238 



EEPORT — 1897. 



Since the ions move with uniform velocity, the frictional forces 

 brought into play must be equal and opposite to the driving forces acting, 

 and therefore these numbers also represent the ionic friction coeflScients 

 in very dilute solution at 18° C. 



Let us now return to the consideration of the velocity. We have seen 

 that the force acting on one gram-equivalent of an ion, when the poten- 

 tial gradient is one volt per centimetre, is 9,653 x 10* dynes, and that, in 

 dilute solution, this gives to the ion its specific velocity u. The velocity 

 it would attain under unit force will therefore be 



U = 



u 



9653 



xlO ^cms. per second. 



In the case of hydrochloric acid, for example, the specific velocity of the 

 hydrogen is 00032, and that of the chlorine 0'00069 cm. per second. 



.•.U=3-32 X 10-^^ and V=7-15 x lO"'" cms. per second. 



the velocities, for convenience, being reckoned in centimetres per day. 



By experiments on diffusion this constant T> can be found experi- 

 mentally,' and the agreement between theory and Scheffer's observations 

 is shown by the table. 



The theoretical numbers are slightly increased by the assumption that 

 the ionisation of the solutions is complete, which is not accurately the 

 case. This correction, then, would make the agreement still better. 



The possibility of thus calculating the diffusion constant must be 

 regarded as very strong evidence in favour of the soundness of the 

 analogy underlying the investigation. 



Further developments for the cases of other solvents and of mixed 

 electrolytes have been traced by Arrhenius,^ who shows, for example, 

 that the rate at which hydrochloric acid diffuses will be increased by the 

 presence of one of its salts. This is confirmed experimentally. Thus, 

 when 1-04 normal HCl diffuses into 0*1 NaCl, D is calculated as 2'43 and 

 observed as 2-50 ; and when the NaCl solution is 0'67 normal, calculation 

 gives 3 58 and observation 3-51. 



Contact Difference of Potential. — As we have seen above, when a 

 solution is placed in contact with water, the water will take a positive or 



' Account in Solution and Electrolysit, p. 49. 

 ' Zeits. physiTtal. Chem. 1892, vol. x. p. 51. 



