A treatise on the theory of solution including the phenomena of electrolysis . let usplace the solution of some substancesuch as sugar in water, and surround theoutside with a large volume of the pure solvent. Water will gradually enter the cell, and, by usingthe glass tube as a pressure gauge, it will be found that thisinflux will continue until a definite internal pressure is gives a measure of what is called the osmotic pressure ofthe solution as it finally exists in the cell. If the membrane has been well prepared, very little if anysugar will escape to the outside, and, altho
A treatise on the theory of solution including the phenomena of electrolysis . let usplace the solution of some substancesuch as sugar in water, and surround theoutside with a large volume of the pure solvent. Water will gradually enter the cell, and, by usingthe glass tube as a pressure gauge, it will be found that thisinflux will continue until a definite internal pressure is gives a measure of what is called the osmotic pressure ofthe solution as it finally exists in the cell. If the membrane has been well prepared, very little if anysugar will escape to the outside, and, although some simplesalts and acids will often leak through to some extent, thereseems little reason to doubt that membranes can be obtainedwhich are practically impervious to solutions of complexchemical substances such as sugar. At first sight the obviousexplanation appears to be that the membrane allows moleculesof water to pass, but will not let through molecules of is an experiment described by Pickering which suggestsJ ner. Deut. Chem. Ges., xxiv. 3639 (1891).. Fig. 36. CH. V] OSMOTIC PRESSURE 97 another possible view. If a mixture of propyl alcohol andwater be placed in a semi-permeable vessel and surroundedwith water, it is found that water enters the cell, and noalcohol escapes. If, however, the same vessel with its samecontents be placed in propyl alcohol, it is the alcohol whichpasses through and enters the vessel, while no water this case it seems clear that the membrane is permeableto either water or propyl alcohol when pure, but is imperviousto tlie combination of the two. Further experimental evidenceis needed to decide whether or not similar phenomena occurwith other solutions. The question is of great interest fromits bearing on the problem of the fundamental nature ofsolution. The mechanism of the passage of the liquid through themembrane is not fully understood. It may be a purely physicalprocess dependent on the relative sizes of the molecu
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