84 WORK OF J. N. PEARCE. 



Nernst, Garrard, and Oppermann, 1 in a study of the concentration changes which 

 take place in an indifferent substance during electrolysis, have calculated that the 



ions SO4, CI, Br, and N0 3 drag with them 9, 5, 4, and 2.5 molecules of water, 

 respectively. 



It is seen from a study of the chlorides and nitrates of the copper group, each of 

 which crystallizes with 6 molecules of water (copper chloride alone separating with 

 two molecules), that the hydration per molecule is approximately the same for all 

 of these salts. If, as Nernst and his coworkers have found, the hydrating power 



of the N0 3 ion is only one-half that of the CI ion, then we should expect the influence 

 of the hydrating power of these two anions to manifest itself in the hydrating power 

 of the salts in question, and especially so since the three cations are so nearly alike 

 chemically. On the basis of this reasoning we are forced to conclude that the 

 hydrating power of any salt is primarily a junction of the cation. 



We do not deny that the anions are capable of forming hydrates; but, if they do, 

 experiments lead us to believe that they have this power only to a relatively slight 

 degree. 



We have noted also this striking relation. It is well known that if the atomic 

 volumes of the elements are plotted as ordinates against the atomic weights as 

 abscissae, there exists between them a periodic relation. At the maxima of the 

 curve are the alkali metals. The three elements having the largest atomic volumes 

 are potassium, rubidium, and caesium. Salts of these metals usually crystallize 

 from aqueous solution in the anhydrous form, and, as experiments have shown, they 

 have very slight hydrating power in solution. Lithium and sodium, some of whose 

 salts crystallize with 2 and 3 molecules of water, have much smaller atomic volumes. 



At the minimum of the third section of the atomic-weight curve we find the ele- 

 ments strontium, iron, cobalt, copper, and nickel. The salts of these metals crystal- 

 lize with large amounts of water, and show great hydrating power in solution. 

 Aluminium, which has less than half the atomic weight of iron, but slightly greater 

 atomic volume, lies at the second minimum. Its salts crystallize with 8 and 9 

 molecules of water and show great hydrating power in solution. 



Comparing the metals of the alkaline-earth group we find that barium, whose 

 salts crystallize with 2 molecules of water or water-free, has the largest atomic 

 volume. The other members of this group form salts which crystallize with 6 

 molecules, calcium nitrate excepted. The magnesium cation, which has the smallest 

 atomic volume, has the greatest hydrating power in solution; the strontium cation, 

 which has the largest atomic volume, has a smaller hydrating power than does the 

 calcium cation, whose atomic volume is slightly less. 



This is conclusive evidence that the hydrating power of the cation is an inverse 

 function of its atomic volume. 



That the velocities of the ions are an inverse function of their mass (and perhaps 

 of their volumes) is an established fact. Experimental evidence, however, seems 

 at variance with this statement. We should expect those ions which have the 

 smallest atomic volumes to have the greatest migration velocities. On the contrary, 



'Gottingen Nachr., 1900, p. 86. 



