222 COMPTON— ARC SPECTRA AND 



The three atomic effects correspond exactly, by the quantum rela- 

 tion eV = hv to the first two lines 1216 A. and 1026 A. and to the 

 convergence wave length 911 A. of the Lyman series. This is, I 

 believe, the first time that evidence has been obtained for the excita- 

 tion of the second member of the series separately. We have some 

 evidence of the separate excitation of the third member, also, and are 

 setting up a new apparatus designed to detect atomic radiation effects 

 very sensitively, with entire freedom from effects of ionization. With 

 this we expect to make a more thorough investigation of the excita- 

 tion of the successive members of the Lyman series. 



The 1 1.3 volt molecular effect is the formation of Hj, an ion 

 which is found by positive ray analysis to be very abundant, although 

 Bohr's theory of the molecule represents it as unstable. The 12.8 

 and 16.2 volt effects correspond to the processes Ho = H + Hr and 

 Hj^H-f H, respectively. The heat of dissociation is given, in 

 equivalent volts, by 12.8 — 10.1=2.7 volts or by 16.2 — 13.6 = 2.6 

 volts. This is in good agreement with the results of the preceding 

 method, but smaller than Langmuir's value. 



The critical potentials reported by previous observers are given in 

 Table IL The considerable lack of consistency among the various 



TABLE II. 



Experimenter Radiation • Ionization 



Davis and Goucher ^ u.o, 13.6, ii.o, 15.8 



Mohl'er and Foote ' 10.4, 12.2, 13.3, 16.5 



Foote, Mohler and Kurth s 10.5, 11. 8, 16.0 



Horton and Davies ^ 10.5, 13.9, 14.4, 16.9 



Franck, Knipping and 



Kriigerio 13.6, 11. 5, 17.1, 30.45 



Compton and Olmstead 11 <io.8, 13.4, >io.8, >I5.8 



Boucher 12 lo.i, 13.6, 15.6 



This Investigation lo.i, 12. i, 12.8, 11.3, 13.6, 16.2 



5 Smyth, Phys. Rev., 14, p. 409, 1919; Compton and Olmstead, Phys. Rev., 

 17, p. 52, 1921. 



^ Phys. Rev., 10. p. loi, 1917. 



"Jour. Optical Soc. of Am., 4, p. 49, 1920. 



s Phys. Rev., 19, p. 414, 1922. 



^ Roy. Soc. Proc, A, 97, p. 23, 1920. 



'^^ Ber. d. D. Phys. Ges., 21, p. 728, 1920. 



11 Phys. Rev., 17, p. 45, 1921. 



12 Phys. Rev., 19, p. 189, 1922. 



