OSMOSIS. 279 
dissociation coefficient (in case of electrolytes) is known. If the 
molecular weight is not known, as in the case of proteids, the substance 
must be removed from the solution, and the difference in the total 
osmotic pressure so produced estimated. 
The coefficients of diffusion must be obtained under the special 
conditions (e.g. diffusion into serum, etc.). 
The permeability of the membrane to dissolved substances, one of 
the most important factors, and one generally not capable of accurate 
estimation, will not only affect the passage of water and dissolved suit- 
stances across the membrane by osmotic action, but also the hydrostatic 
pressure necessary to cause nitration. 
We shall here content ourselves with considering a simple but usual 
case of absorption of a solution by blood, namely, one in which the 
osmotic pressure of the solution is lower than that of the blood, and the 
membrane separating the two permeable to the substance in solution, 
and to one at least of the constituents of the blood, but impermeable to 
others. For convenience the dissolved substance is called x, and that 
constituent of the blood to which the membrane is permeable, y. The 
blood, by virtue of its superior osmotic pressure, tends to take up water 
from the solution, and at the same time x diffuses through the membrane 
into the blood, and y into the solution. If the blood be first supposed 
to be stationary, a time is arrived at when the partial pressure of x and 
y is the same on either side of the membrane : in other words, this 
solution of x and y is now the " solvent " in an osmotic experiment, and 
the substances in the blood to which the membrane is impermeable are 
the " dissolved substances." The whole of x, of y, and the water of the 
original solution, must therefore in the end be absorbed. 1 If the blood, 
however, is circulated, the conditions for absorption are at once improved, 
for the diffusion of x into the blood is favoured by the fact that its 
partial pressure in the blood is kept low by renewed supplies of blood, 
by the stirring action of the corpuscles preventing the formation of 
" wall layers," and by the fact that cells in other parts of the body are 
enabled to take up the substance as it is brought round. It is also 
evident from the above that if, as a rare case, the solution had a higher 
osmotic pressure than the blood, provided only the membrane separating 
the two is permeable to the dissolved substance, and impermeable to 
some constituents of the blood, w'hen once the solution has taken up 
enough water from the blood, and lost enough of its dissolved substance 
to the blood, to lower its osmotic pressure to that of the blood, the 
process described above is gone through, and it is in the end all 
absorbed. 
For such absorption to be carried out completely, it is evident that 
the osmotic pressure of those constituents of the blood to which the 
membrane and capillary wall are not permeable, must exceed the 
pressure necessary to cause filtration across the same structures, for if 
the available osmotic pressure on the inner side of the capillary wall is 
less than the difference between the hydrostatic pressure on the two 
sides of the membrane, filtration must occur, and the solution can never 
be totally absorbed. 
The assumption is here made that the resistance to the passage of 
fluid across the membrane is the same in both directions. It must be 
1 For this explanation to hold »ood, tlie substances in the blood to which the membrane 
is impermeable must be in true solution, and capable therefore of exerting osmotic pressure. 
