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 
d illusion 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 nienibrane 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. Chan., 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 I'Academie royale des sciences," 1748, p. 101. 
4 Ann. d. Flujs. u. Chem., Leipzig, 1815, Bd. Ii. S. 313. 
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 " ilemoires pour servir a Fhistoire anatomique et physiologique des vegetaux 
et des animaux, " Bruxelles, 1837. 
8 Ann. d. Phys. u. Chem., Leipzig, 1848, Bd. lxxiii. S. 519. 
VOL. I. — 18 
