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LITTELL'S LIVING AGE. – NO. 1138.-24 MARCH, 1866.
From the Fortnightly Revie'. nerves of his body those tremors which, ON THE RELATIONS OF RADIANT HEAT when imparted to the nerves of healthy TO CHEMICAL CONSTITUTION, COLOUR,
ears, are translated into sound. There are AND TEXTURE.
various ways of rendering those sonorous
vibrations not only tangible but visible; A LECTURE, DELIVERED IN THE ROYAL and it was not until numberless experiments
INSTITUTION OF GREAT BRITAIN, ON of this kind had been executed, that the FRIDAY EVENING, 19th JANUARY, 1866. scientific investigator abandoned himself
wholly, and without a shadow of uncertainONE of the most important functions of ty, to the conviction that what is sound in physical science, considered as a discipline us is, outside of us, a motion of the air. of the mind, is to enable us by means of the But once having established this fact – tangible processes of nature to apprehead once having proved beyond all doubt that the intangible. The tangible processes give the sensation of sound is produced by an direction to the line of thought; but this agitation of the nerve of the car, the thought once given, the length of the line is not soon suggested itself that light might be limited by the boundaries of the senses. due to an agitation of the nerve of the eye. Indeed, the domain of the senses in Nature This was a great step in advance of that is almost infinitely small in comparison with ancient notion which regarded light as somethe vast region accessible to thought which thing emitted by the eye, and not as anylies beyond them. From a few observations thing imparted to it. But if light be proof a comet, when it comes within the range duced by an agitation of the optic nerve or of his telescope, an astronomer can calcu- retina, what is it that produces the agitalate its path in regions which no telescope tion ? Newton, you know, supposed minute can reach; and in like manner, by means particles to be shot through the humours of of data furnished in the narrow world of the eye against the retina, which hangs like the senses, we make ourselves at home in a target at the back of the eye. The imother and wider worlds, which can be trav- pact of these particles against the tarvet, ersed by the intellect alone.
Newton believed to be the cause of light. From the earliest ages the questions, But Newton's notion has not held its groun'l, “ What is light ? ” and “ What is heat ? ” being entirely driven from the field by the have occurred to the minds of men; but more wonderful and far more philosophical these questions never would have been an- notion that light, like sound, is a product of swered had they not been preceded by the wave-motion. question, “ Wbat is sound?” Amid the The domain in which this motion of light grosser phenomena of acoustics the mind is carried on lies entirely beyond the reach was first disciplined, conceptions being of our senses. The waves of light require there obtained from direct observation, a medium for their formation and propagawhich were afterwards applied to phenome- tion, but we cannot see, or feel, or taste, or na of a character far too subtle to be ob- smell this medium. Still, though thus apserved directly. Sound we know to be due parently cut off from all investigation, its to vibratory motion. A vibrating tuning- exsitence has been established. How has fork, for example, moulds the air around it this been done ? By showing that all the into undulations or waves, which speed phenomena of optics are accounted for with away on all sides with a certain measured a fullness and clearness and conclusiveness velocity, impinge upon the drum of the ear, which leave no desire of the intellect unfulshake the auditory nerve, and awake in the filled, by the assumption of this wonderful brain the sensation of sound. When suffi- intangible ether. When the law of graviciently near a sounding body we can feel tation first suggested itself to the mind of the vibrations of the air. A deaf man, for Newton, what did he do? He set himself example, plunging his hand into a bell when to examine whether it accounted for all the it is sounded, feels through the common facts. He determined the courses of the
