M. W. Beetz on the Colour of Water. 219 



organic substance in the form of mimic acid. If a water does 

 not contain much humic acid, this is not caused by a want of 

 humic acid in the ground, but by the fact that this ground did 

 not give to the water an adequate quantity of alkaline solvent 

 material. 



From these results we may consider the question settled as 

 to why, on chemical principles, some waters are blue, others 

 green, and others brown. I may be permitted to make a few 

 remarks on some physical phenomena which have been observed 

 on coloured waters. 



Formerly water was almost universally classed among those 

 bodies which have a different colour in transmitted, to that 

 which they have in reflected light. Newton says *, " water re- 

 flects the violet, blue, and green rays, but readily transmits the 

 red." Count Xavier de Maistre f considers the colour of water 

 to be blue in reflected, and yellowish orange in transmitted 

 light. AragoJ, that it is blue in reflected, and green in trans- 

 mitted light. The view that the blue of water only occurs in 

 reflected light is common to all three statements. 



In the experiments which Bunsen made to ascertain the 

 colour of distilled water, transmitted light was alone concerned, 

 and yet he found the colour blue. In order to look through 

 still longer columns of water I used the following apparatus : — 

 A box, the bottom and sides of which (a a, fig. 2, Plate I.) are 

 made of plates of gutta percha, is closed at both ends by parallel 

 plates of very white thin plate glass, b V . Directly inside these, 

 two similar glass plates are fixed, which are covered with a silver 

 reflecting surface, by Liebig's method. A narrow slit is scratched 

 in the covering at d and d', as seen in fig. 3, Plate I. If a pencil 

 of direct sunlight is projected upon slit d, this will be reflected 

 several times backward and forward between the two mirrors ; 

 if the box is filled with a liquid, the light is compelled to tra- 

 verse this liquid repeatedly, and it is easy to increase or diminish 

 the length of the layer to be traversed, by altering the angle of 

 incidence. This experiment may be made either objectively or 

 subjectively. If the pencil is allowed to fall into the slit d, 

 so that after a certain even number of reflexions it falls directly 

 upon the slit d 1 , it can be caught upon a screen after its 

 emergence. The number of reflexions may be altered by gra- 

 dually rotating the box. But if the observer uses the illuminated 

 slit d as a self-luminous object, and looks through d 1 into the 

 box, he sees, close to one another, a series of narrow subjective pic- 

 tures of the slit ; they are gradually smaller and nearer each 



* Optices, lib. i. pars 11. Prop. 10. Exp. 17. 



f Salmonia, 3 ed. p. 31 7 ; Pogg. Ann. 1st Supplement, p. 67* 



X Comptes Rendus, vii. 219 ; Pogg. Ann. vol. xlv. p. 470. 



Q2 



