having dissolved in hot water the requisite quantity of cupric sulphate, I
decompose one-fourth of this salt by adding just sufficient of a solution
of carbonate of soda to precipitate the copper, in that quantity of the
sulphate, as carbonate. I then add just sufficient acetic acid to convert
the carbonate into acetate. I have now got in solution--

3CuSO_{4} + Cu(C_{2}H_{3}O_{2})_{2},

and I have to transform it into--

3CuAs_{2}O_{4} + Cu(C_{2}H_{3}O_{2})_{2}.

It is at once seen that I have got the requisite quantity of acetate
formed. I next dissolve the requisite quantity of arsenious anhydride in
an amount of carbonate of soda _rather less_ than is sufficient to
neutralize the acid in the remaining cupric sulphate, and I then bring the
solution to or near the boiling-point by introducing steam into it; the
arsenic is dissolved not in the same vessel as the copper salt, but in a
separate one. When the arsenic solution is fully heated, a small current
of it is allowed to flow into the vat containing the copper salts, and
brisk stirring is kept up in the vat. The emerald green is at once formed;
but if there should be the slightest formation of any arsenite, the flow
of the arsenic solution is at once stopped until every trace of the
arsenite has been converted; the arsenic solution is then allowed to flow
in again, with the same precautions as before; in this way a large batch
of emerald-green can he formed in one or two hours, without containing the
slightest trace of the arsenite. I keep the arsenic solution near the
boiling-point during the whole of the time it is flowing into the other
vessel. By varying the proportions of water I could either make it coarse
or fine, as I wished, which is an important matter to have complete