684 Prof. McLennan and Mr. Dearie on the Infra-red 



of mercury in the non-luminous state. It is desirable, how- 

 ever, that a wider range o£ frequencies should be investigated, 

 especially on the side of the infra-red where but little work 

 on absorption appears to have been done as yet. 



With a view to proceeding in this direction, some preli- 

 minary work has been done by the writers in that region on 

 the emission lines in the spectrum of the mercury arc. It 

 is evident that a knowledge of the lines which characterize 

 this spectrum in the infra-red region, as well as of their 

 exact wave-lengths, would be of great assistance in deciding 

 where to look for absorption by mercury vapour. 



It was found, on examining the work of those who have 

 already investigated the emission spectrum of the mercury 

 arc in the infra-red region, that considerable divergence 

 exists in their results. The first recorded investigations 

 were somewhat cursory attempts by Snow and by Drew, and 

 it was not until 1903 that we have any results in which con- 

 fidence can be placed. 



These are due to Coblentz and Geer*, who worked with a 

 rock-salt prism spectrometer and a radiometer, and found 

 three definite lines between 0'97 jjl and 1*285 fju. In addition 

 to these they were able to identify six lines in the neigh- 

 bourhood of 5*0 yit, and possibly one other near 3*0 \x. 

 Coblentz f repeated this work a couple of years later, and 

 announced that there are no important lines beyond 1*3 /jl 

 except those near 5*0 jjl. W. J. H. Moll { somewhat later,, 

 using a rock-salt spectrometer and thermopile in connexion 

 with an automatic recording device, identified five lines 

 between l'O fi and 1*7 //,. In direct opposition to the results 

 of Coblentz and Geer, Moll states that there is no measurable 

 emission above 1*7 /jl. Probably the most accurate measure- 

 ments on the infra-red spectrum of the mercury arc are 

 those made by Paschen§ with a concave grating and a 

 Rubens thermopile. By means of the better definition and 

 the higher dispersion afforded by the grating, Paschen was 

 able to separate maxima which had previously been recorded 

 as single lines. In all he identified fourteen lines between 

 1'Oyu, and 1'7 fi, and he confirmed the statement by MolL 

 that there are no lines beyond 1'7/a. He jj subsequently 

 repeated his measurements and found a maximum at 4'0 /x, 

 but inasmuch as this maximum came out in the arc spectrum, 



* W. Coblentz and W. C. Geer, Phys. Rev. xvi. pp. 279-286 (1903). 



t Coblentz, Phys. Rev. xx. pp. 122-124 (1905J. 



+ Moll, Kon. Akad. Wet. Amsterdam, Proc. ix. pp. 544-548 (1907). 



§ Paschen, Ann. d. Phys. xxvii. 3. pp. 537-570 (1907;. 



|| Paschen, Ann. d. Phys. xxxiii. 4. pp. 717-738 (1910). 



