ON THE CONSERVATION OF FOKCE. 343 



By conduction and radiation, it can indeed pass from 

 hotter to colder bodies ; but the quantity of heat which 

 the former lose can be shown by the thermometer to have 

 reappeared in the latter. Many processes, too, were 

 known, especially in the passage of bodies from the solid 

 to the liquid and gaseous states, in which heat dis- 

 appeared at any rate, as regards the thermometer. But 

 when the gaseous body was restored to the liquid, and the 

 liquid to the solid state, exactly the same quantity of heat 

 reappeared which formerly seemed to have been lost. 

 Heat was said to have become latent. On this view, liquid 

 water differed from solid ice in containing a certain 

 quantity of heat bound, which, just because it was bound, 

 could not pass to the thermometer, and therefore was not 

 indicated by it. Aqueous vapour contains a far greater 

 quantity of heat thus bound. But if the vapour be pre- 

 cipitated, and the liquid water restored to the state of 

 ice, exactly the same amount of heat is liberated as had 

 become latent in the melting of the ice and in the 

 vaporisation of the water. 



Finally, heat is sometimes produced and sometimes 

 disappears in chemical processes. But even here it might 

 be assumed that the various chemical elements and 

 chemical compounds contain certain constant quantities 

 of latent heat, which, when they change their composi- 

 tion, are sometimes liberated and sometimes must be 

 supplied from external sources. Accurate experiments 

 have shown that the quantity of heat which is developed 

 by a chemical process, for instance, in burning a pound 

 of pure carbon into carbonic acid, is perfectly con- 

 stant, whether the combustion is slow or rapid, whether 

 it takes place all at once or by intermediate stages. This 

 also agreed very well with the assumption, which was the 

 basis of the theory of heat, that heat is a substance 

 entirely unchangeable in quantity. The natural processes 



