486 Dr. Megh Nad Saha on Ionization 



corresponding to 13*6 volts. We have then 



. ^ p 3-2,10 5 j _5 1 K 



lo g l_:, 2 pr= - 4^57lT + 2 log T ~ 6 5 



="6-020 for T=7500 



= 9*279 for T-6000 



These figures show that at a point where T= 6000°, hydrogen 

 can be completely ionized if P = 10~ J1 atmosphere. Thus 

 only at the highest points of the chromosphere, where the 

 partial pressure falls to 10 ~ n atmosphere, can the ionization 

 be complete, and the vanishing of the H- lines be expected. 



Helium. 



The previous work on the ionization of H atoms will have 

 made it clear that the higher the ionization potential of an 

 element is, the less will be its degree of ionization under 

 a given thermal stimulus. This is best exemplified in the 

 case of helium, which has got the highest ionization potential 

 of the elements so far investigated. 



The experimental results, however, are rather discordant. 

 According to Bohr, the ionization potential should be 

 29 volts, while most investigators have detected the com- 

 mencement of a distinct ionization at 20*5 volts. Some 

 investigators have detected two distinct stages of ionization, 

 one at 20 - 5 and another at 25 volts. In addition to this last, 

 Rau detected a rather strong ionization when the potential 

 is raised by 54*6 volts, i. e., to about 80 volts. 



These processes probably take place according to the 

 following schemes : — 



He = He + +Y 1 (A) 1 



Parhe = He + + V 2 (B) 



He = Parhe + V,-V 2 . . . (C) 



He + = He ++ + « + V 8 (D) J 



The distinction between He and the so-called parhelium is 

 taken as one of relative configuration of the steady orbits 

 of the two electrons *. V 2 may be identified with 20*5 volts, 

 Vi with 25 volts, and V 3 with 4. 13'6 = 54 v 6 volts |. 



* See a paper by Lande, Bar. d. JD. Phys. Gesellschaft, 1919. 



f It is not possible to deduce V from the quantum relation e'V=A(l, s), 

 for the fundamental term (1, s) is unknown both for helium and par- 

 helium. What are generally called the principal series of helium are 

 really the series i/ = (2, s) — (m, p) (the leading lines being 10380, for He 

 and !20587 for parhelium). 



