388 SCIENCE PROGRESS 



metallic constituent to unite with a definite number of water 

 molecules with formation of a radical in which the water 

 molecules are so arranged that direct combination between the 

 metallic component and the acid residue is no longer possible." 

 Although in this statement he does not refer to the capacity of 

 the acid residue for combining with water molecules, several 

 instances of this are given in his recent book. 1 



(2) Another argument in favour of the hydration of the ions is 

 based on determinations of their relative migration velocities. 

 The rates of migration of the halogen ions have the relative 

 values : CI = 65*9, Br = 667, I = 667, and those of the alkali 

 metals Li = 35*5, Na = 44*4, K = 65-3. Thus, although of very 

 different weights, the halogen ions migrate at practically equal 

 rates, whilst the speed of the lithium ion is only about half that 

 of the much heavier potassium ion. Bredig (1894) 2 and Euler 

 (1897) 3 suggested as a possible explanation that the ions are 

 associated with a large number of water molecules, so that their 

 size is negligible in comparison with that of the molecular 

 aggregate, but our most exact knowledge on this point is due 

 to Kohlrausch. 4 He showed that the temperature coefficient of 

 conductivity is a function of the mobility of the ions and, further, 

 that the influence of temperature on the electrolytic resistance 

 of dilute solutions of salts approximates to the temperature 

 coefficient of the mechanical friction of water. Kohlrausch 

 points out that these facts are most readily accounted for on 

 the association hypothesis indicated above. 



(3) Further evidence in favour of this view is to be found in 

 the abnormal course of the freezing-point, boiling-point, and 

 vapour pressure curves of many electrolytes with change of con- 

 centration — a phenomenon which has been investigated more 

 particularly by Jones and his co-workers, 5 by Biltz, 6 and by Smits. 7 

 Jones and Biltz find that in many cases the molecular freezing- 



1 Loc. cit. p. 138. 



2 Zeit. physikal. Chcm. 1894, 13, 262. 



3 Ann. der physik. 1897, 64, 273. 



4 Sitzungsber. Preuss. Akad. Wiss. 1902, 26, 579 ; Proc. Roy. Soc. 1903, 

 71, 338; Bousfield and Lowry, Proc. Roy. Soc. 1904, 74, 280; Bousfield, Proc. 

 Roy. Soc. 1905, 74, 563 ; Phil. Trans. 1906, 206A, 101. 



5 Amcr. Chem. Journ. 1904, 31, 356 ; 1905, 33, 534 ; Zeit. physikal. Cheni. 1906, 

 55, 385. 



6 Zeit. physikal. Chem. 1902, 40, 217 ; 1903, 43, 41 ; 1906, 56, 462; Berichte, 

 1904, 37, 3036. 



7 Zeit. physikal. Chem. 1902, 39, 385. 



