700 Professoy Dewar [June 10, 



questions as to tlie emissive power for radiation of short wave-lengths 

 of gaseous substances at comparatively low temperatures. 



Such facts completely modify the inferences which have been 

 drawn as to the continuity of flame spectra and the character of the 

 (specific absorption of the vapour of water. 



Identity of Spectral Lines, 



In Kirchhoff's ' Researches on the Spectra of the Chemical 

 Elements,' p. 10, the following reference is made to the apparent 

 identity of wave-length of some spectral lines. 



" If we compare the spectra of the different metals with each 

 other several of the bright lines appear to coincide. This is especially 

 noticeable in the case of an iron and magnesium line at 1655 • 6 (h^, and 

 with an iron line and calcium line at 1522-7 (E). It seems to me to 

 be a question of great interest to determine, whether these and other 

 similar coincidences are real or only apparent ; whether the lines in 

 question actually fall one upon the other, or whether they lie very 

 close together. I believe that my method of observation does not 

 possess the requisite accuracy for the purpose of answering this 

 (question with any degree of probability, and I think that a large 

 number of prisms and an increased intensity of light will prove 

 necessary."* 



The subsequent investigations of Angstrom and Thalen increased 

 the number of apparent coincidences amongst the spectral lines of 

 different elements. 



The question of the identity of spectral lines exhibited by dif- 

 ferent elements is one of great interest, because it is very improbable 

 that any single molecule should be capable of taking up all the 

 immense variety of vibrations indicated by the complex spectrum of 

 iron or that of titanium, and it might therefore be expected that such 

 substances consist of heterogeneous molecules, and that some mole- 

 cules of the same kind as occur in these metals should occur in more 

 than one of the supposed elements. Further, the supposed identity 

 of certain lines in the spectra of more than one element has been 

 made by Mr. Lockyer the ground of an argument in support of a 

 theory as to the dissociation of chemical elements into still simpler 

 constituents, and in reference to this he wrote : t " The ' basic ' lines 

 recorded by Thalen will require special study, with a view to deter- 

 mine whether their existence in different spectra can be explained or 

 not on the supposition that they represent the vibrations of forms, 

 which, at an early stage of the planet's history, entered into combina- 

 tion with other forms, differing in proximate origin, to produce 

 different ' elements.' " 



Young, on examining with a spectroscope of high dispersion the 



* 'Researches on the Spectra of the Chemical Elements,* by G. Kirchhoff, 

 p. 10. 1862. 



t Troc. Roy. Soc.' vol. xxx. p. 31. 



