292 OXIDES. 



731. HYDRAULIC CEMENT. If, in the 



WJiat is liy- 



drauic ce- preparation of lime, a limestone is used 

 which contains a certain proportion of 

 clay, a double silicate of alumina and lime is produced. 

 The compound has not alone the property of combi- 

 ning with water, like ordinary lime, but of becoming 

 extremely hard and insoluble in the process. Such a 

 lime is called hydraulic cement, and is used for building 

 under water. Silica, magnesia, and some other sub- 

 stances impart the same property to lime. 



ALUMINA, MAGNESIA, (fee. 



Whatisalu- 732. ALUMINA, &c. Alumina, so named 

 mina ' from the corresponding metal, is insoluble, 



and is called an earth. It is, like the peroxide of iron, 

 a sesquoxide, containing three atoms of oxygen to two 

 of metal. Natural alumina, colored blue, is called sap- 

 phire. Colored red, it forms the oriental ruby. The 

 topaz and the emerald are also compounds containing 

 the same oxide. Baryta, strontia, lime and magnesia, 

 are regarded as standing midway between the earth 

 alumina and the alkalies, and are called alkaline earths. 

 They are more or less soluble, and possess the general 

 properties of the alkalies, in a diminished degree. 

 Magnesia is sometimes classed as an earth. 



733. OTHER METALLIC OXIDES. The 



What are the 



properties of remaining metallic oxides are powders of 

 lalhfoxiL% differ ent colors. Most of them are insol- 

 uble. The more important have been 

 already noticed, in the Chapter on Metals. Their 



