12 . INORGANIC EVOLUTION. [CHAP. 



I wrote thus on this subject in 1879 : 



" I am at present engaged in investigating this question of rhythm, 

 and I have already found that many of the first order lines of iron 

 may probably arise from the superposition or integration of a number 

 of rhythmical triplets. All this goes to show how long the series of 

 simplifications is that we bring about in the case of the so-called ele- 

 mentary bodies by the application of a temperature that we cannot as 

 yet define. 



" Indeed, the more one studies spectra in detail, and especially 

 under varying conditions of temperature which enable us to observe 

 the reversal now of this set of lines, now of that, the more complex 

 becomes the possible origin. Some spectra are full of doublets ; others 

 again are full of triplets, the wider member being sometimes on the 

 more, sometimes on the less, refrangible side."* 



Mascartf had noted this recurrence of similar features in spectra 

 ten years earlier. 



Discontinuous Spectra with Dark Lines. 



It is time now to make still another experiment with our needle 

 and prism. 



If we study sunlight (taking care again to shield the prism), by- 

 allowing a sunbeam to illuminate the needle, we get a spectrum of a 

 kind differing from those we have seen before, inasmuch as the con- 

 tinuous band of colour is broken, it is full of dark lines ; that is, some 

 of the coloured rays are lacking ; and hence images of the needle are 

 not forthcoming in places. The positions of some of the chief dark 

 lines lettered by Fraunhofer are shown in Fig. 12. 



We now know that this result is produced by what is termed the 

 absorption of light. To understand it we have only to look at a candle 

 through glasses of different colours : a blue glass absorbs or stops the 

 red light, and only the blue end of the spectrum remains; a red 

 glass absorbs or stops the blue, and only the red end remains. 



In these cases large regions of the spectrum are alternately 

 blotted out as differently coloured glasses are used, but the absorption 

 with which we have to do mostly is of a more restricted character : 

 lines, that is, single images of the slit, are in question. 



One of the most important results that has been gathered from the 



* Proc. Roy. Soc., vol. xxviii, March, 1879. 



t In 1869, he wrote as follows : " Jl semble difficile quo la reproduction d'un 

 pareil phenomene soit tin effet du hasard : ii'est-il pas plus naturel d'admettre que 

 ces groupes de raies semblables sont des harrnoniques qui tiennent a la constitution 

 moleculaire du gaz luniineux ? II faudra sans doule un grand nombre d' observa- 

 tions analogues pour decouvrir la loi qui regit ces harnioniques." 



