Elements in the Sun. 823 



any molecule or molecular compound to exist in the undis- 

 sociated state ? The problems are essentially of the same 

 nature as those which are treated by the physical chemist in 

 the laboratory, only repeated on a scale which is not available 

 on the earth. But thanks to the recent developments in 

 thermodynamics by Nernst, Planck, Sackur, and others, we 

 can now handle these problems from the theoretical side in a 

 very satisfactory manner, if only proper data are available. 

 Examples have already been given ; it has been shown that in 

 the sun, hydrogen and oxygen are in wholly dissociated con- 

 ditions, while some proportion of nitrogen may remain in 

 the molecular state. 



The chief data which are required are heats of dissociation, 

 the specific heat of the components, and the value of the 

 chemical constant of the components. Thus to take a con- 

 crete case, let us consider the chemical equilibrium of NH 3 . 

 The reaction is of the type 



NH^N + aH-Ua 



and the chemical equilibrium is given by the law 



lo « (l + 3«)Vl-r) = -2^CT + ~#l°gT + XC, 



where x is the fraction dissociated. 



Now 2vO p =(CV)ir + 3(e,) M - (C,) N h 3 



and X^C = Cn + 3 Oh — Cuh « 



3 



All of these quantities can be calculated theoretically ex- 

 cepting Cnh. (The theory of the chemical constant for 

 polvatomic gases is yet to be developed — see paper men- 

 tioned below *.) 



It should be remembered that Ui is different from the 

 energy evolved in the reaction, 



2NH 3 =N 2 + 3H 2 -U 2 , 



if Hj-N + N-Uj, 



and H 2 = H + H-U H , 



it is easy to see from the energy principle that 



Ui= — 2 



and cannot be estimated before all these quantities are known. 



* " On the Chemical Constant of Diatomic Gases." Leon Schames 

 Phys. Zeits. vol. xxi. p. 41 (1920). 



