SEPARATION OF ARSENIC. 
65 
and pass the disengaged hydrogen through a narrow glass 
tube, heated to redness in one place, the characteristic film of 
arsenic under the form of a black metallic ring, will not fail 
to appear in the cold part of the tube. This trial gives a 
positive result with .0077 of a grain of sulphuret of arsenic. 
Besides, by this process, there always remains united to the 
zinc, a certain quantity of arsenic which naturally escapes 
this reaction. I believe that I can recommend, in the majority 
of cases, the following method as quite certain, according to 
the results which I have obtained. Dissolve the sulphuretted 
precipitate in a solution of potassa, (if the quantity is so small 
that the sulphuret cannot be detached from the filter, it is to 
be moistened with the solution of potassa,) and add to this 
solution nitrate of silver, until a single drop of the liquid 
will not give a yellow precipitate with acetic acid; the 
potassa should always be in excess in the liquid. The in- 
soluble sulphuret of silver is thus formed, while the arsenic 
remains in solution in the state of arseniate of potassa ; 
hydrochloric acid is to be added to this liquid in a slight 
excess, and the sulphuret and chloride of silver separated by 
a filter; then heat to boiling to separate free carbonic acid, 
and saturate with lime water. The precipitate formed, is the 
arseniate of lime. This having been carefully dried, is mixed 
with powdered charcoal, heated to redness, and reduced by 
the known method. 
In place of precipitating the slightly acid liquor by lime 
water, it may be evaporated to dryness by a salt water bath, 
the residue mixed with charcoal, and the arsenic obtained by 
a red heat. Instead of using nitrate of silver, we may employ 
the nitrate of copper with the same result; only we should 
then, before saturating with hydrochloric acid, separate the 
sulphuret and oxide of copper by the filter. 
Jonrn. de Pharmacie, 
VOL. IV. — no. r. 
9 
