OSMOSIS. 273 



colloidal solution exert an osmotic pressure capable of measurement 

 by our present methods. 



From the above account of osmotic pressure, it is evident that, 

 since it is present in high or low degree in all true solutions, as a result 

 of the kinetic energy of the dissolved molecules, the phenomena of 

 diffusion are most satisfactorily accounted for as directly dependent 

 on the osmotic pressure exerted by the diffusing substance. 1 Substances 

 diffuse from places of higher to those of lower partial pressure, and 

 the differences in rapidity of diffusion of different substances, though 

 present in concentrations exerting the same osmotic pressure, must be 

 accounted for by differences in the resistance met in their passage 

 among the molecules of the solvent. 



When we now turn to the consideration of the interchange of the 

 constituents of solutions through animal membranes, we at once find 

 that, since these membranes are never strictly semipermeable, and are 

 frequently very permeable for dissolved substances, the phenomena are 

 neither those of pure osmose nor pure diffusion, but a complex of 

 the two, in which the relative permeability of the membrane to solvent 

 and dissolved substance is of paramount importance, but, unfortunately, 

 a variable factor with different membranes. 2 All the earlier work upon 

 osmosis was carried out with membranes not fulfilling the condition of 

 semipermeability, so that a double stream of solvent into solution 

 (endosmose) and dissolved substance into solvent (exosmose) was con- 

 sidered as a necessary feature of the process until Traube's discovery of 

 precipitation membranes. 



The first osmose experiment was probably that of the Abbe Nollet, 3 

 in which it was observed that a bladder tied over a vessel of spirits of 

 wine became distended, or even burst, when vessel and membrane were 

 under water. Parrot 4 again called attention to the fact, which 

 had been forgotten, and ascribed the process to " affinity of the first 

 order," which causes all miscible fluids to " wander " into one another. 

 Fischer 5 in Germany and Dutrochet 6 in France again rediscovered 

 the prime fact, and commenced its systematic study. Certainly the 

 main stimulus to subsequent study of the phenomena was given 

 by the work of Dutrochet. 7 Dutrochet's endosmometer was a funnel 

 closed by membrane and provided with a long stem. The body 

 of the funnel was filled with the solution, and the whole immersed 

 in water. The height to which the fluid rose in the stem was the 

 gauge of the osmotic action of the solution. Dutrochet recognised 

 that the concentration of the solution and the temperature affected 

 the results. 



Vierordt 8 improved upon the arrangement used by Dutrochet, by 

 setting the membrane vertical and the stem horizontal, so that filtration 

 error was avoided, and also concluded that the stream of water into the 



1 Nernst, Ztschr. f. physikal. Chem., Leipzig, 1888, Bd. ii. S. 611. 



2 In this connection see a paper by Lazarus Barlow, Journ. PhysioL, Cambridge and 

 London, 1895, vol. xix. p. 140. 



3 " Histoire de 1'Academie royale des sciences," 1748, p. 101. 



4 Ann. d. Phys. u. Chem., Leipzig, 1815, Bd. li. S. 318. 



5 Ibid., 1822, Bd. Ixxii. S. 300. 



6 Ann. de chim., Paris, 1827, tome xxxv. p. 393 ; "Agent imme'diat du mouvement 

 vital," Paris, 1826. 



7 See also " Me"moires pour servir a I'histoire anatomique et physiologique des vegetaux 

 et des animaux," Bruxelles, 1837. 



8 Ann. d. Phys. u. Chem., Leipzig, 1848, Bd. Ixxiii. S. 519. 



VOL. I. 18 



