205 
the Arseniates of Copper , and of Iron. 
contain a known quantity of water of crystallization, and of what 
we formerly called the oxide. But that oxide still retains a quan- 
tity of water, of which when it is deprived, it passes to a very 
dark brown, and changes its chymical nature and properties. 
If, upon that brown oxide, a sufficient quantity of dilute 
sulphuric acid is poured, it yields a blue salt, but in a propor- 
tion greater, by about 24 per cent, than if the green substance 
had been employed. I imagine, therefore, that the first opera- 
tion of this brown oxide is, to assume the quantity of water 
necessary to constitute a hydrate ; and, that the combination of 
sulphuric acid takes place, not between the oxide, but between 
the hydrate and that acid, to form a salt, which, when cry- 
stallized, has taken another portion of water in the act of 
crystallization. It is a well known fact, that there is a state of 
concentration, when an aqueous solution of muriate of copper, 
gently heated, will change from a bluish green to a beautiful 
brown, which, upon cooling, or by the affusion of water, re- 
sumes its former tinge. This brown liquor is probably a solu- 
tion of muriate of copper ; while the blue liquor, like every 
green or blue solution of a cupreous salt, is a combination of 
the acid and the hydrate, or (as we should say in this case) 
a muriate of hydrate of copper. It is true, I have not been 
able to produce, so often as I could wish, this change of 
colour. I can, however, adduce the following instance, as 
being much in favour of my opinion. 
It is evident that oxide of copper (for so I shall henceforth 
call the brown substance) has a very strong affinity for water ; 
because the fixed alkalis, (unless when boiled upon it,) and 
their carbonates, all of which easily decompose the salts of 
copper, cannot dispossess the hydrate of copper of its water. 
