If a mass of copper or other good conductor be set in rapid rotation near a powerful magnet, the motion produces electric currents in the copper, which, being attracted by the magnet, soon bring the mass to rest. It is not so clear in this case into what the mechanical energy of the rotation has been transformed, especially as the electric cur rents cease with the motion; but if we keep up the rotation forcibly, we find in a short tires the copper growing warm; in other words, the energy has been transformed into electric currents, and the latter into heat. This very beautiful experiment is clue to Joule. and has been repeated in a striking popular form,by Foucault.
Advantage has been taken by Faraday of the phentunena of induction, to produce electric currents by aid of the earth's magnetism. His apparatus is simply a revolving disk of metal, and the terminal wires touch, one its axis, the other its edge. The energy Is hich is here transformed into electric currents is the additional effort requisite to turn a conducting disk, instead of an equal non-conducting one. It is a curious conse quence that in all metallic machinery a portion of the energy of the prime mover is lost in producing electric currents, and finally heat, in the moving parts, so that heat in such cases is not entirely, though very nearly, due to friction alone.
Perhaps one of the most singular of these transformations of energy is that already referred to of heat into electric energy. Certain crystals, such as tourmaline, become electrified by heat; but electric currents can be produced by simply heating a junction of two wires or bars of different metals, the other ends also being in contact. Now, if we were to heat the other junction, it is obvious that, as at it the metals are arranged in the opocode order, we should produce a contrary current; conversely, by cooling them we should strengthen the first. But the conservation of energy requires that such a junction should be heated or cooled according to the direction in which a current passes through it. This was discovered by Peltier.
Animal energy is simply a transformation of the potential energy of food. This is well illustrated by the increased diet which is required when man or beast abruptly changes from a state of inactivity to one of toil, as with a polar bear after his winter's sleep; or by the greater amount and better quality of food which are necessary for criminals subject to hard labor, than for those who are merely imprisoned.
Since, then, as far as we have yet seen, there is no such thing as gain or loss of energy anywhere, while it appears that the ultimate transformation of such energy is heat, and that the latter tends to a uniform diffusion or dissipation, in which it is unavailable, as far as we know, for further transformation (see HEAT), whence do we procure the supplies of energy which, are requisite to maintain the economy of life? We answer: Chiefly, or indeed entirely, from the sun, whence they conic as light and radiant heat, perhaps in other forms. Without the sun, where would be vegetation?— without the latter, where animal life? Where would be our stores of fuel, whether wood or coal? It is entirely then, we may say, to the directly supplied energy of the sun that we look for the maintenance of life; and this leads to a question not of much import ance to ourselves, to be sure, but of vast future consequence to the human race: Is this supply finite? Will the sun in time have given off all its energy, Or is it continually receiving accessions itself, and if so, has it an inexhaustible store to draw from.
Now, whether the sun be a hot mass, or be surrounded by an atmosphere in an intense state of combustion, or whether it derives the main part of its heat, as Thomson supposes, from gravitation (in a way presently to be considered), it is certain that, as far as we know, it must at some period be exhausted. Such is the apparently inevitable verdict of the conservation of energy.
The gravitation theory of the origin of energy generally may be given in some such form as tois: The matter in the universe, in a state of coarser or finer division, origin ally filled all space, and possessed, therefore, by virtue of gravitation, a certain amount of potential energy. As particles gradually moved up to each other, and became slowly agglomerated into masses, more and more of this energy was realized in its kinetic form; sonic as heat (that of the sun, or the internal heat of the earth, etc.), some as vis-viva of axial or orbital rotation, etc. There still remains unagglomerated in space (See ZODIACAL LIGHT, AEHOLITES, COMETS, NEBULO much of this original mat ter still falling mainly towards the larger masses, as the sun and stars, and exchanging its potential for kinetic energy. But the latter, as we have seen, tends ultimately to become heat, and to seek a uniform diffusion. This, then, it appears, is to be the. last scene of the great mystery of the universe—chaos and darkness as in the beginning." An immediate consequence of the truth of the conservation of energy is the impos sibility of what is usually understood by the perpetual motion (4. v.) ; for it is to be care fully remembered that perpetual motion, in the literal sense of the words, is not only possible. but very general. If there were no such thing as friction, or if we had a per fectly smooth body, in the form of a teetotum, for example, it would spin forever in vacua with undiminished speed. The earth in its axial rotation affords a good example. Were it a perfect sphere, and of uniform material, the other masses of the system could produce no effect whatever on its rotation, and the latter would remain forever unchanged. And even, as we have already seen, when one form of kinetic energy, as electricity, or ordinary vis-viva, is lost, we find it reappearing in other forms of kinetic energy, such as heat and light. But this is not the technical acceptation of the term, the perpetual motion; it is popularly understood to mean a source of motion which will not only preserve its own vis-viva unchanged. but also do work,. This is, of course, incompatible with the conservation of energy, for wherever work is done, equivalent energy in some form or other is consumed. The ordinary attempts to obtain "the per petual motion " which are still being made in thousands by visionaries, are simply absurd, based as they are for the most part on ignorant applications of mechanics. There is absolute impossibility here; and a "perpetual motionist " of the common herd is far more infatuated than a "squarer of the circle;" for the latter's problem may be solved, though certainly not by_the means usually employed, or in the form usually sought for.