358 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



case of the glasses, the ions consist of the atoms themselves, since, on 

 passing a current through soda-lime glass, the only material transferred 

 is sodium. This and similar cases are the only ones in which it has 

 been definitely demonstrated that an electrolytic ion consists of a charged 

 atom alone. 



In the case of glasses, it is possible to substitute the sodium ion by 

 another positive ion. llb Such a substitution is, in effect, a determina- 

 tion of the speed of the ions by the moving boundary method. Substitu- 

 tion may be quite generally effected but, in the case of most positive ions, 

 the glass disintegrates as the process proceeds. In the case of silver, 

 however, a substitution may be carried out to a considerable depth. If 

 sodium is substituted by silver, the weight of the glass is increased in 

 proportion to the difference in the atomic weight of silver over that of 

 sodium. In the following table are given values of the gain in weight 

 of a sample of soda glass, together with the values calculated from the 

 amount of electricity passed as determined in a coulometer. 110 The tem- 

 perature is given in the first column. 



TABLE CXLI. 



OBSERVED AND CALCULATED GAIN IN WEIGHT OF SODA GLASS ON 

 SUBSTITUTION BY SILVER. 



Gain in Weight Gain in Weight 

 Temperature Calculated Observed 



350 0.0339 g. 0.0347 g. 



350 0.0396 0.0416 



343 0.0732 0.0762 



343 0.0209 0.0209 



By measuring the penetration of the silver boundary into the glass 

 under a given potential gradient, it is possible to determine the volume 

 of the glass which has been affected, and, knowing the composition of 

 the glass, it is possible to determine the fraction of sodium in the glass 

 replaced by silver. This has been done in the case of soda glass with 

 the following results. 110 



TABLE CXLII. 

 RELATIVE AMOUNTS OF SODIUM REPLACED BY SILVER IN SODA GLASS. 



| * 1% 



278 76.5 



295 76.8 



323 77.05 



343 82.3 



ub Heydweiller and Kopfermann, Ann, d. Phya. 32, 729 (1910). 

 Darby, loo. cit. 



