320 Mr. W. Sutherland on the 



that of the liquid, and can write 



A-=27rA/3log — . 

 But our equation for the surface-tension gives 



27r log — 



wX ^ a 



= constant. 



and k 



This is Waterston's law. Now according to thermodj- 

 namical principles the latent heat of evaporation without 

 performance of external work is 



i 





where 6 is the absolute temperature at which evaporation takes 

 place. This cannot be evaluated till we know the character- 

 istic equation for fluids, which bridges in a complete manner 

 the gap between the liquid and the gaseous states. But if we 

 make the assumption that the latent heats of all fluids at their 

 boiling-points are approximately the same fraction of the 

 change of potential energy of the molecules due to evapora- 

 tion, then Waterston's law would still be an approximate de- 

 duction from the law of the inverse fourth power. And this 

 assumption is a natural one, seeing that Eamsay and Young 

 have pointed out that the external work done during evapo- 

 ration at the ordinary boiling-point is approximately the same 

 fraction of the total latent heat for all liquids ; our assumption 

 is, that the fraction of the latent heat due to thermodynamical 

 adjustment is for all liquids the same fraction of the change 

 of mechanical energy. However, that the assumption is not a 

 sound one is shown by the roughness of the approach to con- 



stancy of the following values of ^ . The alcohols and acids 



•^ ^ am-3 



of the fatt}^ series are omitted because of their not following 

 Eotvos^s law. 



C5H10 CeHe CioHie CHCI3 CCl, C,B,Bv C^H.I C^H^Br^ 

 6-7 6-2 4-5 5-8 6-2 6-1 5-9 6-7 



C5H11CI CgHnBr CgHnI CH3I CsHeO (acetone) 

 6-9 6-1 4-9 6-7 4*7 



