362 



A Vapour Pressure Equation, 



in fig. 2. Unfortunately there are no experimental figures 

 to put beside these calculated values. 















Fig. i 



2. 











1-7 























































1-5 







































































Apparently hydrogen is the only gas for which there are 

 experimentally determined values of the molecular heat at 

 low temperatures. It will be interesting to see, when the 

 data become available, whether the kinetic theory will 

 suffice for other gases and to see whether any relations 

 exist between their critical energies and their physical or 

 chemical properties. 



Armstrong College, 



Newcastle-on-Tyne, 

 April, 1920. 



XLL A Vapour Pressure Equation *. 



To the Editors of the Philosophical Magazine, 

 Gentlemen, — 



1. TN the paper by George W. Todd and S. P. Owen on 

 X "A Vapour Pressure Equation," which appeared 

 in the November issue of the Philosophical Magazine, the 

 authors developed the equation 



n„=n 1( t?(i+§\, a) 



and from the fact that N B cc p and 'N 1 cc p at a given absolute 

 temperature they obtained the equation 



- - / B\ 

 P=&pe~ e {l+-g) (2) 



It must be remembered that p is a function of both ]$ v and 



* Published by permission of the Director, U.S. Bureau of Standards. 



