248 3Ir. W. 0. Dampier Whetham [Feb. 16, 



ions. This method is restricted in scope, but the use of jelly solutions, 

 in which the velocities are not markedly different from those through 

 solutions in water, enables us to use traces of precipitates or indicators 

 to show the movement of various ions. Such experiments showed 

 that the observed velocities were in general agreement with those 

 calculated by Kohlrausch. 



If the specific resistances of the two solutions be not equal, inter- 

 esting phenomena occur at the boundary. The electric force will be 

 greater in the solution where the resistance to be overcome is greater. 

 Hence if an ion from the other solution chance to pass the boundary, 

 it finds itself subjected to a greater force, and its velocity is increased. 

 It will thus be pushed further in advance of the junction if it has 

 got in front, and will be brought up into line again if it has straggled 

 behind. We see, then, that if a junction be advancing in the 

 direction of the solution of higher resistance, the junction will become 

 vague and indistinct, while if the advance be towards the solution of 

 lower resistance the junction will keep sharp and well defined. When 

 the solutions have one ion in common, this means that in order to 

 secure a sharp boundary, we must arrange that a specifically slower 

 ion shall follow a faster one. 



Professor Orme Masson recognised that these principles enabled 

 us to dispense with the necessity of choosing two solutions of equal 

 resistance. A salt with quickly-moving ions, such as potassium 

 chloride, is placed in a jelly solution in a horizontal tube. A slow 

 coloured cation is forced electrically into the tube from one end and 

 a slow coloured anion from the other. Thus, blue copper may follow 

 the potassium, and the yellow chromic acid ion Cr04 may follow the 

 chlorine. The higher specific resistance in the two coloured solutions 

 forces their ions to conform to the movement of the potassium and 

 chlorine, and thus the motion of the boundaries gives us the velocities 

 of the potassium and chlorine in a solution of constant and known 

 concentration, and therefore their velocities under a known electric 

 force. 



Further improvements were made by Mr. B. D. Steele and 

 Mr. F. B. Denison. The use of jelly in the tube was dispensed with 

 by placing membranes over the ends of the tube, and removing them 

 when once the junctions had got well within it. The use of coloured 

 solutions also was found to be unnecessary, for, with sharp junctions, 

 the Hue of demarcation was visible, owing to the slight difference 

 in the refractive indices of the solutions, and may be shown by 

 ])rojection on a lantern-screen. Denison and Steele have made a 

 careful series of experiments by this method, and, in their hands it is 

 ])rol)al)le that the results thus obtained are more accurate than those 

 given by the methods of Hittorf and Kohlrausch. 



The general result of these direct measurements of ionic velocity 

 goes to confirm the indirect calculations of the methods of Hittorf 

 and Kohlrausch, in the case, at all events, of simple univalent ions. 



