The wonderful powers of durance which some mortars possess is to be explained with ease ; but before doing so, let us recollect that the mortar and cement found in Herculaneum and Pompeii, now nearly two thousand years old, is as hard and compact as the volcanic rock on which it is found ; and there are many specimens of cements in the museums of Europe, that, after having been under water for centuries, are as good, if not better, than when put down Recollecting also the vast importance of good hydraulic cements in the construction of lighthouses, breakwaters and piers, and all submarine works, perhaps more attention may be given to the subject than otherwise would by noninterested readers These hydraulic cements are such as set under water, and are not decomposed by its action like ordinary mortars They are made either from natural or artificial mixtures of carbonate of lime with silica, or silicate of alumina or magnesia The mineral dolomite, when calcined at a moderate heat, exhibits the property of hydraulic lime; and halfburnt lime (containing still a quantity of carbonic acid,) will set under water From a French engineerM Vicatwe learn that the hardening depends much on the amount of carbonic acid left in the lime ; thus he informs us that a stone that had thirty per cent of carbonic acid left in it after burning, hardened in fifteen minutes, while another, in which there was twentysix per cent, hardened in seven minutes, and one containing twentythree per cent, took nine days to become hard Two varieties in Europe are known as Trass and Puzzolana ; and there is an hydraulic mortar used in England known as "Roman eement," made by burning some nodules found in the tertiary formation Neither clay, (silicate of alumina,) nor lime alone, will set under water, but if an intimate mixture of clay and chalk be calcined at a moderate heat, and afterwards mixed with water, a hydrated silicate of alumina and lime is formed as a hard mass, and this is hydraulic cement If the clay or limestone should contain a little alkali, it seems to aid the solidification There is an excellent cement made near Paris from one part of clay and four of chalk, which are intimately mixed with water, afterwards allowed to settle, and the deposit thus obtained is molded into bricks, which are then dried and calcined at a gentle heat This hydraulic lime, like the best from natural sources, is entirely dissolved by acids All mortars, but especially hydraulic ones, are solidified quicker and better under the influences of pressure andhigh temperature When an hydraulic cement is required, it is advisable to collect specimens of the minerals of the district in which work is to be carried on, and send them to some chemist for analysis This will, in many instances, save much time and money, for we have known cases where Roman cements and other hydraulic cements, have been brought from a great distance to carry on a work, quite close to which there was plenty only wanting the trouble of burning Hi