I^o Mr. Delaval on the Cauje of the 



" their denfities without any confiderable varia- 

 " tion. But the refradive powers of thefe unc- 

 " tiious fubftances, are two or three times great- 

 •* er, in refpedl of their denfities, than the refrac-. 

 " tive powers of the former fubftances in refpef^ 

 «« of theirs." 



The clear conceptions which Sir Ifaac Newton 

 had formed, refpedting the refra(flive power of the 

 inflammable principle, afford an inftance of un- 

 paralleled penetration and difcernment. 



Diamonds, from their apparent refemblance to 

 cryftallinevitrifiableftones,andgems, were univer- 

 fally held, by naturalifts and lapidaries, not to 

 have differed in their conftituent matter, from fuch 

 ftones and gems, except in a greater degree of pu- 

 rity. 



Sir Ifaac Newton, judging of diamonds by their 

 refraflive power, clafTes them with inflammable 

 bodies, and as they are endued with the ftrongeft 

 refractive power, he does not fcruple to rank them 

 at the head of the inflammable bodies, as confift:- 

 ing of the pureft phlogifton. 



This remarkable obfervation has not, 1 believe, 

 ever been noticed, by philofophers, or naturalifts, 

 as an object of their confideration : doubtlefs, be- 

 caufe they were prejudiced by the apparent quali- 

 ties of diamonds, and their refemblance to thofe 

 of other gems, which feem to have with-held their 

 attention from the powers and properties, the 

 difcovery of which was drawn from the more cer- 

 tain teft of optical inveftigation. 



After 



