47o 
Emerald-Green : 
[August, 
higher or lower atmospheric temperature having no effedt in 
hastening or retarding the formation. Furthermore,, it is 
not stated whether the compound forms more readily in an 
acid or neutral solution, or whether it can or cannot be 
formed in a neutral one ; now both these points are im- 
portant to notice in describing its manufacture. As regards 
the former I shall notice it presently, and, as far as my 
knowledge extends, the pigment will not form when the 
solution is neutral. 
The operation is conducted in the following manner in the 
factory: — The requisite quantity of sulphate of copper is 
placed in a large wooden vat, and hot water added to dis- 
solve it ; the requisite quantity of arsenic (arsenious anhy- 
dride) and carbonate of soda, the latter not in quantity quite 
sufficient to neutralise the whole 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 afterwards 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 unsaleable, 
owing to its dirty yellowish green colour. In cold weather 
a much longer time is required for its complete conversion ; 
even at the end of a fortnight or three weeks there fre- 
quently remains sufficient unconverted arsenite to affedt 
seriously the selling price of the colour : 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 utilised, but is 
run into the fadtory 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 : — Alter having 
