218 M. W. Beetz on the Colour of Water. 



in a state of perfect quiescence. Finally, metallic thermometers 

 never give certain indications. Their scales are continually dis- 

 placed by the same causes which determine the displacement of 

 zero for the mercurial thermometer; but the displacement of 

 zero, which does not exceed a few tenths of a degree in the mer- 

 curial thermometer, may reach several degrees in the metallic 

 thermometer. 



I will say, in conclusion, that it is especially interesting to 

 know the law which the temperature follows in the various atmo- 

 spheric layers during the night ; thus a balloon ascent which had 

 this object ought to be made during a calm night by the light of 

 the moon ; and the method I propose is then quite applicable. 



XXXI. On the Colour of Water. By W. Beetz*. 



[With a Plate.] 



IT is only in recent times that explanations based upon actual 

 experiments have been given of the colour of water in the sea, 

 in lakes, and in rivers ; it was previously thought sufficient to 

 conceal the entire ignorance of a daily-observed phenomenon 

 by hypothesis. Bunsenf was the first to state, and establish 

 experimentally, the simple proposition that " chemically pure 

 water is not, as commonly assumed, colourless, but naturally 

 possesses a blue colour." He observed this coloration on 

 looking at a piece of white porcelain through a column of water 

 two yards long. He explained the brown to black coloration 

 of many waters, especially of North German inland lakes, as 

 arising from an admixture of humus ; the green colour of the 

 Swiss lakes, and, still more so, the siliceous springs of Iceland, 

 as arising from the colour of the yellowish base, and of the 

 siliceous sinter surrounding the springs, and which is caused 

 by traces of hydrated oxide of iron. Wittstein {, by careful 

 chemical investigations, has quite recently shown that the green 

 colour also derives its origin from organic admixtures. Accord- 

 ing to him, the less organic substance a water contains, the less 

 does its colour differ from blue. With the increase of organic 

 substances, the blue gradually passes into green, and from this, 

 as the blue is more and more displaced, into brown. Water is 

 softer the nearer it is to brown, and harder the nearer it is to 

 blue ; this does not arise from a greater or less quantity of organic 

 substance, but of alkali, on which, again, the proportion of dis- 

 solved organic substance depends. This alkali dissolves the 



* Translated from Poggendorff's Annalen, January 1862. 

 t Liebig's Annalen, vol. lxii. p. 44. 



% Sitzungsber. der K. bayer. AJcad. der Wissensch, in Munchen, 1860, 

 p. 603. 



