ijo Mr. Delaval en the Cau/c of the 



tbofe particles and the anterior furface of the 

 glafs ; by which a blue colour is compounded. 

 And the lefs refrangible rays, pervading the 

 femipellucid medium, compound a yellow, or 

 orange colour, as has been already explained, 

 "with refpe6l to the glafs combined with arfenic. 



When a -greater degree of heat is applied tq 

 the glafs, by which the opake earthy particles 

 become more copious, the mafs appears almoft 

 white. Becaufe, as in this denfe llate of the 

 opake particles, almoft all the differently refran- 

 gible rays are intercepted, and refleifted, by them, 

 a hue nearly approaching to ^yhitenefs arifes from 

 their mixture. 



The quantity of ferruginous particles, in this 

 mafs, is lb inconfiderable, and the colour pro- 

 duced by them is fo diluted by the numerous 

 reflexions from the white opake particles, that 

 it becomes almoft imperceptible. 



The atmofphere afls upon the rays of light, in 

 the fame manner as the femipellucid media which 

 have here been defcribed. 



The blue colour of the fky is not produced by 

 refiedion from the atmofphere, confidered as an 

 vjniform extended mafs. For fuch a mafs would 

 refle(5t the colour, from its furface. As reflec- 

 tions are made only at the furfaces of bodies, 

 which differ, in denfrty, from the media, which 

 are contiguous to them ; but the atmofphere does 

 not refledt any coloured light from its furface. 



Thus, 



