compound, and still more that of
ammonia, is so trifling as to be of little practical importance. But a
very simple calculation serves to show that, though relatively small,
they are absolutely large, for the carbonic acid contained in the whole
atmosphere amounts in round numbers to

2,400,000,000,000 tons,

and the ammonia, assuming it not to exceed one part in fifty millions,
must weigh

74,000,000 tons,

quantities amply sufficient to afford an abundant supply of these
elements to the whole vegetation of our globe.

_The Soil as a Source of the Organic Constituents of Plants._--When a
portion of soil is subjected to heat, it is found that it, like the
plant, consists of a combustible and an incombustible part; but while in
the plant the incombustible part or ash is small, and the combustible
large, these proportions are reversed in the soil, which consists
chiefly of inorganic or mineral matters, mixed with a quantity of
combustible or organic substances, rarely exceeding 8 or 10 per cent,
and often falling considerably short of this quantity.

The organic matter exists in the form of a substance called humus, which
must be considered here as a source of the organic constituents of
plants, independently of the general composition of the soil, which will
be afterwards discussed.

The term _humus_ is generic, and applied by chemists to a rather
numerous group of substances, very closely allied in their properties,
several of which are generally present in all fertile soils. They have
been submitted to examination by various chemists, but by none more
accurately than by Mulder and Herman, to whom, indeed, we owe almost all
the precise information we possess on the subject. The organic matters
of the soil may be divided into three great classes; the first
containing those substances which are soluble in water; the second,
those extracted by means of caustic potash; and the third, those
insoluble in all menstrua. When a soil is boiled with a solution of
caustic potash, a deep brown fluid is obtained, from which acids
precipitate a dark brown flocculent substance, consisting of a mixture
of at least three different acids, to which the names of humic, ulmic,
and geic acids have been applied. The fluid from which they have been
precipitated contains two substances, crenic and apocrenic acids, while
the soil still retains what has been called insoluble humus.

The acids above named do not differ greatly i