Infra-red Emission Spectrum of the Mercury Arc. 683 



that apart from the crooks at the ends of the alpha-ray 

 tracks, the " scattering " is due to single deflexions through 

 considerable angles. 



4. The experiments failed to bring out evidence in support 

 of ionization by the nuclei of hydrogen atoms projected from 

 the latter when alpha particles collide with the atoms. 



5. The experiments go to show that when alpha particles 

 are projected through hydrogen or through hydrogen con- 

 taining traces of air exceedingly few collisions occur which 

 result in the ejection of a hydrogen nucleus. 



The Physical Laboratory, 



University of Toronto. 

 May 1st, 1915. 



LX XI. On the Infra-red Emission Spectrum of the Mercury 

 Arc. By Professor J. C. McLennan, F.R.S., and Ray- 

 mond C. Dearle, M.A., The University of Toronto*. 



[Plates XIV. & XV.] 



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. Woodf, McLennan and Edwards f, and others in 

 the region between \ = 6000 A.U. and \ = 1800A.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 X=1849 A.U., a 

 moderately strong non-symmetrical one at A, = 2536*72 A.U., 

 and one still less marked, and consisting of four narrow 

 bands, at \= 2338 A.U. 



From this it has been concluded that within the limits 

 mentioned there are three groups of frequencies which cha- 

 racterize the atoms or groups of atoms present in the vapour 



* Communicated bv the Authors. Read before the Royal Society of 

 Canada, May 26th, 1915. 

 t R. W. Wood, 'Physical Optics.' p. 431. 

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



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