1879.] 



On a New Method of Spectrum Observation. 



23 



imagining a higher dissociation. It became, however, necessary to see 

 if the others would also be accounted for. 



I have already given to the Royal Society a preliminary account 

 of the extraordinary, because unexpected, phenomena and changes 

 observed in the spectra of vapours of the elementary bodies when 

 volatilised at different temperatures in vacuum tubes. Many of the 

 lines thus seen alone and of surpassing brilliancy are those seen as 

 short and faint in ordinary methods of observation, and the circum- 

 stances under which they are seen suggest, if we again appeal to the 

 above rule, that mauy of them are produced by complex molecules. 



In this case the appeal lies to the phenomena produced when 

 organic bodies are distilled at varying temperatures ; the simplest 

 bodies in homologous series are those volatilised at the lowest tempera- 

 tures ; so that on subjecting a mixture of two or more liquids to 

 distillation, at the beginning a large proportion of the more volatile 

 body comes over, and so on. 



At any particular heat-level, then, some of the short lines may be 

 due to the vibrations of molecular groupings produced with difficulty 

 with the temperature employed, while others represent the fading out 

 of the vibrations of other molecular groupings produced on the first 

 application of the heat. 



In the line of reasoning which I advanced a year ago,* both these 

 results are anticipated, and are easily explained. Slightly varying 

 fig. 2 of that paper, we may imagine furnace A to represent the 

 temperature of the jar spark, B that of the Bunsen burner, and a 

 temperature lower than that of the Bunsen burner. 



Then in the light of the paper the lines b and c would be truly pro- 

 duced by the action of the highest temperature, c would be short and 

 might be basic, while of the lines h and m, m would be short and 

 could not be basic, because it is a remnant of the spectrum of a lower 

 temperature. 



To make this reasoning valid we must show then that the spark, or 

 better still the arc, provides us with a summation of the spectra of 

 various molecular groupings into which the solid metal which we use 

 as poles is successively broken up by the action of temperature. 



We are not limited to solid metals ; we may use their salts. In this 

 case it is shown in the paper before referred tof that in very many 

 cases the spectrum is one much less rich in lines. 



I have therefore attempted to gain new evidence in the required 

 direction by adopting a method of work with a spark and a Bunsen 

 flame, which Colonel Donnelly suggested I should use with a spark and 

 .an electric arc. This consists in volatilising those substances which 



* "Proc. Roy. Soc," vol. xxviii, p. 16 f, 

 f " Phil. Trans.," 1873, p. 258. 



