and Surface Energy. 41 



From equation (1) we have 



py2__J^ — (V = gramme molecular volume), 



or pY*= ■— . =^r . -x at low temperatures. 



Expressing a T as z . a c and a, in terms of the critical 

 constants we get 



27 R 2 T 6 - 2 1 ^_ 1 



At any (low) temperature T, we may write 

 T c Y c RT C 



whence pV*= ^ XK . T^ . ?/ '.s.^vg^. 



I£ # = pp^ and t = T c — T, then T = - — -; hence 



27 B.r.y.*-* , _^ 

 P * 64 a7-l 6V*' T " 



or ^Z 1 - 27 R • * • y - z • ^ _i? 



• 6 V** 



(5) 



According to the well-known Ramsay-Eotvos empirical 



■XT2 



relationship the left-hand term, - — , is very nearly constant 



over a wide temperature range for " non-associated " liquids. 

 Owing to the limitation of equation (1) to low temperatures, 

 we can draw no conclusions from the right-hand term of 

 equation (5) at higher temperatures. Confining ourselves 

 to low temperatures (boiling-point and under), we may 

 notice that \jr is approximately a constant and that x and y 

 will be the same for many liquids at corresponding tem- 

 peratures ; z has been shown to be a constant (1*4) at the 

 corresponding temperature of the boiling-point, and is 

 probably a constant at other such temperatures. If, there- 

 fore, it can be shown that ==-; is approximately constant for 



different liquids at corresponding temperatures, the constancy 

 of the Ramsay-Eotvos ratio will have been deduced for, at 

 any rate, corresponding temperatures. 



In Table III., V^ and d are compared for a number of 

 substances at their boiling-points. The necessary data were 



