14 REPORTS ON THE STATE OF SCIENCE.—1918. 
ions, as the case may be. ‘There is, therefore, no theoretical 
reason why precipitated hydrous ferric oxide might not adsorb 
chromic oxide, and vice versa. If the precipitated substance will 
do this, there is no reason why the peptised substance should not. 
Nagel®? has shown recently that this does occur, and that it 
accounts for the behaviour of mixtures of chromic and ferric 
salts with excess of alkali. Hydrous chromic oxide is peptised by 
caustic potash, while hydrous ferric oxide is not. If the 
chromium salt is present in large amount relatively to the iron 
salt, the ferric oxide will adsorb the peptised chromic oxide 
and be peptised by it, going apparently into solution. If the 
ferric salt is present in excess, it will adsorb the peptised chromic 
oxide, carrying it out of the liquid phase. It is to be noticed 
that the chromic oxide, when in excess, acts as a so-called protect- 
ing colloid to the iron oxide. Everybody is familiar with the 
fact that gelatine is adsorbed by colloidal gold, for instance; 
but that is usually treated under the heading of protective colloids 
rather than under the heading of mutual action of two colloids. 
The case of chromic and ferric oxides is merely another illustra- 
tion of the fact that the distinction between a suspension colloid 
and an emulsion colloid is now arbitrary and unsatisfactory.** 
Coming back for a moment to the behaviour of two oppositely 
charged colloids, there is a special hypothetical case which is 
perhaps worth mentioning. Suppose we have two sets of finely 
divided particles, neither of which adsorbs the other appreciably, 
and let us also suppose that one set of particles adsorbs a given 
cation very strongly, while the other set of particles adsorbs a 
given anion very strongly. If we take a mixture of these two sets 
of particles and add a small amount of the salt of the given base 
and the given anion, we shall have a colloidal solution which will 
conduct electricity very well, but which will contain no free ionsto 
speak of because, by definition, the cations have been practically 
completely adsorbed by one set of particles and the anions by the 
other set of particles. This particular case has not been realised, 
but an intermediate one seems to have been found by McBain and 
Martin** in sodium palmitate solutions. 
‘Most authors since Kahlenberg and Schreiner*® have, as a 
matter of course, ascribed the conductivity exhibited by soap 
solutions largely to free alkali hydroxide. In previous papers 
from this laboratory the same tentative suggestion was made, but 
it was each time clearly stated that it was only a working 
hypothesis until these experimental data should be ascertained. 
Now it is certain that the conductivity of soap solutions is, only 
to a very minor extent, due to hydroxyl ions. Further, on 
account of the fact that the rise of boiling point in certain soap 
solutions is practically all required to account for the sodium ions 
alone,®® the conductivity cannot be wholly ascribed to simple 
82 Jour. Phys. Chem. 19, 331 (1915). 
83 Bancroft, Jour. Phys. Chem. 18, 556 (1914). 
84 Jour. Chem, Soc. 105, 965 (1914). 
85 Zeit. Phys. Chem. 2'7, 552 (1898). 
86 McBain, Trans. Faraday Soc. 9,99 ; Zeit. Kolloidchemie, 12, 256 (1913) 
