OSMOSIS. 277 
morpha, growing in sea water, is practically isosmotic with that of 
Spirogyra, growing in fresh water, though the osmotic pressure of sea 
water is some 240 times that of fresh. Thus the osmotic pressure of the 
cell sap of Chcetomorpha is far below that of the water in which it lives, 
while that of the sap of Spirogyra is far above that of fresh water. 
One can investigate the permeability of the living protoplast by the 
plasmolytic method already alluded to above. The cell is plasmolysed 
with a solution of some substance indifferent to the protoplasm and known 
not to penetrate (sugar). A solution of the substance to be tested is now 
prepared of the same osmotic pressure as the solution of indifferent 
substance which just causes plasmolysis. If the solution so prepared has 
exactly the same effect as the standard, it cannot pass through the proto- 
plast, for, if it did, there would no longer be ecpaality of osmotic pressure 
on the two sides thereof. If the substance to be tested is only slightly 
soluble in water, or is poisonous to the protoplasm, a small amount of it 
is added to the standard indifferent solution, and the effect of the 
addition on the plasmolysis noted. If it does not pass the membrane, 
then, by virtue of the higher osmotic pressure due to its addition, the 
mixture will produce more plasmolysis than did the standard solution, 
and the effect will be lasting. If no effect results from the addition, it 
must pass cpaickly through the membrane ; if a passing effect, with 
subsequent recovery, it must pass slowly. 
In this way Overton 1 has investigated the permeability of the 
protoplast by a number of chemical substances, and finds that salts 
much dissociated in solution hardly pass the membrane, while many 
complex organic bodies rapidly penetrate, and that the presence of 
certain radicles in these markedly affects the result. 
In animal cells investigations are rather limited (by the fact that 
there is no plasmolysis) to shrinkage and swelling and escape of 
haemoglobin (in red corpuscles), as indices of permeability, under 
conditions of variation of osmotic pressure of surrounding solutions. 
More, therefore, is known about the permeability of the red corpuscle 
than any other cell. A table of substances is given by Gryns, 2 and we 
here confine ourselves to stating that red corpuscles are permeable 
to urea, 3 glycerin, ethyl- and methyl-alcohol, and most ammonium salts 
(not to sulphate, phosphate, and thiocyanate), impermeable to sugars, 
sodium and potassium salts, barium and calcium chlorides, glycin and 
asparagin. 
Thus, as regards action on red blood corpuscles, dilution of an isosmotic 
sodic chloride solution with urea solution produces the same effect as 
dilation with water, because the urea diffuses at once into the interior of 
the corpuscle, while, on the other hand, addition of sugar at once causes 
contraction of the cell. 
Obviously, therefore, a so-called " hyperisotonic " solution does not 
necessarily extract water from a cell, and absorption of water from such 
a solution by the blood may be a purely physical action, if the substance 
in solution can permeate the wall separating it from the blood. Again, 
a drug, by making the wall of a cell less permeable by virtue of its 
chemical action on the protoplasm, may markedly affect the " water 
extracting power" of a salt solution. Possibly the fact that some salts 
1 Ztschr. f. physikal. Chem., Leipzig, 1897, Ed. xxii. S. 189. 
2 Loc. cit., p. 102. 
3 See also Schondorff, Arch. f. d. ges. Physiol., Bonn, 1896, Bd. lxiii. S. 192. 
