21 5 
the Arseniates of Coppery and of Iron . 
I have mentioned. M. Lavoisier obtained his proportions 
by combustion ; and, admitting even that nothing was lost, it 
must have been rather difficult to obtain the sulphuric acid 
in a state proper to value the quantity. Indeed I do not know of 
any direct experiments which prove, in a satisfactory manner, 
that we have ever obtained that acid perfectly free from water ; 
unless when combined with an earth, or an alkali, in some salt, 
and that salt calcined in a very strong red heat. 
To ascertain the quantity of metallic arsenic in mispickel, 
arsenical pyrites, &c. the most advantageous method is, to 
acidify it by nitric acid, and then to combine it with oxide of 
lead. This arseniate of lead (containing, as was before said, 
33,2 percent, of acid,) may be estimated to contain 22 percent, 
of metallic arsenic. If both sulphur and arsenic are present, 
lead may be equally serviceable, after both have been acidified ; 
for sulphate of lead is not materially soluble in any acid; whereas, 
on the contrary, arseniate of lead is very much so. 
When copper and iron are to be separated, one single affu- 
sion of ammonia will not always suffice. That two may be 
sometimes necessary, is an objection to the method I propose, for 
the subsequent ebullition with potash. But, when I use that of 
precipitating the copper by iron, it requires no previous precipi- 
tation by any alkali. It is sufficient to add muriatic acid to the 
original solution of the ore in nitric acid, and evaporate to 
dryness. The nitric acid is dislodged from the oxide of cop- 
per, and the muriatic takes its place. If a single evaporation 
is not sufficient, a second (for the operation is very short, and 
causes no loss upon fibres, &c.) may be attempted ; and, when 
the iron, used for the purpose of revivifying the copper, is put 
in, the liquor may be made to boil ; by which means, the process 
