xii.] SPECTRA OF COMPOUNDS. 159 



4. The Spectroscopic Effects produced ~by the Dissociation of 

 knoivn Compounds. 



There was another way in which the new method of work 

 proved itself very valuable, though at first it did not seem to be 

 connected in any way with solar chemistry. 



With regard to this, I should commence by stating that from 

 a beautiful series of researches carried on by several methods, 

 Mitscherlich concluded, in 1864, 1 that every compound of the first 

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

 which is not decomposed, exhibits a spectrum peculiar to this 

 compound. 



In some experiments of my own, made in 1873, I 

 observed : 2 



First. That whether the spectra of iodides, bromides, &c., be 

 observed 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 temperature, 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 while in air, after the first 

 application 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. 



These results enabled us to make the following statement : 



A compound body, such as a salt of calcium, has as definite 

 a spectrum as that given by the so-called elements ; but while 



1 Phil. Mag. 1864, vol. 28, p. 176. - Phil. Trails. 1873, pp. 650, 651. 



