90 Of the FLEXIBILirr of 



turn ; and this appearance likewife proceeds from the fame kind 

 of fpecular longitudinal plates, arranged in the fame order. 



I now beftowed fome pains in endeavouring to difcover, by 

 the power of the lens, what was the nature of thofe reflecting 

 fpecular laminse ; but I could not fay affuredly, that there were 

 two different fubftances in the conftruction of this done. The 

 irregular quartzy particles, and thefe fpecular bodies, feemed 

 both to be of a perfectly tranfparent cryftalline-like matter. I 

 then had recourfe to the blow-pipe, in order to refolve my 

 doubts ; and this indeed foon made a diftinction of the diffe- 

 rent fubftances contained in this ftratum. Where the fragment 

 of the ftone had received the intenfe heat of the flame, the fo- 

 liated fpecular ftructure totally difappeared ; and here the irre- 

 gular quartzy particles remained feemingly without change. 

 In the other part of the fragment, which had been heated to 

 incandefcence, the doubt with regard to the fpecular bodies 

 was entirely removed, and the tranfparent mica had now af- 

 fumed its natural appearance ; it had become opake with regard 

 to the tranfmiffion of light, at leaft comparatively, and it gave 

 an argentine appearance by reflection, as maybe perceived in 

 the fpecimens which I have here laid before the Society. 



I now could fee, mod evidently, the connection of the ir- 

 regular ftructure of the quartzy particles, with thofe ftratified 

 parallel plates of mica ; and I alfo underftood the reafon why 

 I could not before diftinguifh the proper connection of thofe 

 two fubftances, which was no other than their perfect tranfpa- 

 rency. But being thus fatisfied of the thin flexible platen of 

 mica, we may now confider the particles of quartz, which 

 have little cohefion, as being bound together by thefe thin 

 plates of tranfparent mica ; and thefe connecting plates being 

 flexible, this allows a certain motion of the rigid particles 

 among themfelves, without the fracture or general feparaiion 

 of the ftone. 



Were 



