156 PRACTICAL LESSONS IN SCIENCE. 



plaster of Paris, and at a little distance they have the appearance 

 of stone. Stucco is plaster of Paris sometimes colored, and some 

 cements are made by mixing alum with the plaster. Its property 

 of quickly assuming a solid condition makes it an interesting 

 and very valuable substance. It is interesting to note that 

 plaster of Paris cannot be used a second time, indicating that it 

 is not the same as the gypsum. 



Calcium chloride (CaCl 2 ) is a crystalline substance which has a 

 strong affinity for water and is much used as a drying agent in 

 chemical work. The bleaching powder, called chloride of lime, is 

 an impure hypochlorite of lime which depends for its action on 

 the chlorine it contains. 



MORTAR is a compound of one part of freshly slacked lime 

 and two or three parts of clean sand mixed with water enough 

 to form a stiff paste. When this mixture is exposed to the air 

 the water evaporates, carbon dioxide from the air combines 

 with the calcium oxide forming the calcium carbonate. In this 

 way the mortar becomes very hard so that however massive the 

 structure the mortar can bear its share of the load. 



COMMON GLASS is a silicate of calcium and sodium made by 

 melting together sand (silica) and the carbonates of sodium 

 and calcium. Bohemian glass, used in the manufacture of chem- 

 ical apparatus is a silicate of calcium and potassium. It is not 

 as easily fusible as common glass. Plate glass contains some 

 potassium silicate in addition to the silicate of calcium and so- 

 dium. Flint glass is a silicate of potassium and lead. It has a 

 high refractive power, is easily fusible and easily cut. It is much 

 used for optical instruments, and ornamental glassware. Glass 

 of various colors is produced by melting with it certain metallic 

 oxides, as the oxide of iron gives a green color, the suboxide of 

 copper a red color, the oxide of cobalt a blue, etc, 



BARIUM (Ba), a yellowish malleable metal, is of little value 

 but some of its compounds are interesting. Barium sulphate, 

 called heavy spar, occurs abundantly as an associate of lead 

 ores in the mineral regions of Missouri. It is a white, heavy 

 mineral sometimes used to adulterate white lead paint, and for 



