176 THE CHEMISTRY OF THE SUN. [OHAP. 



the iron gas at another rate. But it was soon found that the 

 differences which could be sharply seen between the spectrum of 

 a particular mass of magnesium vapour and a particular mass of 

 iron vapour extended to the iron vapour itself. 



There were just as many variations in the refrangibility of the 

 lines of iron itself, for instance, as there were between the lines 

 of iron and of other substances : that is to say, we had in the 

 one case magnesium going at one rate and iron going at another 

 rate ; but when we came to deal with the iron lines alone we 

 found one iron line told us the iron vapour was going at one rate, 

 and another iron line told us that the iron vapour was going at 

 another rate, i.e. a thing which could not be divided was going 

 at two rates at the same time. 



Further. The lines on which these determinations of the 

 relative motions of the vapour depended were found to go in 

 sets. In a spot, for instance, we would generally see movement 

 indicated by one set of iron lines, whereas in a prominence we 

 would always see a different set a set in a different part of the 

 spectrum altogether registering this movement for us. Here 

 again was considerable food for thought. 



That was stated very roundly a good many years ago in 

 1869. I will quote what I then wrote on this subject : 1 " Altera- 

 tions of wave-length have been detected in the sodium, 

 magnesium, and iron lines in a spot spectrum. In the case of 

 the last substance, the lines in which the alteration was detected 

 were not those observed when iron (if we accept them to be due 

 to iron alone), is injected into the chromosphere." 



That caveat with regard to iron arose from the fact that of 

 the 460 lines recorded, only three lines had up to that time been 

 seen bright in the solar prominences. 



It will, I think, be clearly gathered from the foregoing, that the 

 more observations were accumulated the more the spectroscopic 

 difficulties increased. 



1 Proc. Roy. Soc. No. 115, 1869. 



