SALTS OF ALUMINA. 127 



erns. It is used, like its associate, in the manufacture of saltpeter 

 (see page 102.) 



Polyhalite. A brick-red saline mineral, with a weak bitter taste, 

 occurring in masses which have a somewhat fibrous appearance. Con- 

 sists of* the sulphates of lime, potash and magnesia, with six per cent, 

 of water. 



Wagnerile. A fiuo-phosphate of magnesia, occurring in yellowish 

 or grayish oblique rhombic prisms. Insoluble. H=5 — 55. Gr=31. 

 From Salzberg, Germany. 



Rhodizite. Resembles boracite in its crystals, but tinges the blow- 

 mpe flame deep red. Occurs with the red tourmaline of Siberia. 



8. ALUMINA. 



The compounds of alumina may often be distinguished by 

 a blowpipe experiment. If a fragment of alumina after 

 having been heated to redness be moistened with a solution 

 of nitrate of cobalt and again heated, it assumes before fu- 

 sion a blue color. This is a good test, and distinguishes 

 aluminous from magnesian minerals, except when the oxyds 

 of the metals are present. 



The sulphates, fluorids and some of the phosphates, (the 

 salts included in this family,) are soluble with more or less 

 difficulty, in the acids ; and some of the sulphates (the vari- 

 ous alums) dissolve readily in water. 



The solution in acids takes place without effervescence, 

 and without forming a jelly like many silicates of alumina 

 (the zeolites, &c.) 



Specific gravities of the species below 3*1. Hardness of 

 some species as high as 6. 



NATIVE ALUM. |\fe 



Monometric. Cleavage octahedral. Occurs in octahe. 

 drons ; but usually in silky fibrous masses, or in 

 efflorescent crusts. Taste sweetish astringent. 



There are several kinds of native alum, dif- 

 fering in one of the ingredients in their consti- 

 tution, but resembling one another in crystalli- 

 zing in octahedrons, and in containing the in- 

 gredients in exactly the same proportions. They all contain 



What blowpipe experiment distinguishes alumina 1 What is said ci 

 the sulphates of alumina 1 What is the composition of the alums 1 



