644 
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 circumstances 
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 adsorption. 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 condition is an equili- 
brium 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 in- 
terest. 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 contact 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 con- 
sequently 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; conse- 
quently 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. 
NO: 23300 VOEwO 3] 
NATURE 
| AUGUST 20, 1914 
The ordinary arrangement for showing osmotic 
pressure is a case such as we are considering, and 
equilibrium throughout the membrane is only obtained 
wnen the necessary difference in pressure exists be- 
tween the two sides of the membrane. 
This condition would eventually be reached no 
matter how thick the membrane was. It is some- 
times helpful to think of the membrane as being very 
thick. It precludes any temptation to view molecules 
as shooting across from one liquid to the other 
through some kind of peep-holes in the membrane. 
The advantage in a thin membrane in practice is 
simply that the necessary moisture is rapidly applied 
to the active surface, thus enabling the pressure on 
the side of the solution to rise quickly, but it has no 
effect on the ultimate equilibrium. 
As far as that goes, the semi-permeable membrane 
or saturated medium might be infinitely thick, or, 
in other words, there need be no receptacle or place 
for holding the pure solvent outside the membrane 
at all. In fact, the function of the receptacle contain- 
ing the pure solvent is only to keep the medium moist, 
and is no more or no less important than the vessel 
of water supplied to the gauze of the wet-bulb ther- 
mometer. Jt is merely to keep up the supply of water 
to the medium. 
The real field where the phenomenon of osmosis 
takes place is the surface of separation between the 
saturated semi-permeable medium and the solution. 
Imagine a large mass of colloidal substance saturated 
with water and having a cavity containing a solution. 
The pressure will now tend to rise in the cavity until 
it reaches the osmotic pressure—that is, until there 
is established an equilibrium of surface transfer of 
molecules from the solution into the medium and back 
from the medium into the solution. 
No doubt, the phenomenon as thus described occurs 
often in nature. It is just possible that the high- 
pressure liquid cavities, which mineralogists find in 
certain rock crystals, have been formed in some such 
manner in the midst of a mass of Semi-permeable 
medium; the pure solvent in this case being carbon 
dioxide and the medium colloidal silica, which has 
since changed into quartz crystal. 
In considering equilibrium between a_ saturated 
semi-permeable medium and a solution there seems 
fo me to be a point which should be carefully con- 
sidered before being neglected in any complete theory. 
That is, the adsorption layer over the surface of the 
semi-permeable medium. We have seen that solu- 
tions are profoundly modified in the surface layers 
adjoining certain solids, through concentration or 
otherwise of the salts in the surface layer, so that the 
actual equilibrium of surface transfer of water mole- 
cules is not between the unmodified solution and the 
semi-permeable medium, but between the altered 
solution in the absorption layer and the saturated 
medium. Actual determinations of the adsorption by 
colloids are much wanted, so as to be able to be quite 
sure of what this correction amounts to or even if 
it exists. It may turn out to be zero. If there is 
adsorption, however, it may possibly help to account 
for part of the unexpectedly high values of the osmotic 
pressure observed at high concentrations of the solu- 
tion, the equilibrium being, as we have seen, between 
the saturated medium and a solution of greater con- 
centration than the bulk of the liquid, namely, that 
of the adsorption layer. In addition, when above the 
critical adsorption point, there may be a deposit in 
the solid state. This may produce a kind of polarised 
equilibrium of surface transfer in which the molecules 
which discharge from the saturated medium remain 
unaltered in amount, but those which move back from 
the adsorption layer are reduced owing to this de- 
= Ree treniesaee.- 
§ 
4 
¥ 
An ceat 
