of the sulphuric acid set free
from the sulphate of copper on the precipitation of the copper as
arsenite, are placed in another wooden vessel; water is then added, and
the formation of the arsenite of soda and its solution are aided by the
introduction of steam into the liquid. When complete solution has been
effected the arsenic solution is run off into the vat containing the
solution of the sulphate of copper, arsenite of copper being at once
precipitated. The necessary quantity of acetic acid is afterward added. In
_warm_ weather the formation of the aceto-arsenite soon commences after
the addition of the vinegar; but, even in that case, it takes a week or
more to have the whole of a big batch of arsenite converted into the
aceto-arsenite; and perfect conversion is necessary, as the presence of a
very minute quantity of unchanged arsenite lowers very much the price of
the emerald pigment, and a by no means large quantity renders the pigment
unsalable, owing to its dirty yellowish-green color. In cold weather a
much longer time is required for its complete conversion; even at the end
of a fortnight or three weeks there frequently remains sufficient
unconverted arsenite to affect seriously the selling price of the color;
when this occurs the manufacturer generally removes these last traces by a
most wasteful method viz, by adding a quantity of free sulphuric acid. The
acid of course dissolves the arsenite, but it dissolves in very much
larger quantities the aceto-arsenite; and this costly solution is not
utilized, but is run into the factory sewer.

By my method of manufacturing it, it can be produced in winter as well as
in summer in one or two hours, and the quantity of free acid required for
its formation is reduced to the lowest amount. I proceed as follows: After
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