planets; he calculated the rapidity of the spaces, bathing the sides of suns and planmoon's fall towards the earth; he consid- ets, but it also encircles the atoms of which ered the procession of the equinoxes, the these suns and planets are composed. It is ebb and flow of the tides, and found all ex- the motion of these atoms, and not of any plained by the law of gravitation. He there- sensible parts of either planets or stars, that fore regarded this law as established, and the ether conveys; it is this motion that the verdict of science subsequently confirm- constitutes the objective cause of what in ed his conclusion. On similar, and, if pos- our sensations are light and heat. sible on stronger grounds, we'found our belief atom, then, sending its pulses through the in the existence of the universal ether. It infinite ether, resembles a tuning-fork sendexplains facts far more various and compli- ing its pulses through the air. Let us look cated than those on which Newton based for a moment at this thrilling ether, and his law. If a single phenomenon could be briefly consider its relation to the bodies pointed out which the etheris proved incom- whose vibrations it conveys. Different petent to explain, we should have to give it bodies, when heated to the same temperaup; but no such phenomenon has ever been ture, possess very different powers of agipointed out. It is, therefore, at least as tating the ether: some are good radiators, certain that space is filled with a medium others are bad radiators; which means that by means of which suns and stars diffuse some are so constituted as to communicate their radiant power, as that it is traversed their motion freely to the ether, producing by that force which holds not only our plan- therein powful undulations; while others etary system, but the immeasurable heavens are unable thụs to communicate their mothemselves, in its unconquerable grasp. tion, but glide through the ether without
There is no more wonderful instance than materially affecting its repose. Experithis of the production of a line of thought ment has proved that elementary bodies, from the world of the senses into the region except under certain anomalous conditions, of pure imagination. I mean by imagina- belong to the class of bad radiators. An tion here, not that play of fancy which can atom vibrating in the ether resembles this give to airy nothing a local habitation and naked tuning-fork vibrating in the air. The a name, but that power which enables the amount of motion communicated to the air mind to conceive realities which lie beyond by these thin prongs is too small to evoke at the range of the senses — to present to it any distance the sensation of sound. But self distinct physical images of processes if we permit the atoms to combine chemiwhich, though mighty in the aggregate be-cally and form molecules, the result in yond all conception, are so minute individu- many cases is an enormous change in the ally, as to elude all observation. It is the power of radiation. The amount of ethe waves of air excited by this tuning-fork real disturbance produced by a compound which renders its vibrations audible. It is molecule may be many thousand times that the waves of ether sent forth from those produced by its constituent atoms when lamps overhead which render them lumin- uncombined. The effect is roughly typified ous to us; but so minute are these waves, by this tuning-fork when connected with that it would take from 30,000 to 60,000 of its resonant case. The fork and its case them placed end to end to cover a single now swing as a compound system, and the inch. Their number, however, compensates vibrations which were before inaudible, are for their minuteness. Trillions of them now the source of a musical sound so powhave entered your eyes and hit the retina erful that it might be plainly heard by at the back of the eye in the time consumed thousands at once. The fork and its .case in the utterance of the shortest sentence of combined may be regarded as a good radiathis discourse. This is the steadfast result tor of sound. of modern research; but we never could A single example will suffice to illustrate have reached it without previous discipline. the vast influence of the coalescence of We never could have measured the waves atoms to oscillating systems upon the radiaof light, nor even imagined them to exist, tion of heat. Supposing a pound of dry had we not previously exercised ourselves oxygen, and also a pound of the transpaamong the waves of sound. Sound and rent vapour of water, to be taken to the light are now mutually helpful, the concep- top of a high mountain where the air is too tions of each being expanded, strength- attenuated to offer any sensible resistance ened, and defined by the conceptions of the to the passage of radiant heat towards stelother.
