1028 
The same could be the case with the surface of the glass tube. 
In the iso-electric point only a minimal concentration of anions 
(the OH ions of water) is present; the concentration of anions 
becomes larger by adding alkali, chlorides, even acids to water. 
Evidently the charge given to the tube by the OH ions and the 
silicic acid of glass is not so large that it cannot increase any more. 
The potential increases more as the added anion is better adsor- 
bed; this explains why we meet with such a great increase towards 
the side of the OH-ions in Fig. 2C and why Powis found that the 
optimum on the right side is higher when the anion is more adsorb- 
ed or polyvalent’). Of course a cation always accompanies the 
anion and the former resists a continuous increase of the electric 
charge, as it is adsorbed itself and gets in a more favourable con- 
dition for electro-adsorption because it bears an electrie charge of 
opposite nature. Consequentely a lowering of the charge must occur 
at higher concentration. 
Stull it is curious that the concentration of anions really present 
plays an important part; for potentially there is in water an amount 
of OH-ions, which is nearly unlimited and we are accustomed in 
questions of this sort to consider as decisive the concentration of 
potential ions. The phenomenon of hydrolysis (e.g. with solutions 
of AICI,) is usually of no importance *). These results warn us howe- 
ver to be prudent with all theory on this account, though on the 
other hand the condition of the ions of water is not always. quite 
comparable with that of salts in solutions. 
POSTSCRIPT. 
Just now a paper appeared of Herstap, Koll. Beih. 8, 399 1916); 
His investigations on the influence of dialysis of goldsols and its 
limit values (ef. fig. 15 and 16) are in striking accordance with 
this communication. The paper induces us to measure the potentials 
in the case of HgCl, solutions. Still more amazing is an investigation 
of Beans and Easriack [Journ. of the Americ. chem. Soc. 87, 2667 
(1915)| on the best conditions for the preparation of highly dispersed 
gold sols. It gives the impression that the concentration of electro- 
lytes necessary to get red sols coincide with those of the optimum 
charge of the glass capillary tube. I hope to revert to this subject 
before long. 
Utrecht, vax ’v Horr- Laboratory. December 1916. 
') Powis it is true made investigations on KCI and K,Fe(CN),, but his conclu- 
sions probably hold as well for acids as for salts. 
*) Cf. FRENDLICcH, Z. f. physik. Chem. 44, 136 (1903). 
