480 



MR. S. CHAPMAN ON THE KINETIC THEORY OF A GAS 



As in the case of the coefficient of viscosity, we proceed to give the values of the 

 molecular radii (according to the two theories of elastic spherical molecules, with and 

 without attraction) calculated from the coefficient of diffusion. From the latter we 

 get <r+ff, the sum of the radii of molecules of each kind considered. By taking three 

 pairs of the same three gases, like O-H, N-H, H-N, we can derive the actual values 

 of a- in each case, which may then be compared with the values obtained from the 

 coefficient of viscosity. I have thought it preferable, however, to compare the values 

 of 0-+0-' obtained from diffusion with the sums <r+</ of the values obtained from 

 viscosity (on the corresponding molecular hypothesis). The values of the coefficient 

 of diffusion (which are in C.G.S. units, and for the temperature C.) are taken from 

 the Smithsonian tables (1910 edition); they are uncertain (partly for reasons already 

 discussed, partly for other reasons) to within about 5 per cent. ; different authorities 

 give values differing by from 5 per cent, to 10 per -cent. ; I have taken the mean, 

 where two values are given in the table cited. The constants C 12 are taken from a 

 table given by SUTHERLAND* ; as they depend only on two values of D 12 , and as C 

 occurs in such a form as to be largely affected by experimental errors, these values 

 are not well determined, though they are probably the best available. In several 

 cases I have not been able to find the value of C. 



Considering the possibility of occasional exceptionally large errors due to the 



* 'Phil. Mag.,' 1895, 40, p. 421; 1894, 38, p. 1. 

 t R SCHMIDT, 'Ann. d. Phys.,' 14, 1904, p. 801. 



