HEAT.*
IT is a common saying that truth is stranger than fiction, but it is seldom realized that the application of the saying is generally least felt where it most applies. The wildest trick of the wildest conjuror sinks into nothing when we think of the great trick of nature by which bodies of inconceivable magnitude moving (some certainly,—probably all) at inconceivable rates and in inconceiv- able orbits, are made to look to the human eye as if they were stationary spangles in a bounded, fixed, and solid setting. To make the trick even mote complete, these stationary spangles, which we can never at any one moment see to be moving, are yet always shifting their places, and never in exactly the same place for two moments together. Yet this portentous hoax—for it comes under the most exact definition of a hoax—is hardly ever, perhaps never, realized half as vividly as the commonest trick of the vulgarest conjuror. In the same way nature might be described from beginning to end as nothing but a series of tricks intended to beguile and elude human perception. Nothing is what it seems. Of course the astronomical trick and puzzle must always remain for its apparent magnitude and grandeur the king and monarch of natural tricks. But as we descend the scale there are others little less astounding, and one is that which Professor Tyndal has so admirably unravelled, of the relation between heat and motion, and the myriad ways in which they pass into one another. Curious, too, it is, looking at the subject from a different though similar point of view, to find how the exactness of science, ap- parently so remote from poetry, has yet given birth to a poetry of its own. Still it must be admitted that, as Professor Tyndal himself hints in a passage we shall quote presently, that it is a bewilder- ing poetry—not a pleasing, but a hard, and garish, a crushing and disturbing poetry, which is to the poetry of human love and hope too much what the poetry of the dissecting-room and the revelations of anatomy are to the cradle and the hearth. The contemplation of one excludes the other, and it requires "a cer- tain force of character" to harmonize the two.
Starting from the apparently paradoxical but now axiomatic
Heal Considered as a Mode of Motion. By John Tyndal, F. Its., do., Professor ot Natural Polloaophy in the Royal Institution and in the Royal School of ?Mum London: Lot:amens. truth that heat is motion and motion heat, we pass from the burning of a candle to the incandescence of the sun, and from a ray of light to the budding of a rose by comparatively easy transi- tions, and the travellers who told with wondering delight how a savage would kindle a fire by rubbing two sticks together, little guessed that the operation they recorded was the simplest illus- tration of a law pervading every apparent phenomenon of which we are conscious. Some great men, it is true, anticipated the modern theories of heat, but it is only within the last twenty years that the subject has assumed that development which en- titles Professor Tyndal to say that the connection of heat with the general energies of the universe is such, that if we master it petu fectly we master all. And nothing perhaps better illustrates the general tendency of recent science, both in this and other subjects, than the gradual abandonment of the material theory of heat for the dynamic theory. The material theory supposes heat to be a kind of matter—a subtle fluid stored up in the inter-atomic spaces of bodies. Hence the corollary of the greater capacity or power of stowage in one body for heat over another. Thus the capacity or stowage of water for this subtle fluid, or caloric, was supposed to be thirty times greater than the stowage of mercury for caloric, because it requires thirty times more heat to raise the temperature of water a given number of degrees, than what is required to pro- duce the same effect on a pound of mercury. So far the theory seemed to square with the facts very nicely, and to solve many problems. "Why," for instance, "does lead when compressed give out heat ?" Answer, "Because uncompressed lead has a larger stowage for caloric. Squeeze the lead, and you diminish the stowage—the caloric must out—argal." But this theory, very neat so far, compelled its advocates to go a step further, and to deny the possibility of generating new heat. You cannot, they said, call new heat into existence, you may pinch it out here and store it up there, or you may drive it out, as Professor Tyndal picturesquely says, into the open light of day, but the quantity of heat in the uni- verse, said they, is a fixed quantity, from which you can neither add nor take away. This corollary exploded the theory. Sir Humphrey Davy generated new heat by rubbing two pieces of ice together. In order to understand the fall significance of his experiment, it is necessary to remember that ice is solid water, and that the solid has only half the capacity for heat possessed by the liquid. If, then, no new heat could be generated by the friction of ice, liquefaction could not possibly ensue, for the ice, " squeeze " it to the utmost, could only give up what it had, and what it had was ex hypothesi only half the quantity of caloric required by the water. Liquefaction in this case conclusively demonstrated a generation of heat, and it was regarded as the first experiment really proving that heat is not material, but dynamical.
