the compounds of chromium. 173 
current of sulphuretted hydrogen may be passed through 
the solution, previously rendered as neutral as possible by 
evaporating it to dryness, and re-dissolving the residual 
matter in water. If the green oxide was pure, no precipitate 
appears ; but if chromic acid was present, a quantity of the 
above described buff-coloured precipitate will be thrown 
down. 
When sulphate of chromium in the state of a dry powder 
is put into a glass tube and heated to redness over a spirit 
lamp, while a current of hydrogen gas is made to pass over 
it, water is at first given off, and afterwards sulphuretted 
hydrogen. When all action is at an end, a black, tasteless, 
insoluble matter remains in the tube, composed (judging 
from the loss of weight) of 
1 atom sulphur - - 2 
2 atoms green oxide of chromium - 10 
12 
It is therefore a disulphuretted oxide of chromium ; or, per- 
haps, a dihydro sulphuret of chromium. 
2. I shall now relate the experiments by which chromic 
acid was converted into green oxide of chromium, by means 
of protosulphate of iron. 
When a solution of protosulphate of iron is mixed with 
one of chromate of potash, a buff-coloured precipitate falls, 
which is a combination of peroxide of iron and green oxide 
of chromium. In this case, the protoxide of iron is peroxy- 
dized at the expence of the oxygen in the chromic acid, which 
becomes green oxide. Thus we have it in our power to 
convert chromic acid at once to green oxide, by mixing it 
