353 



The Committee on Mr. R. A. Tilghman's paper, "On the 

 Decomposing Power of Water at High Temperatures," re- 

 ported, recommending its publication in the Transactions of 

 the Society, which was ordered accordingly. 



From the long known fact that solutions of salts, which require a 

 high temperature for dehydration, frequently underwent partial de- 

 composition before this end was effected, Mr. Tilghman was led to 

 believe, that by exposing the salt, even in its anhydrous state, to a 

 high heat, and passing over it a current of aqueous vapour at the 

 same temperature, these salts might be comipletcly decomposed, and 

 perhaps the action observed even in those which had given no signs of 

 partial decomposition under the treatment before alluded to. Upon 

 trying the experiment, it was found that the anhydrous chlorides of 

 calcium, strontium, and barium, could be rapidly decomposed by ex- 

 posing them, at a high red heat, to a current of steam; hydrochloric 

 acid being copiously evolved, and the oxides of the metals left, the 

 lime remaining anhydrous from the intensity of the heat employed, 

 while the baryta and strontia passed to the state of hydrates. 



In these haloid salts, it is to be observed that the addition oC the 

 elements of water is absolutely essential to the decomposition ; as nei- 

 ther the hydrogen of the acid, nor the oxygen of the base, existed in 

 the anhydrous salt. The action is, therefore, 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 experiments 

 were then extended to the oxysalts, the sulphates of magnesia, lime, 

 strontia and baryta, which contain, even when anhydrous, all the 

 elements generally considered necessary for the separate existence of 

 the acid and bases of which they are composed. The application of 

 the strongest heat causes no liberation of their acid; but, as with the 

 chlorides, this effect is immediately produced by the passage of a cur- 

 rent of steam over them at a high temperature, the baryta and stron- 

 tia being left in the state of hydrates, and the other bases anhydrous. 



The intensity of the affinity between the acid and base of the re- 

 spective salts, is curiously illustrated by the gradual increase of heat 

 necessary for their decomposition by aqueous vapour. Thus sul- 

 phate of magnesia gives off its acid to steam at a low red heat, and 

 consequently a large portion of the acid may be condensed in an un- 

 decomposed state. 



The sulphate of lime requires a high red heat for its decomposi- 

 tion; and on this account the greater part of its acid is resolved into 



