ustible and extinguishes flame. It is a singularly
inert substance, and is incapable of directly entering into union with
any other element except oxygen, and with that it combines with the
greatest difficulty, and only by the action of the electric spark--a
peculiarity which has very important bearings on many points we shall
afterwards have to discuss. Nitrogen is found in plants to the extent of
from 1 to 4 per cent.

_Nitric Acid._--This, the most important compound of nitrogen and
oxygen, can be produced by sending a current of electric sparks through
a mixture of its constituents, but in this way it can be obtained only
in extremely small quantity. It is much more abundantly produced when
organic matters are decomposed with free access of air, in which case
the greater proportion of their nitrogen combines with the atmospheric
oxygen. This process, which is known by the name of nitrification, is
greatly promoted by the presence of lime or some other substance, with
which the nitric acid may combine in proportion as it is formed. It
takes place, to a great extent, in the soil in India and other hot
climates; and our chief supplies of saltpetre, or nitrate of potash, are
derived from the soil in these countries, where it has been formed in
this manner. The same change occurs, though to a much smaller extent, in
the soil in temperate climates.

_Ammonia_ is a compound of nitrogen and hydrogen, but it cannot be
formed by the direct union of these gases. It is a product of the
decomposition of organic substances containing nitrogen, and is produced
when they are distilled at a high temperature, or allowed to putrefy out
of contact of the air. In its pure state it is a transparent and
colourless gas, having a peculiar pungent smell, and highly soluble in
water. It is an alkali resembling potash and soda, and, like these
substances, unites with the acids and forms salts, of which the sulphate
and muriate are the most familiar. In these salts it is fixed, and does
not escape from them unless they be mixed with lime, or some other
substance possessing a more powerful affinity for the acid with which it
is united.

_Oxygen_ is one of the most widely distributed of all the elements, and,
owing to its powerful affinities, is the most important agent in almost
all natural changes. It is found in the air, of which it forms 21 per
cent, and in combination with hydrogen, and almost all the other
chemical elements. In the pure