If limestone be burnt, or rather roasted, in a
kiln, the carbonic acid is given off--as you may discover by your own
nose; as many a poor tramp has discovered too late, when, on a cold
winter night, he has lain down by the side of the burning kiln to
keep himself warm, and woke in the other world, stifled to death by
the poisonous fumes.
The lime then gives off its carbonic acid, and also its water of
crystallisation, that is, water which it holds (as do many rocks)
locked up in it unseen, and only to be discovered by chemical
analysis. It is then anhydrous--that is, waterless--oxide of lime,
what we call quick-lime; that which figured in the comi-tragedy of
"Flour-bag Creek;" and then, as you may find if you get it under your
nails or into your eyes, will burn and blister like an acid.
This has to be turned again into a hard and tough artificial
limestone, in plain words, into mortar; and the first step is to
slack it--that is, to give it back the water which it has lost, and
for which it is as it were thirsting. So it is slacked with water,
which it drinks in, heating itself and the water till it steams and
swells in bulk, because it takes the substance of the water into its
own substance. Slacked lime, as we all know, is not visibly wetter
than quick-lime; it crumbles to a dry white powder in spite of all
the water which it contains.
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