Section III., 1915 [i79] Trans. R.S.C. 



On the Infra-Red Emission Spectrum of the Mercury Arc. 



By Professor J. C. McLennan, F.R.S., and Raymond C. 

 Dearle, M.A. 



(Read May Meeting, 1915.) 



I. Introduction. 



At the present time, when efforts are being directed towards the 

 establishment of relationships between the atomic structure of an 

 element and special features of its spectra, it is desirable to ascertain 

 as fully as possible the frequencies which are associated with the 

 atoms of the element in definite and determinate physical states. 

 The frequencies associated with mercury atoms in the neutral, or 

 supposed neutral state, have been carefully investigated by R. W. 

 Wood^, McLennan and Edwards'^ and others in the region between 

 A = 6,000 A°.U. and A = 1,800 A°.U. In the experiments in which 

 this was done, it has been found that if light of wave-lengths lying 

 within the limits mentioned be passed through non-luminous mercury 

 vapour there is a strong symmetrical absorption band at A = 1,849 

 A°.U., a moderately strong non-symmetrical one at A = 2,536 • 72 A°.U. 

 and one still less marked and consisting of three narrow bands at 

 A =2,338 A°.U. 



From this it has been concluded that within the limits mentioned 

 there are three groups of frequencies which characterise the atoms 

 or groups of atoms present in the vapour of mercury in the non- 

 luminous state. It is desirable, however, that a wider range of 

 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 preliminary 

 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 characterise 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 



iR. W. Wood. Physical Optics, p. 431. 



^McLennan and Edwards, Proc. Roy. Soc. of Canada, 1915. 



