70 Stiles and Jorgensen. — Studies in Permeability. IV. 
It is obvious from these numbers that the exosmosis of electrolytes 
from the cell, and consequently the permeability changes produced by them, 
cannot be referred to surface tension alone. 
The rate at which the external liquid displaces gases in the inter- 
cellular space system may be of importance in this connexion. It is 
a matter of common knowledge that alcohols displace the air in the 
intercellular spaces very rapidly, while aqueous solutions do so slowly. 
Hence cytologists in fixing plant material with aqueous fixatives pump out 
the air from the tissue in order that the fixing fluid may be brought in con- 
tact with the cell membranes bordering on the intercellular spaces. In 
Czapek’s experiments and our own the same considerations hold. The 
lower the surface tension against air, the easier is the air displaced from 
the intercellular spaces (cf. Czapek’s method of measuring surface tension). 
Hence in the lower strengths of solution used the rate of exosmosis may be 
lower than it would be if the air were displaced rapidly as in the higher con- 
centrations of solution. However, with thin sections of tissue the difference 
is not likely to be very great. 
5. The assumption is made that if solutions whose surface tension 
in contact with air is o*68 are just strong enough to produce exosmosis from 
the cell, therefore the surface tension of the outer layer of the protoplasm 
towards air must also be o-68. As far as we are aware there is not a shred 
of evidence in support of this assumption. In his later paper (4) Czapek 
explains this by supposing that at the interface the substance which most 
lowers the surface tension will displace those which lower it less. Even 
if this statement is true, it appears to have no bearing whatever on the 
problem in hand. For methyl alcohol, for instance, lowers surface tension 
less than ethyl alcohol, and according to Czapek’s assumption ethyl alcohol, 
if it were present in the plasma membrane, would not be replaced by methyl 
alcohol in whatever concentration, while if ethyl alcohol were outside and 
methyl alcohol inside the plasma this would be replaced by ethyl alcohol 
even if it were below the critical concentration. In short, it would be 
impossible to accept this step in Czapek’s theory of the plasma membrane, 
even if the previous steps in the argument could be accepted. 
6. It is difficult to understand what Czapek means when he asserts that 
emulsions of neutral fats in water will not lower the surface tension of water 
below o-68, as he himself quotes several examples of fats, emulsions of 
which with water lower the surface tension very considerably below this 
value. 
7. Finally, even if we could accept the evidence that the surface tension 
of the plasma membrane in contact with air is o-68, and if it were a fact that 
fat emulsions cannot lower the surface tension of water in contact with air 
below o*68, this is no reason for concluding that the essential substances in 
the protoplasm as regards its surface tension are neutral fats. For as Czapek 
