PHTSIOLOGT 39 



if the ions of these substances entered and if a 

 Donnan equilibrium were set up in a certain 

 way, but in that case we might expect to find a 

 diflFerent behavior in respect to rate of penetra- 

 tion. We may expect the rate of penetration to 

 rise as the concentration of the penetrating sub- 

 stance increases. If it is the undissociated mole- 

 cules which enter, this is what we actually ob- 

 serve, for as the acidity of the sea water increases 

 the concentration of undissociated molecules and 

 the rate of penetration likewise increase. If, 

 on the other hand, we assume that it is the ions 

 which enter, we should be obliged to conclude 

 that as their concentration increases (with in- 

 creasing alkalinity of the sea water) the rate of 

 penetration falls off. 



It is evident that (except when there is an 

 ionic exchange in opposite directions) a cation 

 cannot pass through the surface without a cor- 

 responding anion, so that in general a cation can 

 pass only when it strikes the surface at the same 

 time as an anion. Hence in many cases a cation 

 must strike the surface without entering but in 

 the case of an undissociated molecule every one 



