£4 On the lest Method of dtcomposlng 



on the contrary we add alkali, we precipitate this chromat?! 

 at the maximum. 



It now remains to explain how oxide of chrome is fomid 

 in this precipitate: — the following experiment seems adapted 

 to give a demonstration of it. When we treat chromate of 

 mercury at the mininmm, by the nitric acid, the solution 

 is eftectod without the extrication of nitrous gas ; but if we 

 reduce the quantity of alkali necessary for the saturation of 

 the acid, we obtain in the first place a brownish-red sedi^ 

 ment formed of chromate of mercury at the rnaximum : 

 the solution by this subtraction becomes areen, and preci- 

 pitates, on the addition of a fresh quantity of alkali, green 

 oxide of chrome, which is easily redissolved in an excess of 

 caustic alkali. In this case, to all appearance, a portion 

 of the chromic acid is de-oxygenated in order to hyper- 

 oxidate the mercury, from which result chromate at the 

 maximum and oxide of chrome. 



It is therefore certain that the chromatti of mercury at 

 the maximum, found in the mother water, may arise from 

 two causes ; either it results from the mercurial solution, 

 if it contains oxide at the maximum, or it proceeds from 

 the solution of the nitrate at the minimum, on account of 

 the excess of acid, and then it is found mixed with oxide 

 of chrome. 



The solubility of the oxide of chrome in the alkali fur- 

 nishes the explanation of what passes, when on lixiviaii ng 

 cold, the product of the decomposition of the chromate of 

 iron by nitre, we obtain agrecn liquor which becomes yellow 

 on ebullition; this is because the green oxide is deposited, 

 which lays us under the necessity of fdtrating these lixi- 

 viums before heating them, in order to separate this oxide 

 in the state of purity. This last phaenomenon inclines us to 

 tliink, with M.Godon de St.Memin, that the chromate of 

 iron, so called in commerce, conta:ins chrome in the state 

 of oxide; for it is improbable that the chromic acid is re- 

 duced at the same time with the nitre. What still confirms 

 the opinion of M. Godon is, that the acids extract a green 

 oxide only from the chromate of iron. 



To return to the subject. We shall observe that it is es- 

 sential to wash the chromate of mercury with a good deal 

 of water, in order to free it completely from the nitrate of 

 potosh, which, by the calcination of mercurial chromate, 

 would again form chromate of potash, which produces a 

 commencement of fusion in the oxide of chrome, conse- 

 quently gives it a deeper bbade^ and renders it heavier, which 



we 



