24 DIFFUSION AND OSMOTIC PRESSURE 



The theory just given is called the theory of dissociation. 

 There are other theories to account for these phenomena, 

 but this has the widest acceptance at the present time and 

 serves the purpose of the physiologist better than any other 

 yet advanced. 



II. IONIZATION OF SOLUTES IN LIQUID SOLUTIONS 



It has been found that a phenomenon similar to the one 

 just described occurs in dilute aqueous solutions of electro- 

 lytes. These solutions uniformly give a higher diffusion 

 tension than the one required by the theory. The explana- 

 tion is the same as that given above ; * if ammonium chlorid, 

 for instance, be put into aqueous solution, it has been shown 

 that some of the molecules ionize, thus increasing the diffusion 

 tension of the solute. The more dilute the solution, the greater 

 is the proportion of molecules dissociated, and at infinite dilu- 

 tion a limit would be reached at which complete dissociation 

 would occur. This theory is named for its originator, Arrhe- 

 nius. In very weak solutions it is found that practically all the 

 molecules are ionized. If several electrolytes are contained 

 in the same solution, ionization occurs in all of them, but 

 not always to the same degree as if there were but one 

 solute; the presence of other molecules and ions seems to 

 influence the amount of dissociation. This subject has not 

 yet been sufficiently investigated to permit the formulation 

 of a general principle. Where a solution contains several 

 different kinds of ions, it is found that the velocity of diffu- 

 sion of some ions is much greater than that of others. 



Only those substances which conduct electricity when in 

 solution are dissociated. The whole theory of primary bat- 

 teries and of electric conduction by liquids depends upon 

 this principle. Enough has been said, however, to prepare 

 the way for what is to follow. 



i S. ARRHENIUS, "Ueber die Dissociation der in Wasser gelOsten Stoffe," Zeit- 

 schr. f. physik. Chem., Vol. I (1887), pp. 631-48. 



