The experiments of Dr. Higgins and Mr. Smeaton
have proved, that if chalk and stone limes be used
equally fresh, their cementitious properties will be
equal ; but as quick-lime absorbs carbonic acid with
greater or less rapidity, in proportion to its spongy or
Bolid texture, so it gradually parts with its cementing na^
ture, and at length becomes totally unfit for the purpose
of mortar. Hence, though stone and chalk limes be
equally good when perfectly burnt and fresh from the
kiln, yet there is an important practical difference be-
tween them, because the latter takes in the carbonic
acid with much greater facility than stone lime. In the
metropolis, there is now no excuse for the use of chalk
lime, except for the commonest purposes. It is re-
ceived from Kent and Essex, and often lies at the dif-
ferent wharfs under open sheds long enough to lose
evfery good property it originally possessed : whereas



50 LIME, SAND, SECT. I.
the stone lime may be had at a short distance from the
inetropolis, not only in abundance, but of the best
quality.
