steam or dry air will not excite electricity whilst rushing through a tube ; this is easily proved by getting the copper tube and globe as hot as possible, and then allowing the steam to issue from the jet. 6 VMMAR Y OF LA WS. 2 7 9 So also with air : the mere fact of allowing air to rub against the inside of the nozzle of a common pair of bellows will not eliminate the electric force ; but if a little whitening or powdered chalk is in- troduced, as a substitute for the watery particles in the steam experi- ment, the electricity is produced, and is shown distinctly if the whitening is blown out on to the cap of the electroscope. VII. By connecting an insulated platinum capsule, containing water, by a wire, with an electroscope, and evaporating the water, no electricity can be rendered evident ; if, however, a piece of red-hot charcoal is placed in the platinum capsule, and a little water suddenly poured upon it, and provided the ebullition is sufficiently violent to cause the particles of water to rub against the sides of the capsule, then elec- tricity is sometimes eliminated. From these experiments it may be concluded that evaporation unattended by friction, as from the surface of the oceans, rivers, lakes, is not a source from whence electricity in nature is obtained, and we must therefore look to some other cause for the explanation of the production of atmospherical elec- tricity. SUMMARY OF THE LAWS OF ELECTRICAL ACCUMULATION. The young students who wish to travel easily through the chapters on voltaic electricity, magnetism, and electro-magnetism will do well to make themselves well acquainted with the laws which relate to frictional electricity, as they will find them reproduced in more complicated forms as they proceed with the consideration of the most important branches of science, with which all well- educated persons should be acquainted. I. Experiments would show, and especially those which relate to the velocity of the passage of an electrical discharge through a copper wire half a mile in length, performed by Sir Charles Wheatstone, p. 267, that the idea of the existence of two forces, the one called " vitreous " and the other "resinous" electricity, seems to be more rational and better capable of proof than the Franklinian theory that supposes the existence of one fluid only; and this idea is further supported by Armstrong's curious experiments with the Polytechnic hydro-electric machine, paragraphs I to 5, page 275. II. Similar electricities repel, dissimilar attract, each other. III. There is no absolute difference between insulators and conductors, it is shown that they may both assume polarity ; but, in the former case, the polarity lasts only so long as the disturbing cause exists ; in the latter, as with glass and resin, the polarity set up is maintained. These are called dielectrics, because they are capable of polarization. IV. Electrical induction means that disturbance of electrical equilibrium which occurs when an electrified body is brought towards another which is in a quiescent state. V. Faraday's theory of induction has overturned all previous hypotheses. " Electrical induction is an action of contiguous particles." Every particle of air between a piece of excited glass and the cap of an electroscope is supposed to be in a polar state. 2 8o ELECTRICITY. As long as the particles maintain their polarization, insulation is secured ; but when the particles discharge themselves one into the other, then a neutralization occurs, and the non-maintenance of polarization is called conduction. Even a Faraday could occasionally write vaguely. It is sometimes better to take the epitome of a philosopher's assumptions through another mind, and this want is admirably supplied by the late Pro- fessor Daniell, of King's College, London : " Up to the date of his discovery, the phenomena of induced elec- tricity were supposed to arise from an action of a charged body upon others at a distance, in straight lines, through non-conducting media, the particles of which were assumed to be unaffected by it ; he has shown induction, on the contrary, to be an action of contiguous par- ticles throughout, capable of propagation in curved lines, and to be concerned in all electrical phenomena ; having in reality the character of a first, essential, and fundamental principle. . . It was formerly supposed that the electric fluid was confined to the surfaces of bodies by the mechanical pressure of the non-conducting air, in the midst of which all our experiments are carried on ; but the fact is that the electric force, originally appearing at a certain place, is propagated to, and sustained at, a distance through the intervention of the con- tiguous particles of air, each of which becomes polarized, as in the case of insulated conducting masses, and appears in the inducteous body, i.e., the body under induction as a force of the same kind exactly equal in amount, but opposite in its directions and tendencies." VI. Electricity is found to reside on the surface of an insulated metallic conductor a natural sequence of the polarization of particles. The difference in form, as between a ball and a point, so far as their rela- tion to an electrical charge is concerned, is explicable by the theory of contiguous particles. " It was," says Daniell, " by an apparatus constructed on similar principles to the electrophorus (p. 245) that