n chemical characters, but
they have been subdivided into the humic, geic, and crenic groups, which
present some differences in properties and composition. They are
compounds of carbon, hydrogen, and oxygen, and are characterised by so
powerful an affinity for ammonia that they are with difficulty obtained
free from that substance, and generally exist in the soil in combination
with it. They are all products of the decomposition of vegetable
matters in the soil, and are formed during their decay by a succession
of changes, which may be easily traced by observing the course of events
when a piece of wood or any other vegetable substance is exposed for a
length of time to air and moisture. It is then found gradually to
disintegrate with the evolution of carbonic acid, acquiring first a
brown and finally a black colour. At one particular stage of the process
it is converted into one or other of two substances, called humin and
ulmin, both insoluble in alkalies, and apparently identical with the
insoluble humus of the soil; but when the decomposition is more advanced
the products become soluble in alkalies, and then contain humic, ulmic,
and geic acids, and finally, by a still further progress, crenic and
apocrenic acids are formed as the result of an oxidation occurring at
certain periods of the decay.

The roots and other vegetable debris remaining in the soil undergo a
similar series of changes, and form the humus, which is found only in
the surface soil, that is to say, in the portion which is now or has at
some previous period been occupied by plants, and the quantity of humus
contained in any soil is mainly dependent on the activity of vegetation
on it. Numerous analyses of humus compounds extracted from the soil have
been made, and have served to establish a number of minor differences in
the composition even of those to which the same name has been applied,
due manifestly to the fact that their production is the result of a
gradual decomposition, which renders it impossible to extract from the
soil one pure substance, but only a variable mixture of several, so
similar to one another in properties, that their separation is very
difficult, if not impossible. For this reason great discrepancies exist
in the statements made regarding them by different observers, but this
is a matter of comparatively small importance, as their exact
composition has no very direct bearing on agricultural questions, and it
will suffice to