Professor Forbes oJi the Colours of the Atmosphere. 35 



Since the red steam occurs only during the critical stage of its 

 partial condensation (and perhaps conversely during evapo- 

 ration), it is evident that it must correspond to a critical state 

 of diffused vapour of the atmosphere. The applications might 

 be very extended ; I will onlj' advert to one, the surest, most 

 consistent, and probably the most ancient of such prognostics. 

 The red evening and grey morning as the signs of fine weather, 

 are recorded in the verses of Aratus*, in the New Testa- 

 mentf , and in one of our most familiar proverbs. It is wholly 

 inexplicable on the theory of Brandes, which considers the 

 redness as due solely to the purity of the atmosphere, since 

 that is usually greater in the morning than the evening. Ac- 

 cording to my view it occurs thus: Soon after the maximum 

 temperature of the day and before sunset, the surface of the 

 ground, and likewise the strata at different heights in the at- 

 mosphere, begin to lose heat by radiation. This is the cause 

 of the deposition of dew, and consequently in severe weather 

 we have vast tracts of air containing moisture in that critical 

 state which precedes condensation, and yet it may be exceed- 

 ingly doubted whether any vapour properly called vesicular 

 is necessarily formed in this process. Be that as it may, every 

 accurate observer of nature in alpine countries will confirm 

 me in stating, that fine weather is almost invariably accom- 

 panied by the formation of dew on exposed surfaces, and by 

 the progressive depression of the moister strata, until at length 

 visible fogs are formed in the bottom of the valleys, and espe- 

 cially over water;]:. This is the surest sign of a following fine 

 day in mountainous regions. Now Saussure in his ascent of 

 Mont Blanc, " observed that the evening vapour which tem- 

 pered the sun's brightness, and half concealed the immense 

 space he had below him, formed the finest purple belt, encir- 

 cling all the western horizon, and as the vapour descended 

 and became more dense, became narrower and of a deeper 

 colour, and at last of a hlood-red\\" Now this phsenomenon 

 corresponds, I imagine, precisely to the development of colour 

 which I have remarked in vapour in the act of being con- 

 densed, and De la Rive's remark, that the nocturnal illumi- 

 nation of Mont Blanc takes place in serene evenings, 'when the 



* Diosemeia, 93. quoted by Kamtz. f Matt. xvi. 2, 3. 



X For the reason why over water, see Davy's Paper, Pliil. Trans. 1819. 



(I Quoted by Harvey in Ency. Metrop. Meteorology, p. *166. The cause 

 of the purple light mentioned here, probably arises from a mixture of the 

 reflected bine of the pure sky {which is always present ivlien purple is seen) 

 with the yellow-orange, which condensing vapour first transmits. I do not 

 think it at all necessary to affinn, however, that pure air has no transmitted 

 colour of its own. 



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