16 Scientific Proceedings, Royal Dublin Society. 



diameter 0'4 mm. gradually dissolved, and was observed for two minutes under a 

 field of 90 volts per cm., but showed no motion. Another bubble 0-08 mm. gave 

 the same result. In the second sample a bubble of 0'16 mm. diameter was 

 watched gradually dissolving for two minutes under the same field, and likewise 

 did not move. In view of these contradictory results, it is of interest to compare 

 results on electric endosmose, reported in the same paper by Quincke, with 

 results obtained by Perrin, and to recall Freundlich's comment on the apparent 

 contradiction : — 



" Perrin fand keine Elektroendosmose bei . . . Terpen tinol 



dagegen fand Quincke bei Terpentinol eine Verschiebung der Fliissig- 

 keit zum po.sitivem Pol ... Da schon beim Wasser kleine Zusatze grosse 

 Anderungen der Elektroendosmose bedingen, ist es fraglos, dass Stoffe 

 verschiedener Pi.einheit ein sehr verschiedenes Ergebnis zeitigen konnen " 

 (Kapillarchemie von H. Preundlich, page 241). 



It is of interest also to note that reference^ is made by Coehn and Mozer to 

 experiments showing that pure turpentine does not acquire a charge in bubbling, 

 while they point out that Lenard found that ordinary unpurified turpentine 

 acquired a charge. 



Discussion. 



The experiments on cataphoresis are meant to be purely of a preliminary 

 nature, and the phenomenon is approached from the qualitative view-point rather 

 than the quantitative. Adopting Lamb's" equation as governing the motion of an 

 air-bubble, then the velocity U nnder field X volts per cm. is given by 



u 



where q = surface density of charge in double layer 

 and h = thickness of double layer, 



M = viscosity of liquid. 



Thus the velocity of the air-bubble under a field in any given liquid is a 

 measure of the charge in the inner double layer, i.e., the free charge which would 

 accompany the bubble in motion in the liquid. This formula is laid down as 

 applicable to all liquids which are not perfect insulators. Thus, absence of 

 cataphoresis, under the conditions of the experiments, means either an entire 

 absence of any double layer, or else that the charge is of a negligible amount as 

 compared with that bound up in the double layer surrounding an air-bubble in 

 distilled water. An air-bubble in nitrobenzene, or in impure liquids, is an 

 exception. Thus, in benzene the presence of impurity brought about the cata- 

 phoresis phenomenon. The question arises, therefore, is the phenomenon in 

 nitrobenzene due to the same cause ? The unstable nature of nitrobenzene and the 

 doubtful nature of its purity in all cases would seem to uphold such belief. In 

 particular the variation in the motion of an air-bubble in different samples from 

 the same source might be explained as due to the instability of the nitrobenzene, 

 which darkens if exposed to daylight. The phenomenon in water might be 

 explained similarly, as distilled water cannot be regarded as perfectly pure. It is 



1 Coehn u Mozer. Ami. der Physik, vol. xliii, p. 1045. 1914. 



2 Lamb, Brit. Ass. Rep., 1887. 



