5 ANALYSIS OF CORUNDUM, AND OF 



modes of anal) lis as were employed with (tones of a fimilar 



nature ; and then prefent a fummary of the remits : laftly, I 



(hall conclude with an enquiry into a much contefted point, 



which lately threatened a revolution in docimaftic chemiftry. 



Principal cha- a principal charader of corundum in general, as may be 

 rader : Extreme c , • xl ^ , n . , . , , ~ . . . 



hardnefs, found in the Count de Bournon's mineralogical defcnption, is 



extreme hardnefs j and thence the difficulty of reducing that 

 fubftance into fine powder will be eafily conceived. We are 

 told by docimaftic chemifts, that the moft advantageous method 

 of pulverizing hard ftones, is to make them red-hot ; and, in 

 that ftate, to plunge them into cold water. But I found that 

 this operation, when performed but once, was by no means 

 P e U e7tedf nition fufficient for corundum. I therefore repeated it, till the ftone 

 and quenching, appeared to be fiffured in every direction. After this, the 

 fpecimen to be pulverized was put into a fteel mortar, about 

 three-fourths of an inch in diameter, and three inches in depth, 

 Efficacy of the j nto wn [ c h a fteel peftle was very clofely adjufted. A few 

 blows upon the peftle caufed the ftone to crumble ; and the 

 fragments were then eafily reduced into an impalpable powder, 

 in an agate mortar, with a peftle of the fame material. The 

 abrafion from the mortar, uiual in the pulverization of hard 

 ftones, was much diminifhed by the above precaution ; rubies 

 and fapphires being, in a fhort time, ground to a powder nearly 

 as minute as the fineft precipitate. 

 The great diffi- M. Klaproth in his analyfis before-mentioned, had obferved 

 corundum wifh Wltn how'much difficulty the ftones were acled upon by potafh 

 alkali, renders or foda. I found that the greateft heat a filver crucible could 

 that procefs ob- rapport, without melting, was not fufficient to produce a fa- 

 tisfa&ory fufion of one part of corundum, with fix parts of 

 either of thofe alkalis ; nor did an expofure to that tempera- 

 ture during feveral hours, feem to render the treatment more 

 efTe&ual. Not more than half the quantity of the corundum 

 was ever rendered foluble in any acid ; and what remained was 

 the powder of the ftone, wholly unchanged. The repeated 

 nitrations and evaporations with which this treatment muft 

 be attended, not only render it tedious, but alfo produce 

 Uncertainty in the refults. Even when very finely powdered 

 corundum was expofed, with fix times its weight of potafh, in 

 a platina crucible, to a heat of 140° of Wedgwood, for two 

 Jiours together, it was not acled upon in fuch a manner as to 



be 



