Laws of Molecular Force. 243 



for saturation-pressures (see Section 6), 



P(t> 8 -«i) = pdv, 





we get 



dP 



dT 



dT ^~ Vl) = } Vi BT 

 Now from thermodynamics we have the relation 



where X is the latent heat, 



.-. J\=C 3 T^dv, 



which of course could have been written down immediately, 

 for if we write it in the form 



JX=( V3 (T^-p)dv + Vv s -Vv 1} 



=»-) 



we remind ourselves that the latent heat of evaporation of a 

 liquid is the heat supplied to neutralize a Thomson and 

 Joule cooling effect. 



We must evaluate the integral in three stages, 



using in each integral the equation that holds between its 

 limits. 



In the first, 



\ V + k/ v + k 



3T p ^ v + k- 



In the second, 



' V B -v-p) v + k' 



