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 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.