252 THE CHEMISTRY OF THE SUN. [CHAP. 



This was one way of regarding the matter, there is another 

 which perhaps is more simple. The analogy between simple and 

 compound bodies which we have heretofore employed, has been 

 that suggested by the manner in which, after a compound of 

 calcium and chlorine has been exposed to the action of a 

 sufficient temperature, the spectra of calcium and chlorine, 

 become visible in addition to that of the compound. 



Now suppose we expose compounds of calcium with chlorine, 

 bromine and iodine to a like temperature, we eventually get 

 the calcium lines visible in all as the compound is broken up. 



This, then, furnishes us with a new analogy, since the common 

 lines seen in the spectra of elementary bodies may be likened 

 to the lines of calcium, common to all the salts of calcium when 

 they are dissociated. 



I am particularly anxious to point out that there was abso- 

 lutely nothing new imported into the consideration of the 

 question. We had simply with regard to change of intensity 

 taken as our guide the behaviour of a known compound body, 

 and then pushed the reasoning three or four stages further. 

 We had gone just the safest possible way, by the easiest possi- 

 ble stages, from the known to the unknown. This had brought 

 home to us another analogy furnished by a group of compound 

 bodies with a common base. No new theory in fact was 

 necessary. The appeal to the law of continuity was open to 

 us, and it seemed to answer our question at once. 



What then was the first thing to be done ? 



In the first part of our inquiry we confined ourselves to the 

 lines seen in the spectrum of one substance. It was obviously 

 now our duty to study the lines seen in the spectra of different 

 substances equally au fond. 



First, it was of importance to see whether an examination of 

 photographs gave us similar results with regard to the coin- 

 cidence of lines in other parts of the spectrum than that first 

 reduced. The question of absolute coincidence being reserved, as 



