34 



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 ex- 

 ported entensively from Italy for making hydraulic limes. 

 It occurs at Pozzuoli, near Naples. A majority of vol- 

 canic 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, Yitruvius, gives the following directions for 

 making this cement : 



Pozzuolana, well pulverized 12 parts. 



Quartoze sand 6 " 



Rich lime, recently slaked 9 " 



Fragments of broken stone 6 " 



The Rev. Mr. Wilson, principal of the academy at 

 Seguin, informs me by letter that the late JDr. Park, for- 

 merly 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 : 



METZ. 



Carbonate of lime 77.3 



magnesia 03.0 



" iron 03.0 



" manganese 01.5 



Clay or silex 15,2 



