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HYDRAULIC CEMENT.

There are many impure limestones in Texas which can be made into hydraulic cements of more or less efficiency. Argillaceous limestones yield hydraulic lime immediately on burning. Such limestones must contain at least from ten to twelve per cent. of clay. Cements made from such rocks require about twenty days to harden under water or in moist places. Cements made from limerocks having twenty or twenty-five per cent. of clay set in two or three days; and those having thirty per cent. harden in a few hours. This last form of lime is sometimes called Roman cement.

However, every neighborhood which has good limerock and clay has all the elements necessary to make a good hydraulic cement. These artificial cements are now extensively made in Europe and America.

Artificial hydraulic limes were used in a majority of the buildings in Paris, France, where hydraulic lime is made by using four parts of chalk and one part of clay, all front Mendon, a few miles from the city. The clay and chalk. are ground by large wheels revolving in a circular track, made into a paste and formed into bricks, which are dried in the sun, and burnt like hydraulic limestones.

The burning of hydraulic limestone requires peculiar care, because, if the temperature be too great, the silex of the clay is melted, and forms a too close combination with the line, and then it will not form a new compound and make a good cement by the addition of water; hence, the