XVIII.] 



AN HYPOTHETICAL DIAGRAM. 



239 



might happen to elementary bodies if they behaved like com- 

 pound ones, struck me with surprise. It seemed to make 

 everything so clear. 



In the first place it presented us with such a simple and 

 sufficient cause for the differences in intensities of lines, not 

 only on passing from one temperature to another in the laboratory, 

 to which Kirchhoff and Angstrom were the first to refer, but 

 also on passing from laboratory to sun. 



It was in 1874 that I first glimpsed the idea that the line 

 spectrum of a substance was probably produced by molecules of 

 different degrees of fineness into which the substance was driven 

 by the temperature employed. 1 



3 



c 



s 



FIG. 87. Hypothetic furnaces. In this case the temperature of A is not so great 

 as in the former one. 



Now these molecules of different finenesses are represented by 

 a, fi, 7, and 8, in the hypothetical diagrams given above ; indeed, 

 it is quite easy to see that, if we change the temperature of 

 the furnaces in such a manner as to produce in turn the lines of 

 each of these hypothetical molecules in their most intense and 

 wide condition owing to the great quantity of vapour rich in 

 that particular kind of molecule, the strong lines produced at 

 these different temperatures would vary; the strongest line 



1 Proc. Hoy. Soc., vol. xxii. p. 380, 1874. 



