144 THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



determined, and the general relationship established between molecular weight 

 and osmotic value. The knowledge of the fact, that in the absence of all exosmosis 

 the membrane exercises no effect whatever upon osmotic pressure, first enabled an 

 exact determination of the osmotic pressure in the cell to be made from physical 

 data l . 



The physics of osmotic pressure. Physiology has to deal with osmotic pressure 

 as a fact, which cannot be altered in the least by theoretical conclusions as to its 

 origin. The whole phenomenon.is most clearly and thoroughly explained by the theory 

 advanced by van 't Hoff on the basis of Pfeffer's researches 2 . According to this 

 theory, osmotic pressure arises in the same way that gaseous pressure does. In 

 a closed semi-permeable cell, the molecules of a dissolved substance exert upon the 

 limiting membrane a pressure similar to that which, according to Mariotte's law and 

 Avogadro's hypothesis, the colliding molecules of a gas exercise upon the walls of 

 the space in which they are confined. A cell filled with CO., and H, through the 

 enclosing membrane of which the CO. 2 cannot diffuse but the H can, might 

 represent a turgid cell ; the stretching is due to the non-diosmosing CO., alone, 

 while the H does not diosmose, because the cell is supposed to be immersed in an 

 atmosphere of this gas. One gramme-molecule of gas (i. e. of CO 2 , 44 grm.) occupies 

 a space of 22-4 litres at 760 m.m. pressure and o C., and hence exerts a pressure 

 of 22-4 atm. when reduced to i litre. The same pressure must be exerted by 

 a solution of 342 grm. of cane sugar in i litre of water. Hence, for a solution of 

 i grm. of cane sugar in 100 c.cm. the pressure will be at o c C., 0655 atm., and at 

 15 C, 0-69 atm. Pfeffer, by direct measurement, obtained values at this tempera- 

 ture of from 0-62 to 0-71 atm. Hence it follows that osmotic values may be 

 calculated directly from physical data with perfect safety and accuracy. 



The osmotic pressure is naturally dependent upon the amount of water 

 present, and can indeed be regarded as the mechanical result of the endosmosis 

 or exosmosis of water. Osmotic pressure is a function of the number of molecules, 

 or of ions, in the unit of volume, and just as with gases, any variations from the laws 

 regulating the relationships between molecular weight and gaseous or osmotic 

 pressure, unavoidably postulate the occurrence of more or less marked molecular 

 association or dissociation 3 . Steam pressure, electric conductivity, and atomic heat 

 show similar molecular relationships. 



The methods employed to determine osmotic pressure are given in the original 

 work by Pfeffer, and reference may also be made to physical and chemical sources 4 . 



1 Pfeffer, Plasmahaut u. Vacuolen, 1890, p. 302 ; Ostwald, Allgem. Chemie, 1891, 2. Aufl., 

 Bd. i, p. 661. L. Meyer's contradictions (Sitzungsb. d. Berl. Akad., 1891, p. 993) rest upon an 

 erroneous basis, as van 't Hoff (Zeitschr. f. physik. Chem., 1892, Bd. IX, p. 447) has shown. Pfeffer 

 (Osmot. Unters.) assumed that the nature of the semi-permeable membrane influenced the osmotic 

 pressure, and hence the conclusions made with regard to the determination of osmotic pressure by 

 reference to isosmotic values were logically correct, although it was also concluded that variations 

 in the plasmatic membrane could have no marked influence upon the turgid pressure. 



2 See Ostwald, Allgem. Chemie, 1891, 2. Aufl., Bd. I, p. 671. See also Pfeffer, Vacuolenhaut, 

 1890, p. 318. 



3 Cf. van 't Hoff, Ber. d. Chem. Ges., 1894, p. 19. 



4 Ostwald, I.e., p. 656; \Vinkelmann, Physik, 1891, Bd. I, p. 624. Incidentally (Plasmahaut 

 u. Vacuolen, 1890, p. 310) it has been pointed out that, for physical experiments, a simpler and more 



