Permanent Colours of Opake Bodies. 257 



which alfo are perfeflly white, when pure, and 

 divefted of adventitious matters. 



All thefe earths conftitute Tranfparent Coloiir- 

 lefs Media, when they are vitrified, together with 

 proper fluxes ; or when they are dilTolved in co- 

 lourlefs mcnftrua. And the faline malTes, which 

 are obtainable from their jbkitions, are tranfparent 

 and colourlefs, while they retain the water which 

 is effcntial to their Civftallization, and are not 

 flawed, or reduced to powder. 



But when, by fuch means, the interftices of thofe 

 mafixs are evacuated, or opened, they are render- 

 ed white and opake, by the admifllon of air, or 

 of a rarer medium. 



The earthy particles, which form the folid parts 

 of bodies, generally exceed, in denfity, the other 

 conftituent particles, which they intercede. For, 

 in the compoficion of bodies, earths are, for the 

 mofi: part, combined with aqueous, aerial, faline, 

 or inflammable matters-, all of which are greatly 

 i-nferior to earths, in their fpeci fie denfity. And, 

 therefore, the furfacesof the earthy particles, which 

 are contiguous to fuch media, muft refiecl the raya 

 of light, with a force which is proportionate to 

 their excefs of denfity. 



The refledive power of bodies does not depend 

 merely upon their excefs of denfity, but upon their 

 diflx-rence of denfity, with refped to the ambient 

 media. I'herefore, Tranfparent Colourlefs Par- 

 ticles, whofe denfity is greatly inferior to that of 

 the media which th<;y intercede, alfo powerfully 



Vol. II. S ' "refidc 



