CHAPTER XIT. 



SOME RESULTS OF THE " LONG AND SHORT " METHOD. 



1. The Short Lines die out ~by mixing. 



IN order to apply this new knowledge to the investigation of 

 the chemistry of the sun, the first thing to be done was to 

 prepare maps showing the long and short lines of those sub- 

 stances stated by Angstrom, Kirchhoff and others, to exist in the 

 solar atmosphere, and then to see if it was possible to get a lead 

 by comparing these with the lines observed either in the general 

 spectrum of the sun, or in the spectrum of any particular por- 

 tion be it spot or prominence. This work was begun in 1870. 



The following facts forced themselves at last upon the 

 attention : x 



1. When a metallic vapour was subjected to admixture with 

 another gas or vapour, or to reduced pressure, I found that its 

 spectrum became simplified by the abstraction of the shortest 

 lines and by the thinning of many of the remaining ones. To 

 observe the effect of reduction of pressure, the metals were 

 inclosed in tubes in which a partial vacuum was produced. 

 In all these experiments it was found that the longest lines 

 invariably remained most persistently. 



2. When we used metals chemically combined with a metal- 

 loid in other words, when we passed from a metal to one of it? 

 salts (I used the chlorides) only the longest lines of the metal 



1 Phil. Trans. 1873, p. 253 et scq. 



