Intelligence and Miscellaneous Articles, 4-83 



Alumina. — This substance combines, as is well known, readily with 

 the alkalies ; these salts are called aluminates. Nature presents us 

 with compounds in which alumina is united with lime, magnesia, or 

 oxide of lead, as in spinelle ruby, gahnite, and plomb gomme. Thus 

 alumina may act the part both of an acid and a base. 



The salts of alumina have been examined by a great number of 

 chemists, but hitherto the aluminates have been incompletely studied ; 

 and the author determined to ascertain the precise capacity of satura- 

 tion of aluminic acid, by analysing a neutral crj'^stallized aluminate. 



Aluminates. 



Aluminate of Potash. — ^This salt may be prepared either by dis- 

 solving alumina precipitated by carbonate of ammonia in potash, or 

 by fusing in a silver crucible anhydrous alumina with excess of 

 potash. If, in both cases, the solution be evaporated under the 

 receiver of the air-pump, the liquor when sufficiently concentrated 

 readily deposits hard brilliant crystals of aluminate of potash. This 

 salt always retains a certain quantity of alkaline solution ; it may be 

 fused and crystallized a second time. 



Aluminate of potash is white, very soluble in water, but insoluble 

 in alcohol ; it has a caustic taste and an alkaline reaction ; a certain 

 quantity of water decomposes it, precipitates alumina nearly pure, 

 and retains in solution a very alkaline aluminate. 



M. Chevreul has observed, that when the acid of a salt is insoluble 

 and the base soluble, the salt is often decomposed by a solvent into 

 an acid salt, which is precipitated, and an alkaline one which re- 

 mains in solution ; water effects, a similar decomposition with alu- 

 minate of potash. The aluminate of potash having been dried in 

 vacuo, was submitted to analysis ; this was effected by dissolving the 

 aluminate of potash in a small quantity of water, and adding excess 

 of sulphuric acid, and then precipitating the alumina by carbonate of 

 ammonia, afterwards evaporating the filtered liquor, and weighing the 

 sulphate of potash, which was ascertained to be neutral and entirely 

 soluble in water. 



100 parts submitted to analysis yielded — 

 Alumina. . . . 40*6 



Potash 37-5 



Water .... 21-2 

 99-3 



Representing the composition of aluminate of potash by the for- 

 mula Ar^ 0», KO + 3 HO, theory gives the following numbers :— 

 Alumina. ... 40-9 



Potash 37-5 



Water 21-6 



It will be seen that by this analysis the salt must be considered 

 as neutral, containing one equivalent of alumina and one of potash, 

 and that in the neutral aluminates, the relation between the oxygen 

 of the acid and that of the base is as 3 to 1 . 



In order to prepare a well-defined aluminate of potash of the above- 

 described composition, the salt ought to be purified by solution in 



2 I 2 



