222 
Mr. Chenevix’s Analysis of 
It is also rather extraordinary, that Mr. Gmelin should have 
taken this ore out of the class of arsenical ores, and left it as 
an unknown species of copper ; when, in fact, it is an arsenical 
ore, but not an arsenical ore of copper. 
I examined some crystals, which are commonly attached to 
the specimens of this ore. They were those which, according 
to M. De Bournon, are in a state of decomposition. By this 
spontaneous decay, they become of a deep brownish red, not 
unlike the substance called colcothar ; but they still retain their 
cubic form. They contain a little acid and water, owing per- 
haps to their having escaped from total decomposition. The 
same theory that accounts for the difficulty of ascertaining the 
quantity of water, will account for the red colour they thus 
assume. 
When green sulphate of iron is exposed in a crucible to a red 
heat, it is well known that sulphureous acid is disengaged in 
great quantities ; and that, if the operation is continued long 
enough, there remains a red powder. In this case, the green 
oxide of iron has taken up oxygen, from the acid ; and this 
latter has been partly decomposed, and almost totally vola- 
tilized. Now, in the species here spoken of, the iron, as in the 
green sulphate, is in the state of green oxide ; therefore, capa- 
ble of receiving an additional portion of oxygen. But arsenic 
acid will, at a high temperature, lose a part of its oxygen, and, 
retrograding to the state of white oxide, will be volatilized; 
and still more easily will those changes take place, when 
oxide of iron, ready to receive, and arsenic acid, ready to yield 
oxygen, are in contact. A less degree of heat, therefore, will 
suffice to drive off this acid, from green arseniate of iron, than 
from arseniate of copper. But we must not from this conclude. 
