194 PROCEEDINGS OF THE AMERICAN ACADEMY 



was then taken from the filter, air-dried, and analyzed (analysis h). 

 Through the remainder (perhaps two teaspoonfuls) of the precipitate 

 five litres of water were passed ; another portion was then dried at 

 100° and analyzed (analysis c). When dried at 100° the precipitate 

 is much less readily soluble in dilute niti'ic acid, than when not so 

 heated, but the difficulty is easily overcome. The method of analysis 

 was that described above. 



a. 

 CrgOa 39.69 per cent. 



CrOs 22.42 " " 



HO (by difF.) 37.89 « " 



A comparison of these three determinations teaches that the abstraction 

 of chromic acid by washing with cold water stops at no definite point, 

 and probably has no limit except the complete change of the original 

 precipitate into chromic oxide. The composition of the substance a 

 must be a close approximation to the composition of the original pre- 

 cipitate, for the slight rinsing which was intended merely to remove 

 the chromates belonging to the filtrate could hardly have abstracted 

 much of its combined chromic acid. In the chroraate of chromium 

 whose formula would be CrjOa CrOs, the ratio of the chromic oxide to 

 the chromic acid would be that of 3 : 2 ; and we find that the ratio of 

 the chromic oxide to the chromic acid in analysis a is but Httle larger 

 than that of 3 : 2. The substance a is probably therefore a chromate 

 of chromium of the formula C\\Oi CrOs, from which a little of the chro- 

 mic acid has been removed by washing. 



Assuming for the present that the precipitate produced by the reac- 

 tion of chrome alum on monochromate of potash is in truth this chro- 

 mate of chromium, let us inquire into the nature of the reaction between 

 these two salts by which this precipitate could be formed, and bichro- 

 mate of potash left in the filtrate. Concentrated solutions of ])ure 

 chrome alum and of pure monochromate of potash were prepared of 

 known strengths, in order to determine the amount of either solution 

 required to produce a precipitate in the other. A single drop of chrome 

 alum solution produces an immediate and permanent precipitate in the 

 chromate solution ; but if the process be reversed and the solution of 

 monochromate of potash be dropped into the alum solution, no perma- 

 nent precipitate is formed till an apparently large excess of chi'omate 



