54® C« the Corundum Stone from Afta. 



to its acute angle 8i° 47' 10", and to its obtufe angle 98° 12' 50"-, the fame as martial vi- 

 triol*. The forms of fragments in, corundum are all acute rhomboids. The cofme of the 

 little angle in corundum is one-feventh of the radius ; but in calcareous fpar, the cofme is 

 one-fifth of the radius ; in fchorl two-fifths of the radius •, in the garnet, one-third ; and in rock- 

 cryftals, one-feventeenth. Thus the application of general laws, to afcertain conftant cha- 

 racter, after they fhall have been fully verified, may be very fimple and general. It will not 

 require perfeft cryftals; for when cryftals feparate into laminae, which fubdivide into frag- 

 ments, and fliew the form or arrangement of their molecules, it is eafy, from fuch frag- 

 ments, to conneft them with their primitive cryftal, and, confequently, with their clafs. It 

 will be a great ftep, to obtain one regular and permanent external charafter. — Attention to 

 other charafters will be neceffary, to afcertain the nature of the fubftance ; and other ex- 

 ternal charafters, fuch as irregular frafture, colour, &c. muft be reforted to, when no per- 

 manent characters exift j but from their nature they are fallible, and, in fa£t, are feldom 

 conclufive. 



The progrefs of cryftallography appearing, to me, of -confequence to the progrefs of mi- 

 neralogy, induced me to defire the Count de Bournon, above mentioned, one of the honour- 

 able vlftims to his allegiance to his king, to defcribe fuch cryftals, in my coUeftion, as Ihewed 

 the different known modifications of corundum ; which will develope the theory of cryftal- 

 lization, fo far as is confiftent with the avowed obje£t of this paper. The fubje£t, I believe, 

 has not hitherto been fubmitted to the confideration of this fociety. The tranflation of the 

 Count de Bournon's defcription has been carefully made, to preferve its clearnefs ; and, I hope, 

 it will be favourably received by the Society, and make fome amends for my tedious intro- 

 duftion. After it, I have added a table, connecting in one view the fpecific gravities of co- 

 lundum, &c. herein mentioned, with thofe given by other authors. 



\An Analytical Defcription of the Cr^aUitie Fonns of Corundum, from the EaJl~IndieSy and from 



China. By the Count D£ BouRNON, 



"THE raoft ufual form of corundum is a regular hixsedral prifm (Plate XXIV. Fig. i.) ; in 

 general, the furface of the cryftal is rough, with little luftre, owing to unfavourable circum- 

 ftances, under which It is cryftallized. 



The cryftals of corundum, hitherto found, were not formed in cavaties, where each cryft J 

 being infulated, its furface could preferve that fmoothnefs, and natural brilliancy, which are 

 common to all fubftances that freely aflume a cryftalline form. Like the cryftals of feldfpar 

 which -we meet with in the porphyroid granites, the corundum cryftals have been enveloped, 

 at the time of their cryftallization, by the fubftance of the rock, which was forming at the 

 fame time with themfclves, in an imperfeft and confufed cryftalline mafs ; and the corundum 

 cryftal, before it had acquired its perfeCt folidity, neceffarily received on its furface the im- 



* This refolt is extrafted from the Journal de Phy fique ; but it appears, from the Journai des Mines, Na 28, 



«hat the Abbe Hauy has fmce reftified t^is mcafurc, and giv<n Sfl" »6' for the acute angle, and 930 34' for 



the obtufe angle. — G. _ . 



preuion 



