A GENERAL ACCOUNT OF THE PROCESSES OF 

 DIFFUSION, OSMOSIS, AND FILTRATION. 



BY E. WAYMOUTH EEID. 

 CONTENTS : Diffusion, p. 261 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, miscible 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 vice versa, 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 3 

 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. Chem., Leipzig, 1887, Bd. i. S. 631. 



2 Kohlrausch, Ann. d. Phys. u. Chem., Leipzig, 1879, 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. 



