290 Red Colour of Morning Clouds y &c. [Book III. 



but the more refrangible rays are intercepted and re- 

 flected by the particles, and the mixture of thofe rays 

 produces a blue colour. 



In air, as well as in the folid femi-pellucid media, 

 \vhen the white particles are more denfely arranged, 

 the intervals which intercede them are diminifhed, and 

 in this Hate of the atmo.fphere a great proportion of 

 all the rays are reflected, fo as to produce the effect of 

 perfect whitenefs, or at leaft an approach towards it. 

 Thus, when the part of the atmofphere, which is near 

 the furface of the earth, is occupied by grofs vapours, 

 this mixture of air with aqueous or orher particles is 

 \vhite: fuch is the common appearance of fogs. When 

 fuch vapours are elevated high in the atmofphere, and 

 form clouds, they reflect the white light of the fun, 

 and appear white, whenever its incident rays fall on 

 them entire and undivided; and as the reflecting par- 

 ticles are not equally diffufed through every part of the 

 pellucid air, of which the atmofphere principally con- 

 fifts, it frequently happens that large tracts of air are 

 only furniftied with fuch a portion as qualify them to 

 reflect a blue colour, while others are fo denfely ftored 

 as to form clouds. 



Of the red and vivid colour of the morning and 

 evening clouds Mr. Melville has fuggefted a caufe 

 upon fimilar principles, which we muft at leaft allow is 

 ingenious and probable. He fuppofes, as well as Mr. 

 Delaval, that a reparation of the rays is made in paf- 

 fing through the horizontal atmofphere, and that the 

 clouds reflect and tranfmit the fun's light, as any half 

 tranfparent colourlefs body would do ; for as the at- 

 mofphere reflects a greater quantity of blue and violet 



air is thick, and [the fun nearly cppofite them, of a warmer co- 

 lour than they otlierwife would, and more approaching to yellow 

 or orange. 



rays 



