ecomes
what is called a cement, and its behavior is different from that of
limestones with less clay. Ordinary limestones are, as you know,
calcined in a kiln. The material which comes from the kiln is called
quicklime, and, on being dosed with water, it slakes, and crumbles to
powder, and in the state of slaked lime is mixed up with mortar. Cement
stones are also calcined; but the resulting material will not fall to
pieces or slake under water. It must be ground very fine, and when
moistened sets rapidly, and as well under water as in air, and becomes
very hard and is very tenacious. Brickwork in mortar will always settle
and compress to some extent. Not so brickwork in cement, which
occasionally expands, but is never to be compressed. This quality and
the rapid setting, tenacity, and strength of brickwork in cement make it
a most valuable material to use in those buildings or parts of a
building where great steadiness and strength are wanted, and in sewage
and dock work, where there is water to contend with. A good many cements
made from natural stones used to be employed, such as Medina, Harwich,
Atkinson's, or Roman cement. The last named is the only one which is now
much employed, except locally. It has the quality of setting with
exceptional rapidity, and is on that account sometimes the best material
to employ; but for almost every purpose the artificial compound known as
Portland cement is preferable.

Portland cement is made largely near Rochester. Its materials are simple
and cheap. They may, without much departure from the truth, be said to
be Thames mud and chalk; but the process of manufacture requires care
and thoroughness. The article supplied, when of the best quality, has
great strength, and is quick setting, and is far better than what was
manufactured from stones in which the ingredients existed in a state of
nature. In England we slake our lime and make use of it while it is
fresh; but it may interest you to know that the custom in Italy and
parts of France is different. There it is customary to slake the lime
long before it is wanted, and to deposit it in a pit and cover it up
with earth. In this condition it is left for months--I believe in Italy
for a year--and when taken out it is stiff, but still a pasty substance.
It is beaten, and more water added, and it is then made into mortar with
sand. It is claimed for mortar made in this way that is exceptionally
strong.

Now that we have considered b