372 THE CHEMISTRY OF THE SUN. [CHAP 



myself as common lines are not truly coincident. My assistants 

 and myself, and others too, have gone over a large number of 

 these lines. My own method was to use a concave grating by 

 Rowland of eleven feet focus, kindly placed at my disposal by 

 Captain Abney, and a powerful electric arc. 



But while this work has been going on other work has 

 entirely endorsed the previous conclusion that these nearly 

 coincident lines certainly differ from other lines of the spectrum 

 taken at random. This previous conclusion and the work 

 which led to it have already been stated in chapter xviii. 

 What I propose to do now is to refer to the later work in 

 which these basic lines have again been noticed to behave 

 differently from others. This difference of behaviour has been 

 observed in the discussion of the observations of sun-spot 

 spectra in connection with the spectra of prominences; and 

 what we have to do is to see what has been the result of this 

 inquiry with regard to the basic nature of some of the lines. 

 Have we, as a matter of fact, or have we not, in these most 

 widened lines in spots, and the most brightened lines in flames, 

 picked out those lines which are common to two substances 

 with the reservation alluded to above ? 



The facts will be gathered from the accompanying map of 

 a limited region of the spectrum. We have under the data 

 for the spectrum of iron formerly given in the first horizon 

 the lines recorded by Angstrom in his first memoir as common 

 to two substances ; the names of the two substances being 

 given below. In the fourth horizon we have the observations 

 of Thalen made a few years after the observations of Angstrom, 

 and in passing from Angstrom to Thalen we pass from the 

 temperature of the arc to the temperature of the induction 

 coil. Now it will be seen that Thalen also gives us lines in 

 some cases agreeing with Angstrom's, in other cases extending 

 the information given by him, and in order to make this work 

 as complete as possible I have gone over this region with the 



