Two features of aqueous polyethylene glycol solutions (molecular weights 285 to 6600) have been noted in experiments with silver membrane osmometers: first, the solutions can be forced through the membranes in a manner which is indicative of capillary viscous flow; second, the osmotic pressures of polyethylene glycols are such that the limits of the reduced osmotic pressure as the concentration tends towards zero are of the formKMagr;, whenKand agr; are positive numbers, instead ofRT/Mas is indicated by thermodynamic theory. The differences between theoretical and observed osmotic pressures are interpreted as being due to diffusion of solute through the membranes. On this basis diffusion coefficients are calculated for the diffusion of polyethylene glycol through the membranes. For an estimated molecular weight of 400 000 the membranes are semipermeable. Similar work has been done on polystyrenes (molecular weights 180 000 to 673 000) and the membrane are shown to be semipermeable for polystyrenes of molecular weights greater than 500 000.
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