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heat should only be sufficient to expel the carbonic acid from the lime and the water in the clay.

Small quantities of magnesia and iron are in many hydraulic limestones, but these are also in many clays which often have a reddish brown color from the presence of iron.

Hydraulic mortars are also made by mixing burnt lime with certain porous rocks resembling in composition burnt clay, such as the well known pozzuolana, formerly exported entensively from Italy for making hydraulic limes. It occurs at Pozzuoli, near Naples. A majority of volcanic tufas possess similar properties. Most burnt clays, when not too highly calcined, are good substitutes for pozzuolana ; as also are common bricks, tiles and common earthenware pounded. Mortars made from burnt lime and pozzuolana are very hard and durable, as is shown in the remains-of buildings at Rome, where the cement has outlasted the rocks used in their construction. The old Roman, Vitruvius, gives the following directions for making this cement:

The Rev. Mr. Wilson, principal of the academy at Seguin, informs me by letter that the late Dr. Park, formerly living there, made a very good hydraulic cement by burning the clays in that neighborhood, and mixing them with the proper proportion of burnt lime.

The following are the analyses of some of the best hydraulic cements of France and of the United States: