258' Hull — Initial Velocities of the Electrons. 



In the case of carbon dioxide, all that I have been able to 

 find about the emission spectrum in this region is the follow- 

 ing statement by Schumann ;* " Its photographic action is 

 uncommonly strong, and it extends far beyond 162/x/x, into the 

 region of the shortest wave-lengths. I doubt not that, were 

 the tubes sufficiently transparent, it could be photographed as 

 far as the hydrogen spectrum extends. For its wealth of lines 

 it stands unrivalled." From this statement it appears that, 

 whatever the cause, the spectrum of carbon dioxide photo- 

 graphed by Schumann did not extend quite as far as the 

 hydrogen spectrum. The value X 1375 is, therefore, a very 

 reasonable one. 



The less refrangible limit of the air absorption band should 

 be about the same for carbon dioxide as for air, except that it 

 might be a little lower for carbon dioxide, owing to its " wealth 

 of lines" in this region. f The results seem to indicate that 

 this is the case. 



The agreement is, therefore, as good as our present knowl- 

 edge of absorption in this region will warrant. For the two 

 cases where this knowledge is most accurate, namely the limit 

 of the hydrogen spectrum due to the absorption of white fluor- 

 ite, and the less refrangible limit of the air absorption band, 

 the agreement is remarkably good.;); An exact comparison of 

 these results with those obtained by Ladenburg for longer 

 wave-lengths is impossible, owing to the difference in method 

 and in the nature of the illuminated metal. In Ladenburg's 

 apparatus there was very little light reflected to the receiving 

 wires (corresponding to our electrode B). Hence his maximum 

 velocities correspond more nearly to the real maxima defined 

 above than to the relative maxima of Table III. The metals 

 investigated by Ladenburg were platinum, copper, and zinc, 

 and for X 2010, his shortest wave-length, he found as maxi- 

 mum energy : for platinum, 1-86 volts ; for copper, 1*69 volts; 

 for zinc, 1*12 volts ; and he observed that the energy was 

 greater the more electro-negative the metal was. If this order 

 holds in general, the maximum energy for carbon for X 2010 

 should be between 1*86 and 1*69 volts, and for X 1710, extra- 

 polating on Ladenburg's hypothesis, it would be between 2*57 

 and 2'33 volts. The value of about 2*5 volts found above for 

 X 1710 lies between these limits. The agreement is, there- 

 fore, qualitatively correct. 



The question of velocities between zero and the maximum 

 requires further investigation. The fact that a positive poten- 



* V. Schumann, Smithsonian Contributions, No. 1413, p: 16. 

 f Hydrogen gives only a weak continuous spectrum between /L 1675 and 

 ?i 3700. Lyman, Astrophys. J., xxiii, p. 199, 1906 ; Schumann, 1. c. p. 28. 

 % The exact agreement is of course accidental. 



