Aluminium. 187 



to the acids they contain ; thus the chlorides act upon it, the 

 sulphates do not. Aluminium fused with nitre is not affected 

 by it, except at a high temperature, and then aluminate of pot- 

 ash is formed. The chlorides of sodium and potassium do not 

 perceptibly act upon it when pure, and only very slowly when 

 impure. Ordinary metals cannot resist the action of common 

 salt — particularly as it is found in sea- water ; even silver is 

 slightly corroded by being boiled in water holding it in solution : 

 and silver articles, which usually contain five per cent, copper, 

 become dangerous when food is allowed to cool in them. Now, 

 supposing silver and aluminium to be even equally affected in 

 such cases, the latter, on account of its low equivalent, will 

 give rise to a much smaller quantity of resulting salt : since a 

 quantity of acid that will dissolve one hundred grains of silver, 

 will dissolve only eight and a half of aluminium; and the 

 physiological effects of the salts are very different. Other 

 metallic chlorides, including even its own, are decomposed 

 by it : and with a facility dependent on the elevated rank 

 of the metal they contain. It does not combine with car- 

 bon : and in this respect has the advantage of platina, in the 

 laboratory. 



Compounds containing silicium are decomposed by alu- 

 minium, at a high temperature : yet we may fuse it in glass, 

 or porcelain, because they are not in contact with it, unless 

 some fusible substance is present ; and hence, when we melt 

 it in vessels which contain silicium, a flux is inadmissible. When 

 aluminium containing silicium is dissolved in hydrochloric 

 acid, siliciuretted hydrogen, a compound not long discovered, 

 is disengaged, and is recognised by its peculiar odour : and 

 the presence of silicium greatly assists the action of that 

 solvent. 



Aluminium combines also with boron : and the latter, like 

 silicium, affects it properties. A very interesting compound, 

 the diamond of boron, is produced by the intervention of alu- 

 minium. Boron assumes three states, corresponding with the 

 three conditions of carbon — amorphous carbon, graphite, and 

 diamond. Boron, in the state of diamond, is obtained by caus- 

 ing aluminium to act on boracic acid : it bears a heat suffi- 

 cient to melt iridium, without change ; and unites with oxygen, 

 at the temperature at which diamond burns — but only on the 

 surface, as it is protected by the boracic acid which is formed 

 externally. It scratches the hardest diamond, by which alone 

 it is exceeded in brilliancy and refractive power. It assumes 

 three forms, having somewhat different qualities : one of them is 

 exceedingly hard, and answers well instead of diamond pow- 

 der, its crystals not being abraded by use : and the least hard 

 is more so than corundum. 



