922 



SCIENCE 



[N. S. Vol. XXIX. No. 754 



a gas, M. Langevin^ has given for the 

 mobility, h, and the eoeiScient of diffusion, 

 D, the equations, 





--'4, 



where e denotes the ionic charge, L the 

 mean free path of the ion, M its mass and 

 V its mean velocity of thermal agitation. 

 Mr. Wellisch in his investigation calcu- 

 lates the mean free path of the ion, taking 

 into account the effect of the ionic charge 

 in increasing the collision frequency, and, 

 substituting in the above equations, reaches 

 general expressions for the two quantities 

 under consideration. If the mass and di- 

 mensions of the ion are taken as the same 

 as those of the molecule the expression for 

 -the mobility becomes at 0° C. 





^PiPi' 



r 



and that for the coefficient of diffusion at 

 the same temperature 



- ' ., (JTi — l)7r^V 1 -' 



^ 2p,Px' J 



D- 



where A (=1.30X10^° electrostatic units) 

 is the product of the number of molecules 

 per cubic centimeter and the ionic charge, 

 7/ the coefficient of viscosity of the gas, E 

 its specific inductive capacity, p the den- 

 sity and p the pressure in dynes per cm.^, 

 the symbols with subscripts referring to 

 values under the standard conditions as to 

 temperature and pressure. 



To test the theory Mr. Wellisch gives the 

 following table of comparison between the 

 observed and the calculated values, the ob- 

 served mobilities, except in the case of air, 

 hydrogen, nitrogen and oxygen, being the 

 results of a series of determinations re- 

 cently made by him. 



Mr. Wellisch further shows that if d 



' Langevin, Ann. de Chimie et de Physique, V., 

 28, p. 289, 1903. 



denote the coefficient of interdiffusion of a 

 molecule through the gas, 



1 + 





and by the following table indicates the 

 nature of the agreement between the cal- 

 culated and observed values. 



Both in the case of the mobility and in 

 that of the coefficient of diffusion the 

 agreement between the calculated and the 

 observed values is, on the whole, quite sat- 

 isfactory, the conclusion being that the 

 behavior of the ion can be explained on the 

 supposition that it consists of a single 

 molecule associated with a charge equal to 

 that carried by the monovalent ion in 

 electrolysis. 



Mr. Wellisch read an account of this in- 

 vestigation of the mobility and diffusion 

 of the ions before the Cambridge Philo- 

 sophical Society at its meeting held on 



' See Jeans, " Dynamical Theory of Gases," p. 

 'Z53. 



•Townsend, Phil. Trans., A, 193, p. 129, 1900. 



