172 
Dr. Thomson on some of 
Green oxide of chromium - 5-02 
Sulphur - - - 0-37 
It was easy to drive off water, but not to determine its 
quantity. The 5-02 of green oxide contained Jth of a grain 
of chromic acid. If we suppose the sulphur to be combined 
with hydrogen, and what is wanting to make up the weight 
to have been water, the constituents of the buff-coloured 
powder will be 
Green oxide - - 4*85 or 185-44 or 37 atoms. 
Chromic acid - - 0-17 or 6-5 or 1 atom. 
Sulphuretted hydrogen 0-39 or 14*9 or 7 atoms. 
Water - - 2*30 or 87-94 or 78 atoms. 
7-71 
The whole chromic acid, owing to an accident, was not 
collected ; but from seme experiments to be related after- 
wards, I am disposed to view the buff-coloured powder as a 
compound of 
5 atoms hexa-chromate of chromium* - - 36-5 
1 atom hydrosulphuret of chromium - - 7-125 
12 atoms water - - 13-5 
57 'i 25 
Green oxide of chromium, when prepared by the usual 
processes, is not always free from chromic acid ; but in con- 
sequence of the property which sulphuretted hydrogen has 
of reducing chromic acid to green oxide, we have it always 
in our power to free it entirely from all such admixture. 
For this purpose it may be dissolved in muriatic acid, and a 
* By hexa-chromate of chromium, I mean a compound of i atom chromic acid, 
and 6 atoms green oxide of chromium. 