Faraday brought to the test of experiment his theoretical anticipation that inductive action, taking place invariably through the intermediate influence of intervening matter, would be found to be exerted, not in the direction of straight lines only, as had always been assumed, but also in curved lines. " A cylinder of solid shellac, of about I in. in diameter and 7 in. in length, was fixed in a wooden foot ; it was made concave, and capped at its upper extremity, so that a brass ball or hemisphere could stand upon it. The upper half of the stem having been excited resinously, by friction with warm flannel, a brass ball was placed on the top, and then the whole arrangement examined by the carrier ball or proof- plane and Coulomb's electrometer (p. 229). For this purpose the carrier ball was applied to various parts of the ball ; the two were uninsulated whilst in contact, or in position, then insulated, separated, and the charge of the carrier examined as to its nature and force. Of course, whatever general state the carrier acquired in any place where it was uninsulated and then insulated, it retained on removal from that place, and the distribution of the force upon the surface of the inducteous body while under the influence of the inductive was ascertained. The charges taken from the ball in this its uninsulated SUMMARY OF LAWS. 281 state were always vitreous, or of the contrary character to the elec- tricity of the lac. When the contact was made at the under part of the ball, the measured degree of force was 512; when in a line with its equator, 270; and when on the top of the ball, 130." FIG. 258. Faraday's Experiment, Proving that the polarization of the particles of air may occur in curved as well as in straight lines. FlG. 259. Faraday's Apparattisfor de- termining the specific or particular inductive power belonging to various substances. A, B, the two brass spheres, one within the other, A being supported by a brass wire, c, passing through a shellac rod, which latter insulates A, and prevents it communicating with n. The space between A and B can be rilled with any solid, liquid, or gas- eous dielectric. E, the stop-cock, which screws into the air-pump, if necessary. The shellac electrophorus with its ball is here exhibited (Fig. 258), together with the positions of the carrier ball referred to. When placed at d, the effect produced was 512 ; at c, 270 ; at , 130. Even in the position e the proof or carrier ball became inducteous ; and at a it was affected in the highest degree, and gave a result above 1000. VII. Specific Induction. If one body capable of maintaining polarization can assume this condition quicker than another, it must be apparent that a resisting force of some kind exists, which causes insulating substances to vary in this respect. Faraday ascertained this variable resistance by means of an appa- ratus (Fig. 259) consisting essentially of two brass spheres, placed one within the other, conducting communication between them being prevented by proper means. The intervening space between one sphere and another could then be filled with a variety of substances, solid, fluid, and gaseous. Faraday used two of the instruments (Fig. 259), and a certain charge having been given to one of these, after the intervening space had been filled with the substance under investigation, it was con- nected with the second instrument, containing air; thus the latter became the standard of comparison used throughout the experiments; and the intensity, as before, was estimated by the carrier or proof 282 ELECTRICITY. VIII ball and Coulomb's electrometer. The inductive apparatus was in effect a Leyden jar, with the advantage that the dielectric, represented in the latter case by glass, could be removed at pleasure, and other bodies substituted. With this apparatus Faraday determined the inductive powers of a number of substances, and his experiments have been extended and verified by Sir William Snow Harris. Substance. Air_ . Rosin Pitch . Beeswax Glass . Sulphur Shellac Comparative Specific Inductive Power. 00 77 80 86 90 '93 '95 All gases, whatsoever may be their nature, have the same specific inductive power- as air; no variation in the moisture, or temperature, or density of the gases affects the uniformity of their property in this respect. Electricity stored in a Leyden jar can be measured into it, if neces- sary, by a beautiful contrivance of Harris, called the unit or standard jar; it is, of course, similar in principle to Lane's discharging electro- meter, page 261. The unit Leyden jar is a very small one, and, mounted on a glass rod, the outside has a brass cap carrying a brass rod, which is placed at any required distance from the wire and ball coming from the interior of the miniature jar. According to the Franklinian experiment, page 251, every charge sent to the outside of the unit jar sets free from the inside an equivalent proportion of" vitreous electricity; and directly the charge in the little jar is of suffi- cient intensity to break through the intervening thickness of air, it discharges itself with the usual snapping noise. IX. With Harris's unit jar (Fig. 260) and balance, the following facts have been ascertained : FlG. 260. Harris's Unit Jar. c, the conductor of the electrical machine connected with the outside of the unit jar, b ; the inside,. a, bting connected ith a large Leyden