The composition of Portland cement is argillocalcareous ; that is, it is formed of clay and limestone, generally containing some silica, the properties of which may vary without injury to the cement The proportion of clay may also vary from 19 to 25 per cent without detriment It is found native at Boulogne, France, in the inferior cretaceous formation The only necessary condition for the formation of a good artificial Portland cement, is an intimate and homogeneous mixture of carbonate of lime and clay, the proportion of clay being as above stated The materials are raised to a white heat in kilns of the proper form, so that they are almost vitrified After the calcination all pulverulent and scorified portions are carefully pricked out and thrown away The remainder is then finely ground and becomes ready for use The amount of water which enters into combination with it in mixing is about 0'3G8 by weight It sets slowly, from 12 to 18 hours being required Ordinary hydraulic or water cements set very quickly, some of them setting in three or four minutes under water at 65 Fah The time of setting varies from this to four hours, according to the characters of the cements They containlarger amounts of silica and alumina than any of the other limes, and also contain magnesia, which Portland cement does not They will not slake after burning, nor shrink in hardening, like the fat limes, and may be used without sand, the latter being only used for economy The slow setting of Portland cement renders its use very convenient for many purposes, as a much larger quantity can be mixed at once than can be done with hydraulic cements When properly made, and this can always be determined by proper tests, it is undoubtedly a durable and strong cement It has been successfully applied to concrete building, road making, lining of iron pipes and cisterns, marine architecture, etc, etc It seems to be growing in favor, and its manufacture is on the increase In England large quantities are made, chalk being the formation from which the carbonate of lime is obtained It has been largely used in the construction of foundations for bridges, some of which have stood from 16 to 18 years, showing no symptom of failure Extensive sea walls have been built with this material in the Mediterranean Mr Hawkshaw, a wellknown engineer of note, says he has used Portland cement in a tideway and has met with no difficulty Neither has he found any difficulty in using concrete blocks in similar situations Many others testify to its good qualities, and there is no doubt in our minds that its use may be greatly extended with advantage