RICHARDS AND BONNET. DISSOLVED CHROMIC SULPHATE. 13 



Thus in about a month the violet and green solutions, kept in a 

 thermostat at 30°, had become identical in their inverting power and 

 probably in every other respect. (Curve F.) The concentration of acid 

 corresponding to this equilibrium is not far from 0.043 normal, judging 

 from the inclinations of the curves at the origin. 



Thus the conclusion which prompted the last experiments is satis- 

 factorily confirmed. Evidently the hydrolytic equilibrium between the 

 green and the violet salts varies continuously with the temperature. At 

 18°, in a fourth-molar solution of salt, the violet salt is chiefly in evidence, 

 the inverting power corresponding to only a small fraction of the green so- 

 lution. At this temperature even the slight hydrolysis which occurs must 

 take place very slowly, and months would be needed to reach the true equi- 

 librium. This conclusion is in accord with the spectroscopic results of 

 Erhard (1875) and Vogel (1878), and explains the fact noted by Lecoq 

 de Boisbaudran (1874) that a violet chromic solution increases slightly in 

 volume during long standing at 20°. At 30° the acid resulting from the 

 hydrolysis attains in equilibrium a concentration of about 0.04 normal ; 

 at 50° of about 0.10 normal, and at 100° of at least 0.13 normal. The 

 last two figures are probably too low because the green solution must 

 obviously begin to " dehydrolyze " as soon as it cools. 



In the light of these facts it is clearly impossible from any direct 

 analysis of the solution to determine the composition of the compounds 

 resulting from the hydrolysis. Evidently, too, the process of neutraliza- 

 tion by alkali adopted by several experimenters will tend to hasten the 

 incomplete hydrolysis by removing the ionized hydrogen ; but at the 

 ordinary room temperature it is unlikely that any definite equilibrium 

 will be reached in a short time. Hence the composition assigned to the 

 basic compound by Recoura and Whitney is by no means certain ; it 

 might have been varied by time and by change of temperature. This 

 indeed is indicated by the disagreeing formula of Siewert. 



It seemed possible, however, that if the acid were in any manner re- 

 moved, without introducing any other substance, the basic substance 

 might finally attain a definite composition. Two methods suggested 

 themselves for this purpose; first, the repeated " shaking out "of the acid 

 by another solvent, after repeated warming; and secondly, the long 

 continued agitation of the green solution with chromic hydroxide. 

 Descriptions of experiments of these kinds are recorded below. 



