164 
Dr. Thomson on some of 
Many methods were tried to determine the atomic weight 
of this oxide. As it does not form crystallizable salts with 
acids, and as acids have the property of combining with it in 
various proportions, we cannot have recourse to the salts of 
chromium for this purpose. But as we know that the atomic 
weight of chromic acid is 6-5, it will be aufficiently satisfactory 
if we can determine exactly the number of atoms of oxygen, 
which must be abstracted from chromic acid, in order to con¬ 
vert it into green oxide of chromium. Now, sulphuretted 
hydrogen gas, and protosulphate of iron, possess the property 
of converting chromic acid into green oxide of chromium. 
I shall relate, in succession, the analysis of chromic acid made 
by means of these two bodies. 
1. A quantity of neutral chromate of potash was dissolved 
in water, and a current of sulphuretted hydrogen gas passed 
through the solution till all action was at an end. A beautiful 
green-coloured precipitate fell: the liquid remained deeply 
coloured; but upon being heated sulphuretted hydrogen gas 
was exhaled, an additional portion of green precipitate fell, 
and the liquid became colourless. It was quite free from every 
trace of chromic acid or oxide of chromium. 
The green precipitate being examined, was found to be a 
compound of one atom of sulphuretted hydrogen and one 
atom of green oxide of chromium. It was therefore a hydro- 
sulphuret of chromium. This hydrosulphuret is tasteless, 
and insoluble in water. It dissolves with facility in acids, 
sulphuretted hydrogen gas being given off and sulphur re¬ 
maining. From the experiment it appears, that this hydro¬ 
sulphuret becomes soluble in water when an additional quan¬ 
tity of sulphuretted hydrogen is combined with it. Probably 
