OSMOSIS. 275 



The attraction of the substance of the membrane for water, at any 

 rate, may then be a factor in the case. Ludwig x demonstrated, indeed, 

 that the concentration of the solution imbibed by an animal membrane 

 may be lower than that of the solution in which it is soaked. 



Fick 2 distinguished between two possibilities for diffusion through an 

 animal membrane a " pore diffusion " in Brlicke's sense, and a 

 diffusion occurring through the spaces between the molecular aggregates 

 of which the membrane may be considered to be built. The latter idea 

 is somewhat of the nature of that formed of the diffusion of a gas 

 through a film of liquid in which it is soluble, or is perhaps better 

 illustrated in the experiment of L'Hermite, 3 in which, when water 

 separates chloroform from ether in a tube, the chloroform increases at 

 the expense of the ether. Tick's " homogeneous " membranes were 

 made of collodion ; but his results show that such a membrane is not 

 unalterable, since the amount of salt passing through increases with 

 time, and it is difficult to escape the conclusion that in many cases 

 some interaction of chemical nature takes place between the membrane 

 and the substances to which it is permeable. 4 



The property possessed by certain substances of imbibing certain liquids 

 (apart from capillary action), must be borne in mind in all considerations of the 

 essential nature of the processes involved in the passage of fluids through 

 membranes. This property can only be ascribed to some " affinity " between 

 the molecules of the imbibing substance and that imbibed; thus gelatin 

 swells in water but not in ether, while the reverse is true of caoutchouc. The 

 retention of a gas, or a colouring matter by charcoal, of water by the silica 

 of the opal, or that of pepsin by fibrin, are instances of the class of phenomena 

 to which attention is here called, and to which the name of adsorption is often 

 applied. When a homogeneous substance imbibes a solution, compounds of 

 the imbibed with the imbibing substance may be formed, which may have a 

 greater affinity for the solvent than the original imbibing substance, but at the 

 same time the osmotic pressure of the solution tends to retard the imbibition 

 of the solvent; hence, with a given pair of substances, the amount of the 

 solution of one taken up by the other will reach a maximum at a certain con- 

 centration, a maximum, however, which may be well above that for imbibition 

 of the pure solvent. 



The "affinity" of the imbibing substance for the solvent and dissolved 

 substance imbibed may be of very different order, for gelatin takes up a more 

 concentrated solution of methyl- violet than that in the dye-bath ; while, on the 

 other hand, a ferrocyanide of copper membrane will take up water while almost 

 absolutely indifferent to dissolved cane sugar. 



Such "affinities" are not purely mechanical, since they vary with the 

 chemical nature of the substances, and yet are not of the nature of chemical 

 affinity in the usual sense of the term, since the "compounds" do not obey the 

 laws of constant and multiple proportion. Ostwald has introduced the term 

 mechanical affinity to meet the case. 



In the complex known as protoplasm there may be imbibing substances of 

 different nature, permeated by a solution of substances whose chemical nature 

 may, directly and indirectly, affect the imbibition of a solution brought in 

 contact with the mass ; and, furthermore, undissolved particles may themselves 



l Ztschr.f. rat. Med., 1849, Bd. viii. S. 1; Ann. d. Phys. u. Chem., Leipzig, Bd. 

 Ixxviii. S. 307. 



2 Untersuch. z. Naturl. d. Mensch. u. d. Thieve, 1857, Bd. iii. S. 294. 



3 Ann. de chim., Paris, 1854. S^r. 3, tome xliii. p. 420. 



4 Tcamman, Ztschr. f. physikal. Chem., Leipzig, 1892, Bd. x. S. 255 ; Walden, ibid.,S. 699. 



