3o8 



INFRA-RED EMISSION SPECTRA. 



curve b; the latter is three times (deflection equals 3.2 cm.) as strong 

 as the band at 4.52 /x. 



As a whole from these curves it will be noticed that no emission lines 

 occur beyond 2 fi, which is entirely unexpected. Beyond this point 

 weak emission lines would be obliterated by the continuous spectrum. 

 The band at 4.52 fx will be noticed shifted to 4.75 fx, in the vacuum-tube 

 radiation. The intensity of the emission lines found depends upon the 

 density of the metallic vapor in the arc. From the fact that no lines 

 were observed beyond those found by Snow it is not to be inferred that 



MM 

 20 



^ 



^ 



7^ 



Fig. 140. 



no lines lie beyond 2 /x. The work simply shows that, for the conditions 

 which produce the lines at the end of the red, no lines of measurable 

 intensity are to be found beyond this point. It is true that Lehmann 

 (loc. cit.) has just shown that Rb and Cs have lines at 1.7 /*; but the 

 emission curves of these two elements, found by Snow, also show zveak 

 lines in this region, and it is only the cumulative effect upon the phos- 

 phor-photographic plate that has enabled Lehmann to map them. 



INFRA-RED EMISSION SPECTRA OP GASES IN VACUUM-TUBES. 



Since the incandescent oxides in the arc and spark emit a continuous 

 spectrum of sufficient intensity to obliterate any weak emission lines, 

 beyond 2 fi, it is impossible to detect them. The vacuum-tube lacks 

 these defects and is adapted to this work, provided one has sensitive 

 apparatus to detect the radiation emitted. 



