68 



LECTURES AND ESSAYS 



to act in the manner indicated ? No 

 doubt of it. Each one of you can 

 submit the question to an experimental 

 test. Water will not dissolve resin, but 

 spirit will dissolve it ; and when spirit 

 holding resin in solution is dropped into 

 water, the resin immediately separates 

 in solid particles, which render the water 

 milky. The coarseness of this precipitate 

 depends on the quantity of the dissolved 

 resin. You can cause it to separate 

 either in thick clots or in exceedingly 

 fine particles. Professor Briicke has 

 given us the proportions which produce 

 particles particularly suited to our present 

 purpose. One gramme of clean mastic 

 is dissolved in eighty-seven grammes of 

 absolute alcohol, and the transparent 

 solution is allowed to drop into a beaker 

 containing clear water, kept briskly 

 stirred. An exceedingly fine precipitate 

 is thus formed, which declares its pre- 

 sence by its action upon light. Placing 

 a dark surface behind the beaker, and 

 permitting the light to fall into it from 

 the top or front, the medium is seen to 

 be distinctly blue. It is not perhaps so 

 perfect a blue as may be seen on excep- 

 tional days among the Alps, but it is a 

 very fair sky-blue. A trace of soap in 

 water gives a tint of blue. London, and 

 I fear Liverpool, milk makes an approxi- 

 mation to the same colour, through the 

 operation of the same cause ; and Helm- 

 holtz has irreverently disclosed the fact 

 that the deepest blue eye is simply a 

 turbid medium. 



The action of turbid media upon light 

 was illustrated by Goethe, who, though 

 unacquainted with the undulatory theory, 

 was led by his experiments to regard 

 the firmament as an illuminated turbid 

 medium, with the darkness of space 

 behind it. He describes glasses showing 

 a bright yellow by transmitted, and a 

 beautiful blue by reflected, light. Pro- 

 fessor Stokes, who was probably the first 

 to discern the real nature of the action 

 of small particles on the waves of ether, 1 



1 This is inferred from conversation. I am 



describes a glass of a similar kind. 1 

 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 varnish with a silk hand- 

 kerchief optical continuity is established 

 and the chill disappears. Some years 

 ago I witnessed Mr. Hirst experimenting 

 at Zermatt on the turbid water of the 

 Visp. When kept still for a day or so, 

 the grosser matter sank, but the finer 

 particles 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 locomo- 

 tive, when favourably observed, exhibits 

 at a certain stage of its condensation 

 the colours 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 

 exhibited 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 trans- 

 mitted light. Such smoke was not in 







not aware that Professor Stokes has published 

 anything upon the subject. 



1 This glass, by reflected light, had a colour 

 "strongly resembling that of a decoction of 

 horse-chesnut bark." Curiously enough, Goethe 

 refers to this very decoction : " Man nehme 

 einen Streifen frischer Rinde von der Rosskas- 

 tanie, man stecke denselben in ein Glas Wasser, 

 und in der kiirzesten Zeit werden wir das vollkom- 

 mensteHimmelblau entstehen sehen." Goethe's 

 Werke, P.. xxix., p. 24. 



