516 J)r. J. R. Mayer on the Mechanical Equivalent of Heat. 



consists of such constituents as undergo no further change of 

 form in chemical processes. 



But a span of that time which stretches both backwards and 

 forwards into eternity is meted out to man here on earth, and 

 the space which his foot can tread is narrowly bounded above 

 and below : so also his scientific knowledge finds natural limits 

 in the direction of the infinitely small as well as of the infinitely 

 great. The question of atoms seems to me to lead beyond these 

 limits, and hence 1 consider it unpractical. An atom in itself 

 can no more become an object of our investigation than a differ- 

 ential, notwithstanding that the ratio which such immensely small 

 auxiliary magnitudes bear to one another may be represented by 

 concrete numbers. In every case, however, the conception of an 

 atom must be regarded as merely relative, and must be considered 

 in connexion with some definite process ; for, as is well known, 

 the particles of an acid and base may play the part of atoms in 

 the formation and decomposition of a salt, while in another pro- 

 cess these atoms may themselves undergo farther division. 



But assuming that, in a chemical sense, atoms have a real 

 existence — an assumption which, among other things, the laws 

 of isomorphism certainly render probable — the further question 

 arises whether, by the continued division of matter, we can at 

 last arrive at molecules which are atoms in relation to the^e- 

 nomena of heat, such that heat cannot penetrate to their interior, 

 and such that, when the whole mass is heated, they for their 

 parts undergo no increase of bulk. But since we are unable to 

 grapple with such preliminary questions as these, we are forced 

 to confess that, whether the existence of an tether and of atoms be 

 admitted or not, we are, so far as regards the nature of specific 

 heat, in a state of ignorance. 



The expression " latent heat " has reference to its correctly 

 recognized property of indestructibility. In all cases in which 

 thermometrically sensible specific heat disappears, it must be 

 assumed that it eludes our perception only by taking on some 

 other state of existence, and that by an appropriate process of 

 inverse transformation the free heat can be reproduced in its 

 original amount. These are the facts on which the doctrine of 

 latent heat rests; and hence, if we have regard to them only, all 

 the connected phenomena may be claimed as so many confirma- 

 tions of the principle of the transformation and conservation of 

 force. 



The conception of latent heat is accordingly nothing else than 

 the conception of something equivalent to free heat, and thus 

 the doctrine of free and specific heat embraces pretty nearly the 

 whole domain of physics. A few examples, chosen from among 

 the abundance of facts, may serve to show how, according to my 



