o a great heat, such as is
practically done on a large scale in lime-kilns, it is converted into
caustic lime or lime proper. Limestone is made up, as we have just
mentioned, of lime and carbonic acid. The latter ingredient is expelled
in the form of a gas, and the lime is left behind. Lime never occurs
naturally as caustic lime, for the simple reason that it is impossible
for it to remain in this state, owing to the great affinity it has both
for water and carbonic acid.

When lime is burnt, and before it is applied to the field, some time is
allowed to elapse in order to permit of its absorbing moisture--or
becoming slaked, as it is technically called. This it does more or less
slowly by absorbing moisture from the air. As, however, the process
would take too long, and as, moreover, the absorption of carbonic acid
gas would also take place at the same time, lime is generally slaked in
another way. This can be done by simply adding water. An objection to
this method is, that the lime is not so uniformly slaked as is
desirable. It becomes gritty. The usual method is to cover it up with
damp earth in heaps, and allow the moisture of the earth to effect the
slaking. When lime absorbs water a new chemical compound is formed,
known as lime hydrate; and so rapidly does the lime unite with water,
that a great deal of heat is evolved in the operation, the temperature
produced being considerably above that of boiling-water. The conversion
of slaked lime into carbonate of lime or mild lime is a slower process.
Sooner or later, however, it takes place, whether the lime is left on
the surface of the soil or buried in it.

A knowledge of these elementary chemical facts is necessary in order
clearly to understand the nature of the action of lime in agriculture.

The respective action of quicklime and mild lime is, on the whole,
similar, although the former is in every case very much more powerful
in its effects than the latter.

_Lime acts both mechanically and chemically._

Lime may be said to act on the soil both mechanically and chemically. It
alters the texture of the soil, and affects its mechanical properties,
such as its absorptive, retentive, and capillary powers with regard to
water. It acts upon its dormant fertility, and decomposes its mineral
substances as well as its organic matter. Lastly, its influence on the
micro-organic life of the soil, which plays such an important part in
the preparation and elaboration o