124 ALUMINA 



Bosjemanite, a Manganeso-alum, found near the Bosjeman Kivor in South Africa, 

 and at Alum Point, noar the Salt Lake, Utah. 



Al/tJlVI, ROMAN" (Alumen Eomanum. AUume di rocca, It.), called sometimes 

 Roch or Rock Alum. See ALUM. 



AlfTTlVX ROOT. The root of the Geranium maculatum, a North American plant. 

 It contains a large proportion of tannin, and is used as a powerful astringent. 



AiiUivt-SH AIj. The chief natural source from which the alum of commerce is 

 derived in this country. It occurs in a remarkable manner near Whitby, in York- 

 shire, and at Hurlet and Campsie, near Glasgow. A full description of the alum- 

 shale, and of the processes by which the crystallisable alum is separated, will be 

 found under ALUM. 



AXiTTM-SItATZI. A clay-slate containing bitumen and sulphide of iron, found 

 in the North of England, in Scotland, in Scandinavia, and other places. The 

 common variety effloresces and acquires the taste of alum. The glossy Alum-slate 

 is of a bluish-black colour, with a semi-metallic lustre. It swells upon exposure 

 to the air, owing to . the saline efflorescence, formed between the foliations, and is 

 eventually disintegrated. 



ALTTivi-STOBrE or AXOmrXTE. (Alun, Fr. ; Alaunstein, Ger.) This mineral, 

 in its purest form, is composed of alumina 37'13, sulphuric acid 38*53, potash 11*34, 

 water 13'00. Silica is also frequently present as an impurity, sometimes to the extent 

 of 60 per cent. It is a white, greyish, or reddish mineral, affording a white streak, 

 and an uneven, flat, conchoidal fracture, which is splintery in the massive varieties. 

 It is transparent or subtranslucent. 



Alum-stone is one of the sources of the alum of commerce, which is obtained from 

 it in crystals after frequent roasting, and lixiviation in water. See ALUM. 



Alum-stone is found at Tolfa, near Civita Vecchia, in the Koman States (sometimes 

 in crystals) ; at Muszaly in Hungary; at Pic de Sancy, in France, and in the Grecian 

 Archipelago. The compact varieties from Hungary are so hard as to be used for 

 millstones. 



AXTTDCnrA. (A1 S 0*, atomic weight 51*4). This is the only oxide which the 

 metal aluminium forms, and it is assumed to be a sesquioxide on account of its 

 isomorphism with sesquioxide of iron. 



Alumina occurs in a native state, forming the gems known as the sapphire and the 

 ruby, and in a less pure form as the minerals corundum and emery. In a state of 

 hydrate, or in chemical combination with water, alumina constitutes the species known 

 as diaspore, gibbsite, and hydrargyllite. Such of these minerals as are of economic 

 value will be noticed under their respective heads. 



Alumina is obtained in the state of hydrate from common alum (KO, SO 1 ; A1 2 S , 

 ,3SO* + 24HO) by adding a solution of ammonia (or bettor, carbonate of ammonia) to 

 the latter salt and boiling. The precipitate is white, and gelatinous in a high 

 degree, and retains the salts, in the presence of which it has been formed, with 

 remarkable pertinacity, so that it is very difficult to wash. 



By drying and igniting this hydrate, the anhydrous alumina is produced; but it 

 may be obtained more readily by heating ammonia-alum (NH 4 0, SO* ; A1 2 0*3SO' + 

 24'HO). All the constituents of this salt are volatile, with the exception of the alumina. 



Alumina. is insoluble in water, but soluble both -in acids and alkalis. Towards 

 the former it plays the part of a -base, producing the ordinary alumina salts ; whilst, 

 with the latter, it also enters into combination, but in this case it is an acid, forming 

 a series of compounds which may be called aluminates. 



The important application of alumina and its compounds in the arts of dyeing and 

 calico-printing depends upon a peculiar attraction which it possesses for organic 

 bodies. This affinity is so strong, that when digested in solutions of vegetable 

 colouring matters the alumina combines with and carries down the colouring matter, 

 removing it entirely from the solution. Pigments thus obtained, which are com- 

 binations of alumina with the vegetable colouring matters, are called ' lakes' 



Alumina has not only an affinity for the colouring matters, but at the sama 

 time also for the vegetable fibres, cotton, silk, wool, &c. ; and hence, if alumina be 

 precipitated upon cloth in the presence of a colouring matter, a most intimate union 

 is effected between the cloth and the colour. Alumina, when employed in this way, 

 is called a ' mordant.' 



Other bodies have a similar attraction for colouring matters,, e..?. binoxide of tin 

 and sosquioxide of iron : each of these gives its peculiar shade to. the qolour or com- 

 bination, alumina changing it least. 



Mr. Walter Crum 1 has discovered a peculiar soluble modification of alumina. The 

 binacetato of alumina has boon found by Mr. Crum to possess tho very curious property 



1 Chemical (Society's Quarterly Journal, Ti. 21C. 



