152 THE CHEMISTRY OF THE SUN. [CHAP. 



is very small, we shall only get the longest line of man- 

 ganese ; if the quantity of manganese is increased, the next 

 longest line will come in ; and so on. So that if the 

 spectrum of any specimen of iron was observed, it was at 

 once easy to see whether there was an impurity of man- 

 ganese in that iron, if we made the admission that the spectra 

 of iron and manganese, and so on, were the spectra of bodies 

 not decomposable at the temperature which we were employing. 

 If, for instance, there was a great quantity of manganese exist- 

 ing as an impurity in the iron, we got a great many lines, and 

 of course with the quantity of admixture the number of lines 

 would go on increasing until we might have fifty per cent, of 

 each, when we should practically have the greatest number of 

 lines of iron and the greatest number of lines of manganese 

 we could ever get together. This was the general statement, but 

 certain exceptions were noted. 



Although we had here the germs of a quantitative spectrum 

 analysis, the germs only were present, because, from the exist- 

 ence of several " critical points," and great variations due to 

 other causes, the results obtained were not constant. 



In a subsequent research on the gold-copper alloys used in 

 the coinage, 1 Mr. Roberts, the Chemist of the Mint, and myself 

 were able to show that the shortening in the length of the 

 lines by reduced quantity was such a definite physical effect 

 that a difference of yo-Juir part of copper in gold could be 

 detected. 



In the case of the metals, the electric spark was passed 

 between metallic poles. For gases of course this method could 

 not be employed. In practice there are very great objections 

 to the use of Geissler tubes, one very valid objection being 

 that the gas becomes much, less luminous as its pressure is 

 reduced ; but in fact, the work connected with gaseous spectra 

 can be done at atmospheric pressure ; we can get the lines down, 



1 Phil. Trans. 1873, Part ii. p. 495 



