176 ON THE DECOMPOSING POWER OF WATER AT HIGH TEMPERATURES. 



"cementation action," through their entire mass: pulverization is, therefore, required to 

 obtain evidence of the internal change which has been produced. 



All the experiments, so far made, would indicate that the following was the general 

 rule applicable to all salts capable of sustaining heat alone without decomposition. 



Whenever a salt, from its own elements alone, or by the addition of those of water, 

 can produce a volatile acid and a fixed base, the evolution of this acid and the liberation 

 of this base will be determined bypassing a current of aqueous vapour over the salt 

 raised to a high temperature. When either the acid or base to be liberated forms a 

 combination with water which can resist decomposition by the heat employed, the ten- 

 dency to form such hydrates adds much to the decomposing power of the aqueous vapour. 

 Although potash and soda are not by themselves fixed bases at high temperatures, yet 

 by the use of the substances before mentioned, they can form combinations which are 

 fixed, and by this means their salts come under the above rule. 



The actual number of salts which have as yet been subjected to this mode of decom- 

 position, is not very large ; yet, from their perfect analogy of composition with many others, 

 there can be but little doubt of the general extension of the principle. 



The chlorides of potassium, sodium, barium, strontium, and calcium, being all thus 

 decomposed, the bromides, iodides, and fluorides of the same and all weaker bases, must 

 probably act in the same manner. The fluoride of calcium has, in fact, been found to do 

 so, by experiment, hydrofluoric acid being freely evolved. In the same manner, from the 

 decomposition of the sulphates, may be inferred that of the seleniates; from the silicates 

 that of the borates. 



The applicability of this simple mode of decomposition to the explanation of a great 

 variety of geological changes, is too evident to escape the attention of those conversant 

 with that science. In a future paper I hope to be able to give a more complete account 

 of some interesting facts which have been observed in connexion with this subject, and 

 to verify, by experiment, many points which must at present be left to inference and con- 

 jecture. In fact, although the existence of this law of decomposition was ascertained in 

 18-12, yet it has only been within a few months that I have been able to give much 

 attention to its investigation, which must be my excuse for the imperfect and hurried 

 manner in which it is now communicated. 



London, Av°;ust 3d, 1847. 



