Popular Science Monthly/Volume 12/April 1878/The Dissipation of Energy
|THE DISSIPATION OF ENERGY.|
SCARCELY had the grand truth been well demonstrated, some thirty years ago, that force can neither be created nor annihilated, when it served as a basis for one of the boldest theories ever conceived in the history of science. Prof. William Thomson (now Prof. Sir William Thomson) in 1853 first broached the theory of the dissipation of energy, and since that time many other eminent men have enlarged it and speculated upon it.
The theory points out, in the first place, that different phases of energy are not transformable into one another with equal ease and completeness. Heat is the only form into which any other can be totally converted. When electricity, mechanical motion, or any kind of energy but heat is sought, an undesired production of thermal effect is unavoidable in the most favorable conditions for efficient conversion known to science. Therefore, in the catalogue of terrestrial forces heat is continually gaining in amount at the expense of every other mode of motion.
Further, not only is it impossible, by any known method, to regain from heat more than one-fourth its theoretical value in useful work, but, as the tendency of all heat is ever to become of uniform temperature, by radiation and conduction, the differences of degree wherein its value as a source of other motion solely lies are being continually abolished. The tendency of energy to appear more and more as uniformly diffused heat is further shown to be true not only on earth but in the heavens. With respect to the solar system, our present information, it is held, indicates that it is gradually drifting toward an utterly lifeless state. The sun is parting with its stores of force most lavishly, and must, at however distant a period, become as cold as its planets are now. The planets are little by little losing their force of axial rotation from the friction of their tides, which transmute it into heat; at some future age they will doubtless present a single side to the sun, as our moon, from similar causes, now does to the earth.
Furthermore, it is thought that the medium which conveys light through space, extremely attenuated though it may be, is capable of opposing some resistance to planetary movements, so that the sun may at last unite with itself all the orbs now circling around it. The collision between the sun and its worlds would render the whole mass fiery hot; but radiation, in the course of time, would slowly bring its temperature lower and lower, until it would cease to shine altogether. The theory then supposes that the fate which shall have overtaken the solar system will then attack the sidereal heavens; that the causes which shall first make the planets unite with their primary will make stars unite with one another, until the ultimate result of all these changes may be that a solitary gigantic ball shall contain all the matter now interfused through space, its enormous store of energy, in the form of equably-diffused heat, being incapable of further change, and utterly unfit for the production or maintenance of life.
All this is assuredly very bold, and deduced most fairly from its premises; but premises in truth and completeness are of much more account and far more difficult of discovery than methods of logical inference. Let us briefly examine the grounds on which it is supposed that Nature is doomed to a death without resurrection, and see if they warrant the tremendous conclusions drawn from them.
The theory illustrates very pointedly the difficulties in which the finite mind of man becomes involved when it attempts to deal with what is not thinkably finite—or, if the term be preferred, infinite. In the first place, the theory under consideration assumes the finiteness in amount of matter and motion; but we do not know, nor can we imagine, that space has bounds, neither can we limit the extent of the orbs and movements which, as far as we can see, occupy it.
Secondly, the theory makes another conjecture in the realm of the absolute, when it presumes that heat is an absolutely homogeneous motion—that particles endowed with it move with so perfect a uniformity that there is an exclusion of any difference of motion which might serve as a starting-point for mechanical changes. But has science advanced far enough to make such a proposition tenable? Our knowledge of the ultimate structure of matter is very restricted; and as to what the modes of motion are which we call heat, electricity, and so on, we are entirely in the dark. Their quantities we know, but their qualities, their peculiar orbits, have scarcely been guessed at as yet. From a variety of reasons, however, the modern opinion, like the ancient one, is that matter is made up of atoms, which in the circumstances are units even if ideally divisible. Approximate measurements of them have been made by Prof. Sir William Thomson himself. (See his paper in Nature, vol. i, p. 551.)
Now, if atoms by virtue of their heat moved uninterruptedly in a simple straight line, their motion would be uniform and undifferenced; but, as neither the position nor the size of a mass undergoes change when temperature does not vary, atomic paths must suffer oft-repeated stops. Elastic particles in this state must have incessantly fluctuating velocities, yet always oscillating about a fixed mean. Matter endued with heat cannot have its particles in absolute contact, or the compressibility or contractibility which is the inseparable property of any mass would not exist. For argument's sake, however, let it be admitted that from absolute contact or any other cause heat-motion is a uniform one. If it be a purely axial rotation, then the equators of the atoms move faster than the poles, and the movement is not homogeneous. Exactly so, if the atom describe as an orbit a circle, ellipse, or other figure recurrently. Such motion would involve axial rotation, the atom would resemble our earth, and different parts of it would move with different velocities. In the case of two tangible spheres of like dimensions it is easy to show that, when swiftly moving at an equal rate, the speed of the one can be accelerated at the expense of the other, by applying it at a point not equatorial to the equator of its neighbor. In some such way it is conceivable that differences in molecular motion may widen from those subsisting between the parts of an individual molecule.
