ESSAY VIII. 



ON SILEX, CLAY, AND ALUM. 1776. 



FROM the writings of Mr. Baume, it appears that he takes 

 the earth of alum to be nothing else than silex, and common 

 clay to be siliceous earth, combined with a little vitriolic 

 acid. Alum he supposes to be the same earth, super- 

 saturated with vitriolic acid. With regard to chemical 

 opinions, it is my custom not to credit any, till I have 

 brought them to the test of experiment. I therefore was 

 obliged to try Mr. Baume's assertion, that silex is soluble in 

 vitriolic acid, by this test. I took 1 oz. of mountain crystal 

 reduced to powder, mixed it with 3 oz. of alkali of tartar, 

 and fused the mixture by a strong fire. This alkaline mass 

 I afterwards dissolved in 2 oz. of water, and poured as much 

 diluted vitriolic acid to it as was more than sufficient to 

 saturate it; I then filtered the liquor, collected, edulcorated, 

 and dried the precipitated silex which remained on the 

 filter. From the filtered liquor, after it was evaporated, I 

 obtained, besides a subacid vitriolated tartar, about 1J drm. 

 of alum. The question now was, Whether the precipitated 

 silex, repeatedly fused with alkali, would still yield alum ? 

 If this was the case, Mr. Baume's system would gain a strong 

 confirmation ; but if it did not, a suspicion would arise that 

 some clay was mixed with the mountain crystal ; I therefore 

 dried the precipitate, and mixed it with a triple quantity 

 of alkali, and, proceeding in the same manner as in the 



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