On the Corundum Stom from Afia. - 539 



formation, perfecllon, and decompofition, as they occur in mines, muft have their qualities im- 

 mediately afccrtained, and be referved for profit, or thrown away on the heap. The pra£tical 

 miner could not, without external characters, make any progrefs. The valuable minerals arc 

 foon pointed out by afiiiy, and their appearance numbered. The accuracy of feleftion depended, 

 in all periods, much on tlie experience of the miners. It remained for Mr. Werner to give 

 tlic utmoft degree of accuracy which irregular external characters can acquire, by fixing 

 appropriate terms to all the characters which occur, and which the fenfes can difcrimi- 

 natc. In 1774, he opened his fyftem of external charafters of minerals; and the per- 

 fection he has fince given to it, has rendered it very general. The Lefkean colleiEtion, 

 arranged after Mr. Werner's metliod, has procured in Mr. Kirwan a powerful fupport 

 to the introduction of that fyltem in this country ; and we Iiave already fome other 

 valuable publications, to recommend and introduce other favourite fyftems of the conti- 

 nent. It is, tlierefore, at this time, the Englifli mineralogiit Ihculd be invited to examine, 

 if not to prefer, permanent chara£ters, fo far as the progrefs of cryftallography has col- 

 lected them, or at leaft to give them a diftinguiilied rank among external characters of 

 bodies. 



If prejudice too long has retarded the union of intrinfic and extrinfic chara£ters, it has 

 alfo occafioned a fchifm among the advocates of cryftallography. 



Rome de L'Ifle, in thejear 1772, publilhed the firft edition of his Eflay on Cryftallogra- 

 phy, wlilch he ftates to be a fupplement to Linnaeus ; and, by the affiftance of a very few 

 friends, he was enabled to increafe the number of cryftals in a degree to affume the appear- 

 ance of a fyftem. He told me that the accuracy of his meafurement of angles of minute 

 cryftals was the acquireaient of great practice ; but that the Count De Bournon, after a 

 fhort practice, attained equal corretStnefs, and afforded him affiftance, which he acknowledges, 

 in his fecond edition, to have received, particularly by the difcovery of cryftals in Dauphinc, 

 Auvergne, Franche-Comte, &c. 



The Abbe Hauy, an accurate and patient obferver, and a good mathematician, confidered 

 cryftallography as founded on certain laws, reducible to demon'ftration by calculation. In 

 the beginning, the differences of Bourguet and Capeller were not more pointed than thofe of 

 "Rome de L'Ifle, and the Abbe Hauy ; but the progrefs of obfervation and calculation having 

 demonftrated their mutual utility, the obferver and meafurer of cryftals will now reft fatisfied 

 only when calculation confirms adual meafurement. To the Abbe Hauy is alfo due a late 

 fcheme to fimplify calculation, by expreffing, according to algebraical formulae, the different 

 laws which determine the modification of cryftals. So far as they are the refult of calcula- 

 tion and meafurement, we may admit the laws of calculation ; for whenever the fuperpofition, 

 or fubtradtion, of fimple or compound molecules, on a nucleus, Ihall, by calculation, give a fc- 

 ries of planes and angles, which correfponds exadtly to the angles and planes meafured on 

 natural cryftals, it will amount to no more nor lefs than a demonflration of the rule or ar- 

 rangement of ele£tive attraction by figures. 



Thefe laws may be reduced to fimple practice ; for inftance, the Abbe Hauy, by meafuring 

 the rhombic plane of corundum, found its two diagonals to be as »wo to three : which gives 



to 