lar space; suppose the gas and the vapour The ether which conveys the pulses of to be there heated to the temperature of light and heat not only fills the celestial boiling water, and afterwards exposed be
neath the cloudless heaven. By the heat- intensely luminous green bands. Here it ing of the gas and vapour a more vigorous is as if two forks only, of slightly different motion - a greater amount of vis viva, as pitch, were vibrating. The length of the we term it is imparted to the ultimate waves which produce this first band is such particles of both. Will the oxygen impart that 47,460 of them, placed end to end, its motion as freely to the ether as the aque- would fill an inch. The waves which proous vapour ? No. The difference between duce the second band are a little shorter; them in this respect is enormous. When it would take of these 47,920 to fill an inch. their temperatures are alike, the amount of In the case of the first band, the number heat radiated, or, in other words, of mole- of impulses imparted in one 'second to cular motion lost by the vapour in a given every eye which now sees it, is 577 millions time, is at least nine thousand times the of millions; while the number of impulses amount lost in the same time by the oxy- imparted in the same time by the second gen. So great is this power on the part of band is 600 millions of millions. I now the vapour, that I profoundly doubt the cast upon the screen before you the beauticorrectness of the simple formula assigned ful stream of green light from which these to it by chemists. The molecule of water bands were derived. This luminous stream represeots a sound-board of vast dimensions, is the incandescent vapour of silver. You otherwise it never could generate waves of cannot by any possibility render that vathe extraordinary magnitude that experi- pour white hot. The rates of vibration of ence has proved it competent to produce.* its atoms are as rigidly fixed as those of two
The pitch of a musical note depends upon tuning-forks; and to whatever height the the rapidity of its vibrations, or, in other temperature of the vapour may be raised, words, on the length of its waves. Now, the rapidity of its vibrations, and consethe pitch of a note answers to the colour of quently its colour, which wholly depends light. Taking a slice of white light from upon that rapidity, remain unchanged. the beam of an electric lamp, I cause that The
of water, as well as the valight to pass through an arrangement of pour of silver, has its definite periods of prisms. It is decomposed, and we have the vibration, and these are such as to disqualieffect obtained by Newton, who first un- fy the vapour from being raised to a white rolled the solar beam into the splendours of heat. The oxyhydrogen Name, for examthe solar spectrum. At one end of this ple, consists of hot aqueous vapour. It is spectrum we have red light, at the other scarcely visible in the air of this room, and violet, and between those extremes lie the it would be still less visible if we could burn other prismatic colours. As we advance the gas in a clean atmosphere. But our atalong the spectrum from the red to the vio- mosphere, even at the summit of Mont let, the pitch of the light — if I may use Blanc, is dirty ; in London it is more than the expression – heightens, the sensation dirty; and the burning dirt gives to this of violet being produced by a more rapid flame the greater portion of its present succession of impulses than that which pro- light. But the heat of the flame is enorduces the impression of red. The vibra- mous. Cast iron fuses at a temperature of tions of the violet are not quite twice as 2,000° Fahr.; the temperature of the rapid as those of the red ; in other words, oxyhydrogen flame is 6,000° Fahr. A the range of the visible spectrum is not piece of platinum is heated to vivid redquite equal to an octave.
ness at a distance of two inches beyond the There is no solution of continuity in this visible termination of the flame. "The vaspectrum; one colour changes into another pour which produces incandescence is here by insensible gradations. It is as if an in- absolutely dark. In the fame itself the finite number of tuning-forks, of gradu- platinum is raised to dazzling whiteness, ally augmenting pitch, were vibrating at and is finally pierced by the flame. When the same time. But turning to another this flame impinges on a piece of lime, we spectrum — that, namely, obtained from have the dazzling Drummond light. But the incandescent vapour of silver — you ob- the light is here due to the fact that when serve that it consists of two narrow and it impinges upon the solid body, the vibra
tions excited by the flame are of periods * Bulk for bulk, that wonderful substance ozone probably transcends ordinary oxygen in radiant different from its own. power a hundred thousand times. This shows that Thus far we have fixed our attention on the atoms of an element can be so grouped as to be atoms and molecules in a state of vibration, pound. May not the molecule of water, from and surrounded by a medium which accepts which its vast radiant power is derived, be a mole their vibrations, and transmits them through cule of molecules, the chemical formula stamping only a single member of the group ?