jnr, every time the little jar discharges itself between b and a, a unit or definite quantity of electrical force has passed into the larger jar. The area of the charged surface remaining constant, the attraction SUMMARY OF LAWS. 283 between the two discs of the balance (see page 231) increases as the square of the quantity. The intensity of the charge being main- tained at one fixed point, and the distance between the discs altered,, the attractive force varies inversely as the square of the distance. Coulomb's laws, already detailed, can only be regarded as general when they are confined to electrized molecules or points ; they are again repeated here for the sake of the student, who may wish to remember the chief laws. First law, " Two electrized bodies attract and repel each other with a force which is inversely proportional to the square of the distance that separates them." The force with which two bodies that possess different electricities attract each other is inversely proportional to the square of the dis- tance by which they are separated. X. The discharge of an electrical accumulation may take place in various ways; viz., 1. By conduction, 2. By disruption, 3. By convection. The first is the most simple, as when a brass rod is held in the- hand, and laid upon the conductor of an electrical machine in full action. The second involves the charge of particles, and their displace- FlG. 261. A Current of Air set in motion from the Electric Point, And, by convection, carrying the electricity to the flame of the candle, when it is dissipated and lost by the heated and rarefied air. ment in a gradual and steady manner, as by brushes or glow; or in a violent degree, as with a spark passing through the air, or causing the fracture of a thin Leyden jar, which has been too highly charged. The third is special and peculiar, and involves motion ; it is, there- fore, called a " carrying discharge." Faraday illustrated it by insu- lating and electrifying a large copper boiler, 3 ft. in diameter, to a limit just within that which would produce the brush or moderate disruptive discharge. A brass ball, 2 in. in diameter, when sus- pended by a silk thread and held within 2 in. of the boiler, became charged, although insulated the whole time. As its electricity was contrary to that of the boiler, the effect would be, with a light ball, that it will be attracted, and then fly off to the nearest conductor, and 284 ELECTRICITY. thus, like dust or any small particles capable of easy motion, would gradually, by convection, carry away the charge. A brush discharge may be frequently changed to a glow, by setting up a current of air in the same direction as that taken by the brush discharge ; and this effect may be reversed, and a glow converted into a brush, by preventing the access of currents of air. LATERAL DISCHARGE. In consequence of the resistance offered, even by metals, to the progress, of electricity, there is always a tendency in any electrical discharge to divide itself if there are many contiguous conductors in the same line or path; and thus sparks or flashes will occur when least expected, and, in the case of ships of war or powder-magazines, may do some harm if they are struck by lightning, although they may be supplied with lightning-conductors. The subject of lateral discharge received considerable attention from the late Sir William Snow Harris and Mr. Charles V. Walker, and the result of their dis- cussions was the more careful protection of Her Majesty's ships by taking care to connect all masses or bars of metal with the main conductor, so that no accidental division shall occur anywhere ; and thus all chance of flashes or sparks are prevented. The following experiment of Dr. Miller* serves to illustrate this point : FIG. 262. Charge a Leyden jar, and arrange a metallic wire, w, from 120 to 150 ft. in length, so as to act the part of discharger ; at the same time open a short path for the discharge to the outer coating, by bringing the balls a, b within a short distance of each other. Under this arrangement a portion of the electricity takes the shorter course from a to b, and overcomes the high resistance of the stratum of air interposed between the balls, owing to the resistance experienced by the discharge to its passage along the continuous conducting wire w. Miller's "Elements of Chemistry," vol. i., p. 432. VOLTAIC ELECTRICITY. 285 FlG. 263. Galvani's Experiment with the Nerves and Muscles of the dead Frog (As exhibited on the disc at the Polytechnic). VOLTAIC, GALVANIC, OR DYNAMICAL ELECTRICITY. It always seems quite natural, and taking things in their right order, to com- mence this subject by speaking of that famous illustration of animal electri- city primarily discovered by Galvani, who ascertained that by touching the lumbar nerves of a frog, or lower part of the spine of a frog, recently killed, with a clean copper wire, and the muscles with a zinc wire, and then bringing the two metals in contact, that a current of electricity was evolved, which was instantly rendered evident by the frog-electroscope, the limbs being always convulsed in the most curious manner. Galvani thought that the nerves and muscles of all animals were in oppo- 2-S6 ELECTRICITY. site states of electricity, and that the effect occurred only at the moment when the two opposite forces rush together and neutralize each other; but it