470 Mr. A\ r . C. M. Lewis : Experimental Investigation 



The values of the tension and concentration are plotted 



Fiff. 1 . 



in fig. 1. 



50 



Z0 



§ 20 

















































^ 



1 

























0-25 0-5 0-75 



Molar Concentration. — Caustic Soda. 



Calculation of T by means of Gills' 's egression : — 

 Take as a particular case the concentration 0*3 per cent. 

 c = 0*003 grm./cc. 



2x4-2xl0 7 



1-0' 



R 



2 x 4*2 x 10' ergs 



20 



molecular weight 



assuming complete dissociation 

 at this dilution. 

 T = 288° abs. 



da 

 dc 



i. e. the tangent to the curve (fig. 1) at the concentration 

 0'3 per cent., 



= _ 9fel^._ =3 ooo. 

 00d grm./cc. 



Hence 



c da „ _, .,„ Q „ 



—• =p7n 3~ — ro x J()~ J grams per cm. 



or 1*9 x 10 -10 gram-moles per cm. 2 



It has been assumed here in using the term gram-moles 

 that the substance has been adsorbed either as undissociated 

 base or in ionically equivalent proportion. Gibbs's expres- 

 sion for surface concentration does not contain any term 

 allowing for the preferential adsorption of a single ion if 

 any should occur. Also the measurement of concentration 

 changes by means of changes in tension embodies the same 

 assumption. 