The dynamical theory stated in simple language is, that heat is a motion of the ultimate particles of matter. This view was held by Bacon. Locke, Mr. Tyndal tells us, said, very happily, that "Heat is a very brisk agitation of the in- sensible parts of the object, which produces in us that sen- sation from whence we denominate the object hot ; so what in our sensation is heat in the object is nothing but motion." How brisk, we may illustrate incidentally by the fact that the waves of a ray of red light, for instance, become apparent to the eye only by striking the retina four hundred and seventy- four million, four hundred and thirty-nine thousand, six hundred and eighty million times in one second. Call it, if you please, for conception's sake, five hundred million million times in one tick of the clock, which is very brisk motion indeed. Now radiant heat and light obey the same laws. Like rays of light, rays of heat proceed in straight lines, and follow the same rule of incidence and reflection both on plane and on curved surfaces, and the " cliather- mancy " of bodies obeys laws similar to the general laws of trans- parency. In fact every luminous ray is also a calorific ray, and a calorific ray which is not also luminous is a ray whose length of wave exceeds the length requisite to produce the impression of light. The difference objectively is a difference in degree, not kind. Subjectively the difference is one of kind, not degree merely. The science of light is therefore only a small and special branch of the more general science of heat, while the science of heat might in certain aspects be looked upon as a branch of the still more general science of motion. The dynamical theory of heat is thus the general of which the undulatory theory of light is the particular. According to the dynamical theory of heat, as it is now generally accepted, heat is the molecular motion of any body in se. This molecular motion is supposed to communicate lie vibrations or pulsations to an infinitely elastic substance pervading space. "Here," says Professor Tyndal, "your conception must be perfectly clear. The intellect knows no
difference between great and small: it is just as easy, as an intel- lectual act, to picture a vibrating atom as to picture a vibrating cannon-ball ; and there is no more difficulty in conceiving this ether, as it is called, which fills space, than in imagining all space filled with jelly. You must, then, imagine the atoms vibrating, and their vibrations you must figure as communicated to the ether in which they swing, being propagated through it in waves ; these waves enter the pupil, cross the ball, and break upon the retina, at the back of the eye. The act, remember, is as real, and as truly mechanical, as the breaking of sea waves upon the shore. The motion of the ether is communicated to the retina, transmitted thence along the optic nerve to the brain, and there announces itself to consciousness as light." The experiments by which Professor Tyndal ocularly demonstrates that when an in- candescent body is allowed to cool until the luminous rays pass into obscure rays, in other words, the radiation continues, but the number of vibrations falls below what is necessary to pro- duce a sense of light as well as of heat, and so the rays are no longer seen, is extremely clear and interesting, but we must refer our readers for them to the book itself. It must be admitted, how- ever, that the "infinitely elastic ether" is only hypothetical, and the dynamical undulation and molecular motion of bodies only a theory. On the other hand, both the hypothesis and the theory have been discussed ever since the days of Newton, have finally rallied round them all the first men of science of the age, and rest upon so vast and varied a foundation of analogy, experiment, and close reasoning, that they may be accepted as not less firmly estab- lished than any other branch of mixed science. It must be re- membered that the way in which the undulatory theory of light and heat is connected with the sense of colour in the mind remains untouched in the opinion of the pure psychologist. The materialist may contend that in the undulatory theory, as now backed by experiment and analogy, he sees the beginning of the end in the controversy respeoting the nature of the perception of colour. It only remains, he thinks, to study the physiological part of the phenomena of light, taking them up at the point where the luminous tide breaks upon the retina, and begins its journey inland up towards the central regions of the brain, and to follow them there. When we have gone thus far scientifically, we shall, he thinks, know all about it. The psychologist, on the contrary, derides the hope of being able to extract the separate entity " colour " out of any series of phenomena. These, he contends, are only constant acces- sories and scientific labels, whereby that entity may be ticketed off and scientifically recognized, but not itself unravelled. This ap- pears to be the view of Professor Tyndal himself, who, while in common with the material school rejecting the idea of "vital force," yet intimates, as far as we understand him, that the ultimate mental phenomena of colour, et similia, will- remain a sealed book to man for ever.
