60 CORRELATION OF PHYSICAL FORCES. 



be no expenditure of the initial force ; and if we suppose that 

 the heat in the condenser was the real representative of the 

 original heat, we should get perpetual motion. Southern sup- 

 posed that the latent heat was constant, and that the heat of 

 vapour under pressure increased as the sensible heat. M. 

 Dcspretz, in 1832, made some experiments, which led him to 

 the conclusion that the increase was not in the same ratio as 

 the sensible heat, but that yet there was an increase ; a result 

 confirmed and verified with great accuracy by M. Regnault, in 

 some recent and elaborate researches. What seems to have 

 occasioned the error in Watt and Clement Desormes* experi- 

 ments was, the idea involved in the term latent heat ; by 

 which, supposing the phenomenon of the disappearance of 

 sensible heat to be due to the absorption of a material sub- 

 stance, that substance, ' caloric,' was thought to be restored 

 when the vapour was condensed by water, even though the 

 water was not subjected to pressure ; but to estimate the total 

 heat of vapour under pressure the vapour should be condensed 

 while subjected to the same pressure as that under which it is 

 generated, as was done in M. Despretz and M. Regnault's 

 experiments. 



M. Seguin, in 1839, controverted the position that derived 

 power could be got by the mere transfer of heat, and by calcu- 

 lation from certain known data, such as the so-called law of 

 Mariotte, viz. that the elastic force of gases and vapours 

 increased directly with the pressure ; and assuming that for 

 vapour between 100 and 150 centigrade, each degree of 

 elevation of temperature was produced by a thermal unit, he 

 deduced the equivalent of mechanical work capable of being 

 performed by a given decrement of heat ; and thus concluded 

 that, for ordinary pressures, about one gramme of water losing 

 one degree centigrade would produce a force capable of raising 

 a weight of 500 grammes through a space of one metre ; this 

 estimate is a little beyond that given conversely by the experi- 

 ments of Mr. Joule, already stated, in which the heat produced 

 by a given amount of mechanical action is estimated. I am 

 not aware that the amount of mechanical work which is pro- 

 duced by a given quantity of heat has been directly established 

 by experiment, though some approximative results in parti- 



