314 



PHENOMENA, ATOMS, AND MOLECULES 



vapor may be reflected. In the case of tungsten at 3000° K., X = 260,000 

 calories, so that we have 



aa = ~ 4-8 



n 



(29) 



n + I 



The following table gives the values of a2 corresponding to different 

 values of n. 



TABLE I 



In this table n gives the effective number of atoms which attract but 

 do not subsequently repel the incident atom. We have seen that there 

 is good reason to believe that the attraction is exerted by three, four, or 

 even more atoms, while the repulsion is exerted principally by one or two. 

 We may therefore expect the value of n to be of the order of 2 or 3. The 

 above table shows, however, that even if n should be as small as o.i, 

 reflection of the incident atom can occur only if it collides with an atom 

 for which 02 = — 0.445, that is, an atom which is already moving by its 

 thermal agitation with a velocity component away from the surface equal 

 to 44 per cent, of the mean thermal velocity. The column headed P gives 

 the probability, according to Maxwell's distribution law, that any given 

 atom on a surface should have a velocity in a given direction exceeding 

 ^2^2- Thus, according to the table, only 23 per cent, of the atoms on the 

 surface have an outward velocity component greater than 44 per cent, of 

 the mean velocity. The probability P is calculated by the equation 



t P = ^ f _^e-^'dx. (30) 



With values of n as large as 2 or 3, the probability is negligibly small 



