THE HYDRATE THEORY OF SOLUTIONS 219 



hydrated character of the ions in aqueous electrolytes would 

 even now have secured general recognition but for the fact that 

 it was necessary to assume the existence of complexes of this 

 type in order to account for the apparently anomalous character 

 of the ionic mobilities, and certain related properties of the 

 solutions. 



Hydration as related to Ionic Mobility 



Having discussed somewhat fully the fundamental problems 

 associated with the application of the hydrate theory to electro- 

 lytes, it only remains now to refer briefly to some of the 

 secondary consequences arising from the hydration of ions. 

 They are of interest, not so much on account of their intrinsic 

 importance, as because of the part they have played in convincing 

 those who accepted the theory of electrolytic dissociation in its 

 original form of the necessity of extending their conceptions, 

 even at the cost of sacrificing the extreme simplicity which con- 

 stituted one of the great attractions of the theory. 



Foremost among the properties which have thus afforded 

 evidence of ionic hydration must be placed the mobilities. If, 

 for instance, the alkali metals are compared, the remarkable 

 result is arrived at that, when acted on by equal forces, the 

 metal of highest molecular weight moves most rapidly, and that 

 lithium with a molecular weight twenty times smaller than that 

 of caesium, moves with only half the velocity of the latter metal : 

 Li Na K Rb Cs 



3344 43'55 64-67 67-6 68-2 



The halogens afford a similar, though less striking, illustration, 



the bromine atom, with an intermediate atomic weight, being a 



trifle more mobile than either chlorine or iodine : 



Cl Br I 



65'44 67*63 66*40 



In the case of elements so similar in character it is difficult to 

 imagine that there is any wide difference in the structure of the 

 different atoms, and the explanation universally adopted is that 

 the ions are not mere naked atoms, as was at first supposed, but 

 are clothed with a " watery atmosphere " — a happy expression 

 for which we are indebted to Kohlrausch. The quantity of 

 water attached to the ion is assumed to vary in the different 

 members of the series, and the sluggish motion of the lithium 

 ion is attributed to the excessive quantity of water with which 



