the Arseniates of Copper , and of Iron. 223 
that the affinity of the latter metal for arsenic acid is superior 
to that of the former; for, the attraction of green oxide of iron 
for oxygen, and of caloric for white oxide of arsenic, deter- 
mine a new order of divelling affinities. 
But, most of the mineral acids that have been tried, have 
been found capable of uniting with iron in two states ; in the 
state of green oxide, and in that of red oxide, the residuary 
powder above-mentioned. I was desirous to know whether I 
could, in any manner, imitate by art, the last natural products 
I have examined, as I had already imitated, in some degree, 
an arseniate of copper. For this purpose, I decomposed green 
sulphate and red sulphate of iron, by arseniate of ammonia ; and, 
having well washed and dried the precipitates, proceeded to 
examine them. 
The green arseniate was acted upon by heat, in the same 
manner as the natural one, and exhibited the same appearances. 
By the usual methods, I found its proportions to be. 
Oxide of iron 
- 
- 43 
Arsenic acid 
- 
- 38 
Water - 
- 
- *9 
100 
This is not the same proportion of acid that is contained in 
the natural arseniate ; however, I state them both as I found 
them. The other artificial arseniate, which is of a pale greenish 
red, afforded, 
Red oxide of iron 
- 
- 3 6 ’5 
Arsenic acid 
- 
- 4 
Water - 
- 
20,0 
98,0 
