126 PROPERTIES OF ELECTRICALLY CONDUCTING SYSTEMS 



for example, the conductance of the lithium ion in ammonia is 3.36 times 

 that of the lithium ion in water, that of the silver ion in ammonia is only 

 2.15 times that in water. So, also, we find that the ammonium ion in 

 ammonia has a conductance of only 2.03 times that of the ammonium ion 

 in water, indicating the formation of relatively large complexes. In this 

 connection it may be pointed out that the ammonium salts form with 

 ammonia saturated solutions whose vapor pressures are extremely low. 

 For example, the vapor pressure of a saturated solution of ammonium 

 nitrate in ammonia is one atmosphere at 26. 



If the complexity of the ions increases with the temperature, we 

 should expect that at higher temperatures the viscosity would be in- 

 creased more largely for a given addition of salt than at lower tem- 

 peratures. This, again, corresponds with observations on the viscosity 

 of solutions. The change of viscosity due to a given addition of 

 salt increases as the temperature rises, and this increase appears to be 

 the greater the higher the temperature. It is to be noted, also, that the 

 increase in viscosity due to the addition of electrolytes is much greater 

 than that due to the addition of non-electrolytes, except in the case of 

 non-electrolytes which have very large molecules. In general, as has 

 already been pointed out, the viscosity effect is the greater the lower 

 the dielectric constant of the solvent. In solvents of very low dielectric 

 constant, the viscosity of some solutions becomes so great, at high con- 

 centrations, that they often become practically solid. 



5. The Influence of Pressure on the Conductance of Electrolytic 

 Solutions. As we have seen, the conductance of the ions is a function 

 of the viscosity of the solution. As the hydrostatic pressure on a solu- 

 tion is increased, its viscosity changes, the sign and magnitude of this 

 change being dependent upon the nature of the solvent medium and 

 upon the concentration of the solution in question. The effect of pres- 

 sure on the viscosity of solutions in water, as well as the effect upon 

 water itself, has been measured by Cohen. 17 In Figure 22 are shown 

 the percentage changes of viscosity for pure water at different pressures 

 and temperatures. From an inspection of the figure it will be seen 

 that with increasing pressure the viscosity of water decreases markedly. 

 As the temperature rises, however, the viscosity effect diminishes and 

 it is evident that at higher temperatures the effect changes sign. From 

 the form of the curves at 15 and 23 it is evident that at higher pres- 

 sures the curves for the viscosity effect will pass through a minimum and 

 that ultimately, therefore, the viscosity change will change sign, the 

 viscosity increasing with increasing pressure. In non-aqueous solvents 



"Cohen. Wied. Ann. J5, 666 (1892). 



