SILICATES OF ALUMINA. 51 9 



placed by metallic oxides isomorphous with it, namely, by per- 

 oxide of iron, deutoxide of manganese, and oxide of chromium. 

 To these salts the generic term alum is applied, and the species 

 is distinguished by the name of the metallic peroxide it con- 

 tains, as iron alum, manganese alum, and chrome alum. 



Alumina dissolves freely in most acids, but like metallic pe- 

 roxides in general, it affords few crystalline salts, except double 

 salts. The oxalate of potash and alumina is the only other of 

 these that has been formed. It is remarkable for its composi- 

 tion, containing 3 eq. oxalate of potash to 1 eq. oxalate of alu- 

 mina, with 6 eq, of water Its formula is, therefore, 



3(KO, C 2 O 3 ) + A1 2 O 3 , 3C 2 O 3 + GHO. 



Like alum it is the type of a genus of double salts. The cor- 

 responding oxalates, containing soda, crystallize with 10HO. 

 (Phil. Trans. 1837, p. 54.) 



Nitrate of alumina is said to crystallize with difficulty in 

 prismatic crystals radiating from a centre. 



An insoluble phosphate of alumina precipitates when phos- 

 phate of soda is added to a solution of alum. By fusion it 

 gives a glass like porcelain. Its composition is 2A1 2 O 3 , 3PO 5 

 (Berzelius). This salt, dissolved in an acid and precipitated by 

 ammonia in excess, gives a more highly basic phosphate, of 

 which the formula is 4A1 2 O 3 , 3PO 5 (Berzelius). The last salt 

 occurs in nature, in combination with fluoride of aluminum, in 

 the form of radiating crystals, containing 27 8 per cent of water. 

 It is the mineral wavellite, of which the formula is A1 2 F 3 -f- 

 3(4A1 2 O 3 , 3PO 5 ) +3bHO. A phosphate of alumina and lithia, 

 containing the same subphosphate of alumina, forms the rare 

 mineral amblygonite, and may be prepared artificially. Its for- 

 mula is 2LO, PO 5 + 1A1 2 O 3 , 3PO 5 . 



SILICATES OF ALUMINA. 



The varieties of clay are essentially silicates of alumina, but 

 composed as they are of the insoluble matter of various rocks 

 destroyed by the action of water, it is not to be expected that 

 they will be uniform in composition. Mitscherlich considers it 

 very probable that the basis of clay is usually a subsilicate of 

 alumina/, of which^ the formula is 2A1 2 O 3 , 3Si O 3 ; and which 

 contains 57-12 parts of silica and 42.58 of alumina in 100 parts. 



