H_{4}OH,

and that it is the substance NH_{4}OH, called ammonium hydroxide, which
has the basic properties, dissociating into the ions NH_{4} and OH.
Ammonium hydroxide has never been obtained in a pure state. At every
attempt to isolate it the substance breaks up into water and ammonia,--

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

~The ammonium radical.~ The radical NH_{4} plays the part of a metal in
many chemical reactions and is called ammonium. The ending _-ium_ is
given to the name to indicate the metallic properties of the substance,
since the names of the metals in general have that ending. The salts
formed by the action of the base ammonium hydroxide on acids are called
ammonium salts. Thus, with hydrochloric acid, ammonium chloride is
formed in accordance with the equation

NH_{4}OH + HCl = NH_{4}Cl + H_{2}O.

Similarly, with nitric acid, ammonium nitrate (NH_{4}NO_{3}) is formed,
and with sulphuric acid, ammonium sulphate ((NH_{4})_{2}S0_{4}).

It will be noticed that in the neutralization of ammonium hydroxide by
acids the group NH_{4} replaces one hydrogen atom of the acid, just as
sodium does. The group therefore acts as a univalent metal.

~Combination of nitrogen with hydrogen by volume.~ Under suitable
conditions ammonia can be decomposed into nitrogen and hydrogen by
passing electric sparks through the gas. Accurate measurement has shown
that when ammonia is decomposed, two volumes of the gas yield one volume
of nitrogen and three volumes of hydrogen. Consequently, if the two
elements were to combine directly, one volume of nitrogen would combine
with three volumes of hydrogen to form two volumes of ammonia. Here, as
in the formation of steam from hydrogen and oxygen, small whole numbers
serve to indicate the relation between the volumes of combining gases
and that of the gaseous product.

COMPOUNDS OF NITROGEN WITH OXYGEN AND HYDROGEN

In addition to ammonium hydroxide, nitrogen forms several compounds with
hydrogen and oxygen, of which nitric acid (HNO_{3}) and nitrous acid
(HNO_{2}) are the most familiar.

~Nitric acid~ (HNO_{3}). Nitric acid is not found to any extent in nature,
but some of its salts, especially sodium nitrate (NaNO_{3}) and
potassium nitrate (KNO_{3}) are found in large quantities. From these
salts nitric acid can be obtained.

[Illustration Fig. 37]

~Preparation of nitric acid.~ When sodium nitrate is treated with
concentrated cold sulphuric acid, no chemical action s