iS6 
F. F. Blackman. 
being able to retain their contents in a medium of lower surface- 
tension than about 0-68. 
In his third paper Czapek proceeds to point out the simplest 
explanation of this general rule is that the living cells of higher 
plants have actually a surface tension of this particular value, 0-68. 
Now as mentioned in the introduction it has been shown by 
Willard Gibbs on thermo dynamical grounds that if substances 
which lower surface-tension are dissolved in a liquid they must 
accumulate in the surface-layer, and that those substances which 
lower it most will displace any less active ones. Czapek concludes 
therefore that the exosmosis is caused by the new substances 
brought into contact with the protoplast being taken up by the 
surface-layer and displacing the normal contents; whereby in 
some way a greatly increased permeability is produced and 
exosmosis starts at once. 
Czapek then turned his attention to colloids, as it had been long 
known that colloidal emulsions of fats, soaps and other lipoid 
bodies in water have a very low surface-tension. These emulsions 
followed exactly the same law as the true solutions and any lipoid 
emulsion with a surface tension as low as 068 caused exosmosis. 
One significant point emerged, namely, that a strong emulsion of a 
neutral fat in water has a minimal limit for its surface-tension, 
which is not passed however strong the emulsion, and this limit is 
just about 068. We are irresistibly driven to think of the theories 
of Quincke, Overton and Meyer, which assumed the presence of 
fats in the surface-layer of the protoplast. Grant the surface-layer 
of the protoplasm to be a saturated emulsion of some neutral fat 
and its surface-tension and relations to exosmosis thereby are 
made comprehensible. 
The lipoid theory of the plasmatic membrane, based, so far, on 
amoeboid movement, the selective uptake of aniline dyes and the 
narcotic action of fat-solvents, thus gains strong support from these 
researches on surface-tension. 
In his fourth publication Czapek discusses the probable nature 
of this surface-layer more in detail and points out that there 
are no grounds for assuming a continuous fatty film round the cell, 
but that an emulsion containing only a few parts per cent of fat 
would give all the required properties. The fat would be sus¬ 
pended as an emulsion in perhaps a continuous colloid complex of 
water and proteid, and thus the uptake of substances insoluble in 
