accounted for when it is known that this compound is a gas composed of
nitrogen and hydrogen. It has already been explained that coal contains
from one to two per cent. of nitrogen, and during the process of
distillation about one-fifth of this nitrogen is converted into ammonia,
the remainder being converted partly into other bases, while a small
quantity remains in the coke.

Ammonia, the "volatile alkali" of the old chemists, and its salts are of
importance in pharmacy, but the chief use of this compound is to supply
nitrogen for the growth of plants. Plants must have nitrogen in some form
or another, and as they cannot assimilate it _directly_ from the
atmosphere where it exists in the free state, some suitable nitrogen
compound must be supplied to the soil. It is possible that certain
leguminous plants may derive their nitrogen from the atmosphere through
the intervention of micro-organisms, which appear capable of fixing free
nitrogen and of supplying it to the plant upon whose roots they flourish.
But this is second-hand nitrogen so far as concerns the plant. It is true
also that the atmosphere contains small traces of ammonia and acid oxides
of nitrogen, which are dissolved by rain and snow, and thus get washed
down into the soil. These are the natural sources of plant nitrogen. But
in agricultural operations, where large crops have to be raised as rapidly
as possible, some additional source of nitrogen must be supplied, and
this is the object of manuring the soil.

A manure, chemically considered, is a mixture of substances capable of
supplying the necessary nitrogenous and mineral food for the nourishment
of the growing plant. The ordinary farm or stable manure contains
decomposing nitrogenous organic matter, in which the nitrogen is given off
as ammonia, and thus furnishes the soil with which it is mixed with the
necessary fertilizer. But the supply of this manure is limited, and we
have to fall back upon gas-liquor and native nitrates to meet the existing
wants of the agriculturist. Important as is ammonia for the growth of
vegetation, it is not in this form that the majority of plants take up
their nitrogen. Soluble nitrates are, in most cases, more efficient
fertilizers than the salts of ammonia, and the ammonia which is supplied
to the soil is converted into nitrates therein before the plant can
assimilate the nitrogen. The oxidation of ammonia into nitric acid takes
place by virtue of a process called