354 



sulphurous acid and oxygen gas. The decomposition of the sulphates 

 of strontia and baryta requires progressively higher heats, which, in 

 the case of the last salt, must be raised even to low whiteness. 



The subphosphateof lime, as it contains an acid much less volalile 

 than the sulphuric, combined with an excess of a powerful base, 

 which adds to its stability, was selected as one of the most difficult 

 tests of this decomposing power of aqueous vapour. By a full white 

 heat, however, its phosphoric acid was slowly disengaged, and the 

 acid, by its white precipitate with nitrate of silver, showed that the 

 excess of aqueous vapour had not prevented the change which heat 

 is known to produce upon this acid. 



It was found, that with the sulphates and muriates of potassa and 

 soda, although the decomposition began freely at a red heat, yet the 

 proportion of alkali set free never exceeded a very small per centage 

 of the residual salt, no matter how long the operation might be con- 

 tinued. This peculiarity being attributed to the volatility of the hy- 

 drates of these bases at high temperatures, substances capable of 

 forming non-volatile combinations with the alkalies were mixed with 

 the salts, before subjecting them to the action of the heated steam ; 

 the acids were then found to be completely disengaged with facility. 



Lime, magnesia, and the subphosphates and subsilicates of lime, 

 baryta, and strontia, produce this effect; and in all these cases the 

 chemical combination is so feeble, that, when cold, the alkali is dis- 

 engaged by the solvent powers of water alone. 



Alumina, which possesses so much of the acid character with re- 

 spect to the strong bases, is proportionally more efficient than any of 

 the preceding substances in aiding the decomposition of the alkaline 

 salts; it remains in combination with the alkali, when cold, as a so- 

 luble aluminate, but is easily precipitated by a current of carbonic 

 acid gas. The fact, long since noticed by Berthier, that the mixture 

 of alumina and sulphate of potassa, formed by the calcination of 

 potash-alum, is converted by heat into aluminate of potassa, was 

 shown to depend, probably, upon the presence of aqueous vapour. 

 When the experiment was repeated, and the presence of this vapour 

 carefully avoided, no decomposition of the sulphate of potassa took 

 place; but by the contact of the vapour produced by the combustion 

 of the fuel or otherwise, even in small quantity, and at much lower 

 temperatures, the decomposition is produced rapidly. 



The powerful action of aqueous vapour upon anhydrous alum, at 

 a high temperature, suggested the possibility that a similar action 

 might take place upon its mineral representative — feldspar. Steam 



