290 MR. W. R. BOURFIELD AND DR. T. M. IX3WRY ON THE ELECTRICAL 



regard the ionic mobilities as constants for concentrated as well as for dilute 

 solutions. 



Correlation between Ionic Mobility and Fluidity. The exact way in which the 

 ionic mobility varies with the temperature and with the concentration of the solution 

 is not known. 



Without doubt the mobility depends chiefly upon the form and dimensions of the 

 ions and the fluidity of the medium through which they travel. When the 

 temperature or concentration of the solution is altered there is usually a marked 

 alteration in its fluidity,* and the experimental data point to the conclusion that 

 this produces a change in the ionic mobilities which may, as a first approximation, be 

 justly regarded as proportional to the change of fluidity, so that u = of, v = bf and 

 M-j-tf = {<*+&)/> where /is the fluidity. 



In certain cases this simple linear relationship undoubtedly exists. Thus, in the 

 case of the excessively dilute solution which constitutes purified water, and in which 

 the process of ionisation must be substantially complete, the conductivity is actually 

 in direct proportion to the fluidity over the range of temperature from C. to 36 C. 

 (BOUSFIELD and LOWRY, loc. cit., p. 48). KOHLRAUSCH has shown that the same 

 relationship holds good in the case of dilute solutions of salts such as sodium valerate, 

 in which the mobility of one of the ions is very small ('Roy. Soc. Proc.,' 1903, vol. 71, 

 pp. 338-356). In other cases the linear relationship is less accurate, and although 

 the ionic mobilities are closely related to the fluidity, they no longer obey a direct 

 linear law. Thus KOHLRAUSCH (' Sitz. der Akad. Wiss. Berlin,' 1902, pp. 572-580) 

 has shown that the temperature coefficients of ionic mobility in dilute solution are 

 usually less than the fluidity coefficient, the greatest difference being found in the 

 case of the mobile hydrogen and hydroxyl ions, for which the coefficients are 0'0154 

 and 0'0179 per degree Centigrade, as compared with 0'0254, for the conductivity and 

 0'0251 for the fluidity of water at 18 C. ; for the remaining ions the coefficients range 

 from 0-0203 for the nitrate ion to 0'0261 for lithium. Furthermore, ' WOLF (' Zeit. 

 Electrochem.,' 1902, pp. 117-119) and RUDORF ('Zeit. Phys. Chem./ 1903, vol. 43, 

 pp. 257-304) have shown that when the viscosity of a salt solution is increased at 

 constant temperature by the addition of acetic acid or a non-electrolyte, the decrease 

 of conductivity is proportionately less than the decrease of fluidity. It is, therefore, 

 clear that the relation between mobility and fluidity is in general only approximately 

 expressed by a linear law, but at present the necessary data are not available for any 

 closer approximation to the form of the function. The linear law, which is an 

 accurate expression in certain instances, we propose to use generally as a first 

 approximation to the relationship between mobility and fluidity. 



Intrinsic Conductivity. The relations A = a(u+v), and A = u,-\-v. l>eing 



* In certain instances, and especially in the case of complex or hydrated ions, the dimensions of the 

 ions may alter with the temperature and concentration of the solution, but usually this effect will he of 

 secondary importance. 



