the Arseniates of Copper, and of Iron . 173 
antique bronze. The green oxide of copper which sometimes 
is observed on it, completes the illusion, by assuming the aspect 
of that fine patina which often covers pieces of antique bronze. 
This ore is likewise found in the form of small cylinders, 
often placed one against the other, and sometimes ramified, 
in the same way as is observed in some kinds of hematites. 
When the broken surface of it is exposed for some time to the 
air, it assumes the colour of tarnished gold. It acquires also, 
by the decomposition of its surface, the same violet, blue, and 
green colours as the kind already described ; but, although these 
colours are frequently very deep, they never are so brilliant as 
in that kind. 
It is very uncommon to find pieces of this ore that are not 
mixed, and frequently even penetrated, with grey vitreous cop- 
per ore. That which accompanies the arseniate of Huel Gorland 
mine, oilers, in this respect, an uncommon and very particular 
appearance. The yellow ore is mechanically mixed with the 
vitreous ore, so as to form a compound, in which, by the as- 
sistance of a lens, the small particles belonging to each of those 
two ores, may be very clearly distinguished. The specific gra- 
vity, as well as the quantity of copper, in this ore, varies con- 
siderably, according to the proportions in which the yellow 
copper ore and grey vitreous copper ore are mixed together ; 
sometimes they seem to be mixed in equal proportions, or 
nearly so. 
Nature has established very remarkable differences between 
the arseniates of copper ; and these take place not only in their 
forms, but likewise in their hardness and specific gravity. These 
differences arise, either from the manner in which the arsenic 
acid is combined with the copper, or from the different proportions 
