THE CHEMICAL ELEMENTS. 607 



Lines due to impurities, if any are thus traced, are marked for omis- 

 sion from the map and their true sources recorded, while any line that 

 is observed to vary in length and thickness in the various photographs 

 is at once suspected to be an impurity line, and if traced to such is 

 likewise marked for omission. I give a case. 



The two lines H and K (3933 and 3968), assigned both to iron and 

 calcium by Angstrom, are proved to belong to calcium in the following 

 way : 



a. The lines are well represented in the spectrum of commercial 

 wrought iron, but are absolutely coincident with two thick lines in the 

 spectrum of calcium chloride with which the iron spectrum was con- 

 fronted. 



b. The lines are represented by mere traces in the spectrum of a 

 specimen of pure iron prepared by the late Dr. Matthiessen, Both 

 poles of the lamp were of iron, the lower pole consisting of an ingot 

 of the metal which had been cast in a lime-mold. 



C. The lines are altogether absent in a photograph of pure iron, 

 where both poles of the lamp were of the pure metal not cast in lime, 

 and they are likewise absent in a photograph of the spectrum of the 

 Lenarto meteorite. 



By eliminating lines due to impurities in the manner just described, 

 a spectrum is at length obtained, of which every line is assignable to 

 the particular element photographed, the same temperature being em- 

 ployed in the case of all the elements observed. 



With regard to the second line of work, I should commence by 

 stating that from a beautiful series of researches carried on by several 

 methods, Mitscherlich concluded in 1864 that every compound of the 

 first order, heated to a temperature adequate for the production of light^ 

 is not decomposed, hut exhibits a spectrum peculiar to this compound. 



In some experiments of my own, communicated to the Royal Societ}- 

 in 1873, I observed : 



First. That whether the spectra of iodides, bromides, etc, be ob- 

 served in the flame or a weak spark, only the longest lines of the metals 

 are visible, showing that only a small quantity of the simple metal is 

 present as a result of partial dissociation, and that by increasing the 

 temperatiu-e, and consequently the amount of dissociation, the other 

 lines of the metal appear in the order of their length with each rise of 

 temperature. 



Secondly. I convinced myself that wdiile in air, after the first ap- 

 plication of heat, the spectra and metallic lines are in the main the 

 same, in hydrogen the spectra are different for each compound, and 

 true metallic lines are represented according to the volatility of the 

 compound, only the very longest lines being visible in the spectrum of 

 the least volatile compound. 



Thirdly. I found that with a considerable elevation of temperature 

 the spectrum of the compound faded almost into invisibility. 



