WORK OF J. N. PEARCE. 77 



We believe that we have found the cause of this phenomenon. Of two ions or 

 ionic complexes of different volumes, that one will meet with less friction on moving 

 through the solution which has the smaller volume. Consequently, it will have 

 the greater velocity. On the other hand, the greater the volume, the greater will 

 be the friction to be overcome by the ion, and, hence, the smaller the velocity. 

 Therefore we should expect to find that those salts which crystallize with little 

 or no water of crystallization give greater values for conductivity than those crystal- 

 lizing with a greater amount. 



It is a well-known law that the conductivity of an electrolyte depends upon the 

 velocities of the ions. These velocities, in turn, depend upon the fluidity and the 

 volume of the ion. The greater the volume, the greater will be the resistance 

 offered to the movement of the ions. 



If we consider the alkalis we find that potassium, rubidium, and caesium, which 

 have the largest atomic volumes and whose salts generally crystallize without water, 

 have the greatest migration velocities, while lithium and sodium, which have smaller 

 atomic volumes and whose salts crystallize with 2 or 3 molecules of water, have 

 very much smaller migration velocities. 



Comparing the members of the calcium group, we find that the atomic volumes 

 increase with increasing atomic weight. The migration velocities of the cations cal- 

 cium and strontium, whose salts usually crystallize with 6 molecules of water, are 

 approximate^ 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, 

 of smaller atomic volume, has a slightly smaller migration velocity, due to the more 

 complex composition of its hydrates. 



The cations of cobalt, copper, and nickel have approximately the same atomic 

 weights, the same atomic volume, and the same hydrating power. Since these 

 cations have the greatest hydrating power of any which we have studied, we should 

 expect them to have the smallest migration velocities, and such is the case. 



ALUMINIUM CHLORIDE. 



Special interest is attached to the study of aluminium chloride, owing to the fact 

 that it is a quaternary electrolyte and crystallizes with 6 molecules of water. 



The hydrolytic effect of water upon this salt, in dilute solutions, can be noted in the 

 first two concentrations. The hydrochloric acid liberated is almost completely dis- 

 sociated at the dilutions in question, thus giving values for C which are considerably 

 higher than would be obtained if the salt were not hydrolzyed. This may be attrib- 

 uted to one or both of two causes : the very high migration velocity of the hydrogen 

 ion or its inability to form hydrates. It is at about 0.075 normal that the influence 

 due to the hydration of the aluminium cation begins to predominate. Just as we 

 should expect, the number of molecules of water held in combination by one molecule 

 of the electrolyte is large and increases very rapidly with increase in concentration 

 (see fig. 27). In the curve representing the hydration per molecule (fig. 26), that 

 part representing concentrations between 0.075 and 0.5 normal represents the abnor- 

 mality in the hydration due to hydrolysis. 



