M 0MP0S1NG I'UW'KB OF WATER AT HIGH TEMPERATURES. 175 



ner as lime and magnesia, and in all these cases the chemical combination is so feeble 

 that, \\h?n cold, the alkali is disengaged by the solvent powers of water alone. 



Alumina, which possesses so much of the acid character with respect to the strong 



, is propoitionably 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 soluble aluminate; but is easily precipitated from its solution by a current of 



carbonic acid ixas. 



The calcination of potash alum leaves a mixture of alumina and sulphate of potash, 

 which Bcrthicr has long since stated to be converted into aluminate of potash by the con- 

 tinued action of heat alone, the sulphuric acid being expelled from the potash by the 

 superior affinity of the alumina at a high temperature. By several careful repetitions of 

 his experiment, in which the accidental presence of aqueous vapours was entirely pre- 

 vented, no decomposition of tin- kind could be effected, even at a white heat. But by 

 the contact of aqueous vapour, produced by the combustion of the fuel or otherwise, even 

 in small quantity, and at much lower temperatures, the decomposition is rapidly produced. 

 It, therefore, seems probable that the accidental contact of aqueous vapour was the actual 

 but unnoticed cause of the decomposition in Berthier's experiment. 



The powerful action of aqueous vapour upon anhydrous alum at a high temperature, 

 -u_u;ostcd the possibility that a similar action might take place upon its mineral repre- 

 sentative, the double silicate of alumina and potash, or common felspar. It will be 

 remembered that this salt, by the simple substitution of sulphuric for the silicic acid 

 which it contains, would he converted into anhydrous alum. To the action of heat 

 alone, felspar presents this difference from alum, that the silicate of alumina is as unaf- 

 fected by it as the silicate of potash itself; so that to produce an effect upon felspar 

 analogous to that upon alum, the silicic acid of both the silicate of alumina and of the 

 silicate of potash would have to be removed. Silicic acid, in a free state, having been 

 long known to be slightly volatile in aqueous vapour at high temperatures, it was though! 

 that, in the present case, it might, like the other acids, be disengaged even from a state 

 of chemical combination, by the same ag< nts. Steam was, therefore, passed slowly, for 

 some time, over small fragments of highly heated felspar. Beyond partial fusion, no 

 other visible change than a considerable degree of vesicularitv in the parts most exposed 

 was produced. These fragments, being finely pulverized and boiled in water, the con- 

 centrated solution was strongly alkaline, and proved, by the usual tests, to consist of 

 aluminate of potash. 



After water ceases to extract aluminate of potash from the powdered mineral, dilute 

 sulphuric acid will produce from the residue a small proportion of alum. The actual 

 analogy between alum and felspar, substances so distinct in their origin and general 

 properties, yet differing only in the nature of their respective acid-;, is rendered -till mor< 

 striking by both thus yielding the same product, when deprived of their acids by the 

 same agent. It i- worthy of remark, that, although the actual contact of the steam in 

 this experiment is confined to ihe mere surface of the small fragments of felspar, yel the 

 chemical decomposition produced by il is not confined to that surface, hut spreads by a 



