to chemical combination but also of that due to the latent heat of gaseity which is evolved when the gas passes from the gaseous to the liquid state. Thus 1. When water unites with a gas and the product is liquid we have great heat developed. 2. When water unites with a liquid (such as sulphuric acid) and the product is liquid, we have generally still heat; but 3. When water unites with a solid and the product is liquid, we have often an absorption of heat; while 4. If the product is solid we again have heat. In the cases now described it would seem to be change of condition as much as chemical action which determines the result as far as heat is concerned. Heat is also produced when a gas condenses on the surface of a solid. Pouillet has also shewn that heat is produced in capillary action. 378. Transmutation of heat into the potential energy of chemical separation. When certain bodies are heated they are decomposed; thus, for instance, when carbonate of lime is heated it gives out its carbonic acid, also when slaked lime is heated it gives out its water and is changed into quick lime. Heat is thus transmuted into the potential energy of chemical separation. Radiant heat (at least those rays which are called chemical rays) may be directly transformed into the potential energy of chemical separation. Thus when such rays fall upon chloride of silver we have a chemical change produced which is made use of in photography. It is believed that this change consists in the decomposition of the salt into its constituents, and that silver is deposited. CONNECTION BETWEEN HEAT AND THE POTENTIAL 379. We have already noticed (Art. 167) that when certain crystals are heated there is a development of statical electricity. The most prominent laws of the relation between heat and other forms of energy are those which have now been given. CHAPTER V. Dissipation of Energy. Sources of Energy. Concluding Problems. DISSIPATION OF ENERGY. 380. It will have become apparent from the preceding chapters that we can no more create energy than we can create matter, and that all we can do is to make the best possible use of the store of energy at present existing in the universe around us. Now some forms of energy are of more service to us than others, and we ought therefore to inquire which of the various forms of energy are the most serviceable and which are the least so. Having come to definite ideas on this subject it becomes one of the most interesting, as well as one of the most important, problems to look around us. and review the various stores of available energy which have been put at our disposal by the Author of the Universe. 381. We have already seen (Art. 317) that a machine only transmutes energy from one form to another, and that in consequence it is impossible for any machine unless supplied with energy of some kind, either continuously or periodically, to go on doing work; and that in this sense perpetual motion is impossible. It will also be seen by Art. 334 that we may modify the usual conception of perpetual motion in a way that will render it not inconsistent with the law of the conservation of energy, although it is nevertheless equally impossible. Indeed, it will appear that the reasoning of Chapter II of this book is founded on the assumption that it is impossible to convert heat into mechanical energy by abstracting it from a substance of lower temperature than the substances around it, because if this were possible a perpetual motion would be possible also; nevertheless, such a perpetual motion is not inconsistent with the principle of the conservation of energy. Now when we come to examine more closely into the results of this chapter we see that the impossibility of this form of perpetual motion is intimately connected with the fact that heat tends to diffuse itself. 382. The following example will make this plain. Suppose a machine to work in a room that neither conducts nor radiates heat to other bodies; that is, in fact, isolated as far as regards the reception or communication of energy with the rest of the universe. Suppose, further, that the source of this machine is supplied with heat, and that in consequence of this the machine does work. Next suppose that this work, by means of friction or otherwise, is immediately reconverted into heat, and then carried again to the source of the engine. Will not such an engine, it may be asked, go on working for ever? There is nothing in the law of the conservation of energy that forbids this result, for the energy of the chamber is supposed to be constant, while a constant proportion of this energy is supposed to exist always in the shape of mechanical work. The possibility of this arrange ment is connected therefore with the possibility of wholly reconverting the heat produced by the mechanical motion into motion, which is again to be converted into heat, and from heat into motion, and so on for ever. To assume the most favourable circumstances, let us suppose that there is absolute zero of temperature in the chamber, except at the source of the engine; then, assuming the truth of the results of Chapter II, we may conclude that if a quantity Q of heat be taken from the source it will be wholly converted into work. Suppose, again, that the work is reconverted into heat in a box similar to the chamber itself that is to say, neither conducting nor radiating heatand that this heated box is taken back to the source, reconverted there entirely into work, and so on. No doubt this arrangement would be, in its literal sense, a perpetual motion-but not in the technical sense, as no external work is produced. When, however, we come to analyse the conditions we have imposed upon the materials employed, we find that they are such as never occur in nature; there is, in fact, no body that neither conducts nor radiates heat, and it is this tendency to diffusion in heat that prevents the arrangement from being possible. Owing to this tendency it is impossible either to procure a chamber which neither conducts nor radiates, or to produce a perfect zero of temperature. Now this latter is quite essential to the perpetuity of our supposed arrangement; for, without such a zero, while all the work is converted into heat only a portion of the heat is reconverted into work. The work will thus at every cycle bear a continually diminishing proportion to the heat, and the final result will be a uniform distribution of this latter form of energy. 383. This example, in which the results of Chapter II are taken for granted, forms of course no new proof of the impossibility of this kind of perpetual motion, because the assumption of this impossibility is the foundation of the argument of this chapter. But the example serves to shew that this impossibility is intimately connected with the diffusive nature of heat. 384. All this has been clearly shewn by Professor W. Thomson, to whom the principle of the dissipation of energy is due. He has shewn that when mechanical energy is transmuted into heat by friction or otherwise there is always a degradation in the form of the energy; and inasmuch as this heat cannot be entirely converted back again into work from its diffusive nature, the final result of continually converting mechanical motion into heat will be that the amount of mechanical motion obtainable from the system will be always growing less, until ultimately all the energy has taken the unavailable form of equally diffused heat. That this form of energy is unavailable will be acknowledged at once by recalling to mind the statement that to get work from heat you must have bodies of different temperatures. 385. Suppose now that we have a ton of water at 212°, while all the other substances around us are at 32°, we have in this ton of water an instrument capable of affording us a certain amount of mechanical work by using it as the source of a perfect engine. There is a certain amount of available work in this ton of water, and do what we may we cannot get it to give more, although it may very probably, if improperly used, give less work. It would appear that by no artifice can we increase this amount any more than we can increase the available work of a head of water of given contents and height of fall. It might perhaps be thought that this would be possible if we could convert the heat of this water into the potential energy of chemical decomposition; could we not use it to |