158 DISCUSSION OF EVIDENCE. 



lithium crystallize is 3. The maximum amount for many of the salts 

 of sodium is 2. The lithium ion is, in general, more hydrated than th3 

 sodium ion, and its velocity is therefore decreased more by hydration. 

 Notwithstanding its smaller volume and lighter mass, on account of its 

 greater hydration lithium moves with about the same velocity as 

 sodium. 



The calcium ion is slightly larger than sodium, but has considerably 

 smaller velocity. This is undoubtedly due primarily to its much 

 greater hydrating power. Within this group the atomic volumes 

 increase with increasing atomic weight. The velocities of calcium 

 and strontium, with many salts crystallizing with 6 molecules of water, 

 are approximately equal to that of barium. Many of the salts of 

 barium crystallize with 2 molecules of water or water-free. The larger 

 mass of the barium ion itself diminishes the velocity. Magnesium, 

 with about half the volume of calcium, has nearly the same velocity, 

 due to its greater hydrating power. The cobalt, nickel, and copper ions 

 have nearly the same volumes and approximately the same hydrating 

 power. They have approximately the same velocities. 



The atomic volumes of the chloride, bromide, and iodide ions are 

 approximately the same. If they hydrate at all we should expect the 

 same order of hydration for all three, as has been made probable. We 

 should expect them to have velocities of the same order of magnitude, 

 and such is the fact. 



The silver ion is the only well-established exception. It has a small 

 volume and many of its salts crystallize without water. Although 

 it has small volume, it apparently has but little hydrating power. 

 Notwithstanding its considerable mass, with its small volume and 

 small hydrating power we should expect it to have a fairly high velocity. 

 The fact is, the velocity of the silver ion is slightly less than that of 

 chlorine, bromine, and iodine. 



The general truth of the relation that the ions with the smallest 

 velocities have the greatest hydrating power is, then, established by 

 the facts, the great hydrating power being one of the factors condition- 

 ing the small velocity. 



DISSOCIATION AS MEASURED BY THE FREEZING-POINT METHOD AND BY 



THE CONDUCTIVITY METHOD. 



When the theory of electrolytic dissociation was proposed, it became 

 a problem to measure accurately the magnitude of dissociation. 

 Arrhenius pointed out, in his original epoch-making paper, that 

 dissociation could be measured either by the freezing-point or by the 

 conductivity method. The conductivity of a few electrolytes was first 

 worked out accurately by Friederich Kohlrausch, by a method which 

 he devised for the purpose. This work was done before the theory 

 of electrolytic dissociation was proposed. Kohlrausch's data were 



