1906.] on the Passage of Electricity through Liquids. 247 



may suppose that they move with unequal velocities. It is now 

 probable that in some cases both these factors come into play, but, to 

 simplify our ideas, let us first imagine that the opposite ions are 

 simple, or at all events loaded with equal amounts of salt or solvent, 

 and that they move with unequal speeds through the liquid. The 

 use of the model to which we have already referred enables us to see 

 clearly that the velocity of the anion is to' that of the cation as the 

 amount of salt lost by the solution near the cathode is to that near 

 the anode. The ratio of the opposite velocities of simple ions could 

 then be deduced from experiment. 



In the year 1879, it was pointed out by F. Kohlrausch that the 

 sum of the opposite ionic velocities might be calculated from a know- 

 ledge of the electrical conductivity of the liquid. The conductivity, 

 that is the amount of current conveyed under the influence of a given 

 electromotive force, is obviously proportional to the number of ions, 

 to the velocity with which they move, and to the electric charge 

 carried by each. On the assumption that all the salt is actively con- 

 cerned in conveying the current, we know the number of gramme 

 equivalents of either ion present from a knowledge of the concen- 

 tration of the solution. Now Faraday, as we have said, discovered 

 that the amount of substance deposited at the electrodes by a given 

 quantity of electricity was proportional to the chemical equivalent of 

 the substance. This means that a given number of ions, whatever 

 their nature, so long as their chemical valency be the same, carry the 

 same amount of electric charge. The charge on a univalent ion is 

 thus seen to be a true natural unit of electricity ; the charge on a di- 

 valent ion consists of two such units, that on a trivalent ion, of three. 

 Faraday's quantitative measurements tell us the charge on a gramme- 

 equivalent of any univalent ion. The concentration of our solution 

 tells us the number of gramme-equivalents present ; thus, by 

 measuring the conductivity, we can calculate the velocity with which 

 the ions move under a given electric force. By this method, 

 Kohlrausch calculated the specific velocity of many simple ions when 

 moving through dilute aqueous solutions. 



In 1886, Sir Oliver Lodge rendered visible the movement of ions 

 which hitherto had been seen by the eye of faith only. By forcing 

 hydrogen ions from a vessel of acid through a tube containing a jelly 

 solution of sodium chloride, he rendered their presence visible by an 

 indicator which changed colour in the presence of an acid, and thus 

 watched their progress through the tube. 



In order to compare the ionic velocities as directly observed with 

 those calculated by Kohlrausch, further modifications of the method 

 are necessary. One arrangement which may be used is to employ 

 two solutions which have a common ion, different densities, a nearly 

 equal specific resistance, and different colours. One of these solutions 

 is placed on the top of the other and a current passed across the 

 junction, the movement of which gives us the velocity of the coloured 



