2 24 ^^'"- Delaval on the Cauje of the 



oil, which is admitted in its ftead, from the 

 nearnefs of its denfity, to that of the ambient 

 powders, reflects no fenfible light, fo that the 

 mafs, which confifts of fuch uniformly denfe 

 media, is black. 



When fmooth furfaces of dark-coloured marble, 

 or flate, or of any other pollfhed coloured fub- 

 flances, are fcratched, the air enters into the 

 interftices which are opened by this operation, 

 and according to the excefs of its rarity, over 

 that of the maffes whofe particles it intercedes, 

 it ftrongly refledls a whiter, or lighter-coloured, 

 hue. But, when by polifhing the Rufacc, or filling 

 the pores with a denfer medium, the air is re- 

 moved from them, the darker hue is reftored. 



From all the preceding experiments and ob- 

 fervations, it appears that vegetable, animal, 

 and mineral Coloured Matter is tranfparent ; that 

 it does not refledt colours, Kut exhibits them by 

 tranfminion only, that Opake Coloured Bodies 

 confifl; of tranfparent matter, which covers 

 opake white particles, and tranfmits the light, 

 which is refledled from them. 



I fliall, in the next place, examine the nature 

 of Semipellucid Coloured Subilances, which 

 appear of one colour, when viewed by incident 

 light, and of another, when viewed by tranf- 

 mitted light : and I fhall endeavour to fhew, 

 from fever?l experiments and obfervations, that, 

 in thefe, as well as in Opake Coloured Bodies, 



no 



