152 Papers from the Department of Marine Biology. 
other hand, the weak alkalies penetrate without affecting the surface, 
without killing the cell, and before functional activity is appreciably 
affected. The easily penetrating group of alkalies are also lipoid 
soluble and active in lowering the surface tension of water. Data for 
a quantitative comparison with lipoid solubility and capillary activity 
are not available at present. 
On comparing the acids with the alkalies several facts of interest 
appear. The acids form a series much more evenly graded in prop- 
erties. Consequently, two classes of acids, easily penetrating and 
difficultly penetrating, are not so easily recognized. The acids which 
correspond to NH,OH and the amines, the lipoid-soluble acids, are 
benzoic and salicylic with possibly valeric. The remaining acids all 
meet a resistance at the living cell-surface (abolished on death of the 
cell) which varies specifically with the acid. Once the pigment is 
turned red orange by the acid, removal of the tissue to sea-water does 
not reverse the color change. The tissue is also killed and the pigment 
diffuses away. Since the testis epithelium is an inert tissue, 7. @., 
contains no cilia or other indicators of functional activity, the relation 
between entrance of an acid and change in functional activity could not 
be observed. 
With acids as with alkalies ability to penetrate the cell-surface 
appears to determine the toxicity of the acid. I am inclined to think, 
but can not be certain of this, that, as in the case of the strong alkalies, 
the lipoid-insoluble acids must destroy the normal impermeability of 
the cell-surface before they enter. The most toxic of the lipoid-insol- 
uble acids would therefore be those which destroy the surface most 
rapidly, and they would penetrate the cell most readily for this reason 
also. Destruction of the cell-surface appears to depend largely on the 
strength of the acid, 7. e., its ability to combine with proteins of the 
cell-surface, but in part also on some specific, as yet unrecognized, 
peculiarity of the acid anion. 
A few remarks in regard to Traube’s Haftdruck theory may not be 
out of place. This theory depends essentially on the Gibbs-Thompson 
principle that a substance tending to lower the surface tension of water 
will collect in the surface. If a membrane is at the surface the sub- 
stance will tend to pass the membrane, and always in the direction of 
the solution with the greatest surface tension. Difference in surface 
tension, ‘‘Oberflaschendruck” or “‘ Haftdruck,’”’ determines the direc- 
tion and velocity of osmosis. The more a substance lowers or increases 
the surface tension of water the less or the greater is its Haftdruck, and 
it will pass from regions of low to regions of high Haftdruck. 
Originally Traube claimed that the direction and velocity of passage 
of a substance through a membrane were independent of the membrane 
and depended entirely on the Haftdruck of the solution. Had this been 
the case Traube’s theory would have been of real value as a simple 
