FLAMES OF ATOMIC HYDROGEN 129 



and M. Randall ^ and others have pointed out that the third law of 

 thermodynamics gives a relation between the heat of dissociation and the 

 degree of dissociation. 



DEGREE OF HYDROGEN DISSOCIATION 



With this relationship and the new temperature scale of Forsythe and 

 Worthing,''' the degree of dissociation of hydrogen has been recalculated 

 from the experimental data of 1914. The results are expressed by the 

 equation 



logioi^ = -^^^ + 1.765 logioT - 9.85 X 10-^ 7 -0.256 (2) 



where K is the equilibrium constant defined by 



K^P.yp, (3) 



pi being the partial pressure of atomic hydrogen and p2 the pressure of 

 molecular hydrogen, both expressed in atmospheres. Then if P is the total 

 pressure, pi and p2 are given by 



Pi = 2Px/ {i^x);P2= {i-x) P/ {i+x) (4) 



where x is the degree of dissociation. 



The equilibrium constant K in equation (2) is also given by 



K = ^xy{i-x^) (5) 



Table I gives the equilibrium constant K and the degree of dissociation 

 at various temperatures as calculated from equations (2) and (5). 



Note — This revision in the calculation of x changed the values of x calculated in 

 1915 to the following values: 



From equation (2), by applying the Clapeyron equation, we can 

 calculate H, the heat absorbed by the dissociation of the molecular hydrogen 

 (at constant pressure), as follows: 



H = 97,000 -\-2,-ST - 0.00045 T- (6) 



This is expressed in small calories per gram-molecule of hydrogen (2.016 



grams). The heat of reaction Q at constant volume is 



® "Thermodynamics," p. 470, McGraw-Hill Book Co., New York, 1923. 

 '' Astro phys. Jour., 61, 146 (1925). 