To follow Professor Tyndal through all the windings of his vast and beautiful subject would be impossible. But the convertibility of heat and motion can be exemplified in count- less ways. For instance, we all know that friction begets heat and expansion, and that if we stop the friction the heat and expansion subside. This, according to the theory, seems to mean that by the vibration caused by friction we add and can add indefinitely to the molecular vibration of the thing rubbed and to the molecular repulsion of the particles, and as that vibration increases the object grows hotter and hotter, its molecular motion being greater. When the friction ceases these molecular vibrations in the object having been artificially raised to a point in excess of the circumambient medium, are returned upon that medium until the excess is spent, and the equilibrium between the vibrations of the medium and the object are restored. Of course in the act of friction force is expended, and that force itself is in fact a transformation of heat. And reasoning back step by step we are driven to the conclusion that this heat, again, was itself simply the "primitive force " in another form. Thus, although it is true that new heat can be gene- rated, it can be so only by the transformation or transference of primeval motion or force. And here we open in full view upon the convertibility and indestructibility of force, of which we will only give one of Professor Tyndal's illustrations. "Suppose a train to approach a station, say at the rate of thirty miles au-hour, and a break to be applied ; smoke and sparks issue from the wheel on which it presses. The train is brought to rest. How? Simply by converting the entire moving force which it possessed at the moment the break was applied into heat." It must be observed, however, that in the application of heat by whatever means to a body, two distinct kinds of work are performed. First, in altering the capacity of the body ; second, in making it hotter.
This means that part of the force applied goes to push asunder, or otherwise disturb, the particles in their molecular arrangement, and part to create the additional molecular movement which in its communicated vibrations is expressed to our sensation as heat. Few phenomena are more puzzling to grasp than that of combustion. Yet it is very simple upon the dynamical or vibratory theory. We have said that incandescence is only another word for that amount of vibration which when ap- plied to the eye produces the sense of light. Combustion accord- ing to the dynamical theory arises when the molecular clash between the atoms of different bodies raises the vibration to the point of creating that sense of light. Thus if steel filings are scattered in a flame, starlike scintillations announce the clash and as it were bombardment of the atoms of steel by the atoms of oxygen, for which the heated steel has an affinity. There is a weird poetry in the conception which makes this minute bombard- ment of atoms only a miniature of the bombardment of the sun by minor planets crashing into its surface, whereby it is believed, according to the meteoric theory, the heat and incandescence of the sun may be sustained.
The way in which by small, almost indefinite, steps, science rises to the consideration of the most colossal problems, until the seen is swallowed up in the unseen, is apt at first to produce a certain sense of intellectual alarm and discomfort,—one of the least pleasant effects of science upon the uninitiated. The dis- section of the universe has its own vague horrors, no leas than the dissection of the body. When every change, organic and inor- ganic, in our globe is realized as being due to heat, and that heat de- rived from the sun, yet no more than an infinitely small fraction of the sun's radiation ; when it is added that the heat given out by the sun per hour is equal to that which would be produced by the combustion of a layer of solid coal ten feet thick entirely sur- rounding the sun, and the heat emitted in a year is equal to that which would be produced by the combustion of a layer of solid coal seventeen miles in thickness, covering the entire surface of the sun; when we are further told that if the sun were a solid block of coal it would be consumed in oxygen in five thousand years by combustion, or if simply incandescent, would, at the present rate of the emission of heat, cool down 15,000 deg. Fahr. in five thou- sand years, and we then ask ourselves how the heat and light of the sun can be maintained through ages, which to human appre- hension are as eternity itself, we are apt to feel as if thought were expatriated from comfort for ever, and human feeling an idle and insignificant mockery. Some such phase and agony of intel- lectual expatriation may have affected the mind of the great Pascal, in whom the scientific nature seemed to live in a long conflict side by side with the yearning after faith and moral supremacy, when he cried, "If the universe were to conspire to crush me, I should be greater than the universe, for I should know that I was crushed, and crushed by brute force."
