100 INORGANIC EVOLUTION. [CHAP. X. 



So far there has been no definite pronouncement touching the 

 possibility that each series may represent vibrations of similar mole- 

 cules, but the facts as they stand are in favour of this view so long as 

 we consider a series as representing the simplest result of atomic vibra- 

 tion. There are facts which suggest that even a series is not a simple 

 result. 



I am glad to be able to complete this chapter, which Professor 

 Kayser has kindly read over for me, with the following expression of 

 his opinion, which he allows me to publish. 



" I quite agree with your opinion, that the molecules of elements 

 .are in general very complicated systems of atoms, and that their 

 complexity is very variable with temperature and perhaps other 

 conditions. I think that at the highest temperature every molecule 

 has the simplest structure ; is perhaps a single atom ; and that in this 

 condition it will emit a very simple spectrum consisting of one, or 

 perhaps three, series of doublets or triplets. If the temperature is not 

 high enough above the melting point to dissociate all the molecules, 

 nevertheless some will be dissociated, and we shall have always a 

 mixture of molecules, from the most complex ones that can exist at 

 this temperature to the most simple ones. When the temperature 

 gets lower and lower, more and more complex molecules will be added, 

 while the simplest ones gradually disappear. In the same degree the 

 simplicity of the spectrum is lost, of the series only the strongest lines 

 or none remain, and the spectrum is the sum of more or less lines of 

 a great many different spectra. I expressed the same opinion in the 

 first publication of Kayser and Runge (Abhandl. d. k. Akad., Berlin, 

 1888), and I think our researches have shown nothing that contradicts 

 it." 



