SOLUTIONS 177 



about in the liquid as independent particles, their freedom of 

 movement being complete in every sense but one. Any attempt 

 to remove one kind of ion is fruitless, for if the liquid is evaporated 

 and crystallisation of solid salt ensues every positive ion uniter 

 with a negative ion to reproduce molecules of the ordinary 

 neutral kind. And when a current is passed through the liquid 

 for every positive ion removed at one electrode a negative is 

 withdrawn at the other. 



Neither is it possible by the process of liquid diffusion through 

 a membrane or a porous partition to separate sodium ions from 

 chlorine ions even temporarily. 



If a solution of common salt is placed in a porous pot or bag 

 of parchment paper which is immersed in pure water, a portion of 

 the salt will diffuse into the water. It is an eminently good 

 diffuser, as compared with sugar and especially with gum or 

 albumen, all of which move in a similar way, but far more slowly. 

 But the salt which passes through to the water as the result of 

 this spontaneous movement consists of sodium ions and chlorine 

 ions in exactly the same proportions as in the salt which remains 

 behind, so far as chemical analysis can determine. No doubt, 

 as will be mentioned presently, all sorts of ions do not move 

 about at the same rate, some being very much faster than others, 

 but there are electrostatic forces at work between the positive 

 and negative particles which prevent them from wandering 

 beyond the range of each other's attraction, and thus becoming 

 separable to anything more than an almost infinitesimal extent 

 far beyond recognition by chemical analysis. There is in fact no 

 reason to suppose that it will ever be possible to separate a 

 neutral salt solution into a positive portion and a negative 

 portion. There may be membranes in animal and vegetable 

 tissues which possess to a small extent this kind of semipermeable 

 property, but very little is definitely known in this direction. 

 At the same time it is possible that some of the electrical effects 

 observed in leaves and other living parts may be traced to this 

 cause. Evidence of the existence of free ions is obtained from 

 other considerations. 



If this hypothesis is adopted it will be apparent why salts in 

 general exert an osmotic pressure so much greater than that 

 which is observed in the case of sugar. The following figures 

 show some of the results of Pfeffer's work with different 

 solutions : 



