HYDROGEN INTO ATOMS 147 



quired to heat the wire could be expressed as the sum of two terms, 

 thus : 



W = Wc + Wd (i) 



Here W is the total power (in watts per centimeter) required to main- 

 tain the wire at a given temperature T2. 



Wo represents the part which is carried by ordinary heat conduction 

 and follows the usual laws of heat convection, namely : ^ 



PT, 

 Wc=S\ kdV^S(cp2—<Pi) (2) 



S is called the shape factor and depends on the diameter of the wire 

 and the nature of the gas, but is independent of the temperature of the 

 wire, k is the coefficient of heat conductivity of the gas, Wd represents 

 that part of the heat loss which is dependent on the dissociation of the gas. 

 It was shown that Wd could be expressed thus : 



Wd = SD^iCi (3) 



where S is the shape factor, D is the diffusion coefficient of hydrogen 

 atoms through molecular hydrogen, qi is the heat of formation of hydrogen 

 molecules from i g. of hydrogen atoms, and Ci is the concentration of 

 hydrogen atoms (grams per cc.) in the gas immediately in contact with 

 the hot wire. 



The experiments gave W directly (after subtracting the heat radiated 

 from the hot wire). The value of Wc could be calculated by (2) and thus 

 equation (i) led to a determination of Wd- By substituting in (3) the 

 values of S and Wd, the product DqiCj was obtained from the results of 

 the experiments. 



It was then necessary to estimate the approximate value of D. This 

 was done by guessing the probable free path of hydrogen atoms through 

 hydrogen molecules by analogy with other gases and by then substituting 

 this value in an equation, derived from the kinetic theory, which expresses 

 the diffusion coefficient in terms of the normal free path. 



Having thus chosen D and dividing this into the known value of D^iCi, 

 the product qiCi was found. Van't Hoff's equation gives a relation be- 

 tween qi and the temperature coefficient of Cj. Since qi is nearly inde- 

 pendent of the temperature, the temperature coefficient of Cx must be 

 practically equal to that of qiCi, so that in this way qi could be found. 

 From this, the value Ci and the degree of dissociation was then calculated. 



The degree of dissociation of hydrogen into atoms (at atmospheric 



"* Phys. Rev., I. c. 



