174 ON THE DECOMPOSING POWER OF WATER AT HIGH TEMPERATURES. 



In these haloid salts, it is to be observed, that the addition of the elements of water is 

 absolutely essential to the decomposition; as neither the hydrogen which is contained in 

 the acid, nor the oxygen in the base, existed in the anhydrous salt. The action is, there- 

 fore, the result of a double decomposition between the steam and the chloride, as well as 

 of the affinity of the liberated acid and base for water. 



The oxysalts, the sulphates of magnesia, lime, strontia, and baryta, unlike the haloid 

 salts just mentioned, contain, even in the anhydrous state, all the elements generally con- 

 sidered necessary for the separate existence of the acid and bases of which they are com- 

 posed. The application of the strongest heats to these salts causes, however, no liberation 

 of their acid; but, as with the chlorides, this effect is immediately produced by the passage 

 of a current of steam over them at a high temperature, the baryta and strontia being left 

 in the state of hydrates, and the other bases anhydrous. 



The intensity of the affinity between the acid and base of the respective salts, is curiously 

 illustrated by the gradual increase of the heat necessary for their decomposition by the 

 aqueous vapour. 



Thus the sulphate of magnesia gives off its acid to the current of steam at a low red 

 heat, and consequently a large portion of the acid may be condensed in an undecomposed 

 state. The sulphate of lime requires a high red heat for its decomposition, and on this 

 account the greater part of its acid is resolved into 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 subphosphatc of lime, as it contains an acid much less volatile than sulphuric, com- 

 bined 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. This phosphoric acid gave a white 

 precipitate with nitrate of silver, showing that its liberation and subsequent condensation 

 in contact with a great excess of aqueous vapour, had not prevented that change which 

 heat is known to produce upon this acid. 



It might be expected from the decomposition of the salts of baryta, that the sulphates 

 and muriates of potash and soda would undergo the same change with even greater 

 facility. But it was found, by experiment, that although the decomposition of these last 

 salts commenced with facility, when they were exposed to steam at a red heat, yet the 

 proportion of alkali thus liberated never exceeded a very small per centage of the residual 

 salt, however long the operation might, be continued. Attributing this peculiarity to the 

 volatile nature of the liberated hydrates of potash and soda at high temperatures, sub- 

 stances capable of forming non-volatile combinations with the alkalies were mixed with 

 their salts, previously to subjecting them to the action of the steam; the acids were then 

 found to be completely disengaged with facility. The fact that both lime and magnesia, 

 substances capable of forming chemical combinations of but the most feeble character 

 with potash and soda, were found to produce the above effect, was considered as con- 

 firming, in a great measure, the hypothesis that the volatility of their hydrates was the 

 cause of the apparent difficulty of completely decomposing the salts of these alkalies. 



The subphosphates and subsilicates of lime, baryta, and strontia, act in the same man- 



