ELECTROLYSIS AND ELECTRO-CHEMISTRY. 245 



molecules. Neither does it throw any light on the fundamental nature 

 of solution. It has been very generally assumed that the dissociation 

 theory of electrolytes was necessarily bound up with a view of solution 

 which considers the dissolved matter to be in a state dynamically similar 

 to that of a gas, and to produce osmotic pressure by the impacts of its 

 molecules, just as a gas produces pressure on the walls of its containing 

 vessel. 



Now Poynting ' has shown that the phenomena of osmotic pressure 

 can, on certain not improbable assumptions, be completely represented by 

 the hypothesis that chemical union occurs between the solvent and the 

 dissolved matter. In the present state of our knowledge the dissociation 

 ttheory of electrolytes seems perfectly compatible with such an explanation.'^ 

 All that follows from the facts is the essential freedom of the ions from 

 each other. "Whatever be the cause of osmotic pressure, it certainly de- 

 pends, to a first approximation, at all events, on the number of dissolved 

 anolecules, and not on their nature, and thus, whether it be due to impact 

 or to chemical union, it will have an abnormally great value when, as in 

 the case of electrolytes, the number of effective molecules is increased by 

 •dissociation. 



Again, the theory does not forbid the assumption that complex mole- 

 ocular aggregates, formed of two or more solute molecules, may exist, 

 especially in concentrated solutions, as well as dissociated ions. Such 

 molecules would be electrolytically inactive, unless an odd ion was linked, 

 to them. They would also, as has been suggested by Wildermann and 

 others, explain a lowering of the freezing point less than that calculated 

 ii'om the conductivity. 



It has been found that the specific resistance of many liquids, including 

 ■water (i.e. a dilute solution of electrolytes), increases when the electrodes 

 are brought within a certain critical distance of each other.^ Similar 

 phenomena have been observed in the case of gases, through which an 

 electric discharge was passing, by Lord Kelvin, Bailie, and Peace,^ and 

 this has been explained by J. J. Thomson on the hypothesis that a com- 

 plete chain-like structure is necessary for electrolytic conduction, which 

 cannot occur unless there is room for such a chain to form. It is possible 

 that the same explanation may hold good for liquids, the necessary 

 electrolytic unit being a complex structure formed of a dissociated ion 

 and several solvent molecules. From what has been said, it will be seen 

 ■that there is nothing inconsistent with this idea in the dissociation 

 theory. 



To sum up the results of this section, we may say that, whatever may 

 be the ultimate nature of solution, it seems certain that the electrolytic 

 ions migrate in accordance with Kohlrausch's theory, and, in a homo- 

 geneous solution, are free to travel independently of each other through 

 the liquid. 



' Phil. Mag. 1896, vol. xlii. p. 289. 



2 See letters in Nature, 1896, voL liv. p. 571 ; vol. Iv. pp. 33, 78, and 150. 



^ Koller, Wien. Ber. 1889, vol. 98, ii. p. 201. 



' See J. J. Thomson's Recent Researclies in Electricity, p. 72. 



