D4 J. A. EWING AND J. G. MACGREGOR ON THE 
An inspection of column III. shows that in this case the rate of expansion is 
as nearly as possible uniform from the first. 
When solutions of sulphate of zinc and sulphate of copper were mixed it 
appeared as if a slight contraction took place. The density of the mixture 
seemed generally to be slightly greater than the mean of those of the two com- 
ponents, equal volumes of both being taken. But to make a satisfactory series 
of experiments on this subject great precautions would be necessary; such as 
to take the densities of the components immediately before mixing them, and 
that of the mixture immediately after, in order to prevent error due to evapo- 
ration; also to mix-the two in absolutely equal volumes. Such precautions as 
these we did not attempt to take, as the main object of our inquiry was the 
electrical resistance, and hence we cannot speak positively on this point. 
IL. Electrical Conductivity. 
The electrical conductivity of saline solutions is a subject which has received 
the attention of numerous physicists at intervals during the last thirty-six 
years. The earliest observer was PovuILLET, whose investigations are given in 
the ‘Comptes Rendus,” vol. iv., 1837, and may also be found in his “ Traité 
de Physique,” vol. i. He does not appear to have taken any account of the 
polarisation of the electrodes produced by the transmission through the solution 
of the electric current, without which it is impossible to measure its conductivity. 
His results, which are few in number, are rendered untrustworthy by this 
circumstance alone. 
The next deserving of notice is HANKEL (Pogg. Ann., Ixix., 1846). He 
divided the current of two or three DANIELL’s cells into two parts, the one passing 
through a rheostat, and the other through a tube containing the solution whose 
resistance was to be measured, and then passed them through the coils of a 
differential galvanometer. It will be seen that this method neglects polarisa- 
tion—except in the case of sulphate of zinc, in which it was at least partially 
avoided by using zinc electrodes. But, though for this reason HANKEL’s figures 
cannot be even approximately correct, his paper is interesting on account of his 
having made some general discoveries of importance, such as that the effect of 
increase of temperature on a saline solution is to diminish its resistance. It 
will be remarked that this is exactly the opposite of what holds in the case of 
metallic conductors, but is similar to what takes place with glass, india-rubber, 
gutta-percha, and, as Farapay showed, with dry sulphide of silver and other 
substances. HANKEL also observed, that though, generally speaking, the con- 
ductivity increases as the density of solution increases, there is a solution of 
sulphate of zinc which conducts better than the saturated one. 
In the same year (1846) E. BecqurereL published a paper (Ann. de Chimie, 
