64 
SELECTED ARTICLES. 
when we pass the hydrogen gas, liberated from a liquid con- 
taining arsenic, through a tube, one line in width, and of glass 
difficult of fusion, heated to a redness by means of a spirit 
lamp, for a space of two inches from the orifice, where the gas 
is disengaged; we will then see all the arsenic deposited in 
the form of a bright black metallic ring, beyond the red part 
of the tube. It is not necessary to hurry the disengagement 
of the gas, as we would under other circumstances, to give a 
certain size to the flame of the gas; on the contrary, it is pre- 
ferable that it should form as slowly as possible. We have 
then a particular advantage, that the particles of foreign metals 
carried off shall be reduced in the red part, or be disposed in 
the form of black rings, which cannot be driven from place to 
place, like arsenic, by a low red heat. 
I made some experiments to reduce in a similar manner 
the sulphuret of arsenic, and obtained very satisfactory results. 
All chemists agree, that arsenic is separated with more 
certainty and completeness from an acid liquid, by means of 
sulphuretted hydrogen, since we cannot always, by the method 
of Rose, prevent the formation of ammonia during the boiling 
of the animal matters. On saturating the alkali with hydro- 
chloric acid, a salt of ammonia is always formed, which 
hinders, as is known, the precipitation of thearseniate of lime. 
When we have obtained, by the method of Berzelius, a sul- 
phuretted precipitate from a similar liquid, the solution of 
the question then consists in disengaging the arsenic with all 
its properties. 
If we dissolve this precipitate in a solution of potassa, and 
add to the liquor acetate of lead, taking care that the alkali 
shall remain in excess, the sulphur of the sulphuret of arsenic 
will unite with the lead, and all the arsenic will be found in 
the solution in the state of arsenious acid. The salt of lead 
should be added until the liquid no longer gives any precipitate 
with acetic acid; that is, until the sulphuret of arsenic has 
undergone the decomposition pointed out above. If we now 
add to this liquor, without separating the sulphuret of lead, an 
excess of weak sulphuric acid, and place in it a piece of zinc, 
