348 THE MECHANICAL EQUIVALENT OF HEAT. 



one another may be represented by concrete numbers. In 

 every case, however, the conception of an atom must be re- 

 garded as merely relative, and must be considered in connec- 

 tion 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 

 process these atoms may themselves undergo further 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 phenomena 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 



hese, we are forced to confess that, whether the existence of 

 in aether and of atoms be admitted or not, we are, so far as 



gards the nature of specific heat, in a state of ignorance. 

 The expression "latent heat" has reference to its correctly 



ccognized property of indestructibility. In all cases in which 



iiermometrically 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 

 confirmations of the principle of the transformation and con- 

 servation 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, cho- 

 sen from among the abundance of facts, may serve to show 



