HYDRAULIC CEMENT. 35 



polish. Only 3-4 per cent, of silica, absorbed bj the chalk, 

 impart these properties. This material is well adapted to 

 sculpture and various ornaments. Plaster of Paris (sulphate 

 of lime), treated with soluble glass, is similarly silicated, and 

 even plaster casts become hard and smooth on the surface. 

 But the solution must be very dilute, or otherwise the surface 

 cracks and scales off. If the articles to be hardened are to 

 be exposed to the weather, the glass must be made with potassa, 

 and not with soda, as the latter is more apt to effloresce. (See 

 also Journ. Fr. Inst. (3) xvii. 201.) 



The carbonic acid of the air acts an important part in the 

 induration of these compounds, by abstracting the alkali of 

 the silicate, and thus freeing the silica, which, by contracting, 

 promotes the solidification. 



Some of the most important principles in Kuhlmann's essay 

 were published by Fuchs, in his excellent essays on lime and 

 mortar (Erdmann's Journ. of Techn. Chem. vi.) ; on the pro- 

 perties and constituents of hydraulic cements (Polytech. 

 Journ. xlix. 271) ; on soluble glass (Polytech. Journ. xvii. 

 465), &c. 



A good essay on the action of carbonic acid in hydraulic 

 cements, by Villeneuve, will be found in the Lend. Journ. 

 Sept. 1850. 



Prechtel gives the following simple mode of making hy- 

 draulic cement. Common burned lime is slacked with a solution 

 of copperas, instead of with water, and then mixed with sand. 

 (It may also be used without sand.) It hardens readily in the 

 air or under water, and becomes very hard. Experiments 

 made with it on a large scale proved very satisfactory. When 

 freshly prepared, it has a greenish color, from the segregation 

 of protoxide of iron, passing into peroxide, when its color is 

 yellowish ; and to this oxidation Prechtel ascribes its harden- 

 ing property. Sulphate of lime is also formed at the same 

 time, and probably a double carbonate. (Polytech. Notizbl. 

 1846.) 



A slag, from an iron furnace, which forms, with lime, a hy- 

 draulic cement, has been found to consist of: 



