230 



REPORT — 1897. 



centrations. The following table shows Kohlrausch's latest ' values for 

 the ionic velocities of three chlorides of alkali metals in 10"'' cms. per 

 sec. at 18° C, calculated for a potential gradient of 1 volt per cm. : — 



These numbers clearly show the increase in ionic velocity as the dilu- 

 tion gets greater. Moreover, if we compare the values for the chlorine 

 ion obtained from observations on these three different salts, we see that, 

 as the solutions get very weak, the velocity of the chlorine ion becomes 

 the same in all of them. Similar phenomena appear in other cases, and, 

 in general, we may say that, at great dilution, the velocity of an ion is 

 independent of the nature of the other ion present. This at once leads 

 to the idea of specific ionic velocities, the values of which for different ions 

 are given by Kohlrausch in the following table : — 



Having once obtained these numbers, we can calculate the molecular 

 conductivity of the dilute solution of any salt, and the comparison of such 

 values with observation furnished the first confirmation of Kohlrausch's 

 theory. Some exceptions, however, are known. Thus, acetic acid and 

 ammonia give solutions of much lower conductivity than is indicated by 

 the sum of the specific ionic velocities of their ions as determined from 

 other compounds. 



Professor Oliver Lodge was the first to directly measure the velocity 

 of an ion.2 In a horizontal glass tube connecting two vessels filled with 

 dilute sulphuric acid, he placed a solution of sodium chloride in solid agar- 

 agar jelly. This solid solution was made alkaline with a trace of caustic 

 soda to bring out the red colour of a little phenol-phthalein added as 

 indicator. A current was then passed from one vessel to the other along 

 the tube. The hydrogen ions from the anode vessel of acid were thus 

 carried along the tube, forming hydrochloric acid as they travelled, and 

 decolorising the phenol-phthalein. By this method the velocity of the 



» Wied. Ann. 1893, vol. 1. p. 385. 



- Bntish Association Revert, 188fi p, 389. 



