﻿392 Prof. H. A. McTaggart on the Electrification 



hydrolysis occurs with the formation of thorium hydroxide 

 thus — 



Th(N0 3 ) 4 + 4 HOH— >Th(OH) 4 + 4 HN0 3 . 



There is present in the solution some of the original salt, 

 some acid, and the hj-droxide in colloidal form. The pre- 

 sence of the last-mentioned was suspected as one of the 

 causes producing the reversal of sign, and experiments were 

 then made to test its activity in altering the charge. 



A colloidal solution of thorium hydroxide as free as 

 possible from salt and acid was prepared by dialysis (Burton, 

 4 Physical Properties of Colloidal Solutions,' 2nd Ed., p. 16). 

 A dialysing " sleeve " shaped in the form of a test-tube was 

 made of " parlodion " (sold by the Du Pont Chemical Co., 

 New York). A solution of the parlodion in ether and 

 alcohol was used to coat the inside of a test-tube of suitable 

 size. After the solvent had evaporated the parlodion re- 

 mained as a thin but strong film which when detached from 

 the glass served very well as a dialysing vessel. 



For this experiment a solution containing about 2 gm. of 

 salt in 50 c.c. of water was dialysed for a period of three 

 weeks, after which an estimate was made of the colloid pre- 

 sent. A sample of 10 c.c. evaporated over sulphuric acid 

 gave a residue of '0034 gm. The residue formed a thin 

 layer of gelatinous material on the bottom of the evaporating 

 dish, with drying cracks across it in all directions. 



The effect of this colloid on the charge on small spheres 

 of air in water was then examined, the dialysed solution 

 above mentioned being diluted as shown in the following 

 examples : — 



No. 



C.c. colloid 



solution in 



100 c c. water. 



Diameter of 

 sphere 

 in mm. 



Sign 

 char 



1 



10 



0-21 



+ 







0-12 



+ 







0-07 



4- 



2 



5 



017 



+ 

 + 



3 



25 



0-17 







0-07 



+ 



It is seen that the surface is charged positively by the 

 presence of very small amounts of the colloid. 



The following examples show the gradual reversal of the 



