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.

Then it must be made to set, that is, to return to limestone, to
carbonate of lime, by drinking in the carbonic acid from water and
air, which some sorts of lime will do instantly, setting at once, and
being therefore used as cements. But the lime usually employed must
be mixed with more or less sand to make it set hard: a mysterious
process, of which it will be enough to tell the reader that the sand
and lime are said to unite gradually, not only mechanically, that is,
by sticking together; but also in part chemically--that is, by
forming out of themselves a new substance, which is called silicate
of lime.

Be that as it may, the mortar paste has now to do two things; first
to dry, and next to take up carbonic acid from the air and water,
enough to harden it again into limestone: and that it will take some
time in doing. A thick wall, I am informed, requires several years
before it is set throughout, and has acquired its full hardness, or
rather toughness; and good mortar, as is well known, will acquire
extreme hardness with age, probably from the very same cause that it
did when it was limestone in the earth. For, as a general rule, the
more ancient the strata is in which the limestone is found, the
harder the limestone is; except in cases where volcanic action and
earthquake pressure have hardened limestone in more recent strata, as
in the case of the white marbles of Carrara in Italy, which are of
the age of our Oolites, that is, of the freestone of Bath, etc.,
hardened by the heat of intruded volcanic rocks.

But now: what is the limestone? and how did it get where it is--not
into the mortar, I mean, but into the limestone quarry? Let me tell
you, or rather, help you to tell yourselves, by leading you, as
before, from the known to the unknown. Let me lead you to places
unknown indeed to most; but there may be sailors or soldiers among my
readers who know them far better than I do. Let me lead you, in
fancy, to some island in the Tr