Permanent Colours of Opah Bodies. 23 3 



grofs vapours, this mixture of air with aqueous, 

 or other particles, is white : and, the anterior 

 furface, as well as the interior parts, of the com- 

 pounded medium, appears to be denfely ftored 

 with white particles. Fogs, which confifV of 

 grofs vapours, feem chiefly to differ from 

 clouds, in their deprelTion, and contiguity to 

 the furface of the earth. 



When fuch vapours are elevated higher in 

 the air, and form clouds, they reflect the white 

 light of the fun, and thereby appear white, 

 whenever its incident rays fall upon them incire 

 and undivided. 



As the reflexive particles are not equally 

 diffufed through every part of the pellucid air, 

 of which the atmofphere principally confifl;s, 

 it frequently happens, that large trads of air 

 p,re furni Plied with only fuch a portion of fub- 

 tile contents, as qualify them to refietfl a blue 

 colour; whiin others are fo denfely ftored with 

 refledive particles, that they conftitute clouds. 



The inferior parts of the atmofphere which 

 are adjacent to the furface of the earth, con- 

 ftantly abound in denfe exhalations, more than 

 the higher regions. 



Naturalifts have not hitherto obtained any 

 confiderable infight into the nature, and origin, 

 of the particles by which the tranfparency of 

 the air is impaired, except as far as relates to 



aqueous 



