Permeability 13 
by the membrane (adsorption potential). These three ways in which 
potential differences may arise have been described in Chapter III. 
There are nine possible cases of electrification, and these are 
clearly set out in diagrammatic form by Bart ell and Madison. These 
are here shown in tabular form in Table XVI. 
Table XVI 
Possible Cases of Electrification of Membranes and 
the Effect on Osmosis 
Charge on 
solution side 
No. of membrane 
1 None 
2 None 
3 None 
4 + 
5 + 
6 + 
7 
8 
9 
Charge on 
liquid in 
capillaries 
Osmosis 
None 
Normal 
+ 
Normal 
- 
Normal 
None 
Normal 
+ 
Abnormally low 
- 
Abnormally high 
None 
Normal 
+ 
Abnormally high 
- 
Abnormally low 
It is to be observed that all these expected results have been 
realised in practice, as the summary of Bartell’s work already given 
here has indicated. In cases 5 and 9, where we have respectively 
positively charged water moving to the side of the membrane nega¬ 
tively charged, and vice versa , the tendency for liquid to move from 
the solution into the solvent may exceed the tendency for normal 
osmosis to take place, in which case negative osmosis results. It is 
to be noted that both Girard (1919) and Bartell and Madison (1920) 
measured the potential difference between the two sides of the mem¬ 
branes they employed, and have shown that this is such as the theory 
requires. It is also possible that the electrification of the system 
might arise in other ways (cf. Freundlich, 1916). The phenomenon, 
it will be observed, is similar to that of electrical osmosis, in which 
water passes from one solution to another across a membrane as a 
result of a potential difference applied to the solutions on either 
side of the membrane. (See especially Briggs, 1919). 
A curious case of negative osmosis has recently been noted by 
Loeb (1920 a). When solutions of aluminium chloride or of lantha¬ 
num salts are separated from water by a collodion membrane treated 
with gelatine, water diffuses into the solution, although such diffusion 
does not take place when an untreated membrane is used. If such 
