﻿M. L. Soret on the Colour of the Lake of Geneva. 345 



segregated out at certain faces of the lead crystals. By the ad- 

 dition of a small quantity of arsenic or antimony the pink colour 

 was replaced by a dull purple; and a clear pink tint was only ob- 

 tained when all the oxidizable metals had been removed. 



I come now to the discussion of the state in which the silver 

 exists to cause a pink or reddish reflection of light. Silver does 

 not oxidize under the conditions of exposure to acetic-acid vapour 

 and oxygen of the air. Moreover oxide of silver and silver car- 

 bonate are themselves decomposed and reduced to a metallic 

 state by a heat below that attained in the stacks of fermenting 

 tan. The silver must consequently be in the metallic state. As 

 confirming this statement I made the following experiments : — 

 Silver carbonate was triturated with white lead and water and 

 then dried. Upon increasing the temperature, a delicate pink 

 tint became visible upon the reduction of the oxide of silver. If 

 a small quantity of silver carbonate be precipitated along with 

 lead carbonate, the colour upon drying and heating is more uni- 

 form, and it may be obtained exactly resembling the tint seen on 

 white-lead corrosions. 



The colour of the photographs obtained by means of silver- 

 salts is also evidence in favour of the metallic state of the silver; 

 and I may also adduce the fact that a ray of light, when reflected 

 ten times from a polished silver surface, is distinctly of a reddish 

 colour. 



Collegiate Laboratory, Sheffield, 

 April 19, 1869. 



XLVIII. On the Colour of the Lake of Geneva. By L. Soret*. 

 My DEAR Tyndall, Geneva, March 31, 1869- 



IN thanking you for your letter and the pamphlet you sent 

 me, I take the opportunity of communicating to you an 

 observation which may interest you. 



Whilst dealing with a different subject, I was led to consider 

 whether the blue colour and the absorption of certain rays of light 

 by water are due to the liquid itself or to the therein suspended 

 solid particles. This question has been often discussed by others, 

 as well as by yourself in your work ' The Glaciers of the Alps.' 

 Your memoirs on the polarization of the blue light of the sky 

 have suggested to me the idea that if the blue colour of the 

 water be due to suspended solid particles, phenomena of polari- 

 zation will be produced analogous to those observed by you on 

 the light of the sky. 



The water of the Lake of Geneva, owing to the well-known 

 * Coinmimicated by Professor Tyndall, F.R.S. 



