48 INORGANIC EVOLUTION. [CHAP, 



and which is indicated in the accompanying temperature curve, will be 

 greatly changed in its essential points ; it rests upon so wide a basis of 

 induction. 



When this view of celestial evolution was first formulated as the 

 result of the wide spectroscopic inquiry to which I have referred, most 

 of the lines in the nebulae, and in the stellar groups III, IV, and V r 

 were of unknown origin; the groups were established by accepting 

 their presence as criteria, without any reference to chemistry. In the 

 lower groups I, II, and VI, the chemistry was obvious, and the identi- 

 fication of many metallic flutings made it clearer still. 



When engaged later on, in 1893, in the classification of stars, accord- 

 ing to their photographic spectra* I came across two very important sets 

 of lines of unknown origin, one in the hottest stars, the other in stars 

 of intermediate temperature. 



After the discovery of a terrestrial source of helium by Professor 

 Ramsay, I showed in a series of seven notes communicated to the Royal 

 Society,! May September, 1895, that the cleveite gases, which I 

 obtained by the process of distillation, accounted to a very great 

 extent for the first set. 



This result proved to be the key to the chemistry of groups III and 

 IV, which contains the hottest stars. 



In 1897, in a series of three communications to the Royal 

 Society,! I pointed out that some of the other set of unknown lines in 

 the stars of intermediate temperature, taking a Cygni as an example, 

 were due to the enhanced spark lines of iron and other metals, the arc 

 lines being almost entirely absent. 



The recent developments of this research, and the ultimate forma- 

 tion of a " test-spectrum," have been referred to in Chapter III. The 

 result of this has been to greatly strengthen the argument based upon 

 the first observations. 



In the accompanying photograph, a comparison is shown between 

 the lines of a Cygni and the enhanced lines of the substances thrown 

 together to form the " test-spectrum." The extraordinary number of 

 coincidences is seen at a glance. The facts are as follows : 



The number of lines measured in the spectrum of o Cygni at 

 Kensington between \ 3798'! and A 4861 '6 is 307 



Of these the number which approximately coincides with the 

 enhanced metallic lines so far observed is . .. .. ..120 



* Phil. Trans., A, vol. clxxxiv, p. 675. 



f 1st note, Proc. Roy. Soc., vol. Iviii, t>. 67; 2nd, ibid., vol. Iviii, p. 113 ; 3rd, 

 ibid., vol. Iviii, p. 116 ; 4th, ibid., vol. Iviii, p. 192 ; 5th, ibid., vol. Iviii, p. 193 ;. 

 6th, ibid., vol. lix, p. 4 ; 7th, ibid., vol. lix, p. 342. 



J Proc. Roy. Soc., vol. Ix, p. 475; ibid., vol. hi, p. 148 ; ibid., vol. Ixi, p. 441. 



