mortar. The discovery was probably very ancient; and
made, probably like most of the old discoveries, in the East,
spreading Westward gradually. The earlier Greek buildings are
cyclopean, that is, of stone fitted together without mortar. The
earlier Egyptian buildings, though the stones are exquisitely squared
and polished, are put together likewise without mortar. So, long
ages after, were the earlier Roman buildings, and even some of the
later. The famous aqueduct of the Pont du Gard, near Nismes, in the
south of France, has, if I recollect right, no mortar whatever in it.
The stones of its noble double tier of circular arches have been
dropped into their places upon the wooden centres, and stand unmoved
to this day, simply by the jamming of their own weight; a miracle of
art. But the fact is puzzling; for these Romans were the best mortar
makers of the world. We cannot, I believe, surpass them in the art
even now; and in some of their old castles, the mortar is actually to
this day harder and tougher than the stones which it holds together.
And they had plenty of lime at hand if they had chosen to make
mortar. The Pont du Gard crosses a limestone ravine, and is itself
built of limestone. But I presume the cunning Romans would not trust
mortar made from that coarse Nummulite limestone, filled with gritty
sand, and preferred, with their usual carefulness, no mortar at all
to bad.

But I must return, and tell my readers, in a few words, the chemical
history of mortar. 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