264 DIFFUSION, OSMOSIS, AND FILTRATION. 



T is the ratio of the diffusion coefficients of the two salts, with 

 separate diffusions. 



T is the ratio with simultaneous diffusions. 



R, the ratio of the amounts diffused of the same salt in separate 

 and in simultaneous diffusion, i.e. the alteration of the coefficient of 

 diffusion produced by the presence of the other salt. 



As a rule, as seen in R, the more diffusible salt is accelerated, the 

 less diffusible delayed. In the two last pairs both members are delayed, 

 but the less diffusible more markedly. 



In the body it is rare to find the conditions present for a free 

 diffusion between the constituents of two solutions ; a membrane, 

 whether composed of cells or the surface layer of the protoplasm of 

 a cell, as a rule intervenes, and obviously the permeability of the 

 membrane affects the result. If pig's bladder separates methyl alcohol 

 and ether, the methyl alcohol diffuses into the ether, but if a caoutchouc 

 membrane separates the two liquids, the ether diffuses into the alcohol. 1 



OSMOSIS. 



The term osmosis is applied to diffusion taking place between two 

 liquids separated by a membrane. 



The simplest case of this is that in which a solution of a substance 

 is separated from the pure solvent by a membrane permeable by the 

 solvent but impermeable by the dissolved substance. Such membranes 

 were first prepared by Traube, 2 in the form of colloidal precipitates, 

 such as tannate of gelatin and ferrocyanide of copper, but Pfeffer 3 

 was the first to thoroughly study the process of osmosis under such 

 conditions. The name " semipermeable " has been given to such mem- 

 branes, but it must be noted at once that this expression is seldom 

 strictly accurate and must always be used relatively to some particular 

 substance. Tamman 4 has pointed out that such membranes are by 

 no means the " molecule sieves " that Traube imagined, 5 and in experi- 

 mental work the membrane must be chosen to suit the substance, or 

 vice versd. Copper ferrocyanide forms one of the best of such mem- 

 branes, and is nearly impermeable to cane sugar. 



1 Raoult, Ztschr. f. physical. Chem., Leipzig, 1885, Bd. xvii. S. 735. 



2 Arch.f. Anat. u. Physiol., Leipzig, 1867, S. 87 and 129. 



3 "Osmotische Untersuch.," Leipzig, 1877. 



4 Ztschr. f. physikal. Chem., Leipzig, 1892, Bd. x. S. 255. 



5 See also Walden, ibid., 1892, Bd. x. S. 699. 



