HYDROGEN INTO ATOMS 205 



Table XXI. The diffusion coefficient of hydrogen atoms through nitrogen 

 is found to be 



D = 2.5(T/273)^^^ 



which is in excellent agreement with a value calculated by the kinetic 

 theory. 



PART THREE 



THE DISSOCIATION OF HYDROGEN INTO ATOMS 

 THE MECHANISM OF THE REACTION 



In part II of this paper ^ the degree of dissociation of hydrogen into 

 atoms has been calculated from data ^ on the heat losses from highly heated 

 wires in hydrogen at various pressures. In this calculation it was necessary 

 to make certain assumptions regarding the mechanism of the processes 

 occurring on and around the wire. The remarkable agreement between the 

 theory thus derived and the experimental results, furnished ample justifica- 

 tion for the assumptions, but did not necessarily indicate that the particular 

 mechanism assumed was the only one which would lead to similar agree- 

 ment. 



The writer has recently developed a theory ^ of heterogeneous reactions 

 which throws a great deal of light on the mechanism of such reactions. 

 Applied to the dissociation of hydrogen around a heated wire, this theory 

 leads to a mechanism which may seem radically different from that pre- 

 viously used, but curiously enough it results in an equation identical with 

 that obtained by the original theory. 



The object of the present paper is to compare the older and the newer 

 theories and to point out the advantages of the latter, x^t the same time 

 this will serve as an illustration of the principles of the new theory and will 

 indicate how it may be of use in the study of other heterogeneous reac- 

 tions. 



In the theory previously described, it was assumed that the surface of 

 the tungsten wire contained atoms and molecules of hydrogen in chemical 

 equilibrium with each other. The atomic and molecular hydrogen escaped 

 from the wire at rates respectively proportional to their concentrations in 

 the wire, while the absorption of the atoms or molecules by the wire was 



^ Jour. Amer. Chem. Soc, 37, 417 (1915). 



^ Part I, Langmuir and MacKay, Jour. Aiiicr. Chem. Soc, 36, 1708 (1914). 



^ This theory has, as yet, been only very briefly described {Jour. Amer. Chem. 

 Soc, 38, 1139 (1915) and Jour. Ind. Eng. Ch-em., 7, 348 (1915) : Phys. Rev., 6, 79 

 (1915)), but will serve as the basis of a series of papers to appear in Jour. Atner. 

 Chem. Soc. and in the Phys. Rev. 