We have lingered as long as we could over this most beautiful but, paradoxical though it may sound, most truly harrowing book. Mr. Tyndal's treatise on heat is likely to live side by side with Sir John Herschel's famous work on astronomy as one of the classics of science. It is impossible to over-estimate the value of such works in sowing the seed and hastening the harvest of further dis- covery, and the peculiar combination of qualities required in order to be at once popular without triviality, and accurate without being unintelligible, is perhaps one of the rarest gifts vouchsafed to an author. Mr. Tyndal is not only a most exact and original philosopher, he is a consummate artist in the arrangement of his materials. It would be difficult to find a grander climax to any book, looked at as an artistic whole, than the concluding passage of his treatise, which we need make no apology for quoting entire.
After saying that "when we have exhausted physics," and "reached its very rim," the real mystery of thought "looms, and will ever loom—ever beyond the bourne of man's intellect," justi- fying the lines,—
"We are such stuff
As dreams are made of, and our little life Is rounded with a sleep,"
—.he proceeds, "Still, presented rightly to the mind, the discoveries and generalizations of modern science constitute a poem more sub- lime than has ever yet been addressed to the imagination. The natural philosopher of to-day may dwell amid conceptions which beggar those of Milton. So great and grand are they, that in the contemplation of them a certain force of character is requisite to preserve us from bewilderment. Look at the integrated energies of our world,—the stored power of our coal-fields, our winds and rivers, our fleets, armies,. and. guns. What are they ? They are all generated by a portion of the sun's energy, which does not amount to one two thousand three hundred millionth of the whole. This is the entire fraction of the sun's force intercepted by the earth, and we convert but a small fraction of this frac- tion into mechanical energy. Multiplying all our powers by millions of millions, we do not reach the sun's expenditure. And still, notwithstanding this enormous drain, in the lapse of human history we are unable to detect a diminution of his store. Mea- sured by our largest terrestrial standards, such a reservoir of power is infinite ; but it is our privilege to rise above these standards, and to regard the sun himself as a speck in infinite extension—a mere drop in the universal sea. We analyze the space in which he is immersed, and which is the vehicle of his power. We pass to other systems and other suns, each pouring forth energy like our own, but still without infringements of the law, which reveals immutability in the midst of change, which recognizes incessant transference or conversion, but neither final gain nor loss. This law generalizes the aphorism of Solomon, that there is nothing new under the sun, by teaching us to detect everywhere, under its in- finite variety of appearances, the same primeval force. To Nature nothing can be added ; from Nature nothing can be taken away ; the sum of her energies is constant, and the utmost man can do in the pursuit of physical truth, or in the applications of physical knowledge, is to shift the constituents of the never-varying total. The law of conservation rigidly excludes both creatiou and an nihilation. Waves may change to ripples, and ripples to waves— magnitude may be substituted for number, and number for mag- nitude--aeteroids may aggregate to suns, suns may resolve them- selves into jhrz and fauna, and „flora and fauna melt in air— the flux of power is eternally the same. It rolls in music through the ages, and all terrestrial energy—the manifestations of life as well as the display of phenomena—are but modulations."
He who can read this passage for the first time with undivided attention, and not feel his pulse beat quicker under the almost cruel and savage grandeur of the picture unfolded before him, may lead a "little life rounded by a sleep," but assuredly he at all events is not "made of such stuff as dreams are made of."
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