﻿460 Prof. A. W. Porter on the 



might be found between them. As a matter of fact, it is 

 found that if the logarithm of the mobility be plotted against 

 the logarithm of the vapour-pressure, a curve differing very 

 little from a straight line is obtained both in the case of 

 mercury and in that of water. These curves are shown 

 in figs. 2 and 3. 



Fig. 2.— Water. 



U 











/ u ■ 











/o 



teg- ("s>-ir~vi£c£j } 





0/ 



y 



•' / 



o 











• J -6 -9 



.-a 



hS 



,0 



tetf ■ (vafauns -£>re<>&<*rcj 



'■7 



The greatest deviation in the case of mercury only corre- 

 sponds to less than 6 parts in 1000 in the value of the 

 mobility. It is scarcely likely that the experimental values 

 are correct to this degree of accuracy, although the smooth- 

 ness of the curve which may be drawn amongst them would 

 seem to indicate otherwise. It must be remembered that the 

 corrections for kinetic head &c. which are usually applied in 

 connexion with Poiseuille's method are of uncertain validity* 

 and they may easily introduce a fictitious curvature. 



The equation to tbe straight line for mercury (fig. 3) is 



logM = -0403 log/? +1-938, 



and for water (fig. 2) is 



logM = -362 log;? + 1-54. 



* L. R. Wilberforce, Phil. Mag. 1891; Hagenbach, Pogg. Ann. c\x. ; 

 Franz Neumann, JLecons (1858-59: published after his death); 

 Couette, Journal de Physique, 1890, p. 414; Menneret, Journal de 

 Physique, October 1911. 



