WORK OF J. N. PEARCE. 85 



we find that potassium, rubidium, and csesium have the greatest migration velocities 

 + 

 (H and OH excepted), while the ions of the iron and copper groups, with very small 



atomic volumes, have the smallest migration velocities. 



A glance at the two curves representing the relation between atomic volume and 

 atomic weights, and between migration velocities and atomic weights, shows at once 

 the cause of this apparent anomaly. It has been found that those elements which 

 have the smallest atomic volumes have the greatest hydrating power, and vice versa. 

 We see, then, that those ions which have the smallest migration velocities have also the 

 greatest hydrating power. 



A somewhat detailed comparison of the members of the different groups will bring 

 out this idea more clearly. 



The atomic volumes of potassium, rubidium, and caesium increase rapidly with 

 increasing atomic weights ; and, as a rule, their salts crystallize without water. We 

 should expect, then, the potassium ion to have the greatest migration velocity and 

 the csesium ion to have the smallest. Experiments show that they have approxi- 

 mately the same migration velocities. Sodium and lithium, whose atomic volumes 

 are less than half that of potassium, have migration velocities which are only about 

 two-thirds that of potassium. It will be remembered that sodium and lithium form 

 salts which may crystallize with 2 and 3 molecules of water, respectively. Hence, 

 we may assume that the increase in volume of the sodium and lithium ions, due to 

 the formation of a relatively large hydrate, decreases the velocity of those ions to a 

 far greater extent than the slight hydration of the large potassium ion decreases the 

 velocity of that ion. 



The atomic volume of lithium is about one-half that of sodium, and the maximum 

 amount of water with which lithium salts crystallize from solution is 3 molecules, 

 whereas the maximum for sodium salts is 2 molecules. Since the ratio of 2: 3 repre- 

 sents approximately the ratio of the hydrating power of the two ions in solution, 

 we should expect the effect upon the velocity of the greater increase in the volume 

 of the small lithium ion, due to its hydration, to compensate somewhat for the 

 smaller increase in the volume of the larger sodium ion. Experiment shows that 

 the migration velocities are nearly equal. 



The same relation holds for the metals of the alkaline-earth group. The atomic 

 volumes increase with increasing atomic weight. The migration velocities of the 

 cations calcium and strontium, whose salts crystallize with G molecules of water, 

 are approximately equal to that of the barium cation, whose salts crystallize either 

 with 2 molecules of water or water-free. On the other hand, the magnesium cation, 

 which has one-half the atomic volume of the calcium ion, has nearly the same migra- 

 tion velocity, due to compensation between the atomic volumes and the hydration 

 of the ions. 



The calcium ion has a slightly greater atomic volume than sodium, yet, owing to 

 its much greater hydrating power, its migration velocity is considerably less. 



The cations copper, cobalt, and nickel have nearly the same atomic volumes and 

 the same hydrating power. We should expect them to have the same migration 

 velocity, and such is the case. 



