no PHYSICAL SCIENCE 



justifies the use of such solid solutions in ex- 

 periments on the migration of ions. Lodge 

 determined the velocity of the hydrogen ion by 

 watching the rate at which, passing along a glass 

 tube, it changed the colour of an indicator, while 

 the present writer has measured the velocity of 

 many other ions by tracing the formation of 

 opaque precipitates, formed in minute quantity 

 by the ions in their path. 



These methods have been improved and 

 extended by Orme-Masson, B. D. Steele, G. N. 

 Lewis, and Lash Miller. The result of the 

 experiments is to confirm the values for the ionic 

 velocities calculated from the theories of Kohl- 

 rausch and Hittorf. 



The velocities with which the ions travel, even 

 when driven forward by intense electric forces, 

 are very small. Hydrogen, the most mobile ion 

 known, moves over a distance of lo centimetres, 

 or 4 inches, in one hour, when the applied electro- 

 motive force is i volt per centimetre. Most 

 other ions travel at about one-tenth this rate. 



These comparatively small velocities must not 

 be confounded with an entirely different thing : 

 the velocity with which an electric impulse, started 

 at one end of a tube filled with an electrolyte, 

 reaches the other end. This velocity is very 

 great, closely approaching the rate at which an 

 electro-magnetic wave travels through free space, 

 that is, the velocity of light, about 180,000 

 miles a second. 



If we accept for the moment the common con- 

 ception of an electric current as analogous to the 

 flow of a liquid through a conducting pipe, the 



