On the interpretation of experiments on osmotic pressure.

Scholander and his coworkers have described in These Proceedings several experiments in which the osmotic pressure exerted by colloidal particles appears to be altered when the particles are subject to an external force; they interpret the experiments as showing that the traditional explanation of osmotic pressure is wrong (Proc. Nat. Acad. Sci. USA 68, 1093-1094; 68, 1569-1571; 70, 124-128). (In their view the osmotic pressure is determined by the concentration of the colloidal particles at the free surface rather than at the semipermeable membrane.) This note shows that one must also consider the way in which the external force changes the total hydrostatic pressure in the liquid. When that is done, the experiments of Scholander et al. are quantitatively consistent with the traditional interpretation of osmotic pressure.