WORK OF J. N. PEARCE. 83 



solutions, passes through a very pronounced minimum, and then inereases as the 

 concentration increases. A glance at the tables of the hydrates shows that in every 

 case the observed molecular lowering produced by any salt is greater than the cal- 

 culated lowering based on conductivity measurements. 



If there were no hydration, we should expect the observed and the calculated 

 molecular lowerings to be equal, except for the difference due to the influence of the 

 friction between the ion and the solvent. The nearest approach to this condition 

 which we have met is found in the most concentrated solutions of barium nitrate. 

 Here the observed molecular lowering is about 1 per cent greater than the calcu- 

 lated value. An equally satisfactory agreement was found by Jones and Stine for 

 solutions of potassium chloride, which, likewise 1 , crystallizes without water. 



The values of M and H also show that the abnormality of the freezing-point 

 lowering in the dilute solutions is greatly augmented by the relatively great hydrat- 

 ing power of the ions. 



Since, then, the hydration of the ion increases with increase in dilution, the volume 

 and mass of the ionic complex arc greater the more dilute the solution, and, therefore, 

 the greater will be the resistance to be overcome by the ion as it moves through the 

 solvent. This being the case, the dissociation as measured by the conductivity 

 method will be less than the true dissociation, and the abnormality in the dissocia- 

 tion measured will increase with increasing dilution. 



The effect produced by adding more of the given electrolyte will be to break down 

 these larger hydrates into simpler ones with smaller volume, thus decreasing the 

 resistance to the motion of the hydrated ion. 



This agrees well with the results of Jones and Uhler. 1 They found that the 

 number of ether waves of different wave-length with which a given particle will 

 vibrate in resonance, decreases with increasing dilution, thereby producing a nar- 

 rowing of the absorption bands. On the other hand, the addition of more of the 

 same electrolyte, or a strong dehydrating agent, decreases the complexity of the 

 hydrate, thereby decreasing its period. As a result, the particles are free to vibrate 

 in resonance with a greater number of wave-lengths, and the absorption bands widen. 



It will be seen that, with the exception of magnesium chloride, the value of M, 

 the total amount of water held in combination by one molecule of the electrolyte, 

 decreases rapidly in the dilute solutions, passes through a minimum, and then 

 becomes a linear function of the concentration. 



The hydration per molecule decreases rapidly to approximately the same con- 

 centration which corresponds to a minimum in the freezing-point lowering, and then 

 remains practically constant as the concentration increases. Eliminating the hydra- 

 tion due to the ions, the hydration per molecule in solution over a given range of 

 temperature is constant, just as the amount of water with which that same salt 

 will crystallize from solution is constant for a given range of temperature. This 

 relation is best illustrated by the curves representing the values of H. They arc 

 almost asymptotic to the coordinates. 



Having found that the ions of a salt are hydrated, the next question which arises 

 is this: Is it the cation or the anion which has the greater hydrating power? 



'Ainer. Chem. Journ., 37, 126 (1907); Carnogie Institution of Washington Publication Xo. 60. 



