August 26, 1910] 



SCIENCE 



283 



In 1894 " Professor Traube announced his 

 conviction that the direction and velocity of 

 the osmotic current (and consequently the 

 driving force in osmosis) is due to a difference 

 in surface-tension between the two fluids on 

 either side of the colloidal or semi-permeable 

 membrane. He bases this conclusion upon 

 some years of experimental investigation, to 

 which he was originally led by a consideration 

 of Overton's work on plasmolysis and absorp- 

 tion in vegetable cells. Overton found that 

 all substances whose watery solutions can 

 penetrate the walls of plant cells lower the 

 surface-tension of the solvent water, while 

 those which can not penetrate the cell-walls 

 raise the tension of the water in which they 

 are dissolved. Traube's investigations of cap- 

 illary constants tallying in every particular 

 with Overton's plasmolytic data, the natural 

 inference was that there is an equation be- 

 tween velocity of osmotic diffusion and degree 

 of surface-tension. In Traube's theory, the 

 driving force in osmosis is a superficial (or 

 interfacial) pressure (Oherfldchendruch) ob- 

 tained by subtracting the surface-tension of 

 one fluid from the tension of the fluid into 

 which it diffuses. His view is thus entirely at 

 variance with the theory of van't Hoff and his 

 followers who contend that the motor power 

 in osmosis is the kinetic energy of the dis- 

 solved molecules, the osmotic pressure being due 

 to the impact of these molecules against the 

 walls of the semi-permeable membrane, and 

 obeying the laws governing pressure relations 

 in gases. Traube compares the two fluids on 

 either side the semi-permeable membrane with 

 two parallel chains of men and women holding 

 hands and facing each other. Suppose each 

 man to loosen his hold and grasp the hands of 

 the woman opposite, and a " tug of war " to 

 ensue: the physically weaker women will be 

 pulled towards the stronger men. So, in 

 osmosis, the fluid having the lower surface- 

 tension must inevitably be drawn towards that 

 having the higher. 



In 1898 ^ Professor Traube proceeded to 



" Pfliiger's Arch. f. d. ges. Physiol., Bonn, 1904, 

 CV., 541, 559. 

 ^Pfliiger's Arch., 1908, CXXIII., 419-432. 



rectify and clarify some of his views in regard 

 to the selective action which the semi-per- 

 meable membrane may acquire through deposi- 

 tion of lipoid solvents upon it, and he was 

 further led to fortify his theory of osmosis in 

 another way. To quote his own language: 

 " I attribute this clarification of my views to 

 the fact that my attention was called to an 

 important theorem of which I had not previ- 

 ously been aware, the thermodynamic demon- 

 stration of which is originally due to the great 

 mathematical physicist, Willard Gibbs. . . . 

 The theorem of Gibbs amounts virtually to 

 this, that all substances which lower the sur- 

 face-tension of a solvent tend to collect at the 

 surface of discontinuity." Traube then re- 

 states the Gibbs theorem in a form more prac- 

 ticable for physiological chemists as follows : 

 '' The more a substance increases or decreases 

 the surface-tension of the pure solvent, the 

 larger or smaller is its ' Haftdruch ' " — mean- 

 ing by ' Saftdruclc ' the " attraction-pressure " 

 with which the solute (dissolved substance) 

 tends to remain in solution, a quantity which 

 Professor Traube identifies with the " cohe- 

 sion-constant " of van der Waals. The attrac- 

 tion pressure Traube holds to be the pressure 

 corresponding to the (chemical) union of the 

 solute and the solvent. It is thus the " inten- 

 sity factor" of the solution-energy, as op- 

 posed to the number of dissolved molecules, 

 which is its " capacity factor."" 



Now the theory of osmosis which Traube so 

 honestly and conscientiously traces back to the 

 Gibbs theory of surface-tension affords at the 

 same time an interesting confirmation of 

 the view of osmosis propounded by Gibbs 

 himself. According to Gibbs," the force 

 that drives the fluid through the semi- 

 permeable membrane is not an initial " os- 

 motic pressure," but either a difference in 

 temperature or a difference in chemical poten- 

 tiality between the two fluids bathing opposite 

 sides of the membrane. If the fluids be iden- 

 tical in composition but have different tem- 



" Traube, J. Phys. Chem., Ithaca, 1910, XIV., 

 452-470, 471-475. Also, Pfliiger's Arch., 1910, 

 CXXXII., 511-538. 



" Tr. Connect. Acad. Arts and Sc, New Haven, 

 1874-8, III., 138-140. 



