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IV. On the Nature of the Diamond. By Smithson Tennant, 
Esq. F.R.S. 
Read December 15, 1796. 
Sir Isaac Newton having observed that inflammable bodies 
had a greater refraction, in proportion to their density, than 
other bodies, and that the diamond resembled them in this 
property, was induced to conjecture that the diamond itself 
was of an inflammable nature. The inflammable substances 
which he employed were camphire, oil of turpentine, oil of 
olives, and amber; these he called “fat, sulphureous, unctuous 
“ bodies and using the same expression respecting the dia- 
mond, he says, it is probably “ an unctuous body coagulated.” 
This remarkable conjecture of Sir Isaac Newton has been 
since confirmed by repeated experiments. It was found that, 
though the diamond was capable of resisting the effects of a 
violent heat when the air was carefully excluded, yet that on 
being exposed to the action of heat and air, it might be en- 
tirely consumed. But as the sole object of these experiments 
was to ascertain the inflammable nature of the diamond, no 
attention was paid to the products afforded by its combustion ; 
and it still therefore remained to be determined whether the 
diamond was a distinct substance, or one of the known in- 
flammable bodies. Nor was any attempt made to decide this 
question till M. Lavoisier, in 1772, undertook a series of 
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