52

gradually transformed into carbonic acid and water, whilst
that part of the fibre which remains, in the course between the
first start of decay and final destruction, runs through many
phases of decomposition, and represents in this form that
which we call humus.

The term "putrefying," on the other hand, is applied to
the decay of such organic compounds as are noted for the
disagreeable and penetrating smell they evolve, and this is
the case with all those which contain, besides the above
named elements, nitrogen as a constituent part. Small quan-
tities of sulphur and phosphorus also generally accompany
them. Urea, the chief constituent of urine, may be regarded
as a representative of this class of organic matters, and we
may find at once, that its decay or putrefaction differs some-
what from the process mentioned before, as it does not alto-
gether consist in an oxydation of its carbon, hydrogen and
nitrogen, at the expense of the atmospheric oxygen. Urea
becomes gradually transformed into gaseous carbonic acid
and ammonia, of which the former carries away all the ori-
ginal carbon, whilst the latter embraces the whole of its
nitrogen and hydrogen. But there is originally not enough
of hydrogen in urea so as to combine with the whole of its
nitrogen to form ammonia, and the balance must therefore
necessarily be furnished by some source from without. In
accordance with this, practical observations have shown that
it is water to which this important function has to be attrib-
uted; the presence of moisture being, as we know, one of
the conditions for putrefaction. The water, most remarkably,
suffers a gradual decomposition itself, when in contact with
the putrefying substance, and becomes thus divided into its
elementary constituents, hydrogen and oxygen, of which the
former, at the moment of its being liberated, unites with any
excess of nitrogen to form ammonia, whilst the latter, in
common with the oxygen of the atmospheric air, transforms
the whole of the carbon into carbonic acid.

Returning to barn-yard manure, it is evident that we have
both classes of organic matters, the rotting and putrefying
principles, fairly represented in its composition. There is
the litter-straw of wheat or Indian corn, nearly identical
with wood-fibre; and on the other side, intimately mixed
with the former, the various nitrogenous compounds, (includ-
ing urea,) as they occur in the solid and liquid excrements of
animals.

Left to itself for a sufficiently long time of exposure,
barn-yard manure may, therefore, become wholly converted
into water, carbonic acid and ammonia, and will then,