DISCUSSION OF EVIDENCE. 157 



ion has the greater density of charge upon its surface; this might 

 enable it to hold more molecules of water in combination with itself. 

 There seem, however, to be certain physical objections to this explana- 

 tion of the relation in question. Whatever the explanation, the fact 

 remains. 



HYDRATION OF THE IONS AND THE VELOCITIES WITH WHICH THEY MOVE. 



Certain apparent discrepancies presented themselves in the velocities 

 of the different ions, which, for a time, could not be explained. It was 

 known that the lithium ion, under the same driving force, moves more 

 slowly than potassium; and yet it has smaller volume and smaller 

 mass. It was not until it was shown 1 that the lithium ion is more 

 strongly hydrated than sodium or potassium that this fact could be 

 explained, and other apparent discrepancies presented themselves. 

 A relation between the migration velocities of the ions and their hydrat- 

 ing power was worked out by Jones and Pearce. 2 Their discussion is 

 repeated here to bring out the point in question. 



The velocities of the ions in moving through any given medium is 

 known to vary inversely as their mass, the driving force being constant. 

 Their velocities would also vary inversely as their volumes. Mass 

 being constant, we should expect the ions with the smallest atomic 

 volumes to move the swiftest under a constant driving force, while 

 the facts are often the opposite. Leaving out of account the hydrogen 

 and hydroxyl ions, potassium, rubidium, and caesium have very great 

 velocities and the largest volumes; while the ions of the iron and 

 copper group have the smallest volumes and very small velocities. The 

 meaning of this apparent discrepancy can be seen at once by com- 

 paring the atomic volume curve and the migration velocity curve. 



The ions with the smallest volumes have the greatest hydrating 

 power. The ions with the smallest volumes frequently have the 

 smallest velocities. Therefore, the ions with smallest velocities fre- 

 quently have the greatest hydrating power. To discuss the relations 

 somewhat in detail, the atomic volumes of potassium, rubidium, and 

 caesium increase rapidly with increasing atomic weight, and their salts 

 generally crystallize without water. The atomic volumes of sodium 

 and lithium are less than half that of potassium, and yet their velocities 

 are only about two-thirds that of potassium. It will be recalled that 

 salts of sodium and lithium may crystallize with 2 or 3 molecules of 

 water. We may therefore assume that the increase in the volume 

 and mass of the lithium and sodium ions, due to the formation of a 

 hydrate, decreases the velocity of these ions below that of potassium. 



The small velocity of the lithium ion was, as we have seen, for a long 

 time unexplained. It has a volume only about half that of sodium, 

 and the largest ascertained amount of water with which the salts of 



Carnegie Inst. Wash. Pub. No. 60. z lbid., 180, pp. 84-86. 