infinite space. But suppose the waves generated by one system of molecules to im- | brations. Here are three small gas-flames pinge upon another system, how will the inserted in three glass tubes of different wares be affected? Will they be stopped, lengths. Each of these flames can be caused or will they be permitted to pass ? Will to emit a musical note, the pitch of which is they transfer their motion to the molecules determined by the length of the tube suron which they impinge, or will they glide rounding the flame. The shorter the tube round the molecules, through the intermo- the higher is the pitch. The flames are now lecular spaces, and thus escape ? The silent within their respective tubes, but each answer to this question depends upon a con- of them can be caused to respond to a propdition which may be beautifully exempli- er note sounded anywhere in this room. I fied by an experiment on sound. These two have bere an instrument called a syren, by tuning-forks are tuned absolutely alike. which I can produce a powerful musical They vibrate with the same rapidity, and note. Beginning with a low pitch, and asmounted thus upon their resonant stands, cending gradually to a higher one, I finalyou hear them loudly sounding the same ly reach the note of the flame in the longest inusical note. I stop one of the forks, and tube. The moment it is reached, the flame throw the other into strong vibration. I bursts into song. I stop and re-excite the now bring that other near the silent fork, syren, to enable you to hear that its note but not into contact with it. Allowing them and the flame's note are identical. But the to continue in this position for four or five other flames are still silent within their tubes. seconds, I stop the vibrating fork; but the I urge the instrument on to higher notes ; sound has not ceased. The second fork has the second flame has now started, and the taken up the vibrations of its neighbour, third alone remains. But a still higher and is now surrounded in its turn. I dis- note starts it also. Thus, as the sound of the mount one of the forks, and permit the syren rises gradnally in pitch, it awakens other to remain upon its stand. I throw every flame in passing, by striking it with a the dismounted fork into strong vibration, series of waves whose periods of recurrence but you cannot hear it sound. Detached are similar to its own. from its stand the amount of motion which Let us apply these facts to radiant heat, it can communicate to the air is too small taking as before the vapour of water as a to make itself sensible to the ear at any dis- representative case. The molecules of this tance. I now bring the dismounted fork vapoar have definite periods of vibration to close to the mounted one, but not into ac- which they are as rigidly bound as a tuning, tual contact with it. Out of the silence fork is to its periods. Recurring then to rises a mellow sound. Whence comes it ? our experiment on the mountain top: inFrom the transferred vibrations of the dis- stead of exposing our hot vapour in the mounted fork. That the motion should manner described, with nothing above it, thus transfer itself through the air it is ne- let us suppose a stratum of aqueous vapour cessary that the two forks should be in per- to be spread out between it and the firma. fect unison. If I place on one of the forks ment. The light of the stars is unaffected a morsel of wax not longer than a pea, it is by this stratum, which I suppose to be true varendered thereby powerless to affect, or to pour, and, therefore, perfectly transparent. be affected by, the other. It is easy to un- But the case is different as regards the rays derstand this experiment. The pulses of issuing from the hot vapour underneath. the one fork can affect the other, because The molecules of this vapour and of the they are perfectly timed. A single pulse stratum overhead are, if I may use the excauses the prong of the silent fork to vi- pression, tuned to precisely the same note, brate through an infinitesimal space. But and instead of the direct transference of just as it has completed this small vibration, the vibratory motion into space, we have it another pulse is ready to strike it. Thus, transferred to the molecules of the vapour the small impulses add themselves together. above. The motion is thus intercepted – In the five seconds during which the forks in technical language the heat is absorbeil. were held near each other, the vibrating The upper stratum of vapour having thus fork sent 1.280 waves against its neighbour, become warmed, first at its under surface, and those 1,280 shocks, all delivered at the and then, by a gradual progression, through proper moment, all, as I have said, perfectly its entire mass, it would radiate in all directimed, have given such strength to the vi- tions, returning a portion of the heat combrations of the mounted fork as to render municated to it to the source from which it them audible to you all.
