t section of this book,
lime usually exists in nature, in the form of carbonate of lime, as
limestone, chalk, or marble (being lime and carbonic acid combined), and
when this is burned, the carbonic acid is thrown off, leaving the lime
in a pure or caustic form. This is called burned lime, quick-lime, lime
shells, hot lime, etc. If the proper quantity of water be poured on it,
it is immediately taken up by the lime, which falls into a dry powder,
called _slaked lime_. If _quick-lime_ were left exposed to the weather,
it would absorb moisture from the atmosphere, and become what is termed
_air slaked_.

[What is air slaking?

If slaked lime be exposed to the air, what change does it undergo?

What is the object of slaking lime?

How much carbonic acid is contained in a ton of carbonate of lime?

How much lime does a ton of slaked lime contain?

What is the most economical form for transportation?]

When _slaked lime_ (consisting of lime and water) is exposed to the
atmosphere, it absorbs carbonic acid, and becomes carbonate of lime
again; but it is now in the form of a very fine powder, and is much more
useful than when in the stone.

If quick-lime is applied directly to the soil, it absorbs first
moisture, and then carbonic acid, becoming finally a powdered carbonate
of lime.

One ton of _carbonate of lime_ contains 11-1/4 cwt. of lime; the
remainder is carbonic acid. One ton of _slaked lime_ contains about 15
cwt. of lime; the remainder is water.

Hence we see that lime should be burned, and not slaked, before being
transported, as it would be unprofitable to transport the large quantity
of carbonic acid and water contained in carbonate of lime and slaked
lime. The quick-lime may be slaked, and carbonated after reaching its
destination, either before or after being applied to the land.

[What is the best form for immediate action on the inorganic
matter in the soil?

For most other purposes?]

As has been before stated, much is gained by slaking lime with _salt
water_, thus imitating the lime and salt mixture. Indeed in many cases,
it will be found profitable to use all lime in this way. Where a direct
action on the inorganic matters contained in the soil is desired, it may
be well to apply the lime directly in the form of quick-lime; but, where
the decomposition of the vegetable and animal constituents of the soil
is desired, the correction of _sourness_, or the supplying of lime to
the crop, the mixt