36 HYDROPLASTICS. [H, 



Jacohi. Grashof. 



Silica 40.12 40.44 



Alumina 15.37 15.38 



Lime 36.02 33.10 



Protoxide of manganese. 5.80 4.40 



" iron 1.25 1.63 



Potassa 2.25 2.07 



Sulphur 0.70 0.76 



101.51 97.78 



From these results, Eisner has deduced the formula 2(3CaO, 

 Si03+A],03, Si03) + 3CaO, 2Si03. 



See essays by Eisner on Puzzolan, &c., in Journ. f. Pract. 

 Chem. 1844-1845, and on slags of blast-furnaces as hydraulic 

 cements, in Verhandl. d. Gewerbv. f. Preussen, 1847. Those 

 slags, decomposable by muriatic acid, are chiefly applicable to 

 cements. The best method of testing them is to pulverize a 

 piece very finely, and pour over it strong muriatic acid. If 

 it become gelatinous in a short time, it is adapted to the pur- 

 pose. A slag which was proved to be good for making the 

 cement, had the composition : Silica, 40.12 ; alumina, 15.37 ; 

 lime, 36.02 ; protoxide of manganese, 5.80 ; protoxide of iron, 

 1.25; potassa, 2.25; sulphur, 0.70 =101.51. 



Plaster, or gypsum, may be boiled or deprived of its mois- 

 ture by highly-heated steam, as described by Violette. (See 

 Lend. Journ. p. 424, 1849.) See the apparatus for charring 

 described in the present Report, under Pyrotechny, which is 

 varied for adaptation to gypsum. 



Plaster hardened hy Salts. — Boiled plaster, when mixed with 

 a solution of alum, becomes remarkably hard, as shown by 

 Eisner, (Verb. d. Gewerbv. in Preussen, 1843.) A solution 

 of 1 pt. borax in 9 pts. water has the same effect (Keating.) 

 Gay-Lussac observes that raw, unboiled plaster in fine pow- 

 der becomes similarly hard when mixed with solutions of car- 

 bonate and bicarbonate, sulphate and bisulphate of potassa, 

 and even caustic potassa. Soda salts, nitrate and muriate of 

 potassa, are ineffective. 



