300 ON THE CONSERVATION OF FORCE. 



physicists designate that view of Nature corresponding to the 

 law of the conservation of force with the name of Mechanical 

 Theory of Heat. 



The older view of the nature of heat was that it is a sub- 

 stance, very fine and imponderable indeed, but indestructible, 

 and unchangeable in quantity, which is an essential fundamental 

 property of all matter. And, in fact, in a large number of 

 natural processes, the quantity of heat which can be demon- 

 strated by the thermometer is unchangeable. 



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 disappeared at any rate, as regards the ther- 

 mometer. 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 precipitated, 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 va- 

 porisation 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 composition, 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 car- 

 bon into carbonic acid is perfectly constant, whether the com- 

 bustion is slow or rapid, whether it takes place all at once or 



