to
phosphine and ammonia, is formed. The reaction when oxide of arsenic is
so treated is

As_{2}O_{3} + 12H = 2AsH_{3} + 3H_{2}O.

Arsine is a gas with a peculiar garlic-like odor, and is intensely
poisonous. A single bubble of pure gas has been known to prove fatal. It
is an unstable compound, decomposing into its elements when heated to a
moderate temperature. It is combustible, burning with a pale
bluish-white flame to form arsenic trioxide and water when air is in
excess:

2AsH_{3} + 6O = As_{2}O_{3} + 3H_{2}O.

When the supply of air is deficient water and metallic arsenic are
formed:

2AsH_{3} + 3O = 3H_{2}O + 2As.

These reactions make the detection of even minute quantities of arsenic
a very easy problem.

[Illustration: Fig. 72]

~Marsh's test for arsenic.~ The method devised by Marsh for
detecting arsenic is most frequently used, the apparatus being
shown in Fig. 72. Hydrogen is generated in the flask A by the
action of dilute sulphuric acid on zinc, is dried by passing
over calcium chloride in the tube B, and after passing
through the hard-glass tube C is ignited at the jet D. If a
substance containing arsenic is now introduced into the
generator A, the arsenic is converted into arsine by the
action of the nascent hydrogen, and passes to the jet along
with the hydrogen. If the tube C is strongly heated at some
point near the middle, the arsine is decomposed while passing
this point and the arsenic is deposited just beyond the heated
point in the form of a shining, brownish-black mirror. If the
tube is not heated, the arsine burns along with the hydrogen at
the jet. Under these conditions a small porcelain dish crowded
down into the flame is blackened by a spot of metallic arsenic,
for the arsine is decomposed by the heat of the flame, and the
arsenic, cooled below its kindling temperature by the cold
porcelain, deposits upon it as a black spot. Antimony conducts
itself in the same way as arsenic, but the antimony deposit is
more sooty in appearance. The two can also be distinguished by
the fact that sodium hypochlorite (NaClO) dissolves the arsenic
deposit, but not that formed by antimony.

~Oxides of arsenic.~ Arsenic forms two oxides, As_{2}O_{3} and
As_{2}O_{5}, corresponding to those of phosphorus. Of these arsenious
oxide, or arsenic trioxide (As_{2}O_{3}), is much