Mr. Croft on a new Oxalate of Chromium and Potash. 199 



form, but become of a bright dark green colour : as soon as 

 the decomposition of the oxalates commences they fall into a 

 light green powder, which when stronger heated becomes 

 brown. In closed vessels carbonate of potash is formed; in 

 open ones, when the heat is continued for a length of time, 

 cnromate is produced. This chromate must be extracted by 

 water, reduced, and the oxide of chromium precipitated by 

 ammonia : in this operation, however, it is better to evaporate 

 the ammoniacai solution to dryness, as the ammonia always 

 dissolves a small quantity of the oxide. This method is pre- 

 ferable to that usually employed (Heinrich Rose's Analytical 

 Chemistry) : the ammoniacai and potash salts must be dis- 

 solved out, evaporated, the ammonia driven off, and the potash 

 determined either as chloride or by means of platinum. 



The oxalic acid may be determined by boiling the salt with 

 sulphuric acid, as proposed by Prof. Graham. 



The salt being excessively difficult to crystallize, it seldom 

 happens that a perfectly homogeneous substance can be ob- 

 tained for analysis : the method of analysis is moreover some- 

 what complicated, and consequently the analyses do not agree 

 so perfectly as could be desired. 



i. a. in. iv. v. vi. 



Cr 2 O s 21*80 21-83 23*11 22'05 21-10 24*11 

 KO 13-18 13-11 12-22 12-92 

 C 2 O a 37*00 36-98 40-89 



The water as obtained by other experiments, is 

 H O 26*42 26-27 28*22 



The most plausible formula is KO, C 2 3 + Cr 2 9 3 C 2 O a 

 -1- 12 HO. 



4754-80 100*000 



This differs from the black salt in containing one atom of 

 basic oxalate instead of three. It may be said to be related 

 to the black salt in the same way as metaphosphates are to 

 phosphates. It is evident, therefore, that if we add two atoms 

 of oxalate of potash to one atom of the red salt, we ought 

 to obtain the black salt, which is indeed the case. 



2*37 grammes of red salt were mixed with 1*15 gr. of oxalate 

 of potash (these are the atomic proportions), the solution 

 boiled and evaporated, they yielded 3*119 grs. of the black 

 salt in good crystals, and perfectly pure : according to theory 

 it ought to have given 3*070. The weight of the black salt 

 must be equal to that of the red salt, plus two atoms of anhy- 



