X.] THE " SERIES " EVIDENCE. 99> 



It seems as if the two series are of the " subordinate " type, and 

 that the principal series is wanting if Rydberg's conclusion be not 

 accepted ; because while in subordinate series the lines for large values 

 of n lie very near to one another, the similar lines of the principal series 

 on the other hand are always more refrangible. It seems, therefore, 

 probable that one or two of the many unknown lines recorded in stellar 

 spectra still awaiting identification may belong to the principal series 

 of hydrogen. 



If we are dealing in this case with a single molecule of hydrogen 

 vibrating in a previously unknown way in consequence of a higher 

 temperature, why is it that the molecules of other bodies do not put on- 

 similar transcendental vibrations and appear in the same stars so that 

 we shall get new forms of the other chemical elements ? The fact that 

 we do not do so is, I claim, an argument in favour of the view that the- 

 principal and subordinate series are produced by molecules of different 

 complexities, and that the finer molecules can alone withstand the- 

 action of the highest temperatures, and require high temperatures to- 

 produce them. 



In this way we can easily explain the visibility of the new form of 

 hydrogen in connection only or mainly with the lines of the cleveite 

 and other similar gases (for there is already evidence of the existence 

 of other similar gases) in the hottest stars. 



From the admirable work done on such substances as lithium,, 

 sodium and potassium, which apparently are reduced to their finest 

 atoms at relatively low temperatures, and more recently on the series of 

 oxygen seen at low temperatures, we are bound to consider that when the- 

 research includes the complicated spectrum of iron that that also must 

 follow suit ; but it is already obvious that a principal and two sub- 

 ordinate series will never do ; there will be very many series involved. 



Now these series must include both the arc and the enhanced lines, 

 and as these are visible each without the other in stars of different 

 temperatures, in one case associated with the cleveite gases, in another 

 without them, we have another argument in favour of molecular com- 

 plexity. 



I may here point out that it is always the hot line which avoids 

 " series." The argument that lines in series represent the vibration 

 of one molecule proves that lines not in series are produced by the 

 vibrations of some other molecule. 



Finally then, I stated in 1878 that the spectrum of a substance was 

 the integration of the spectra of various molecular groupings. 



It has now been definitely established that the spectrum of some 

 substances is the integration of " series." 



H 2 



