IMBIBITION 203 



functions of one and the same property, namely, osmotic pres- 

 sure." This may be true, and an attempt wiU be made to show 

 why some think that it is; but there are those who regard imbibi- 

 tion as a process quite distinct from osmosis. The latter term 

 has heretofore always been applied to the forces involved in a 

 system where a membrane encloses a solution, Freundlich 

 states emphatically that it is fundamentally false to regard 

 osmotic pressure as identical with imbibition pressure. There is 

 heat of imbibition, but no heat of osmosis. (This may be merely 

 a matter of degree.) More significant are the facts that osmotic 

 pressure is proportional to concentration, while imbibition pres- 

 sure is proportional to a high power of the concentration. 



Botanists, concerned as they are with the swelling pressure of 

 gels (cellulose, starch, and agar), on the one hand, and the turgor 

 of the cell vacuole, on the other, regard the two as distinct because 

 the respective systems in which they take place are mechanically, 

 i.e., structurally, different. 



The Donnan Equilibrium. — We shall have occasion a number 

 of times to refer to what is known as the Donnan equilibrium. 

 Here it serves us as an interpretation of imbibition. The 

 mathematical analysis of a simple osmotic equilibrium by Willard 

 Gibbs, who stated that the chemical potential of a solution is of 

 the same value on both sides of a differentially permeable mem- 

 brane, and the suggestion of Wilhelm Ostwald that significant 

 relationships must exist between ions when a selectively perme- 

 able membrane separates two solutions served F, G. Donnan of 

 London as a basis for his theory of membrane equilibriums. In 

 its simplest form, the Donnan equilibrium is as follows: If a 

 membrane separates a solution of sodium chloride from one of 

 sodium albuminate, then there will be the following distribution 

 of ions at the beginning of the experiment (the albumin anion is 

 represented by A~) : 



After osmosis has been allowed to act, the Cl~ ion on the one side 

 will have diffused to a greater or lesser extent to the other side. 

 As the albumin ion A~ cannot pass through the membrane, the 

 following condition will exist at the end of the experiment: 