was soon shown that the convulsions were due to the effect of a current of elec- tricity, however feeble, set up when the two metals touched each other in the presence of a third element, viz., the liquid, containing chloride of sodium, with which the limbs of the recently killed frog would necessarily be moistened ; it was, in short, the oxidation of the zinc wire that produced the current, and the prepared limbs of the frog represented only the electroscope that rendered the electrical disturbance evident. The biographer of Lewis Galvani, in " Rees's Cyclopaedia," states that he was born in 1737, at Bologna. In his early youth he showed a great propensity to religious austerities ; but, being dissuaded from entering into an order of monks, whose convent he fre- quented, he directed his attention to the study of medicine. He pursued this study under able masters, and gained their esteem, especially that of Professor Galcazzi, who received him into his house and gave him his daughter in mar- riage. In the year 1762, after having sustained an inaugural thesis, " De Ossibus," he was appointed public lecturer in the University of Bologna and reader in anatomy to the Institute in that city. By the excellence of his method of teaching, he obtained crowded audiences. FIG. 264. The prepared Frog's Limbs. Then follows the story of the soup made of frogs, which had been recom- mended to his dearly loved wife, who was in a declining state of health, and the accidental discovery that the limbs of the frog were affected by the point of a scalpel held near the prime conductor of an electrical machine in action. Matteuchi,. however, denies the originality of the experiment, and declares that it was performed many years before the time of Galvani, in the presence of the Grand Duke of Tuscany, by the celebrated Swammerdam. His first publication on the subject was printed for the Institute at Bologna, 1791, and entitled " Aloysii Galvani de viribus Electricitatis in motu musculari Commentarius." This work immediately excited the attention of philosophers, both in Italy and other countries, and the experiments were repeated and extended. In conjunction with his physiological inquiries, the duties of his professor- ship and his employment as a surgeon gave full occupation to the industry of Galvani. In addition to a number of curious observations on the organ of hearing in birds, which were published in the memoirs of the Institute of ALDINPS EXPERIMENTS. 287 Bologna, he drew up various memoirs on professional topics, which have re- mained unedited. He regularly held learned conversations with a few literary friends, in which new works were read and commented upon. He was a man of a most amiable character in private life, and possessed of great sensibility, insomuch that the death of his wife, in 1790, threw him into a profound melancholy. His early impressions on the subject of religion remained unimpaired; he was always punctual in practising its minutest rites ; and from this cause, no doubt, he steadily refused to take the civic oath exacted by the then new con- stitution of the Cis-Alpine republic, and was consequently deprived of his posts and dignities. In a state of melancholy and poverty, he retired to the house of his brother James, a man of very respectable character, and fell into an extreme debility. The republican governors, probably ashamed of their conduct towards such a man, passed a decree for his* restoration to his professorial chair and its emoluments ; but it was too late. He expired on the 5th of November, 1798. But the good philosopher's name and works were not to lie dead and forgotten : his nephew, the Professor Aldini, of Bologna, seeing the grief and the sad end of his uncle, determined to rescue his name from obscurity, and to defend Galvani's theories, which had been attacked and repudiated. For this purpose, Aldini travelled through France and England, demon- strating the remarkable physiological experiments of Galvani, and so pleased the professional authorities at Guy's Hospital, in 1803, that they presented him with a gold medal. For a very complete epitome of organic electricity, the reader is referred to another work.* It may be sufficient here to state that Aldini maintained that " Muscular contractions are excited by the development of a fluid (electric) in the animal machine, which is conducted from the nerves to the muscles, without the concurrence or action of metals. " All animals are endowed with an inherent electricity, appropriate to their economy, which electricity, secreted by the brain, resides especially in the nerves, by which it is communicated to every part of the body. " The principal reservoirs are the muscles, each of which he regarded to have two sides in opposite electric conditions. ." When a limb is willed to move, the nerves, aided by the brain, draw from the interior of the muscles some electricity ; discharging this upon their sur- face, they are thus contracted and produce the required change of position." It is a remarkable fact, that when an acid and an alkaline solution are so placed that their union may be effected through the substance of an animal membrane or, indeed, any other porous diaphragm, a current of electricity is evolved, the causes of which disturbance of electric equilibrium have already been investigated. Now, with the exception of the stomach and caecum, the whole extent of the mucous membrane is, in the human subject, bathed with an alkaline mucous fluid, and the external covering of the body, the skin, is as constantly exhaling an acid fluid, except in the axillary and, perhaps, pubic regions. The mass of the animal frame is thus placed between two great * "The Elements of Natural Philosophy," by Golding Bird, M.A., and Charles Brooke, M.A. John Churchill, New Burlington Street. 