206 On a new Method of obtaining pure Silver. 
therefore precipitated. But the copper of commerce always 
contains other metals ; hence we must examine if any of them 
inquinate the product. Lead, antimony and arsenic have been 
found in copper. The first and last dissolve in the nitrous acid. 
Antimony scarcely dissolves in dilute nitric acid, but { find that 
when alloyed with alarge portion of copper it dissolves with 
sufficient facility. Hence none of the above metals will be found 
in the reduced silver. When copper is dissolved in strong nitric 
acid, the solution when saturated lets fall a brown powder 
which Fourcroy considered oxide of copper. I collected a small 
quantity of this powder, heated it to a bright red, and found it 
magnetic. Its saturated solution also struck a black with tinc- 
ture of galls. This powder is therefore iron, and probably a 
sub-oxynitrate. But when copper is precipitating the silver from 
its nitrous solution, the iron, as I have found, will remain dis- 
solved, provided that no heat be used, and that the acid used in 
forming the nitrate did not exceed s. G, 1:2. 
The only metal, therefore, which adulterates the precipitated 
silver is copper, proceeding no doubt, as Vauquelin observes, 
from the galvanic action between the precipitant and precipitated 
metals. It might be doubted that ammonia will dissolve the 
copper, as it is commonly supposed that this metal in its me- 
tallic state is very little, or not at all, acted on by that alkali. The 
observation, however, does not apply to copper minutely divided. 
I boiled ammonia on copper powder precipitated from its solu- 
tion by iron, and well cleansed by washing with dilute muriatic 
avid: the ammonia in a minute or two assumed a deep blue 
colour. The silver resulting from the above described process 
does not afford the least trace of copper to the strictest scrutiny. 
if the solution of 240 grains of silver be made to amount to two 
ounces measure, the silver precipitates speedily when the copper 
is immersed; and the particles are large enough to subside, and 
to permit the pouring off of the washings without loss, 
The quantity of copper necessary to precipitate 100 grains of 
pure silver is 28-7 grains, according to a great number of trials 
which all agreed. Bergman makes it 31: but in this case the 
difference is of no consequence. 
The solution from which the silver has been separated af- 
fords no precipitate with muriate of soda: hence the separation 
is complete: and I find that, with a little care, the loss of silver 
by filtration and decantation need not exceed 3 per cent. 
The boiling with ammonia merely takes up the copper: when 
ammonia is boiled on silver, even in powder, and afterwards 
saturated with muriatic acid, the transparency of the fluid is 
scarcely impaired. 
I have been thus minute in my account of so simple a process, 
because 
