154 THE CHEMISTRY OF THE SUN. [CHAP. 



length in all the spectra to which they are common, especially if 

 the volatility of the metals in question is about the same : those 

 cases are much more common in which they are long in one 

 spectrum and shorter in the others. 



2nd. As a rule, in the instances of those lines of iron, cobalt, 

 nickel, chromium, and manganese (substances volatilised with 

 difficulty), which are coincident with lines of calcium (which 

 volatilises easily), the calcium lines are long. Hence we are 

 justified in assuming that lines seen in the spectra of iron, 

 cobalt, nickel, chromium, and manganese, coincident with long 

 and strong lines of calcium, are really due to traces of the latter 

 metal occurring in the former as an impurity. 



The question then naturally arose whether, in cases of coin- 

 cidences of lines found between the lines of various spectra, the 

 line may be fairly assumed to belong really to that one in 

 which it is longest and brightest, and make its appearance 

 merely as an impurity in all the others. Indeed, a pro- 

 longed examination of various spectra was not required to 

 afford evidence not only of the great impurity of most of the 

 metals used, but of the fact that many of the coincidences 

 observed by Thalen and others might be explained without 

 having recourse to the idea of physical coincidences. 



A study of Angstrom's map of the solar spectrum, to take an 

 instance, shows many cases in which a line has been observed 

 to be common to two or more spectra ; and this is especially the 

 case with the lines of iron, titanium, and calcium, nearly every 

 other solar metallic spectrum exhibiting one or more cases of 

 coincidence with the latter. In those cases which were exa- 

 mined in the light of the " long and short lines," it was 

 frequently found that a line coincident in different spectra was 

 long and bright in only one of them, and that in others it was 

 short, or faint, or both ; or even, in certain specimens of the 

 substances, altogether absent from the spectrum. 



As an instance of this difference of behaviour, the following 



