ft3'2 Mr. Velaval on the Caiife of the 



from thf air. And, as they are, for the moft 

 part, colourlefs, they refledl a white light, and 

 therefore may be confidered as white particles, 

 diifufed throughout the pellucid colourlefs air. 



In this refpefl, the atmofphere is fimilar to 

 the feniipellucid mediunn, which is formed by 

 difFufing the white particles of arfenic, through- 

 out pellucid colourlefs glafs. In both thefe 

 fubftances, whilft the white, or colourlefs, par- 

 ticles are rarely diffeminated through the tranf- 

 parent medium, the lefs refrangible rays are 

 tranfmitted through the intervals which intercede 

 the particles, and compofe an orange, or red, 

 colour : but, the more refrangible rays are inter- 

 cepted, and reflected, by the particles, and the 

 mixture of thofe rays produces a blue colour. 



In the air, as well as in the folid femipellucid 

 media, when the white particles are more denfe- 

 :ly arranged in the pellucid medium, the intervals 

 which intercede them are diminifhed, and the dif- 

 tance, between the furface of the pellucid medi- 

 um., and the white particles, is alfo reduced. 



In this fl:ate of the atmofphere, as the greateft 

 part of the yellow, orange, and red rays, are 

 intercepted, and reflefled, together with the 

 more refrangible rays, they, by their union, 

 exhibit a hue nearly approaching towards whice- 

 refs. 



When the part of the atmofphere, which is 

 near the furfi^ce of the earth, is occupied by 



grof% 



