422 Boiling-points and Molecular Volumes of Liquids. 



together with certain of Ramsay and Young's results (Phil. 

 Mag. 1885 and 1886), leads to the following conclusion : — 



If different compounds of the same group be compared when 

 boiling at equal pressure^ the latent heats of vaporization are 

 inversely proportional to the molecular weights and directly 

 ■proportional to the absolute temperatures of ebullition : or 



— pl — = a constant for each group, pressure being equal. 



This may be demonstrated in the following manner, starting 

 with the thermodynamic formula 



L _ T dp 

 S!-!S 2 ~ J'dt' 



where Si and S 2 represent the specific volumes of the same 

 substance at the boiling-point in the gaseous and liquid con- 

 ditions respectively. 



Ramsay and Young have shown that the value of T. ~ 



is a constant for closely allied substances such as the halides 

 of ethyl and phenyl, i. e. for different members of the same 

 group; whence also (if we distinguish quantities belonging to 

 two such compounds by dashes) 



L L' 



(1) 



Si— S 2 S'i— S's 





Substituting molecular weights 



for unit weights, we 



get 



M.L 



MM/ 





Vt — V T - 



«V-YV " ' ' 





where v T and Y T are used as in 

 has there been shown that 



the foregoing paper 



. Bi 



7j = y" and 



Y T V' T . 

 T ~ T ' 





Hence 



v T — Y T v 



T ' V rp/ 





T 



T , .... 





Combining (1) and (2), we get 







M.L MM/ 





T 



T' 





(2) 



(3) 



The molecular latent heats and the moleular volumes, both 

 liquid and gaseous, therefore follow the same law as regards 

 their relation to temperature. 



