288 TRANSACTIONS OF SECTION A. 
as being saved from thirst in a place where the water was charged with alkali 
by filtering the water through bags of sand. Whether this is a practical thing 
or not is doubtful, or even if it has ever been tried; for it is only the first part of 
the liquid to come through the filter which is purified, and very soon the surface 
has taken up all the salt it can adsorb, and after that, of course, the solution 
comes through intact. It is interesting, however, to know that so long ago as 
Defoe’s time the phenomenon of adsorption from salt solutions had been 
observed. It is not so well known that in the case of some salts under the cir- 
cumstances mentioned above, the first of the solution to come through the sand 
filter is stronger instead of weaker. This, as already mentioned, is because 
water, or at least a weaker solution, forms the adsorption layer. 
Most of the alkali chlorides as the temperature is raised show this anomalous 
adsorption, provided the strength of the solution is below a certain critical value 
differing for each temperature. For strengths of solution above these values 
the normal phenomenon takes place. 
No investigations seem to have been made on the effect of pressure on adsorp- 
tion. These data are much to be desired. 
The investigation of adsorption and absorption should throw light on Osmosis, 
as in the first place the phenomenon occurs across a surface necessarily covered 
with an adsorption layer, and in the second place, as we shall see, the final con- 
dition is an equilibrium between the absorption of water by the solution and that 
by the membrane. 
The study of the conditions of absorption of water throughout the mass of the 
colloidal substance of which osmotic membranes are made is of much interest. 
Little work has been done on the subject as yet, but what little has been done is 
very promising 
It is convenient to call the material of which a semi-permeable membrane is 
made the semi-permeable medium. The ideal semi-permeable medium will not 
absorb any salt from the solution, but only water, but such perfection is probably 
seldom to be met with. If a semi-permeable medium such as parchment paper 
be immersed in a solution, say, of sugar, less water is taken up or absorbed than 
is the case when the immersion is in pure water. The diminution in the amount 
absorbed is found to increase with the strength of the solution. It is at the 
same time found that the absorption or release of water by the semi-permeable 
medium according as the solution is made weaker or stronger is accompanied by 
a swelling or shrinkage greater than can be accounted for by the water taken up 
or rejected, 
The amount of water absorbed by a semi-permeable medium from a solution 
is found by experiment to depend upon the hydrostatic pressure. If the pressure 
be increased the amount of water absorbed by the semi-permeable medium is 
increased. It is always thus possible by the application of pressure to force the 
semi-permeable medium to take up from a given solution as much water as it 
takes up from pure water at atmospheric pressure. 
It is not possible for a mass of such a medium to be simultaneously in con- 
tact and in equilibrium with both pure water and with a solution all at one and 
the same pressure, seeing that the part of the medium in contact with the pure 
water would hold more water than that part in contact with the solution, and 
consequently diffusion would take place through the mass of the medium. 
If, however, the medium be arranged so as to separate the solution 
and the water, and provided the medium is capable of standing the necessary 
strain, it is possible to increase the pressure of the solution without increasing 
the pressure of the water on the other side. Thus the part of the medium which 
is in contact with the solution is at a higher pressure than that part in contact 
with the pure solvent; consequently the medium can be in equilibrium with both 
the solution and the solvent, for if the pressures are rightly adjusted the moisture 
throughout the medium is everywhere the same. 
The ordinary arrangement for showing osmotic pressure is a case such as we 
are considering, and equilibrium throughout the membrane is only obtained when 
the necessary difference in pressure exists between the two sides of the 
membrane. 
This condition would eventually be reached no matter how thick the mem- 
brane was. It is sometimes helpful to think of the membrane as being very 
