P€^-mane'iit Colours of Opeke Bodies. 26 1 



Many marcafites, which fcarcely yield to any 

 of the rtiecals, in luftre, do not contain any me- 

 tallic particles, but confift principally of fulphur, 

 which itfelf is compofed of phlogifton, and vi- 

 triolic acid. 



Nor is the luftre, and metallic appearance, con- 

 fined to fubftances which abound in phlogifton : 

 for, it is exhibited by the furfaces of feveral Pellu- 

 cid Colourlefs Media, which vividly refledl the 

 light; from the difference between their denfity, 

 and that of the ambient media; from their glofiy 

 fmoothnefs or polilli ; or from various other caufes. 



Mr. Melvill has clearly explained the filver- 

 like appearance, which drops of water exhibit, 

 •when they He on the leaves of colewort, and fome 

 other vegetables. He obferved, that the luftre 

 of thofe drops, is produced by a copious refleclion 

 of light, from the flattened part of their furfaces, 

 contiguous to the plant; that the drops, when 

 they have that appearance, do not touch the 

 plant, but are kept at fome diftance from it, by 

 the force of a repulfive power, or medium, which 

 occupies the intervals between the drops and the 

 leaves : that, as the furfaces of the leaves are con- 

 fiderably rough and unequal, the under furfaces 

 of the drops become rough likewife, and fo by 

 rtfleding the light copiouOy, in different direc- 

 tions, affurne the refplendent whitenefs of un- 

 pol idled filver. * 



» Phjfical EfTays: Edinburgh. 17^6. vol. IF. p, 2j. 



S 3 When 



