- Treussart on Hydraulic Cemenls. B25 



ments of his own, related in a late number of the Memorial de 

 VOfflcier de Genie, states, that he has since then established an 

 important fact, which he had previously been led by Raucourf s 

 remarks to anticipate, with regard to the preparation of artificial 

 pozzolan mortar, or hydraulic cement ; namely, that the access 

 of air, during the calcination of the argillaceous cement, is of 

 great consequence to the tenacity of the mortar, and the quick- 

 ness with which it hardens. He first refers to his former expe- 

 riments (which we have not yet seen) as proving, that, contrary 

 to what is generally supposed, neither the oxide of iron, nor that 

 t)f manganese, nor magnesia, can communicate to lime the pro- 

 perty of hardening under water. He then observes, that, on 

 calcining an argillaceous earth, procured near Frankfort (and 

 consisting of silica and alumina, a 66th part of magnesia, and a 

 trace of iron), and mixing it with half its weight of lime to form 

 a mortar, he found, that, if it had been calcined under free ex- 

 posure to the air, it hardened under water in two or three days, 

 and at the end of a year required a weight varying from 390 

 to 530 pounds to break it ; while, if the clay had been calcined 

 out of reach of the air, the mortar took thirty days to harden, 

 and broke with a weight of 40 or 50 pounds. Analogous re- 

 sults were obtained with a clay from Holzheim, near Strasburg ^ 

 and in this instance he also found that it was useful to mix a 

 50th part of lime with it before calcination. It is not easy to 

 account for these differences ; but the General himself is dispos- 

 ed to ascribe them to the absorption of oxigen by the alumina. 

 In proof of this, he mentions, that the same difference is observ- 

 ed, if, instead of impure clay and lime, the purest alumina, and 

 the lime of white marble, be employed. The alumina, when 

 calcined under a current of air, makes a mortar which hardens 

 sooner, and is much stronger than when the calcination is con- 

 ducted in a close furnace. Another fact in support of his con- 

 jecture is, that alumina, when calcined in the air, dissolves 

 more easily in sulphuric acid. The results of his latest in- 

 vestigations are, that the clay to be chosen for the best hy- 

 draulic mortars should contain a little lime ; that it should be 

 calcined under exposure to a current of air, contrived according 

 to the nature of the furnace ; that, after being reduced to a fine 

 powder, it should be mixed with paste of Hme in the proportion 

 of one of the atter to two, or two and a-half, of the former ; 