288 ELECTRICITY. envelopes, the one alkaline and the other acid, meeting only at the external outlets. This arrangement has been shown by Donne to be quite competent to the evolution of electricity, and, accordingly, he found that if a platinum plate, connected with the galvanometer, be held in the mouth, whilst a second be pressed against the moist perspiring surface of the body, the needles will instantly traverse, as they did in the experiment just shown with an acid and an alkali. The current thus detected by Donne' at once explains the cause, and con- firms the accuracy, of the celebrated experiment of Aldini, in which he excited convulsions in a frog by holding its foot in the moistened hand, and allowing the sciatic nerve to touch the tongue. There is also another remarkable expe- riment of Aldini, explicable on the same principle, and shown in Fig. 265. FIG. 265. Aldini's Battery, Formed of the heads of recently decapitated oxen. A, c, c. One of the ears of the first head, A, is well moistened with salt and water, and connected, through the tongue, by a silver wire with the ear of B ; the tongue of B is in like manner connected with the ear of C. The ear of A and the tip of the tongue of c form the terminals of this "bovine battery;" silver wires brought round from both are now connected with the prepared limbs of a frog, just killed, so that the portion of the spine still connected with its lumbar nerves touches the wire from the tip of the tongue, which had been previously drawn out of the mouth of the ox, and the skinned legs touch the wire from one of the ears. The frog's legs instantly contract, and the contraction ceases when the circuit is broken. Dr. Wilkinson estimated that the irritable muscles of a frog's leg were no less than 56,000 times more delicate, as a test of electricity, than the most sensitive condensing electroscope (p. 243). " About forty years prior to Galvani's discovery,* a person of the name of Sultzer gave an account of the following fact : * "Rees's Cyclopaedia," article Galvanism. VOLTAIC ELECTRICITY. 289 " If a piece of lead and a similar piece of silver be laid together, and the edges of both be brought in contact with the tongue, a taste is perceived similar to that of vitriol of iron ; at the same time that the metals applied separately produce no effect. " The observer of this fact does not appear to have been surprised at the effect. At that time the doctrine of vibrations was employed to explain all natural phenomena. " He, therefore, concluded that some peculiar vibration took place from the contact of the metals, which produced the peculiar sensation on the tongue. " All the world were satisfied with this explanation ; and thus a prominent fact had slept in obscurity from the time of Sultzer to the time of Galvani." The excitation of galvanic electricity is traceable to chemical action. It has already been stated that the combustion of a piece of charcoal will eliminate the electric force, and can be discovered by a delicate condensing -electro- scope. In galvanic experiments another instrument is required, in order to detect the feeble currents of electricity of low tension or intensity. This instrument admits of wonderful refinement, as will be seen presently in the description of Sir William Thompson's reflecting galvanometer needle ; but for ordinary experiments an instrument constructed as follows will suffice : FIG. 266. The ordinary Galvanometer Needle. It will be seen presently, that a single wire conveying an electric current causes a magnetic needle to be deflected, and to take up a position at right angles to the current. If one wire can produce this result, it is clear that, by twisting the wire and increasing the number of convolutions, the effect of the single wire is multiplied ; and by covering the wire with silk or cotton, so as to prevent lateral communication, a much greater surface of electrified wire is brought to bear by induction upon the magnetic needle. These conditions are fulfilled in Fig. 266, which will answer remarkably well for any ordinary lecture-table experiment: it consists of a magnetic needle, c, properly sus- pended and placed inside a coil of wire, d, the two ends of which terminate at a b. The whole is levelled by three screws. The instrument, Fig. 267, is carefully levelled by three screws and spirit- levels; it contains a coil of fine wire,-the two ends of which are brought out to two screw connections. The magnetic needle is made astatic 19 290 ELECTRICITY. just balanced), by being connected with another magnetic needle, the north pole of which is placed opposite the south pole of the other, and vice versa, and is thus unaffected by the earth's magnetism. When a current of electricity, however feeble, is passed through the coil, the astatic needle is deflected according to laws which will be fully explained in the article on Electro- Magnetism. FIG. 267. The Galvanometer Multiplier. With this instrument