That this is the real state of the case—and that the original setting fire to the com bustible has nothing to do with the matter, as is frequently imagined—will be made evident by considering any spontaneous combination, say that of chlorine and copper filings, or of mercury and sodium, etc., in which cases the potential energy lost by the compound appears as heat, light, and sometimes sound.
The equivalents of the other forms of energy have not been even approximated to, with the exception of that of light. Thomson has determined the energy of a cubic m. of sunlight at the earth to be somewhere about 12,000 foot-pounds, giving about 10,000 as the horse-power (q.v.) of each sq. ft. of the sun's surface. There are some additional difficulties in the way when we seek the equivalent of electric energy, for here the question arises; "Is there a special substance which is, or the energy of whose motions is, electricity, or does it depend upon motions and distortions of the luminifer ous ether?" for we can scarcely suppose it to he due to motions of the particles of mat ter. If the first, we have as yet no means of estimating its energy; if the latter, we may consider it as within the reach of experiment. It may merely be remarked here, that Weber's exquisite theoretical statement of electric laws—resting on the fundamental assumption that there arc two electric fluids—requires the admission of mutual forces, which vary with the relative velocity of its particles, and for which, therefore, the con servation of energy does not necessarily hold.
Helmholtz, in an admirable paper ((Aber die Erhaltung der Kraft, translated in Tay lor's Scientific Memoirs, new series; a), starting from the assumptions above explained, has applied the principal of conservations of energy to the investigation of many recondite problems connected with the physical energies. We cannot, of course, enter into his work in detail, as it is somewhat analytical, but we may freely borrow such of its con tents as we have not already alluded to, at least such as will suit the plan of this article.
A very good example of the conservation of energy is found in the increasing veloc ity of a planet or comet as it approaches the sun, and thus loses potential energy; and also in the fact, that in the case of these bodies the mere distance from the sun, and the velocity at that distance; enable us to tell at once the nature of the orbit described—i.e.,
which of the conic sections it is.
Latent heat is probably a form of potential energy, depending on the physical state of the substance in which it is stored up. The same may be said of those substances which, when mixed, produce heat or cold, as water and sulphuric acid, or nitrate of ammonia. It is easily seen that here the heat or cold depends upon a change of mole cular arrangement of some kind; that is, a change of the potential energy.
In magnetism and statical electricity, of course, the conservation of energy holds, as we know that all the phenomena can be explained by attractions and repulsions, follow ing the law of gravitation. In the discharge of a Leyden battery, the potential energy lost is reproduced as heat in the connecting wires, and as light, heat, and sound with the disruptive spark. In charging a Leyden jar by means of the electrophorus, the charge is directly produced by the expenditure of mechanical work in overcoming the attraction of the negative electricity of the resinous plate for the positive electricity of the cover.
In the ordinary voltaic battery, the excess of loss of potential energy in the cells by the chethical union, say of zinc and oxygen, and of sulphuric acid and oxide of zinc, over that gained by the decomposition of water, produces the kinetic energy of the cur rent, which may be transformed into heat, light, magnetism, or motion, or two or more. Or it may be employed.to reproduce potential energy by chemical decomposition, say that of water. This again, by a spark, can be reconverted into kinetic energy as an explosion accompanied by heat, light, and sound. While an electric current causes the motion of a magnetized needle, our general principle should lead us to infer that the current itself will be weakened. This is found to be the case, but, as it should be, only during the motion of the needle. The needle in a permanent state of deflection produces no effect whatever. Now, the diminution of an electric current is simply equivalent to the addition of a weaker current passing in the opposite direction. We should expect, then, that the motion of a magnet near a conducting wire will in general produce a cur rent in the latter, and this is, in fact, Faraday's great discovery of magneto-electric induction. In this case, the current ceases so soon as the magnet ceases to move relatively to the wire.