PHYSICS NINETEENTH CENT.SPECTROSCOPY. 491 



depend upon the temperature. Thus, while lithium compounds in the 

 flame of the Bunsen burner give only the splendid red and the feeble 

 yellow lines already referred to, these same compounds, volatilized in 

 the intense heat of the electric arc between carbon points, yield an 

 additional line of a beautiful blue colour. The spark-spectra of barium, 

 of strontium, and of calcium exhibit many lines in addition to those 

 which are visible in their flame-spectra. Thallium, for instance, in 

 addition to the intense green line of its flame-spectrum, shows five 

 other lines. In examining spark-spectra, the induction coil (the de- 



FIG. 225. APPARATUS FOR SPARK SPECTRA. 



scription of which will be found in Chapter XX.) furnishes a very 

 convenient apparatus. The poles of the " secondary coil " are formed 

 of slender rods of the metal to be examined, or sometimes the sparks 

 are drawn from a solution containing the metal. In Fig. 225 is a simple 

 and effective arrangement for obtaining spark-spectra, either with solu- 

 tions or pieces of metal. The spectra of metals examined in this way 

 are found to contain a very great number of lines. Thus the spark- 

 spectrum of calcium exhibits in a powerful spectroscope at least 75 

 lines, while that of iron shows nearly 500. 



It was soon observed that these spark-spectra exhibited two sets of 

 bright lines; one set special to the particular metal of which the poles 

 are found, and the other set belonging to the gases which surrounded the 

 poles. Thus, if the sparks are taken in air between poles of platinum, 

 some of the bright lines due to the oxygen and to the nitrogen of the 



