OSMOSIS 117 
solution through the membrane to the stronger. Sometimes 
some of the dissolved substances may pass through the membrane, 
but often the membrane permits only the water to pass, in which 
case it is known as a semi-permeable membrane. In case of the 
pig’s bladder not much sugar is allowed to pass through its wall, 
which is, therefore, semi-permeable in reference to this particular 
solution. When a membrane will allow a dissolved substance 
to pass, it is said to be permeable to that substance. Most 
membranes are permeable to some substances and impermeable 
to others. 
The causes of the movement of the water or other solvents from 
the less dense to the denser solution are not thoroughly under- 
stood. Some think that it is due to the affinity of the dissolved 
supstances for the solvent, which is pulled to the substances with 
a force increasing with the amount of the substances in solution. 
Others think that it is due to the checking of the diffusive power 
of the dissolved substances by the membrane, which permits the 
two liquids to approach an equilibrium only through the passing 
of more of the solvent to the denser solution. 
In comparing osmosis in the cell with the illustration, the cell 
membrane corresponds to the wall of the pig’s bladder, the cell 
sap to the solution within the bladder, and the solutions around 
the cell correspond to the water or solutions in the jar. If the 
cell sap in denser than the solution on the outside of the cell mem- 
brane, then water with those dissolved substances to which the 
membrane is permeable will pass in; but, on the other hand, if the 
cell sap is less dense than the solution without, water and prob- 
ably some dissolved substances will pass out. Thus the passing 
of liquids through the cell membrane from a less dense to a denser 
liquid is also the chief feature of osmosis in cells. It should also 
be noted in connection with osmosis in cells: (1) that the more 
the two solutions separated by the cell membrane differ in con- 
centration, the more rapid is the process of osmosis; and (2) that 
the solvent, which is water in case of cells, passes through the 
membrane independently of its dissolved substances, which are 
either carried along or left behind according to whether or not 
the membrane is permeable to them. 
However, it is only in principle and not in practice that osmosis 
as demonstrated with the pig’s bladder is identical with that in 
the cell. In the first place, instead of a solution containing only 
