the Arsen iates of Copper, and of Iron. 227 
a very dark greenish precipitate, approaching to dark brown. 
What the state of that precipitate is, I have not yet determined ; 
but I imagine it to be a mixture of black and red. Brown and 
yellow oxides, I am confident, are mixtures of simple oxides, 
and neither of them is an oxide sui generis . The red oxide is 
the extreme of oxidation ; and affords many beautiful colours in 
nature and in art. 
It is much to be desired, for the advancement of analytic 
chymistry, that experiments upon the proportions of oxygen 
with which metals are capable of uniting, under different circum- 
stances, and upon the combination of those oxides with all the 
known acids, together with many others of their properties, 
would attract the notice, and engage the labours, of accurate 
manipulators. Experiments of this kind have been despised, 
from an idea of their resembling a mere mechanical employ- 
ment ; but, so far is that from the truth, they may justly be 
considered among the most difficult problems of chymistry ; 
and it is only from the rigid and constantly similar results of 
such experiments, that we can hope to attain an intimate know- 
ledge of the principles with which nature has originally operated. 
SECTION III. 
ANALYSIS OF THE RED OCTAEDRAL COPPER ORE, IN WHICH THE 
METAL EXISTS IN A STATE HITHERTO UNKNOWN IN NATURE. 
In the course of the experiments which have been stated in 
the preceding sections, I have had occasion to examine a great 
number of copper ores, and particularly of copper ores from 
Cornwall; but, the only one which has afforded any interesting 
Gga 
