﻿at the Boundary between a Liquid and a Gas. 393 



sign of the charge accompanying the absorption of the 

 bubble : — 



No. 



C.e. colloidal 



solution in 



100 c.c. water. 



Diameter of 



sphere 

 in nun. 



Sign of 

 charge. 



1-0 



0-17 



— 





0-14 



+ 



0-5 



0-35 



_ 





0-26 



— 





012 



+ 





005 



+ 



0-25 



032 



- 





0-17 



- 





0-08 



— 





0-05 



4- 



The experiments show that the colloidal thorium hydroxide 

 gives both the effects observed with the ordinary solution. 

 It not only charges the surface positively if present in suffi- 

 cient amount, but it also exhibits the reversal of charge with 

 diminishing size of the bubble, and this, too, in concentrations 

 of thorium of about the same order as in the case of the salt. 



Discussion. 



The state of the matter and the nature of the electric 

 forces in surface layers of liquids is still a subject on which 

 no very clear ideas exist. Experiments on electro-endosmosis 

 all point to a selective action in such layers so far as the 

 ions in the solution are concerned. But (he observations 

 are always complicated by the presence in contact with the 

 liquid surface of a solid whose role in the selecting we are 

 ignorant of. The same is true of cataphoresis experiments 

 with solids, as, for example, in the study of the electrical 

 charge on colloidal particles. This difficulty is avoided, 

 however, in similar experiments with small spheres of air — 

 or any gas — and in such cases we can safely regard any 

 effects observed as due largely to the properties of the liquid 

 and its free surface. In particular, the electrical charge 

 existing at any air-liquid surface may be considered as the 

 result of forces residing altogether in the liquid. It ought 

 to be possible, then, in considering potential differences at 

 solid-liquid junctions to isolate the contribution of the liquid. 



In the case of thorium nitrate in solution the selective 



