(sulphate of lime) very carefully introduced and intimately mixed. The
result is that the mortar so made sets rapidly, and is very hard.

It is claimed that a larger proportion of sand can be used with
selenitic lime than with ordinary, thus counterbalancing the extra
expense occasioned by royalty under the patent and special care in
mixing. When a limestone contains 20 to 40 per cent, of clay, it becomes
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