d, and can now be purchased
in liquid form in steel cylinders. The gas is colorless and has a
strong, suffocating odor. It is extremely soluble in water, 1 l. of
water at 0 deg. and 760 mm. pressure dissolving 1148 l. of the gas. In
dissolving this large volume of gas the water expands considerably, so
that the density of the solution is less than that of water, the
strongest solutions having a density of 0.88.

~Chemical properties.~ Ammonia will not support combustion, nor will it
burn under ordinary conditions. In an atmosphere of oxygen it burns with
a feeble, yellowish flame. When quite dry it is not a very active
substance, but when moist it combines with a great many substances,
particularly with acids.

~Uses.~ It has been stated that ammonia can be condensed to a liquid by
the application of pressure. If the pressure is removed from the liquid
so obtained, it rapidly passes again into the gaseous state and in so
doing absorbs a large amount of heat. Advantage is taken of this fact in
the preparation of artificial ice. Large quantities of ammonia are also
used in the preparation of ammonium compounds.

~The manufacture of artificial ice.~ Fig. 36 illustrates the
method of preparing artificial ice. The ammonia gas is
liquefied in the pipes X by means of the pump Y. The heat
generated is absorbed by water flowing over the pipes. The
pipes lead into a large brine tank, a cross section of which is
shown in the figure. Into the brine (concentrated solution of
common salt) contained in this tank are dipped the vessels A,
B, C, filled with pure water. The pressure is removed from
the liquid ammonia as it passes into the pipes immersed in the
brine, and the heat absorbed by the rapid evaporation of the
liquid lowers the temperature of the brine below zero. The
water in A, B, C is thereby frozen into cakes of ice. The
gaseous ammonia resulting from the evaporation of the liquid
ammonia is again condensed, so that the process is continuous.

[Illustration Fig. 36]

~Ammonium hydroxide~ (NH_{4}OH). The solution of ammonia in water is found
to have strong basic properties and therefore contains hydroxyl ions. It
turns red litmus blue; it has a soapy feel; it neutralizes acids,
forming salts with them. It seems probable, therefore, that when ammonia
dissolves in water it combines chemically with it according to the
equation

NH_{3} + H_{2}O = N