290 THE REALITIES OF MODERN SCIENCE 



For example, if a mass of some substance other than 

 water was heated and mixed with a mass of water at 

 a lower temperature, the final temperature was found 

 to be lower than equation (2) indicated. The sub- 

 stance was, therefore, said to have a smaller capacity 

 for the intangible fluid. For calculations the mass 

 of the substance was therefore reduced to that of an 

 amount of water, equivalent in heat capacity, by 

 multiplying it by a factor called its specific heat. 

 The general relation of which equation (2) is a special 

 case is then 



M l S 1 (t 1 -t)=MA(t-ti) (3) 



where Si and S 2 are the specific heats 1 in calories per 

 gram. 



It is convenient in discussing gases to deal with the 

 molecular specific heat, that is, the heat per degree 

 per mole, as we have in equation (1). Let us retain 

 the same symbols but express C v and C p in calories; 

 since A equals R, 2 we have 



C P =C V +R/J (4) 



1 The calorists found a curious state of affairs when one of the 

 components of the mixture passed through a change of state. For 

 example one gram of steam at 100 C. in changing from steam to 

 water without change of temperature would liberate about 536 

 calories. To-day we recognize in this change a conversion of poten- 

 tial energy into kinetic. The calorists, however, considered that 

 there was a hidden store of the fluid which was released when steam 

 condensed and called the amount per gram the "latent heat of 

 vaporization." Similarly when water freezes about 80 calories 

 per gram are released merely by the change of state. This they 

 called the "latent heat of fusion." These two unfortunate terms 

 remain in the literature to-day. 



2 Consider 1 mole, at pressure p and temperature T, to be haated 

 1 C r and to do external work, A, against a piston of area, a. Lot 



