HYDROGEN INTO ATOMS 209 



Let 6 represent the ratio of the number of vacant elementary spaces to 

 the total number of spaces No- Thus 6 is the fraction of the surface which 

 is bare. The rate at which atomic hydrogen condenses on the surface is 

 thus equal to aOyc. 



Consider the case of hydrogen molecules condensing on the surface. 

 If each hydrogen molecule occupies only one elementary space, then tlie 

 rate of condensation will still be given by the product ^ aB\i. 



If, on the other hand, each hydrogen molecule should occupy two 

 elementary spaces, then the rate at which molecular hydrogen would 

 condense on the surface is equal to a6^\i. The exponent 2 for the quantity 

 is due to the fact that two adjacent spaces must be vacant simultaneously, 

 in order that the molecule may condense. The chance that a given space, 

 towards which a gas molecule may be moving, shall be vacant, is 6. 

 The chance that two given spaces shall be simultaneously vacant is 0^. 



The adsorbed atoms or molecules on the surface evaporate at a definite 

 rate. Let the rate of evaporation in gram molecules per sq. cm. per second 

 from a completely covered surface, be represented by v. If di is the fraction 

 of the surface which is covered by the atoms or molecules in question, then 

 the actual rate at which the hydrogen evaporates is v^i. 



In considering the mechanism of the dissociation of hydrogen in contact 

 with a heated wire, we may make two alternative hypotheses. 



First Hypothesis. — Hydrogen exists on the surface in the form of 

 atoms only. Molecules, formed by the combination of adjacent atoms, leave 

 the surface immediately. 



Second Hypothesis. — Hydrogen can exist on the surface in either 

 molecular or atomic condition. 



FIRST HYPOTHESIS 



We assume that out of all hydrogen atoms striking a bare surface, the 

 fraction ai condenses and that the corresponding fraction for the mole- 

 cules is a2. 



Let 0) be the velocity with which the dissociation of hydrogen is brought 

 about by the heated wire. We shall express O) in gram molecules of 

 hydrogen (H2) dissociated per second per sq. cm. of surface. 



The rate at which atomic hydrogen leaves the wire is Vi^i. The rate 

 at which it is taken up by the wire is ai^Ui. The net rate at which it is 

 produced is the difference between these two, and this must be equal to 2(0. 



^ In calculating vl in this case by ( i ) the value M = 2 will have to be used instead 

 of M = I. 



