526 Professor Thomas R. Merton [March 10, 



WEEKLY EVENING MEETING, 



Friday, March 10, 1922. 



Colonel E. H. Grove-Hills, C.M.G. D.Sc. F.R.S., 

 Secretary and Vice-President, in the Chair. 



Thomas R. Mertox, M.A. D.Sc. F.R.S., Professor of Spectroscopy 

 in the University of Oxford. 



Problems in the Variability of Spectra. 



[Abstract.] 



It has been known for many years that the radiations which an 

 element emits in the state of a luminous gas are not invariable, but 

 depend on the presence of other elements, the manner in which the 

 substance is excited to luminosity, and other circumstances. It was 

 recognised in some of the earliest investigations that many band 

 spectra were to be associated with compounds, and that a spectrum 

 might be due partly to such compounds and partly to uncombined 

 atoms. Thus, for example, if strontium chloride is introduced into 

 the flame of the Bunsen burner we find lines associated with the 

 element, bands due to strontium oxide, and also bands due to the 

 chloride, and when strontium bromide is substituted for the chloride 

 the spectrum is the same as regards the lines due to the element 

 and the oxide bands, but bands peculiar to the bromide are found 

 to have replaced those due to the chloride. Minute quantities of 

 substances can sometimes be detected by means of these characteristic 

 bands due to compounds, a familiar example being the blue flame 

 which is seen when common salt is thrown on to a coal fire, and 

 which is due to the copper chloride formed from the chlorine in the 

 common salt and the minute trace of copper which is present in the 

 coal. A number of different elements are present in most flames, 

 and the reactions which occur are probably very complex. In gases 

 contained in vacuum tubes which are excited to luminosity by elec- 

 trical discharges, it is possible to work with pure substances, and a 

 discussion of the spectra observed is simpler. 



In the case of gases in vacuum tubes the spectrum sometimes 

 consists of bands, and the band spectrum from the negative pole 

 may be different from that seen in the positive column. Thus nitro- 

 gen, when excited by uncondensed discharges, shows in the visible 



