380 SCIENCE PROGRESS. 



acid should give 316-8, while the observed number is 4/6 for 

 the strength given above, and only rises to 1 38 '6 for a strength 

 of one hundred thousandth of a gram-equivalent per litre. 



The first direct measurement of the velocity of an ion 

 under a definite potential gradient was made by Oliver 

 Lodge {British Association Report, 1886). A glass tube 

 contained a jelly in which sodium chloride was dissolved 

 with just enough caustic soda to make it alkaline and bring 

 out the colour of a little phenol-phthallein, which was added. 

 The tube ended in two vessels filled with dilute solutions of 

 sulphuric acid. A current of electricity was then passed 

 from one vessel to the other along the tube. The hydrogen 

 ions of the sulphuric acid travel with the current, and when 

 they enter the tube displace the sodium ions, which are also 

 moving in the same direction, and form hydrochloric acid. 

 The presence of this acid decolourises the phenol-phthallein 

 and thus the movement of the hydrogen along the tube 

 can be traced. Lodge found that the velocity produced by 

 a potential gradient of one volt per centimetre came out 

 from three experiments, as '0029, "0026 and '0024 centi- 

 metres per second respectively. If this be compared with 

 the number ("00282) calculated on p. $77, for a decinormal 

 solution, or with Kohlrausch's value, '0032 for infinite dilution, 

 it will at once be seen how remarkable the agreement is, and 

 what strong evidence it gives for the truth of the theory. 



Some further experiments, in which the motion of barium 

 and chlorine ions were traced by the precipitates produced 

 by contact with sulphuric acid, did not give such good re- 

 sults, but the disturbances produced by the precipitates, and 

 the uncertainty of potential gradient introduced by the vary- 

 ing resistance of the solution at the places where they are 

 formed, are quite enough to explain the discrepancy. 



The present writer has determined the velocities of a 

 few other ions by a method which gets over some of the 

 difficulties inherent in the use of indicators or precipitates 

 {Philosophical Transactions of the Royal Society, 1893, A, 

 P- 337)- -A- P 3 -* 1 * °f solutions, with one ion in common, is 

 used, one at least of which is coloured. The solutions must 

 be of the same equivalent strength, nearly equal specific 



