355 



was therefore passed slowly, for some time, over small fragments 

 of highly heated feldspar. Beyond parted fusion, no other visible 

 change than a considerable degree of vesicularity in the parts most 

 exposed was produced; but when the fragments were finely pul- 

 verized and boiled in water, the concentrated solution was strongly 

 alkaline, and proved, by the usual tests, to consist of aluminate of 

 potassa ; and after water ceases to extract this salt from the powdered 

 mineral, dilute sulphuric acid will produce from the residue a small 

 proportion of alum. It is worthy of remark, that although the con- 

 tact of the steam in this experiment is confined to the mere surface of 

 the small fragments of feldspar, yet the chemical decomposition pro- 

 duced by it is not confined to that surface, but spreads by a "cemen- 

 tation action" through their entire mass; pulverization is therefore 

 i-equired 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 deter- 

 mined by passing a current of aqueous vapour over the salt, raised 

 to a high temperature. When either the acid or the base to be libe- 

 rated forms a combination with water, which can resist decomposition 

 by the heat employed, the tendency 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 these salts come 

 under the above rule. 



The actual number of salts which have been as yet subjected to 

 this mode of decomposition 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 applicability of this simple mode of decomposition to the ex- 

 planation of a great variety of geological changes, is too evident to 

 escape the attention of those conversant with that science. The au- 

 thor expresses the hope to be able, in a future paper, 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 conjecture. 



