340 THE CHEMISTRY OF THE SUN. [CHAP 



spectrum of the spot is not only simpler than the spectrum of 

 the arc, but simpler than the spectrum of the spark. We shall 

 call attention to other points later on. 



4. Comparison of Iron Lines in Spots and Prominences. 



Now the importance of these statements depends on other 

 statements which we can bring to confront with them relating 

 to other phenomena in the present case the solar prominences. 



To facilitate this confronting, what was next done was to 

 mass the observations in hundreds, the individual observations 

 of the sun-spots enabling us to plot out on another map the 

 number of times each line was seen in a hundred observa- 

 tions. This number then gives us in the case of each line, its 

 "Frequency in Sun-spots." The individual observations of 

 prominences can naturally be treated in the same manner. 



Next these individual observations both of spots and 

 flames are treated in a certain way with reference to the 

 discussion. I will explain what that certain way is : we have 

 treated the sun-spot observations so that the lengths of the 

 lines will represent the number of times they have been seen in 

 100 sun-spots ; the line at wave-length 491 9'5, for instance, has 

 been seen seventy-two times ; that line, in fact, has been seen 

 more than any other ; the one 5005*0 some forty times, and so 

 on ; very many lines having been seen less than ten times. 



It is now the turn of the prominences, and the individual 

 results obtained from Tacchini's observations of prominences 

 have been summarised in exactly the same way. The next 

 diagram shows the spot observations compared with those of 

 100 prominences seen between the years 1872 and 1876. 

 The line 5017*5 was seen in 66 prominences out of 100. It is 

 known that prominences exist in a region of the solar atmo- 

 sphere not very far from that occupied by the spots, but we 

 have already seen that whereas the spots are produced by a 

 downrush of cool material, prominences are produced by an 



