22 



PBOCEEDINGS OF THE AMERICAN ACADEMY. 



were provided below with carefully tied diaphragms of parchment paper 

 made tight at the sides with collodion. The middle cup contained a 



green chromium solution which had been freed 

 from acid by shaking with chromic hydroxide, 

 while the others contained cadmic sulphate. 

 Cadmium electrodes were used to prevent the 

 formation of ionized hydrogen or hydroxyl. 



Because gravity caused a slight filtration, only 

 the presence of chromium in the cup above the 

 green solution could be taken as evidence of 

 certain electrolytic migration. Hence the posi- 

 tive current was run first downwards in order to 

 determine if there was any anodic migration of 

 chromium, and then in subsequent experiments 

 upwards in order to determine if there was any 

 cathodic migration. In order to determine the 

 chromium, the cadmium in the cell was first 

 removed as sulphide. No trace of chromium was 

 found in the anode solution after 340 coulombs of 

 electricity had been passed through the apparatus, 

 as indicated by the deposition of 0.1119 gram of 

 copper in a serially connected coulometer. 



On the other hand, in an experiment in which 



the cathodic migration was tested, 183 coulombs 



carried 36.6 rag. of chromium, corresponding to 



I 19.3 grams of chromium for each Faraday's 



/ 



equivalent of 96,580 coulombs.* The cation 

 was green. The quantitative result is surpris- 

 ingly large. Since the atomic weight of chro- 

 mium is only 52, it is clear that each atom of 



V J I chromium cannot be associated with over two 



charges, and pi'obably with not more than a 



single charge. Assuming from the absence of 



chromium in the anodic fluid that the sulphuric 



Figure 2. ion alone migrates in that direction, and that the 



mobility of this anion is 70, f the mobility of 



the chromium group must be 41 on the assumption that each chromium 



has a single charge and 243 on the assumption that each has a double 



* The weights of chromic oxide found were 53.5 and 48.2 uig. respectively. 

 t Kohlrausch and Holborn, Leitvermugen der Electrolyte, p. 200. 