is derived.. We are here manifestly dealLet me give you one other illustration of ing with that great principle which lies at the influence of synchronism on musical vi- the basis of spectrum analysis, and which
has enabled scientific men to determine the withdraw the cell of water; the ice imsubstances of which the sun, the stars, and mediately liquefies, and you see the water even the nebulæ, are composed : the prin- trickling from it in drops. I re-introduce ciple, namely, that a body which is competent the cell of water; the fusion is arrested and to emit any ray, whether of heat or light, is the drops cease to fall. The transparent competent in the same degree to absorb water of the cell exerts no sensible absorpthat ray. The absorption depends on the tion in the luminous rays, still it withdraws synchronism which exists between the vi- something from the beam, which, when perbrations of the atoms from which the rays, mitted to act, is competent to melt the ice. or more correctly the waves, issue, and This something is the dark radiation of the those of the atoms against which these electric light. Again, I place a slabof pure ice wares impinge.
in front of the electric lamp; send a luminous To its incompetence to emit white light, beam first through our cell of water and then aqueous vapour adds incompetence to ab- through the ice. By means of a lens I cast sorb white light. It cannot, for example, an image of the slab upon a white screen. absorb the luminous rays of the sun, though The beam, sifted by the water, has no pow. it can absorb, and that with mighty power, er upon the ice. But observe what occurs the non-luminous rays of the earth. This when the water is removed: we have here incompetence of aqueous vapour to absorb a star and there a star, each resembling luminous rays is shared By water and ice- a flower of six petals, and growing visibly in fact, by all really transparent substances. larger before your eyes. As the leaves enTheir transparency is due to their inability to large their edges become serrated, but there absorb luminous rays. The molecules of is no deviation from the six-rayed type. such substances are in dissonance with the We have here, in fact, the crystallization of luminous waves, and hence such waves pass the ice inverted by the invisible rays of the through transparent substances without dis- electric beam. They take the molecules turbing the molecular rest. A purely lumin- down in this wonderful way, and reveal to ous beam, however intense may be its heat, us the exquisite atomic structure of the subis sensibly incompetent to melt the smallest stance with which nature every winter roofs particle of ice. can, for example, con- our ponds and
es. verge a powerful luminous beam upon a sur- Numberless effects, apparently anomalous, face covered with hoar frost without melt- might be adduced in illustration of the aca single spicula of the ice-crystals." How tion of these lightless rays. Here, for exthen, it may be asked, are the snows of the ample, are two powders both white, and unAlps swept away by the sunshine of sum- distinguishable from each other by the eye. mer? I answer they are not swept away The luminous rays of the lamp are unabby sunshine at all, but by solar rays which sorbed by both powders, — from those rays have no shine whatever in them. The they acquire no heat; still one of the subluminous rays of the sun fall upon the snow- stances is heated so highly by the concenfields and are flashed in echoes from crystal trated beam of the electric lamp that it first to crystal, but they find no lodgment within smokes violently and then intiames, while the crystals. They are not absorbed, and the other substance is barely warmed at the hence they cannot produce fusion. But focus. Here, again, are two perfectly transa body of powerful dark rays are emitted parent liquids placed in a test tube at the by the sun, and it is theso rays that cause focus; one of them boils in a couple of the glaciers to shrink and the snows to dis- seconds, while the other in a similar posiappear; it is they that fill the banks of the tion is hardly warmed. The boiling point Arve and Arveyron, and liberate from their of the first is 78° C., which is speedily captivity upon the heights the Rhone and reached; that of the second liquid is only the Rhine.
48° C., which is never reached at all. Placing a concave silvered mirror behind These anomalies are entirely due to the unthe electric light I converge its rays to seen element which mingles with the lumina focus of dazzling brilliancy. I place in the ous rays of the electric beam, and indeed path of the rays, between the light and the constitutes 90 per cent. of its calorific power. focus, a vessel of water, and now introduce I have here a substance by which these at the focus a piece of ice. The ice is not dark rays may be detached from the total melted by the concentrated beam which has emission of the electric lamp. This ray-filpassed through the water, though matches ter is a black liquid that is to say, black are ignited at the focus and wood is set on as pitch to the luminous, but bright as fire. The powerful heat then of this lumin- a diamond to the non-luminous radiation. ous beam is incompetent to melt the ice. I It mercilessly cuts off the former, but allows