ALUMINIUM 469 



cate fabrics (cloth) are rendered waterproof by saturating them 

 with aluminium acetate solution and boiling to promote the 

 hydrolysis. The aluminium hydroxide is precipitated in the 

 capillaries of the cotton or linen, rendering them non-absorbent. 



Aluminium Oxide (Alumina) A1 2 O3. Corundum, and the 



impure variety emery, are next to the diamond in the scale of 

 hardness, and are used as abrasives. Ruby and sapphire are also 

 crystallized aluminium oxide, containing traces of impurities 

 (iron and titanium in the one case and chromium in the other) to 

 which they owe their colors. By ingenious methods of fusing the 

 oxide, " synthetic " sapphires to the extent of six million carats 

 and rubies to the extent of ten million carats are now made annu- 

 ally. The artificial gems are chemically identical with the natural 

 ones, and can be distinguished only by the fact that they are free 

 from microscopic bubbles and other defects. Alundum, an arti- 

 ficial abrasive, and refractory material for crucibles and muffles, 

 is made by barely melting the oxide in the electric furnace. 



Aluminium Sulphate A1 2 (SO 4 ) 3 ,18H 2 O. The sulphate is 

 manufactured by the action of sulphuric acid on bauxite. It 

 crystallizes in leaflets, which usually have a faint yellow tinge due 

 to the presence of iron (Fe(OH) 3 ) derived from the mineral. The 

 salt is used in fireproofing cloth, since, when heated, it melts in its 

 water of hydration. It is used as a source for precipitated alu- 

 minium hydroxide hi paper-making, water purification, and 

 dyeing. 



Alums. When aluminium sulphate and potassium sulphate 

 are dissolved together in molecular proportions, the solution 

 deposits transparent octahedral (Fig. 39, p. 94) crystals of 

 potash-alum K2S04,AJ2(S04)3,24H 2 O. This salt is more easily 

 freed from impurities (e.g. compounds of iron) by recrystallization 

 than is aluminium sulphate, and is therefore used instead of the 



