ALUMINATE OF SODA 125 



of parting -with its, acetic acid until the whole is expelled, by the long-continued 

 application of heat to a solution of this salt ; the alumina remains in the solution, in 

 a soluble allotropic condition, forming -what has boon called metalumina. Its coagulum 

 with dyewoods is translucent, and entirely different from the opaque cakes formed by 

 ordinary alumina ; hence this solution cannot act as a mordant. But this solution of 

 alumina, which is perfectly colourless and transparent, has the alumina separated from 

 it by the slightest causes. A minute quantity of either an acid, an alkali, even of a 

 neutral salt, or of a vegetable colouring matter, effects the change. The precipitated 

 alumina is insoluble in acids, even boiling sulphuric ; this shows another allotropic 

 condition. But it is dissolved by caustic alkalis, by which it is restored to its 

 common state. 



ALTTlKIurA, ACETATE OF. The acetates of alumina are extensively used in 

 the arts on account of the property which they possess of being readily decomposed 

 with deposition of their alumina on the fibre of cloth ; hence they are used as mor- 

 dants, in the manner described under CALICO-PRINTING ; and so/netimes in dyeing 

 they are mixed with the solution of a colouring matter ; in this the textile fabric is 

 immersed, -whilst, on heating, the alumina is precipitated upon the fabric, which, in 

 consequence of its affinities before alluded to, carries down the colouring matter with 

 it, and fixes it on the cloth. 



The acetate of alumina thus employed is obtained by treating sulphate of alumina 

 with neutral acetate of lead, and filtering off the solution from the precipitate of sul- 

 phate of lead. Acetate of lime is also used ; but the sulphate in this case does not 

 leave the solution so clear or so rapidly. 



According to Mr. Walter Crum, 1 the solution resulting from the decomposition of 

 sulphate of alumina APO*, 3S0 1 (Al a 3SO 4 ) by monobasic acetate of lead contains 

 the salt APO 1 , 2(C 1 H S 8 . HO) [A1 2 O. 4C 2 H 4 O 2 ] (binacetate of alumina), together 

 with one equivalent of free acetic acid, the compound APO 3 , SC'H'O 1 (Al 2 6C 2 H 8 O a ) 

 not appearing to exist. By evaporating this solution at low temperatures, e.g. in a 

 very thin layer of fluid below 38 C. (100 F.), Crum obtained a fixed residue 

 completely soluble in water, the composition of which, in the dry state, approached 

 APO", 2C 4 H0 + 4HO (A1 S O S . 4C 2 H 4 O S + 2H 2 O). 



AXiOTMCXirA, SILICATES OF. Silicate of alumina is the chief constituent of 

 common clay ; it occurs also associated with the silicates of iron, magnesia, lime, 

 and the alkalis in a great variety of minerals, which will be found described elsewhere. 

 The most interesting of these are the felspars and the zeolites. Sea CLAY, FELSPAB, 

 ZEOLITE. 



Of course, being present in clay, silicate of alumina is the essential constituent of 

 porcelain and earthenware. See EABTHENWAKE and POBCELAIN. 



AXiVMIDTA, SULPHATE OF. The neutral sulphate of alumina, APO', 

 3SO* + 18HO (A1 2 3SO 4 + 18H 2 O), which is obtained by dissolving alumina in sul- 

 phuric acid, crystallises in needles and plates ; but sulphuric acid and alumina combine 

 in other proportions, e.g. a salt of the formula APO 3 , 3S0 3 + APO ( A1-3SO 4 + Al'O") 

 was obtained by Mons, and the solution of this salt, when largely diluted with water, 

 splits into the neutral sulphate and an insoluble powder containing APO 1 , 3S0 1 

 + 2APO' + 9HO (A1 2 3S0 4 + 2A1 2 O 3 + 9H 2 O). This subsalt forms the mineral 

 aluminite, or Websterite, found near Newhaven, and was found by Humboldt in the 

 schists of the Andes. 



The sulphate of alumina is now extensively used in the arts instead of alum, under 

 the name of 'concentrated alum' (seep. 122). For most of the purposes for which alum 

 is employed, the sulphate of potash is an unnecessary constituent, being only added 

 in order to facilitate the purification of the compound from iron ; for in consequence of 

 the ready crystallieability of alum, this salt is easily purified. Nevertheless, Wies- 

 mann has succeeded in removing the iron from the crude solution of sulphate of 

 alumina obtained by treating clay with sulphuric acid, by adding ferrocyanide of 

 potassium, which throws down the iron as Prussian-blue ; the solution, when evapo- 

 rated to dryness, is found to consist of sulphate of alumina, containing about 7 per 

 cent, of potash-alum. About 1750 tons of this article were produced at Nowcastle-on- 

 Tyne alone in the year 1872. See also ALUM. 



; ALtnvxzurATE OF SODA. 3 NaO. APO 9 (3XTa*O. Al-O 3 ). This salt is now 

 manufactured on a large scale for use ns a mordant in dyeing and calico-printing.; 

 in. the preparation of lakes for pigments ; in the manufacture of certain forms of 

 artificial stone ; and in candle and soap manufacture. The salt is generally obtained 

 by heating Bauxite with carbonate of soda in a reverberatory furnace. The 

 alumina present in the Bauxite combines with the soda, while carbonic 1 acid is 

 expelled. On lixiviating the product, the aluminate of soda is dissolved out ; tho 



* Chemical Society's Quarterly Journal, vi. 210. 



