HYDROGEN INTO ATOMS 203 



The resulting general Equation 31 is of the form (neglecting constant 

 factors) 



K - ( W/g )^ ( P/D + i/aiY 

 ' P — (W/g) (P/D + i/aa) 

 where P is the total pressure. 



9. By comparing this equation with the experimental data, it is found 

 possihle to so choose the quantities K, q, D, ai and a2 that the resulting 

 equation agrees excellently with the experiments at all temperatures and 

 pressures. At the same time, the values of K conform to the thermo- 

 dynamical requirement given by van't Hoff's equation 



d In K/rfT = g/RT2. 



The agreement between the theory and the experiments is shown in 

 Tables XVII and XVIII, where the observed and calculated values of W 

 are placed side by side. 



10. The values of the quantities K, q, D, ai and a2 which were used in 

 these calculations, and which were found to give the best agreement 

 between experiment and theory, are as follows : 



Dissociation Constant K.- — Let us define the dissociation constant K 

 by the equation : 



K = Pi'/po (22) 



Pi and p2 being the partial pressures (in mm.) of atomic and molecular 

 hydrogen, respectively. Then it is found that 



logio K = 7.77 - 19700/T. (65) 



Values of K calculated from this equation for temperatures from 1200 

 to 4000° have been given in Table XV, together with the corresponding, 

 degrees of dissociation at pressures of 760 mm. and i mm. The following 

 is a comparison of these results with those obtained previously (in the 

 year 1912) : 



Degree of Dissociation at 760 mm. 



Temp. ** K 2000 2300 2500 3100 3500 



Results in 19 12 o.*ooi3 0.012 0.039 o-44 o 84 



Pesults in 1915 0.0033 0.014 0.031 0.17 c ■4 



In the range from 2200-2600°, the results are nearly identical, but at 

 higher and lower temperatures the differences become large. 



Heat of Formation of Hydrogen Molecules. — By applying van't Hoff's 



equation to the foregoing data, we find the heat of reaction for two grams 



of hydrogen to be 



84000 calories at constant volume 

 90000 calories at constant pressure 