the following experiments, all demonstrating that chemical action is a source of electricity, can be performed : Into a small clean iron ladle, well scraped inside with a file to secure a metallic and not a rusted surface of iron, are placed some crystals of nitre ; the ladle is supported by a tripod stand above a Bunsen's burner, and, when melted by the heat, a wire is wound round the clean metallic surface of the handle, and connected with one of the connecting screws of the galvanometer, and the other with a second wire, bound round a piece of hard charcoal, such as would be used for the electric lamp. Of course all metallic connections must be bright and clean, and, directly the charcoal is dipped into the nitre, the oxidation of the charcoal occurs ; the nitre gives oxygen to the charcoal, and converts it into carbonic acid, which unites with the remaining potash, producing carbonate of potash, and at the same moment a current of electricity is liberated, which violently affects the galvanometer needle. The writer gives a drawing of the arrangement which will always be found most simple and effective at the lecture-table. Moreover, it illustrates another fact that one of the elements of a voltaic series must be in a liquid state, if a notable current of dynamical electricity is desired to be shown or used. The writer has always felt that when coal or charcoal could be oxidized, and used VOLTAIC ELECTRICITY. 291 FIG. 268. The Oxidation of Carbon, An instance of the evolution of electricity by true chemical action. A, iron ladle, containing the nitre; B, the charcoal ; c, the Bunsen burner; r>, the galvanometer needle. in the galvanic battery, the cheapest source of electricity will have oeen attained ; and he learns from Mr. Crookes that a plate of platinum and one of charcoal placed in fused soda or potash give a very good current. The same experiment repeated, and a condensing electroscope used as the test of electrical excitation, with the precaution of supporting the charcoal on a glass rod, is very satisfactory ; and thus, by the oxidation and slow burning of charcoal, both current or dynamical electricity and static electricity may be obtained. The usual mode of showing that charcoal in a state of combustion elimi- nates electricity is by twisting a piece of copper wire round a bit of charcoal some inches in length, and then connecting it with the lower plate of the con- densing electroscope, whilst the upper plate is connected with the ground. The charcoal is now ignited by a spirit-lamp, and if blown on with bellows, and the top plate of the electroscope raised and lowered several times, any rubbing of the two plates one against the other being carefully avoided, the gold leaves will be seen to diverge with negative electricity ; and sometimes one movement of the upper plate of the condensing electroscope is found to be sufficient. The experiments already quoted form a sort of connecting link between frictional and voltaic electricity, and are further supported by some excellent experiments of Faraday, who shows by a simple arrangement that the electri- city of high tension obtained from the electrical machine will do all that a voltaic circuit may effect. Faraday says :* " Chemical Decomposition. The chemical action of voltaic electricity is characteristic of that agent, but not more characteristic than are the laws under which the bodies evolved by decomposition arrange themselves at the *" Experimental Researches in Electricity " by Michael Faraday. 19 2 2 9 2 ELECTRICITY. poles. Dr. Wollaston showed * that common electricity resembled it in these effects, and that i they are both essentially the same ; ' but he mingled with his proofs an experiment having a resemblance, and nothing more, to a case of voltaic decomposition, which, however, he himself partly distinguished; and this has been more frequently referred to by others, on the one hand, to prove the occurrence of electro-chemical decomposition, like that of the pile, and, on the other, to throw doubt upon the whole paper, than the more nume- rous and decisive experiments which he has detailed. " I take the liberty of describing briefly my results, and of thus adding my testimony to that of Dr. Wollaston on the identity of voltaic and common electricity as to chemical action, not only that I may facilitate the repetition of the experiments, but also lead to some new consequences respecting electro- chemical decomposition. " I first repeated Wollaston's fourth experiment,t in which the ends of coated silver wires are immersed in a drop of sulphate of copper. By passing the electricity of the machine through such an arrangement, that end in the drop which received the electricity became coated with metallic copper. One hundred turns of the machine produced an evident effect ; two hundred turns a very sensible one. The decomposing action was, however, very feeble. Very little copper was precipitated, and no sensible trace of silver from the other pole appeared in the solution. FIG. 269. "A much more convenient and effectual arrangement for chemical decompo- sitions by common electricity is the following: " Upon a glass plate (Fig. 269) placed over, but raised above, a piece of white paper so that shadows may not interfere put two pieces of -tinfoil, , b ; connect one of these by an insulated wire solvo. N. Electrolyte ; V. Electrolyze. t aviov (hat ti