TABLES 620-631. 

 IONIC MOBILITIES AND DIFFUSIONS. 



405 



The process of ionization is the removal of an electron from a neutral molecule, the molecule thus acquiring a result- 

 ant -f charge and becoming a + ion. The negative carriers in all gases at high pressures, except inert gases, consist 

 for the most part of carriers with approximately the same mobilities as the -f- ions. The negative electrons must, 

 therefore, change initially to ions by union with neutral molecules. 



The mobility, U, of an ion is its velocity in cm/sec, for an electrical field of one volt per cm. The rates of diffusion, 

 D, are given in cm 3 /sec. U = DP/Ne, where P is the pressure, N, the number of molecules per unit volume of a gas 

 and e the electronic charge. 



Nature of the gas and the mobilities: (i) The mobilities are approximately proportional to the inverse sq. rts. of 

 the molecular weights of the permanent gases; better yet when the proportionality is divided by the 4th root of the 

 dielectric constant minus unity; (2) The ratio U + /U seems to be greater than unity in all the more electro- 

 negative gases. 



Mobilities of Gaseous Mixtures: Three types: (i) Inert gases have high mobilities; small traces of electro- 

 negative gases make values normal. (2) Mixed gases: lowering of mobilities is greater than would be expected from 

 simple law of mixture. (3) Abnormal changes produced by addition of small quantities of electro-negative gases: 



e.g.: normal mobility 

 6 mm CzHsBr gave 

 6 mm CzHsI 

 10 mm CjHsOH " 

 9 mm CsHsO " 



U+ = 1.37 

 1-37 

 1-37 

 0.91 

 i IS 



U - 



Wellisch, Pr. 

 Roy. Soc. 8zA, 

 p. 500, 1009. 



Temperature Coefficient of Mobility: There is no decided change with the temperature. 



Pressure Coefficient of Mobility: Mobility varies inversely with the pressure in air from 100 to i/io atmosphere 

 for ion, to i/iooo, for + ion; below i/io atmosphere all observers agree that the negative ion in air increases 

 abnormally rapidly. 



Free Electrons: In pure He, Ar, and N, the negative carriers have a high mobility and are, in part at any rate, 

 free electrons; electrons become appreciable in air at 10 cm pressure. 



TABLE 520. Ionic Mobilities. 



TABLE 521. Diffusion Coefficients. 



The following table gives the observed and computed (D = sooUP/Ne = very nearly 0.023610 value* of the 

 diffusion coefficients. The diffusion coefficients are given for some neutral molecules as actually determined for some 

 gases into gases of nearly equal molecular weight. Table taken from Loeb, " The Nature of the Gaseous Ion, J. Franklin 

 Inst. 184, p. 775, 1917. 



* COs into COs. t Ethyl formate. J Estimated. 



SMITHSONIAN TABLES 



