Permanent Colours ofOpake Bodies. 191 



After Sir Ifaac Newton had publiihed his Op- 

 tical Works, almoft a century had pafled, dur- 

 ing which no experiments were made, whereby 

 the conftituent matter of diamonds could be 

 afcertained. Several eminent chymlfts, and phi- 

 lofophers, in France, have recently applied great 

 fkill and induflry in the examination of that 

 fubject. 



From their inquiries, it appears that diamonds, 

 when enveloped in powdered charcoal, and in- 

 clofed in crucibles carefully luted, refift the force 

 of fire, and remain unaltered, in the fame manner 

 as charcoal, and other folid inflammable bodies. 



When expofed, under a receiver, to the focus 

 of a large burning glafs, they impart phlogiftoa 

 to the air which is contained in the receiver. 



When placed, under a muffle, upon a cupel or 

 ftand of white calcined refradlory earth, in a de- 

 gree of heat fufficient to melt pure filver, the 

 diamonds fend forth a continued flame, and thus 

 entirely burn away. 



When expofed to the fire, in thick vefTels of 

 white porcelain, perfedly clofed with floppies of 

 the fame fubftance, fo that their cavities contain 

 only air and the inclofed diamonds, they are en- 

 tirely diflipated, and difappear, without leaving 

 the leaft trace of the matter, of which they were 

 formed. 



Thcfe experiments prove, that diamonds confift 

 ♦f phlogiftonj and they are the only natural fub- 

 ftance. 



