276 DIFFUSION, OSMOSIS, AND FILTRATION. 



exert surface action, so that the possibilities for purely physical absorption are 

 quite unknown, and so-called vital elective action may be the result of specific 

 adsorptive affinity. Hofmeister * has shown that gelatin has an " elective " action, 

 for common salt, the concentration of the solution imbibed exceeding that of 

 the surrounding solution ; and, further, that the combination of sodic chloride 

 with the gelatin favours the uptake of water. Again, gelatin takes up more 

 water from '5 to 2 per cent, solution of ethyl-alcohol in water than from pure 

 water. 



With salts that undergo electrolytic dissociation in solution, permeability 

 must be a function of ions. Thus, according to Ostwald, 2 copper ferrocyanide 

 is permeable to potassium chloride, because both chlorine and potassium ions can 

 pass ; it is impermeable to barium chloride, because the barium ion is stopped ; and 

 impermeable to potassium sulphate, because the sulphuric acid ion cannot pass ; 

 and, under ordinary circumstances, on account of opposite electrical charges, if 

 one ion is stopped, so must be the other. There are, however, conditions 

 under which an ion, stopped on account of the impermeability of the membrane 

 to its fellow in a salt, may pass the membrane. 



If the negative ion of a salt is prevented from passing through the 

 membrane, only because it is impermeable to its positive fellow, the addition of 

 another salt, whose positive ion can pass the membrane, will allow the negative 

 ion of the first salt to pass in company with it. Or a salt whose negative ion 

 can pass the membrane may be placed on the opposite side, the two negatives 

 exchanging with their positive fellows across the membrane, and equal numbers 

 of the two negative ions passing in opposite directions in a given time. This is 

 of interest to the physiologist, since it opens a possible physical explanation of 

 the fact that a cell may hold back a substance under certain conditions, while 

 under others, when surrounded by a differently constituted fluid, the same 

 substance may be given up. 



Koeppe 3 has attempted to apply this to the formation of hydrochloric acid 

 in the stomach from sodium chloride, maintaining that the stomach wall is 

 impermeable to chlorine ions, but that the sodium ions are exchanged for 

 hydrogen ions from the blood. That free hydrogen ions are present in the 

 alkaline blood is, however, hardly possible. 



Whether permeability be a function of physical or chemical nature, it 

 is obvious that in the case of a living membrane the complex to which 

 the term " physiological condition " is applied must affect the property, 

 so that one and the same membrane in the body may, under different 

 circumstances, be more or less permeable by the same substance. 



The simplest living membrane with which experiments can be made 

 is probably the differentiated outer layer of the protoplast of the vege- 

 table cell (PlasmaJiaut). There is no doubt that the permeability of this 

 membrane for different chemical substances is very variable. It is pene- 

 trated by some dye-stuffs but not by others, very impermeable to 

 many simple salts, though easily permeable by certain complex organic 

 substances. 4 Since this membrane is in its living condition so 

 slightly permeable to salts, the osmotic pressure within vegetable cells is 

 high (3 to 4 atmospheres). This special relative impermeability to salts 

 is obviously regulated in some manner by the " physiological condition " 

 of the membrane. Jansen 5 found that the cell sap of the alga, Cliceto- 



1 Arch. f. cxper. Path. u. PharmakoL, Leipzig, 1891, Bd. xxviii. S. 210. 

 8 Ztschr. f. physical. Chem., Leipzig, 1890, Bd. vi. S. 71. 

 s Arch.f. d. ges. PhysioL, Bonn, 1896, Bd. Ixii. S. 567. 



4 Pfeffer, Abhandl. d. math.-phys. Cl. d. k. sacks. Gesellsch. d. Wissenscli., 1890, Bd. 

 xvi. S. 149. 



5 Verhandl. d. Jc. Akad. v. Wetensch., Amsterdam, 1888, vol. iv. p. 345. 



