126 FRAGMENTS OF SCIENCE 



tal 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 mi- 

 nute 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 Yisp. When kept still ior a day or so, the 

 grosser matter sank, but the finer particles remained sus- 

 pended, 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 re- 

 markable 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 by observing, on a calm day at Killarney, 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. 

 Briicke's fine precipitate above referred to looks yellow- 

 ish by transmitted light; but, by duly strengthening the 



