RICHARDS AND BONNET. DISSOLVED CHROMIC SULPHATE. 25 



prominently, to assist the hydrolytic effect, as judged by the greenness of 

 the solutions — but these changes were subordinate in magnitude, and 

 inessential. 



Qualitative though they were, these experiments furnished final evi- 

 dence that the effect is hydrolytic. 



In this paper no attempt has been made to study the colloidal acid sul- 

 phates of Recoura and Whitney, made by heating the dry salts to a 

 higher temperature. These introduce a new complication into the theo- 

 retical interpretation of the behavior of chromium ; but they have been 

 shown by Whitney to be unstable in solution, and hence probably do not 

 enter into the equilibrium under consideration. 



Summary. 



Our results, which amplify rather than contradict the best of the pre- 

 ceding work, may be summarized as follows : — 



1. It is shown that the immediate yield of violet chromic sulphate 

 from chromic anhydride is much increased if the reducing reaction is 

 allowed to take place at low temperatures. 



2. Violet chromic sulphate was found to dialyze in normal atomic pro- 

 portions, while green chromic sulphate allows an excess of sulphuric acid 

 to pass through the diaphragm. This result agrees with that of van 

 Cleef, and indicates hydrolysis as the cause of the color-change. 



3. With the help of the inversion of sugar it is showu that the extent 

 and speed of the hydrolysis is dependent upon the temperature. 



4. The acid formed by hydrolysis may be removed by successive 

 shaking with a mixture of ether and alcohol, or by digestion with chromic 

 hydroxide. This accelerates the hydrolysis. 



5. Excess of acid accelerates the reversion to violet, and retards the 

 formation of the green salt. 



C). The formation of the green sulphate compound requires time, either 

 from the double decomposition of the green chloride, or from hydrolytic. 

 action. 



7. At 100°, a constant result is reached when the solution saturated 

 with chromic hydroxide attains the atomic ratio Cr : (S0 4 ) ; at 20°, the 

 ratio is Cr 5 : (S0 4 ) 4 . 



8. The green substance occluded by baric sulphate from green chromic 

 solutions also seems to be highly basic. 



9. Migration experiments indicate no anion containing chromium in a 

 green solution. 



