ON THE CLEARING OF TURBID SOLUTIONS. 63 



If by the influence of light more spreading fluid is formed on the 

 light side than on the shaded side of the suspended particles the sus- 

 pended particles will go towards the light. I call this phenomenon posi- 

 tive photodromy. 



If the influence of light stops the formation of the spreading solution 

 or the spreading fllm, the flocks would go to the shaded side, or will show 

 negative photodromy. 



A retarding influence of the light is not probable, but many physicists 

 suppose with E. Becquerel a retarding or stopping influence of red light 

 in the case of the fluorescence of Sidot-Blende. I think that the negative 

 photodromy may also be explained by the heating efiect of the light and 

 the formation of air bubbles on the light side of the suspended particles. 

 The air bubbles will hinder the spreading of the newly formed solution on 

 the surface of the suspended particles, and the vortices of suflicient 

 energy will only exist on the shaded side, and the flocks will go away from 

 the light or show negative photodromy. 



Turbid solutions of gum mastic, silica, sodium or potassium silicate, 

 kaoline, gummi gutti, shellac, soap, proteid, can remain apparently 

 unchanged for months or years, but after some weeks or months we can 

 always And flocks at the bottom of the solution. Moreover horizontal 

 layers are formed with more or less suspended particles. 



What is the reason of the stability of the turbid solution 1 Hardy ' and 

 J. J. Thomson see the reason for the stability in the electromotive force 

 at the boundary of the suspended particles and the surrounding fluid, 

 which hinders the movement of the solid particles, while, according to 

 Dorn,^ electric work is done by the displacement of the particles. The 

 action is the same as if the viscosity of the fluid had been increased. 



That electric work is done by the displacement of suspended particles, 

 or by the displacement of fluids over the solid walls of porous bodies, and 

 that electromotive force exists at this boundary was known before the 

 researches of Dorn, and is a consequence of my old researches on capillary 

 electric currents.^ If the explanation of Hardy and J. J. Thomson should 

 be right, the turbid solutions must have the greatest stability if the sus- 

 pended particles show the greatest electromotive force in contact with the 

 surrounding fluid — i.e., sulphur, silica, shellac, suspended in water. But 

 shellac gives turbid solutions of little stability. It may be that the 

 electromotive force at the boundary of liquid and suspended particles may 

 increase the stability of the suspension, but the principal reason of the 

 stability may be that the velocity of the falling particles is not constant, 

 but variable or periodic. The impulses of the periodic velocity are 

 propagated with the velocity of sound, and will be reflected inside or at 

 the bottom of the turbid solution. The direct impulse will interfere with 

 the reflected impulses, and the particles will be collected in horizontal 

 layers at distances of half a wave length. 



The air also separated at the common surface of the suspended 

 particles and the surrounding liquid has in many cases an important 

 influence, and will be attached to it or will cover it. The diameter of the 

 air bubbles or thickness of the thin air cover may be so small that it is 

 not possible to see it with the best microscope, but it forms the condensa- 

 tion nuclei for masses of absorbed air previously separated. 



' Hardy, Proc. Roy. Soc, 1900, vol. Ixvi. p. 123. 



* Dorn, Wiedemann's Anrialen, 1880, vol. x. p. 70. 



' G. Quincke, Poggevdorff's Annalen, 1860, vol. ex. p. 56; 1861, vol. cxiii. p. 546. 



