II.] PIONEERING DIFFICULTIES. 25 



tained suggested that the various terrestrial and solar phenomena were 

 produced by a series of simplifications brought about by each higher 

 temperature employed. That is, that- the new instrument, the spectro- 

 scope, showed that higher temperatures than those previously em- 

 ployed were doing for chemistry what previous similar inquiries had 

 done, namely, indicating the existence of finer constituents in matter 

 supposed at each point of time to be elementary. 



This was the first glimpse of dissociation in relation to the produc- 

 tion of changes in the line spectrum. 



By the year 1872 the work of Rutherfurd and Secchi on stellar 

 spectra enabled the base of the inquiry to include the stars as well as 

 the sun. In some of the stars the existence of hydrogen, magnesium, 

 and carbon were beyond question. The point that first struck me 

 was that in white stars like a Lyrse and Sirius, with continuous spectra 

 extending far into the violet stars therefore hotter than their fellows 

 of a yellow or red colour we had to do with hydrogen almost alone. 



It was in 1873 that I first called the attention of the Royal Society 

 to the very remarkable facts which had even then been brought to- 

 gether regarding the possible action of heat in the sun and stars. 

 Referring more especially to the classification of stars by Rutherfurd, 

 I wrote as follows : * 



"I have asked myself whether all the above facts cannot be grouped 

 together in a working hypothesis which assumes that in the reversing 



FIG. 19. Spectrum of a sun-spot as compared with the general spectrum, 

 showing that certain metallic lines (sodium and calcium in this instance) 

 are widened. The darker portion represents the spectrum of the spot. 



layers of the sun and stars various degrees of ' celestial dissociation * 

 are at work, which dissociation prevents the coming together of the 

 atoms which, at the temperature of the earth and at all artificial tem- 

 peratures yet attained here, compose the metals, the metalloids and 

 compounds." 



Subsequently in a private letter to M. Dumas, who took the 



* Phil Trans., vol. clxiv, Part IF, p. 491. 



