1917] BROOKS—PERMEABIL ITY 235 
tained unless there is either a change in the tendency of the water 
to enter, or a change in the tension of the membrane. The latter 
has not been shown to occur, but may be responsible for certain 
as yet unexplained phenomena observed in plasmolytic experiments. 
The former will be produced by alterations in the concentration of 
the solution bathing the cell, an increase in its concentration 
causing a loss of water from the cell, with consequent shrinkage, 
and a decrease causing an intake of water with accompanying 
increase in volume. These changes will proceed until a new 
equilibrium is established at which the internal osmotic pressure 
is again equal to that of the external solution plus the pressure 
produced by the tension of the membrane. 
The rate at which the exchange of water will occur is a function 
of the difference in osmotic pressure between the intra- and extra- 
cellular solutions and of the permeability of the membrane to 
water. There appears to be a tendency among physiologists to 
confuse the effect of the rate of penetration of a solute on that of 
water (which is produced by the resultant progressive change in 
the osmotic gradient), with a hypothetical effect, independent of 
the osmotic gradient, produced by the simultaneous passage of 
both solute and solvent through the membrane. There is no 
physical justification for the latter assumption, and the two ideas | 
should be carefully distinguished. The change in volume of the © 
cell is the sum of the change in volume due to diffusion of water 
and that due to diffusion of the solute. In cases where the solute 
is a substance like alcohol, the. latter factor may be of consider- 
able importance; but protoplasm is in general so much less per- 
meable to inorganic salts than to water that their diffusion ~~ 
be neglected in so far as their volume is concerned. — : - 
The i intra and paey Caen 71.. 4 
2 ei octyl carapace osmotic p 
equalized by passage of water through 
eo 
- of ‘equilibrium is reached, which, if the membrane i is pen neab co 
