358 M. Arfwedson's Examination of certain Minerals. 



peating its ignition with potassa, the portion insoluble in mu- 

 riatic acid was lessened to 0,108, which on trial was found to 

 be pure silica* {a). 



All the solutions, together with the water used for edulco- 

 rating the precipitates, were then precipitated with caustic ani- 

 nionia in tlie least possible excess. The well-washed precipi- 

 tate, after exposure to a red-heat, weighed 0,507 gram.; these 

 were dissolved in sulphuric acid, leaving a residue of 0,007 

 gram, [b) of sihca, and the solution gave a precipitate with 

 caustic potassa, which was dissolved by addmg more potassa, 

 and only left a few unappreciable flakes of oxide of iron be- 

 hind. "That portion, therefore, which was dissolved in sul- 

 phuric acid was alumina, the quantity of which, after deduct- 

 ing the separated silica, amounts to 0,500 gram. (c). 



For the sake of certainty, the solution in caustic potassa was 

 saturated with muriatic acid, until the precipitate became re- 

 dissolved ; after which carbonate of ammonia was added to 

 great excess, but no glucina or magnesia could be discovered ; 

 and the filtered fluid remamed perfectly clear even when boil- 

 ing, and after the excess of ammonia had been expelled. 



The fluid, precipitated by caustic ammonia, was neutraUzed 

 with muriatic acid, and mixed with a few drops of oxalate of 

 ammonia ; but after the lapse of twelve hours, not the least 

 sign of turbidity appeared, and on boiling it with subcarbonate 

 of potassa, no precipitate could be produced. 



0,61^ gramm. of this fossil have therefore yielded, 



Silica (a)... 0,108 lu 100 Parts. 



{b)... 0,007 0,115 18,73 



Alumma (c) 0,500 81,43 



0,615 100,16 



1 8,73 parts of silica contain 9,42 of oxygen, and 81,43 parts 

 of alumina contain 38,03; but 9,42x4=37,63; by this ratio 

 the formula of chrysoberyl becomes A •* S. 



Boracite from Lunehurg. 

 Professor Stromeyer mentions in Gilbert's Annals, vol. xviii. 

 p. 215, that he had found tliis mineral to be compounded of 

 67 boracic acid and S3 magnesia. As the analytical expe- 

 riments of Professor Stromeyer have in general gained so 

 just a confidence, one could not well doubt the correctness of 



* In order to coimnce myself that the silica I obtain in my experiments 

 is pure, I am accustomed to dissolve it by fusion in a good quantity of sub- 

 carbonate of potassa. If the mass dissolve in water without any residue, 

 I take it for granted, that the silica is not contaminated by any other 

 earth. 



the 



