OXY-SALTS AND HALOID -SALTS. 55 



nesia, called boracite^ is a compound of boracic acid and 

 magnesia. 



108. Chlorid of Ammonium. This was formerly sup- 

 posed to be composed of muriatic acid and ammonia, 

 but modern chemistry has discovered that it is a com- 

 pound of chlorine and ammonium, a metal which has 

 never yet been seen, but whose existence has been satis- 

 factorily proved, as shown in 230, Part II. The com- 

 mon name of this mineral is sal ammoniac. It occurs 

 in the vicinity of volcanoes, and is the result of volcanic 

 action. 



109. Alum. Common alum is a double salt, a combi- 

 nation of sulphate of alumina and sulphate of potash. 

 But there are other alums a soda alum, in which there 

 is sulphate of soda in place of the sulphate of potash ; a 

 magnesia alum, in which there is sulphate of magnesia; 

 an iron alum, in which there is sulphate of iron ; an am- 

 monia alum, in which there is sulphate of ammonia ; and 

 a manganese alum, in which there is sulphate of manga- 

 nese. Then there is what is called feather alum, which 

 is not a double salt, but a simple hydrous sulphate of 

 alumina, and therefore can not, strictly speaking, be call- 

 ed an alum. This is more abundant in nature than any 

 of the true alums. This salt, the potash alum and the 

 iron alum, often impregnate the rocks called clay slates, 

 and when thus charged they are termed aluminous slates 

 or shales. Alum is often obtained from these rocks by 

 lixiviation. 



110. Phosphates of Alumina. Wavellite, a mineral 

 which is found adhering to rocks in small hemispheres, 

 is a phosphate of alumina, having combined with it a 

 small amount of fluorid of aluminum. Turquois, an 

 opaque greenish-blue mineral, much used for ornament, 

 is a phosphate of alumina, having combined with it 

 phosphates of copper and iron in small quantities. This 

 gem is often imitated, and the counterfeits are some- 

 times so good that they can be detected only by chemi- 

 cal tests. 



