ufacture of coal gas. As will be shown later in the chapter,
the group NH_{4} in this compound acts as a univalent radical and is
known as _ammonium_. When ammonium chloride is warmed with sodium
hydroxide, the ammonium and sodium change places, the reaction being
expressed in the following equation.

NH_{4}Cl + NaOH = NaCl + NH_{4}OH.

The ammonium hydroxide (NH_{4}OH) so formed is unstable and breaks down
into water and ammonia.

NH_{4}OH = NH_{3} + H_{2}O.

Calcium hydroxide (Ca(OH)_{2}) is frequently used in place of the more
expensive sodium hydroxide, the equations being

2NH_{4}Cl + Ca(OH)_{2} = CaCl_{2} + 2NH_{4}OH,

2NH_{4}OH = 2H_{2}O + 2NH_{3}.

In the preparation, the ammonium chloride and calcium hydroxide
are mixed together and placed in a flask arranged as shown in
Fig. 35. The mixture is gently warmed, when ammonia is evolved
as a gas and is collected by displacement of air.

[Illustration: Fig. 35]

2. _Commercial method._ Nearly all the ammonia of commerce comes from
the gasworks. Ordinary illuminating gas is made by distilling coal, as
will be explained later, and among the products of this distillation a
solution of ammonia in water is obtained. This solution, known as _gas
liquor_, contains not only ammonia but other soluble substances. Most of
these combine chemically with lime, while ammonia does not; if then lime
is added to the gas liquor and the liquor is heated, the ammonia is
driven out from the mixture. It may be dissolved again in pure, cold
water, forming _aqua ammonia_, or the ammonia water of commerce.

~Preparation from hydrogen and nitrogen.~ When electric sparks
are passed for some time through a mixture of hydrogen and
nitrogen, a small percentage of the two elements in the mixture
is changed into ammonia. The action soon ceases, however, for
the reason that ammonia is decomposed by the electric
discharge. The reaction expressed in the equation

N + 3H = NH_{3}

can therefore go in either direction depending upon the
relative quantities of the substances present. This recalls the
similar change from oxygen into ozone, which soon ceases
because the ozone is in turn decomposed into oxygen.

~Physical properties.~ Under ordinary conditions ammonia is a gas whose
density is 0.59. It is therefore little more than half as heavy as air.
It is easily condensed into a colorless liqui