382 Prof. G. N. Antonoff on Interfacial 



molecular doublets, which are in fact continually in a state 

 of vibration and of translatory motion with a definite free 

 path. The surface force in either direction on a row of 

 doublets is a mean value and actually must on the average 

 be the same in every direction, thus producing the ordinary 

 phenomenon of surface tension. 



The number of doublets in unit area of the surface is -y 2 , 

 so that the inward pull per unit area is 



The inward pull 6e 2 l 2 v 2 on unit surface is the molecular 

 pressure, which we denote by the symbol P. The surface 

 tension is a. Thus 



F = 6e 2 l 2 p 2 , cc=3e 2 iy^ (1) 



Thus ¥ = kupW, 



where k is a numerical quantity, practically equal to 2 

 if the magnetic forces are negligible compared with those of 

 electric origin. As an example we may calculate the value 

 of P for benzene, assuming the following data : — 



Weight of an atom of hydrogen = 1*64 x 10 ~ 24 gr. 



Molecular weight of benzene ... = 78. 



Specific gravity of benzene at 



ordinary temperature = 0'890. 



Thus 



°' 890 *a inn 



P = 78xl-64xl0-* = 68xlQ " 



At ordinary temperature, the surface tension of benzene is 

 32 dynes per cm. Therefore 



P = 32* (6-8 x 10 21 ) 1/3 = 12 x 10 6 dynes per sq. cm., 



with k = 2. This is approximately 1200 atmospheres, and 

 its order of magnitude is in accord with indirect evidence. 

 The expression for the molecular pressure can be somewhat 

 modified. Write 6<? 2 = J. The length I of a doublet is a 

 magnitude which cannot exceed the molecular dimension. 

 Some evidence exists which tends to show that / is the same 

 for various liquids at corresponding temperatures, and in 



