204 HELMHOLTZ 



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. Aque- 

 ous 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 vaporisation of the water. 



Finally, heat is sometimes produced and sometimes dis- 

 appears 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 quan- 

 tity of heat which is developed by a chemical process for 

 instance, in burning a pound of pure carbon into carbonic 

 acid is perfectly constant, whether the combustion is slow 

 or rapid, whether it takes place all at once or by inter- 

 mediate stages. This also agreed very well with the assump- 

 tion, which was the basis of the theory of heat, that heat 

 is a substance entirely unchangeable in quantity. The natural 

 processes which have here been briefly mentioned, were the 

 subject of extensive experimental and mathematical investi- 

 gations, especially of the great French physicists in the last 

 decade of the former, and the first decade of the present, 

 century; and a rich and accurately-worked chapter of 

 physics had been developed, in which everything agreed 

 excellently with the hypothesis that heat is a substance. 

 On the other hand, the invariability in the quantity of heat 

 in all these processes could at that time be explained in no 

 other manner than that heat is a substance. 



But one relation of heat namely, that to mechanical work 

 had not been apcurately investigated. A French engineer, 

 Sadi Carnot, son of the celebrated War Minister of the 

 Revolution, had indeed endeavoured to deduce the work 



