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the blueness of certain Alpine lakes is in part due to 

 this cause. Professor Roscoe has noticed several strik- 

 ing cases of a similar kind. In a very remarkable paper 

 the late Principal Forbes showed that steam issuing 

 from the safety valve of a locomotive, when favorably ob- 

 served, exhibits at a certain stage of its condensation 

 the colors of the sky. It is blue by reflected light, and 

 orange or red by transmitted light. The effect, as 

 pointed out by Goethe, is to some extent exhibited by 

 peat smoke. 



More than ten years ago I amused myself at Killar- 

 ney, by observing on a calm day, the straight smoke col- 

 umns rising from the chimneys of the cabins. It was 

 easy to project the lower portion of a column against 

 a bright cloud. The smoke in the former case 

 was blue, being seen mainly by reflected light ; in 

 the latter case it was reddish, being seen mainly 

 by transmitted light. Such smoke was not in ex- 

 actly the condition to give us the glow of the Alps, 

 but it was a step in this direction. Briicke's fine pre- 

 cipitate above referred to looks yellowish by transmitted 

 light, but by duly strengthening the precipitate you may 

 render the white light of noon as ruby colored as the 

 sun when seen through Liverpool smoke or upon Alpine 

 horizons. 



I do not, however, point to the gross smoke arising 

 from coal as an illustration of the action of small parti- 

 cles, because such smoke soon absorbs and destroys the 

 waves of blue instead of sending them to the eyes of the 

 observer. 



These multifarious facts, and numberless others which 

 cannot now be referred to, are explained by reference to 

 the single principle that where the scattering particles 



