350 On th£ Cdcnir of the Atmosphere and Deep Water, 



mixed in the ordinary paste of white glass, in such quantity 

 as to produce in the mass a shght bluish tinge, without altering 

 materially its transparency ; this powder appears in a state of 

 extreme division, or as if it had undergone a slight degree of so- 

 lution, which does not disperse the transmitted light. 



The colour of the light transmitted by opahne bodies, varies 

 according to their size ; it is yellow if the body is thin, and be- 

 comes successively orange and red as the thickness is increased. 



The analogy between the air and opaline substances is not 

 only shewn by the blue reflection, but also by their action on the 

 transmitted light, which becomes successively yellow, orange, 

 and red, according to the volume of air that transmits it, and the 

 nature of the aqueous vapours with which it is impregnated. 

 When the sun is risen above the horizon, and his light traverses 

 the purest and least dense part of the air, the rays are white, 

 with a slight tinge of yellow ; as he sinks by degrees, they some- 

 times appear yellow and orange ; and when the light falls aslant 

 on the earth, and is transmitted by air of the greatest density, 

 and charged with the vapours of the evening, they are perceived 

 to be of a red colour, or even purple. 



But it often happens, that the colours are not observed, and 

 the sun sets without producing them. It is not, therefore, to 

 pure air alone that we must attribute the opaline property of 

 the atmosphere, but to the mixture of air and aqueous vapour 

 in a particular state which produces an effect analogous to that 

 of the powder of calcined bones in opaline glass. Neither is it 

 the quantity of water which the air contains that occasions these 

 colours, for when it is very humid it is more transparent than it 

 is in an opposite state, the distant mountains then appearing more 

 distinct — a well known prognostic of rain — and the sun then sets 

 without producing colours ; in the fogs and vapours of the 

 morning, the light of the sun is white, but the red colour of the 

 clouds at sunset is generally regarded as the forerunner of a 

 fine day, because these colours are a proof of the dryness of the 

 air, which then contains nothing more than the particular dis- 

 seminated vapours to which it owes its opaline property. In 

 this state of the air, the disk of the sun sometimes appears like 

 a globe of fire, and deprived of rays. 



According to the nature of the vapours disseminated through 

 1 



