ere originally derived.

[What is the cause of odor?

When we perceive an odor, what is taking place?

Why do manures give off offensive odors?

How may we detect ammonia escaping from manure?]

It must be recollected that every thing, which has an _odor_ (or can be
smelled), is evaporating. The odor is caused by parts of the body
floating in the air, and acting on the nerves of the nose. This is an
invariable rule; and, when we perceive an odor, we may be sure that
parts of the material, from which it emanates, are escaping. If we
perceive the odor of an apple, it is because parts of the volatile oils
of the apple enter the nose. The same is true when we smell hartshorn,
cologne, etc.

Manures made by animals have an offensive odor, simply because volatile
parts of the manure escape into the air, and are therefore made
perceptible. All organic parts in turn become volatile, assuming a
gaseous form as they decompose.

We do not see the gases rising, but there are many ways by which we can
detect them. If we wave a feather over a manure heap, from which ammonia
is escaping, the feather having been recently dipped in manure, white
fumes will appear around the feather, being the muriate of ammonia
formed by the union of the escaping gas with the muriatic acid. Not only
ammonia, but also carbonic acid, and other gases which are useful to
vegetation escape, and are given to the winds. Indeed it may be stated
in few words that all of the organic part of _plants_ (all that was
obtained from the air, water, and ammonia), constituting more than nine
tenths of their dry weight, may be evaporated by the assistance of decay
or combustion. The organic part of _manures_ may be lost in the same
manner; and, if the process of decomposition be continued long enough,
nothing but a mass of mineral matter will remain, except perhaps a small
quantity of carbon which has not been resolved into carbonic acid.

[What remains after manure has been long exposed to
decomposition?

What gaseous compounds are formed by the decomposition of manures?]

The proportion of solid manure lost by evaporation (made by the
assistance of decay), is a very large part of the whole. Manure cannot
be kept a single day in its natural state without losing something. It
commences to give out an offensive odor immediately, and this odor is
occasioned, as was before stated, by the loss of some of its fertilizing
parts.

Animal manure contains, as will be