_Tests._--In _solution_, arsenic may be detected by the liquid tests.
(1) Ammonio-nitrate of silver gives a yellow precipitate (arsenite of
silver). (2) Ammonio-sulphate of copper gives a green precipitate
(Scheele's green). (3) Sulphuretted hydrogen water gives a yellow
precipitate.
_Marsh's Process._--Put pure distilled water into a Marsh's apparatus
with metallic zinc and sulphuric acid. Hydrogen is set free, and should
be tested by lighting the issuing gas and depressing over it a piece of
white porcelain. If no mark appears, the reagents are pure, and the
suspected liquid may now be added. The hydrogen decomposes arsenious
acid, and forms arseniuretted hydrogen. The gas carried off by a fine
tube is again ignited. A piece of glass or porcelain held to the flame
will have, if arsenic be present, a deposit on it having the following
characters: In the centre a deposit of metallic arsenic, round this a
mixture of metallic arsenic and arsenious acid, and outside this another
ring of arsenious acid in octahedral crystals. The deposit is dissolved
by a solution of chloride of lime, turned yellow by sulphide of ammonium
after evaporation; on the addition of strong nitric acid, evaporated and
neutralized with ammonia and nitrate of silver added, a brick-red colour
is produced--arseniate of silver.
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