194 PHENOMENA, ATOMS, AND MOLECULES 



with the third hypothesis, we should not expect the bulb temperature to 

 materially affect the velocity of the reaction. It is to be expected, however, 

 that the Inilb temperature might have a slight effect on the coefficient 02 

 and thus slightly change the velocity of the reaction. 



Some experiments were undertaken to determine whether cooling the 

 bulb in liquid air had any effect on the values of Wd obtained at low 

 pressures (from o.oi to o.i mm.). The results showed conclusively that 

 the bulb temperature was without material effect on the velocity of the 

 reaction. This fact affords additional evidence that the velocity depends 

 primarily on some equilibrium condition within the surface of the wire. 



Specific Heat of Hydrogen at High Temperatures. — When hydrogen 

 is heated to a temperature at which appreciable dissociation occurs, the 

 heat absorbed by the reaction may be very large compared with that which 

 would be required if there were no dissociation. We should therefore 

 expect a very large increase in the apparent specific heat of hydrogen at 

 very high temperatures. In Part I 'we have already briefly discussed Pier 

 and Bjerrum's experiments on the specific heat of hydrogen by the ex- 

 plosion method. We saw that the earlier calculations of the dissociation 

 of hydrogen (paper of 1912) led to apparent specific heats too high to 

 reconcile with the results of the explosion method, unless we assumed that 

 equilibrium was not reached during the short period of the explosion. 



In order to compare our present results with the data of the explosion 

 method, we shall calculate the increase in apparent specific heat caused by 

 the dissociation as given in Table XV. 



If we heat a gram molecule of hydrogen from 0° K. to T, the number 

 of gram molecules of hydrogen dissociated will be .r/(2 — x) where x is 

 the degree of dissociation at the temperature T, x being expressed in terms 

 of partial pressures according to {6y). 



The heat absorbed by this dissociation will be 



q2x/{i—x). 



In comparison with this we may neglect the increase in specific heat 

 due to the difference in the specific heat of the molecular hydrogen and 

 that of the atomic hydrogen formed (2X2.98). 



Pier -° gives for the mean specific heat of hydrogen from room tem- 

 perature up to ^° C. 



Cv = 4.70 + 0.00045; 



or in absolute temperatures 



Cv = 4-578 + 0.00045T 

 '^^ Z. Elektrochem., 15, 536 (1909). 



