LEWIS. — THE LAW OP PHYSICO-CHEMICAL CHANGE. 63 



Equation (16), uamely, 





holds true for the two states which we have just considered, one of 

 which is the vapor in the state of a perfect gas at the low pressure P^. 

 By the aid of equation (24) we may therefore write 



•As 



h^- R\n P^. 



According to (22) 



5 r • 



and the last two equations give by addition 



which is equation (21). 



I think, therefore, that we are justified in considering h the true 

 capacity dimension of heat, and in considering equation (21) the special 

 form of equation (19) applied to heat energy. The replacement of 

 entropy in general energy equations by the quantity h will have a 

 further advantage on account of the much greater simplicity of the 

 latter, the approximate value of which may be in all cases very easily 

 determined by assuming that the vapor of the substance in question may 

 be regarded as a perfect gas, in which case equation (24) evidently 

 becomes 



h = ^ + R\np, (25) 



where Q is the total heat absorbed in the evaporation of one gram- 

 molecule and p is the vapor pressure.* 



We have now obtained equations of the form of (19) for two of the 



* This approximate equation is a special form of a general and rigorously exact 

 equation, 



h=^ + R\n^, (25a) 



in which 4' is the escaping tendency of the substance and Q' is the heat absorbed 

 when one gram-molecule is allowed to evaporate irreversibly against an infini- 

 tesimal vapor pressure. Since this equation will not be used in this paper its 

 demonstration may be postponed. 



