IMBIBITION 205 



in the case of albumin immersed in a sodium chloride solution.) 

 The same unequal distribution of ions results, with an imbibition 

 or osmotic pressure within the gelatin. 



The application of the Donnan equilibrium to experimental 

 (not theoretical) imbibition conditions has met with some 

 objections. The ratio of the concentration of the diffusible ions 

 appears to be an assumption. So also is the statement that a 

 salt of gelatin is formed when the latter is in an acid or alkaline 

 medium. If a salt is formed and if its formation is a necessary 

 prerequisite of the equilibrium and consequent swelling, we then 

 have no interpretation of the swelling of gelatin in pure water or 

 of the effect of salts on the degree of swelling of isoelectric gelatin. 

 The Donnan membrane equilibrium, sound when applied to 

 systems where its demands are met, does not interpret all phases 

 of the general problem of the swelling of gels. 



In spite of doubts and difficulties, the concept of membrane 

 equilibrium offers interesting suggestions for a possible 

 mechanism of a number of biological processes. A high concen- 

 tration of the nondiffusible ion A~ will inhibit diffusion of the 

 sodium chloride to such an extent that if sodium chloride is 

 added to the side containing the nondiffusible ion, there may be 

 diffusion to the other side against an opposing osmotic pressure — 

 in other words, excretion of sodium. It is possible that glands 

 may function in this manner and thus be able, as they are, to 

 excrete against concentration gradients (see also page 378). 



The problem of the identity of imbibition and osmosis is one 

 that cannot yet be solved. While it still seems desirable to 

 distinguish them, certain criticisms are not fully justified. It 

 is stated that imbibition pressure should not be confused with 

 osmotic pressure, because the former may reach values greatly 

 in excess of the latter. Actually, both are infinite. There is no 

 force that can keep dry sugar from becoming wet. The same 

 is true of dry gelatin. No force could keep it from absorbing 

 some water. 



Plants sometimes make use of one method, osmosis, and some- 

 times of the other, imbibition, for getting water. Gortner cites 

 the case of seeds which take up water from a saturated lithium 

 chloride solution (osmotic pressure, 1,000 atmospheres), although 

 their salt content is sufficient to account for only a few atmos- 

 pheres of osmotic pressure. Cacti with osmotic pressures not in 



