23() 



BOOKS AND CURRENT LITERATURE 



pressure is conccriKMl.- Of course hydration is to be considered as 

 altering the nature of the active solute particles, making them larger, 

 giving them other new properties and changing the value of the effec- 

 tive molar fraction. Granted that there are particles of some kind 

 which ma.y be considered as effective solute particles, and granted that 

 these particles can be effective and retain their peculiar properties only 

 within the limits of the solvent, it follows that an attraction of solvent 

 by solute, causing entrance of the former into the solution through 

 the membrane, must add to the "driving force of diffusion" of the 

 whole liquid solution, solvent and solute particle? alike. Since, how- 

 ever, the solvent particles are free to pass the membrane in both direc- 

 tions, no pressure upon the membrane can be exerted by these, and 

 we arrive at the conclusion that the pressure developed must be imme- 

 diately due to the "driving force of diffusion," or diffusion tension, 

 of the effective solute particles. The latter may be pictured as pressed 

 more strongly against the membrane or compelled to bombard it more 

 vigorously, by the crowding into the solution of solvent particles, the 

 latter entering because of attraction. Thus, in a solution in contact 

 with the pure solvent, there is developed a certain diffusion tension of 

 the effective solute particles, and this may act to produce osmotic 

 pressure when a suitable membrane is interposed. It makes little 

 difference whether we suppose this diffusion tension to be due to the 

 temperature of the dispersed solute, emphasizing, as we must, the fact 

 that this tension cannot be effective beyond the limits of the solvent, 

 which alone allows dispersal; or whether we maintain that the tension 

 here considered is due to the entrance of solvent into the - solution 

 because of attraction, thus producing hjxlrostatic pressure in the latter, 

 which pressure must be transmitted in all directions, to the solute par- 

 ticles and thence to the semipermeable membrane. Since such theo- 

 retical considerations serve us only as mental pictures, and since the 

 gas-pressure theory is so much simpler, and hence so much more valu- 

 able, in physiological considerations, it seems that physiology may 

 retain this theory without apology, and thus make for clarity in a field 

 in which it.s publications have not usually been characterized by this 

 quality. 



An excellent review of the various theories which have been advanced 

 to account for the action of the osmotic membrane is included in this 

 last chapter of the book before us. Of course no final and general 

 conclusion can yet be reached in this connection. — B. E. L. . 



- Uvinpston, B. E., T he role of diffusion and osmotic pressure in plants. 

 Chicago, 1903. 



