HYDROGEN INTO ATOMS 167 



The first column gives the pressure of hydrogen in mm. of mercury. In the 

 third cokimn are the vakies of T^ calculated by Equation ii. 



The accommodation coefficient a was calculated from the data at the 

 lowest pressure (0.015 nim.) by substituting Ta = 300;^^ To =1500; 

 W'c = 0.004; and p = 0.015 in Equation 11. This gave a = 0.19, which, 

 when substituted in (11) gave 



T2 — T, = 260 (Wc/p) Vf^. (13) 



The values of T„ given in the third column of Table V were obtained 

 by solving these quadratic equations. 



The free path of hydrogen molecules at 0° C. and i mm. pressure is 

 0.0135 cm., according to O. E. Meyer. The free path varies with the 

 temperature according to Sutherland's formula, but for the present pur- 

 pose it is accurate enough to assume that it is inversely proportional to 

 the density. The fact that at low pressures the molecules leaving the 

 filament travel through groups of molecules having a much lower tempera- 

 ture, introduces the necessity for a rather special calculation of the free 

 path leading to the formula ^^ 



X = ^9-5 X io-« rjg_ (,4) 



p ^Ti + Ta 



This equation was used to calculate the free paths given in Col. 4 of 

 Table V. 



'* That it is justifiable to place T„ = 300 at these lowest pressures is shown by the 

 results obtained below and given in the seventh column. 



*^ Derived from the equations given on page 260 of Meyer's "Kinetische Theorie 

 der Gase," German edition of 1899. 



