34 HYDROPLASTICS. [H. 



C. Morfit gives the composition of a fresh oyster-shell, as 

 follows : 



Water.... 2.25 



Organic matter 0.90 



Carbonate of lime 93.89 



Matters soluble in water : 

 Alumina, magnesia, and phosphoric acid 



with lime 0.70 



Chloride of sodium, with traces of sul- 

 phates of soda and lime 2.20 



99.94 

 Hydraulic Cement. — According to 0. Ostermeier (Jahrb. 

 f. Prac. Chem. xiv. 259), when finely powdered marble, lime- 

 stone, or chalk, is mixed to a paste with milk of lime, it har- 

 dens rapidly, like hydraulic lime, has a feeble alkaline reaction, 

 and resists water tolerably well. The mass is plastic and may 

 be used to take large or small impressions. It forms a basic 

 carbonate of lime, or, rather, a hydrocarbonate, which takes 

 up water of crystallization. The analysis of a genuine Roman 

 mortar from Pompeii leads to the inference that the Romans 

 prepared their mortar from a mixture of caustic and carbonate 

 of lime, with the addition of pulverized calcareous spar. 



Kuhlmann's essay, in the Ann. de Chim. et de Phys. Nov. 

 1847, treats of the part performed by potassa and soda in 

 hydraulic cement. He observes that most limestones, of what- 

 ever geological age, contain these alkalies, whence the fertility 

 of a lime soil, and from which we can explain the alkaline 

 efflorescence on newly-constructed walls. He states that a 

 hydraulic cement is made when powdered chalk is moistened 

 with a solution of silicate of potassa (soluble glass) ; that when 

 exposed to the air, it gradually becomes harder than hydraulic 

 cement ; that there is formed some silicate of lime and car- 

 bonate of potassa. When chalk, mixed with water to a dough, 

 is brought in contact with a solution of soluble glass (of soda 

 or potassa), it becomes so hard in a few days as to scratch 

 gome marbles, exhibits a close grain, and admits of a fine 



