OSMOTIC PRESSURE OF CELL-CONTENTS 265 



of energy by the cell must rapidly lead to its disintegration. Further- 

 more, the solutions which Overton, in the results cited above, found 

 to be isotonic with the cell-contents of spirogyra filaments exerted 

 an osmotic pressure of some four and a half atmospheres, and the 

 corresponding pressure in the cell-contents themselves, can only 

 have been due to diffusible water-soluble substances which must 

 therefore have penetrated the protoplasm at some period of its devel- 

 opment. The semipermeability of cell-membranes is in fact, even 

 in the most typical instances, apparent and not real. It. is purely a 

 question of Relative Permeability, of the rapidity with which dissolved 

 substances and water relatively penetrate the cell. In the case of 

 Bacillus cholera, for example, the relativity of the semipermeability 

 of cells can very clearly be seen, for these organisms, as well as certain 

 other bacteria, are temporarily plasmolyzed by hypertonic salt solu- 

 tions or sugar-solutions, but not at all by Glycerol solutions. Even 

 the plasmolysis observed in salt- or sugar-solutions disappears in the 

 course of an hour or two, because, after the lapse of this time, a suffi- 

 cient proportion of the salt or sugar has penetrated the cells to restore 

 isotonicity between the inner and outer fluids. Evidently, therefore, 

 in the case of these cells water and glycerol penetrate the exterior 

 limiting membrane almost instantaneously, salt and sugar more 

 slowly. The disparity of the velocities of penetration for water and 

 dissolved substances is greater in spirogyra filaments than in the 

 above-mentioned species of bacteria, and this constitutes the origin 

 of the apparent semipermeability of the protoplasm in spirogyra. 



It is a rather remarkable, and certainly a regrettable fact that 

 physical chemists have hitherto paid so little attention to the investi- 

 gation of the Time-relation of Osmosis. The comparative neglect of 

 this and other fields of inquiry which would naturally suggest them- 

 selves to the student of pure physics or mechanics, is undoubtedly 

 attributable to the bias toward purely thermodynamical reasoning 

 which has been communicated to the students of physical chemistry 

 by the past generation of chemists. The thermodynamical relation- 

 ships and equations contemplate only attained equilibria, not fluctuat- 

 ing or kinetic phenomena. Hence, the relationship between the lapse 

 of time and the degree of penetration of a 'membrane by various 

 solvents or dissolved substances, which would seem to present a most 

 obvious subject for inquiry, is as yet very imperfectly known. One 

 would expect, however, that the quantity of penetration would be an 

 exponential function of the time, and that this function would contain 

 specific parameters or constants, characteristic for the membrane, 

 the particular solvent employed, and the dissolved substance respec- 

 tively. The evaluation of these parameters in the case of living cell- 

 membranes would afford an accurate quantitative measure of Per- 

 meability, for the estimation of which we must rely at present upon 

 qualitative rather than upon quantitative data. 



In the plasmolytic method of estimating isotonic solutions we regard 



