238 THE CHEMISTRY OF THE SUN. [CHAP. 



Let us further assume that in A there exists a substance a, 

 competent to form a compound body ft } by union with itself or 

 with something else, when the temperature is lowered. 



Then we may imagine a furnace, B, in which this compound 

 body exists alone. The spectrum of the compound ft would be 

 the only one visible in B, as the spectrum of the assumed 

 elementary body a would be the only one visible in A. 



A lower temperature furnace, c, will provide us with a more 

 compound substance 7, and the same considerations will hold 

 good. 



Now, if into the furnace A we throw some of this doubly- 

 compounded body 7, we shall get at first an integration of the 

 three spectra to which I have drawn attention ; the lines of 7 

 will first be thickest, then those of B ; finally a will exist 

 alone, and the spectrum will be reduced to one of the greatest 

 simplicity. 



This is not the only conclusion to be drawn from these 

 considerations. Although we have, by hypothesis, ft, 7, and 8, 

 all higher, that is more compound, forms of a, and although 

 the strong lines in the diagram may represent the true spectra 

 of these substances in the furnaces B, c, and D respectively, 

 yet, in consequence of incomplete dissociation, the strong lines 

 of ft will be seen in furnace c, and the strong lines of 7 will be 

 seen in furnace D, all as thin lines. Thus, although in c we 

 have no line which is not represented in D, the relative 

 intensities of the lines in c and D are entirely changed. 



Here is another diagram representing the facts on the sup- 

 position that the furnace A, instead of having a temperature 

 sufficient to dissociate ft, 7, and 8 into a. is far below that stage, 

 although higher than B. 



It will be seen from the diagram (Fig. 87) that then the 

 only difference in the spectra of the bodies existing in the four 

 furnaces ivould consist in the relative thicknesses of the lines. 



I confess that the result of this simple projection of what 



