274 Caufe of the "permanent [Book III. 



which it enters. Thus, tranfparent bodies reduced to 

 powder appear white, which is no other than a co- 

 pious reflection of the light from all the furfaces of the 

 minute parts, and from the air which is interpofed be- 

 tween thefe particles. 



The general appearance of a flrong infufion of co- 

 chineal is black; but when agitated, its furface is co- 

 vered with a red froth. The reafon is, that the light 

 is reflected from the globules of air inclofed in each 

 of the bubbles which conftitute the froth, and is tranf- 

 mitted through the films of red liquor which cover 

 them. Several vitreous fubftances in like manner ap- 

 pear black in a folid mafs, but when powdered, of a 

 different colour. The action of thefe powders on the 

 rays of light is the effect, of the difcontinuance of their 

 parts, and the air being admitted into the interfaces, 

 the light is tranfmitted through the thin tranfparent 

 particles of the glafs, which give it that tinge the 

 powder exhibits. If oil, inftead of air, intercedes 

 the interftices of powdered fubftances, in proportion as 

 it approaches to the denfity of the fubftances them- 

 felves, and as it exceeds air in this refpect, it renders 

 the colour proportionably darker. " Thus when in- 

 digo, and other tranfparent paints, are united with oil, 

 the air is expelled from their interftices, and the oil 

 which is admitted in its ftead, from the nearnefs of its 

 denfity to chat of the powder, 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 any other polifhed fub- 

 ftance, 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 mattes which it 

 intercedes, it reflects a whiter or lighter coloured hue. 

 . By 



