144 FRAGMENTS OF SCIENCE. 



Capital specimens of such glass are to be found at Salviati's 

 in St. James's Street. What artists call "chill" is no 

 doubt an effect of this description. Through the action of 

 minute particles, the browns of a picture often present the 

 appearance of the bloom of a plum. By rubbing the var- 

 nish with a silk handkerchief optical continuity is estab- 

 lished, and the chill disappears. Some years ago I wit- 

 nessed Mr. Hirst experimenting at Zermatt on the turbid 

 water of the Visp, which was charged with the finely-divided 

 matter ground down by the glaciers. When kept still for 

 a day or so, the grosser matter sank, but the finer matter 

 remained suspended, and gave a distinctly blue tinge to 

 the water. The blueness of certain Alpine lakes has been 

 shown to be in part due to this cause. Professor Roscoe has 

 noticed several striking 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 observed, exhibits at a certain stage of its con- 

 densation the colors of the sky. It is blue by reflected 

 light, and orange or red by transmitted light. The same 

 effect, as pointed out by Goethe, is to some extent ex- 

 hibited by peat-smoke. More than ten years ago I amused 

 myself at Killarney by observing on a calm day the straight 

 smoke-columns rising from the cabin chimneys. It was 

 easy to project the lower portion of a column against a 

 dark pine, and its upper portion 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 

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

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

 cipitate above referred to koks yellowish by transmitted 



von der Rosskastanie, man stecke denselben in ein Glas Wasser, und in 

 der kiirzesten Zeit werden wir das vollkommenste Himmelblau entstchcn 

 sehen." Goethe's Werlce, b. xxix., p. 24. 



