268 Dr. Megh Nad Saba on the Problems of 



According to many physicists, the flame, the arc, and the 

 spark represent gradually increasing stages of temperature, 

 viz. 2000° K, 4000° K, and 5000° K. It is well known that 

 spectra produced under these conditions are widely different 

 in their general characteristics, but the hypothesis that these 

 variations can mainly be attributed to the varying values 

 o£ a single physical variable, viz. the temperature, is not 

 generally accepted. 



I wish to point out that the value of the ionization potential, 

 as obtained experimentally by Franck and Hertz, McLennan, 

 and others, or theoretically from the quantum relation 

 eY = h(l, s), has a great bearing on the problem. As a rule, 

 the higher the Ionization-potential of an element, the greater 

 is the difficulty with which it can be excited to emit its line- 

 spectrum. This will be apparent from the following tables*: 









Table I. 











Element. . Mg 



Ca 



Sr 



Ba 



Na 



K 



Rb 



Cs 



Ionization 1 ,_ fi _ 

 potential./ rb0 



6-12 



5-70 



5-21 



5-11 



4-32 



4-16 



3-88 



Element. . Zn 



Cd 



Hg 



II 



He 



Ne 



A + 



NJ 



Ionization 1 Q . 

 potential. J 



9 



10-45 



136-17-1 



20-5-25 



•6 17-23f 



16 



17-18 



The line or lines v= (1, s) — (2,jt?) form the most important 

 lines of an element, and experiments on the ionization 

 potential have shown that when the vapour of an element is 

 bombarded by electrons, this is the line which is the first to 

 be excited, other lines appearing only when the stimulus is 



substantially greater. The potential V= — [(Is) — (2/?)] is 



therefore called the resonance potential, and may be taken 

 to be a measure of the stimulus which is required so that 

 an element may be just excited to radiation of its funda- 

 mental line. Abetter name would probably be "Badiation- 

 Potential." 



We may give a number of interesting examples. It is 

 well known that generally it is very difficult to excite helium, 

 the smallest trace of a foreign gas tending to quench the 

 He-lines. According to the present theory, this is due to 

 the fact that helium has the highest ionization and radiation 

 potential of all elements, so that when it is subjected to a 



* McLennan, Proc. Lond. Phys. Soc. — Guthrie Lecture, Dec. 1918. 

 Franck and Hertz, Verh. d. Deutsch. Ges. vol. xx. (1919). 



f Rentschler, Phys. Reo. vol. xiv. p. 503 (1919). Horton and Davies, 

 Proc. Roy. Soc. Lond., vol. xcviii. p. 124. 



\ Davis & Goucher, loc. cit. Jan. 1919; also Smyth, loc. cit. (1919). 



