A GENERAL ACCOUNT OF THE PROCESSES OF 
DIFFUSION, OSMOSIS, AND FILTRATION. 
By E. Waymouth Reid. 
Contexts :— Diffusion, p. 201— Osmosis, p. 264 — Filtration, p. 280. 
Diffusion. 
By current hypothesis the molecules of a liquid are considered to be in 
constant motion, so that if two liquids, miscihle without chemical inter- 
action, are placed in contact, a mutual interpenetration, without the 
action of any external force, takes place; or, in other words, a diffusion 
of the molecules of one among those of the other, and rice versd, occurs, 
the process tending to continue until in the final state a homogeneous 
mixture of the two exists. In physiological problems we deal with the 
diffusion of substances in dilute aqueous solution, and it must at once 
be noted that the condition of the molecules of a substance in dilute 
aqueous solution is probably different in the case of different substances, 
and by no means necessarily the same as that of the undissolved 
substance ; that, in fact, the solvent and dissolved substance in many 
cases interact, with a resultant alteration of physico-chemical pro- 
perties. 
In the case of substances acting as electrolytes in aqueous solution, 
it is believed that dissociation into the ions takes place to a greater or 
less extent of the total number of molecules, according to the degree of 
dilution. 1 There will thus be at lower degrees of dilution a mixture of 
molecules, active as regards electrolytic conduction and chemical action, 
and inactive molecules, the latter tending to become active by ionic 
dissociation as dilution is increased, so that at infinite dilution only 
active molecules exist in the solution. The coefficient of activity will be 
the number expressing the ratio of active molecules to the total of 
active plus inactive, and is unity at infinite dilution. The electrical 
conductivity of a solution of an electrolyte is dependent on the velocity 
of migration of its ions, 2 so that the ratio of the molecular conductivity :: 
of a solution of an electrolyte at given dilution, to the limiting value 
1 Arrhenius, Bijhang. till k. Svens. Vet.-Akad., Stockholm, 1884, Bd. viii., Nos. 
13 and 14 ; Ztschr. f. physikal. C'hem., Leipzig, 1887, Bd. i. S. 631. 
- Kohlrausch, Ann. d. Phys. u. C'hem., Leipzig, 1S79, Bd. vi. S. 1, 145 ; 1885, Bd. xxvi. 
S. 161. 
3 The molecular conductivity is the ratio of the conductivity to the molecular concen- 
tration of the solution, the latter being the ratio of grammes per litre to the molecular 
weight in grammes. 
