1026 
4. In the chemistry of colloids one often meets with facts which 
can be explained by the result of these investigations, viz. that the 
contact potential in a very diluted solution is higher than in pure 
water. 
The stability of suspensoids is a function of the rece change 
of the suspended particles *). In literature “stabilising ions” are often 
mentioned; it is well known that a suspensoid sol free from all 
electrolvtes has but little stability, a long dialyzing often causes 
floculation.*) The importance of these traces of electrolytes can now 
easily be understood in connexion with this investigation: though 
greater amounts of electrolyte lower the potential of contact and 
so cause floculation, when the critical potential is reached, extremely 
low concentrations increase the electric charge and at the same time 
the stability. Of course they are absolutely necessary when the 
contact potential in pure water is lower than the critical potential of 
the sol and when the optimum in the potential curve is higher 
than that critical value. As the presence of some electrolyte generally 
seems to be necessary for a stable sol, we may conclude the potential 
in pure water to be ordinarily rather low. The current potential, it 
is true, may be high, this is only a consequence of the enormous 
electrical resistance of this liquid. In the Tables above-mentioned we 
E 
have not calculated ¢ for pure water as the value zet ca 350 m.V., 
is not exact, the enormous resistance in the cell lowering the ex- 
actness of the measurement too much. When we put * to 9107 
(this was indead the case with the water when freshly prepared) 
the electric charge of the tube would be less than 0.01 e.g.s units. 
5. The result of this investigation, that an optimum in the curve 
for the contact potentials may be generally received, draws our 
attention to the change of our knowledge about the iso-electric 
phenomenon. In fig. 2 A, B, and C the concentrations are given as 
abscissae, the contact potentials being ordinates. The axis of ordinates 
is erected in. the iso-electric point, so we have the alcalic 
liquids on the left, acid liquids on the right. Fig. 2 A 
1) 1 think Powrs [Z. f, physik. Chem. 89, 186 (1914)] is quite right when he 
concludes that spontaneous floculation does not occur at the potential zero but 
at a certain “critical” value. In fig. 1 of my communication [these Proceedings 
23, 623 (1914)] the same supposition is represented graphically. 
2) A great many examples may be found in Svepsere’s work: Methoden zur 
Herstellung kolloider Lésungen anorganischer Stoffe (Dresden 1909). — - 
