the Arseniates of Copper , and of Iron. 211 
in a large quantity of alcohol. It loses about 43 per cent, of 
carbonic acid, by solution in a stronger acid ; and, prepared in 
the manner I have mentioned, contains no more than 5 per cent, 
of oxide of copper ; although carbonate of potash, when poured 
into a solution of any salt of copper, appears to retain a much 
greater quantity. This, therefore, is a sufficient proof of the 
inaccuracy of attempting to precipitate copper from its solutions, 
by an alkaline carbonate ; for, carbonate of soda has, like car- 
bonate of potash, the property of dissolving and crystallizing, as 
a triple salt, with hydrate of copper. 
In addition to this source of error, we may add, the very 
uncertain operation of one metal upon the saline combination 
of another, whatever may be their affinity for oxygen. Indeed 
I have so often remarked this very great uncertainty, that I 
was pleased to find the observation had occurred to Mr. Proust ; 
and I have constantly found that more dependence can be 
placed upon the method I have adopted, than upon any attempts 
to precipitate the metals by each other. 
With regard to efficacy and promptness, tin or zinc is pre- 
ferable to iron ; but, with any of the three metals, a phenome- 
non occurs, for which I have not been able to account, in any 
manner the least satisfactory. As the effects are more striking 
with zinc than with the other metals, I shall enter into particu- 
lars respecting the use of that metal only. 
When a bit of zinc (or tin, or iron,) is immersed in a solution 
of muriate of copper, a precipitation begins, and all the copper 
is finally separated in a few hours. * But, if a little muriatic acid 
is added to this solution, and the zinc then immersed, a violent 
* If any quantity of neutral salt is present in the solution, the precipitation is much 
retarded, and is seldom complete. 
E e 2 