The imperfect homogeneity of thermal motion, which is here contended for, has some palpable parallels in the distribution of two other phases of energy—electricity of high tension and magnetism; these forces are cumulative in their manifestations, increasing in intensity toward the poles of the masses presenting them.
Thirdly, it is not strictly an accurate premise in the theory that, when heat is produced from any other force, it is unaccompanied by any phase of energy not thermal. Increments of heat invariably alter the dimensions of bodies, as a rule expand them, and thus part of the original energy applied appears as gravity. The sun in warming the earth's atmosphere lifts it, and, when the air cools, its fall is of no insignificant dynamic value. What is so evident in this extreme case is true of any mass whatever when heated. Not only is heat pitted against gravity, but at times against cohesive and crystalline forces, which, though overcome, must modify and diminish its effects.
There is a check to the continuous increase of temperature which is of much more importance than those just noted, but akin to them. A compound substance receives additions of heat with tolerable evenness up to a certain point, when it is resolved into two or more simple constituents, according to its complexity. These if compound are in turn decomposed into their chemical elements if more heat be applied. Now, chemical energy is a motion quite distinct by itself, and we find that heat in its higher degrees must coexist with it. So that on this account we cannot accept the notion that heat is ever to become the only kind of motion in the universe. In so doing we recognize another reason for believing that Nature will never attain absolute equilibrium, from the variety of forces ever abiding together within her sphere.
Fourthly, our present inability to obtain the movement of masses from, the motion of molecules or small masses—that is, the derivation of work from uniformly heated matter—does not decide that such conversion is impossible in Nature, or even to science in the future. While it is perfectly right to reason from what we know, it is of yet higher importance to constantly bear in mind how little we know. It would require infinite knowledge to say that the motion of uniform heat may not be transformable into phases of energy quite as diverse as high and low temperatures.
Such a change would not contravene the truth of the uncreatability or indestructibility of force, but would simply be an enlargement of the known, which every one feels to be indefinitely small as compared with the knowable. The possibility here suggested may be conceivable as depending on the definiteness in size of atoms; or, on the variety of motions to which the differences between atoms, as chemical elements, may give rise—differences in size and form; or, on the variety of motions implied by the checks offered to steady accessions of temperature, already explained in this paper.
One of the first principles to which the mind clings as fundamental is, that every truth has its converse. Although this may seem an axiom, yet its demonstration may be often very difficult, and, at times, even impossible. A knife-blade held over a gas-flame for a moment shows that hydrogen and oxygen combine to form watery vapor; yet the proof of the converse—the decomposition of water into its elements—demands extensive and powerful apparatus. Oersted, in a happy hour, noticed that an electric wire moved a magnetic needle; but years of experiment had to elapse before the electro-magnet and the magneto-electric machine established the complementary principles in a practical form. The analysis of compounds, chemically, is vastly easier than the building them up from their elements. We know the exact percentages of carbon, hydrogen, and oxygen, that go to make up sugar, and can express to a nicety the dynamical relations of the compound to its elements; but how to bring about the changes desired, with economy, is what puzzles us.
When we see high and low temperatures coming to an equality, it is certainly permissible to entertain faith in the possibility of the converse; in a change equivalent to a mass becoming, in its several parts, hotter and colder. To have recourse to such a supposition is less straining to the mind than the alternatives usually proposed by the theory under consideration.
If that theory be true, the question suggests itself, "Why has not the universe come to death by this time, for limits cannot be imagined to past duration?"
To this two replies have been given by the maintainers of the theory: That the universe has either had a beginning in time; or that, if it be really eternal, there are revolutions in its laws unknowable to man—interpositions of Creative Will!
These men of science are plainly not afraid of carrying out their opinions rigorously to their logical conclusions, but is their information as to the nature and relations of the phases of energy wide and deep enough to warrant them in framing an hypothesis so lofty as to include the cosmos and eternity? Hardly.
At the present stage of science, a student pondering the subject so briefly presented here may be compared to a judge before whom a few witnesses in an important case have appeared. As he hears each one, he makes, for convenience sake, a provisional summing-up, and tacks the testimony together in one directive line. But it would be a most injudicial act to mistake a provisional opinion for a final judgment, and, with an indefinite number of witnesses unheard, to pronounce sentence of death.