306 SCIENCE PROGRESS. 



creases more slowly than the negative temperature coefficient 

 of dissociation, it is evident that, at a certain temperature, 

 the two will become equal, and that, beyond that tem- 

 perature, any further heating will decrease the dissociation 

 more than it increases the fluidity, so that the conductivity 

 will again become less. The temperatures of maximum 

 conductivity were calculated by Arrhenius for solutions of 

 hypophosphoric and phosphoric acids, and shown to be 

 57 and 78° respectively. An experimental investigation 

 actually gave maxima at 55° and 75". More recently 

 Sack * has found a maximum for copper sulphate solution 

 at 96 . Calculation by Arrhenius' method gives 99° for a 

 similar solution. Thus it appears that the thermal pheno- 

 mena of electrolytic solutions are all satisfactorily explained 

 by the dissociation theory. 



An equally striking success has attended the applica- 

 tion of the theory to the phenomena of the diffusion of dis- 

 solved electrolytes. A satisfactory account of the diffusion 

 of non-conductors has been given by Nernst 2 and Planck. 3 



In any solution, the dissolved molecules are continually 

 moving about in all directions. Therefore, in a given time, 

 more molecules cross any fixed plane from the side on which 

 the concentration is greater to that on which it is less than 

 in the opposite direction. The more dilute parts of the 

 liquid will thus grow stronger at the expense of those which 

 were originally more concentrated, and this process will go 

 on till the concentration is uniform throughout. 



Now, the osmotic pressure of a solution depends on the 

 concentration, and is approximately proportional to it, so 

 that we may, for convenience, consider the variation in 

 osmotic pressure, which exists in a solution of non-uniform 

 concentration, to be the cause of the diffusion which goes 

 on. Thus, if we put a layer of water over a solution of 

 sugar, the osmotic pressure of the solution can be con- 

 sidered as driving the molecules of sugar into the water, 



1 Wied. Ann., xliii., p. 212, 1891. 



- Zeits. fiir physikal. Chemie, ii., p. 613, 188S ; or, Nernst's Theoretische 

 Chemie. 



3 Wied. Ann., xl., p. 561, 1890. 



