from volcanoes, and
in solution in the so-called sulphur waters of many springs. It is
formed when organic matter containing sulphur undergoes decay, just as
ammonia is formed under similar circumstances from nitrogenous matter.

~Preparation.~ Hydrosulphuric acid is prepared in the laboratory by
treating a sulphide with an acid. Iron sulphide (FeS) is usually
employed:

FeS + 2HCl = FeCl_{2} + H_{2}S.

A convenient apparatus is shown in Fig. 41. A few lumps of iron sulphide
are placed in the bottle A, and dilute acid is added in small
quantities at a time through the funnel tube B, the gas escaping
through the tube C.

[Illustration: Fig. 41]

~Explanation of the reaction.~ Iron sulphide is a salt of
hydrosulphuric acid, and this reaction is therefore similar to
the one which takes place when sulphuric acid acts upon a
nitrate. In both cases a salt and an acid are brought together,
and there is a tendency for the reaction to go on until a state
of equilibrium is reached. This equilibrium is constantly
disturbed by the escape of the gaseous acid set free, so that
the reaction goes on until all of the original salt has been
decomposed. The two reactions differ in that the first one is
complete at ordinary temperatures, while in the case of
sulphuric acid acting upon sodium nitrate, the reacting
substances must be heated so as to secure a temperature at
which nitric acid is a gas.

~Physical properties.~ Hydrosulphuric acid is a colorless gas, having a
weak, disagreeable taste and an exceedingly offensive odor. It is rather
sparingly soluble in water at ordinary temperatures, about three volumes
dissolving in one of water. In boiling water it is not soluble at all.
In pure form it acts as a violent poison, and even when diluted largely
with air produces headache, dizziness, and nausea. It is a little
heavier than air, having a density of 1.18.

~Chemical properties.~ The most important chemical properties of
hydrosulphuric acid are the following:

1. _Acid properties._ Hydrosulphuric acid is a weak acid. In solution in
water it turns blue litmus red and neutralizes bases, forming salts
called _sulphides_.

2. _Action on oxygen._ The elements composing hydrosulphuric acid have
each a strong affinity for oxygen, and are not held together very
firmly. Consequently the gas burns readily in oxygen or the air,
according to the equation

H_{2