SPECTRUM ANALYSIS. 283 



some chemical substances burn with colored flames ; po- 

 tassium with a violet flame, and sodium with a yellow 

 flame, as seen when burning on water, or when common 

 salt is thrown into a fire. Then, again, strontium com- 

 pounds burn with a beautiful red flame, and barium with 

 a green flame, so that they are used in making fireworks. 



Now in all these cases the flames are colored by the 

 bodies named in the state of gases. 



If you examine the light from burning sodium by means 

 of a prism -that is, allow the light from incandescent sodi- 

 um vapor to pass through a prism you will obtain a dis- 

 continuous or broken spectrum: only one color will be 

 seen, viz., yellow, and this yellow color will fall at the 

 same point in the spectrum that the yellow rays of the 

 sun spectrum would strike. It appears, then, that sodium 

 vapor heated red-hot gives out rays of a particular re- 

 frangibility ; now this illustrates a well-defined law : That 

 every chemical element in the state of gas, when heated 

 until it becomes luminous, gives off a peculiar light. In 

 the example taken, light of all one color was given out by 

 the substance heated ; this is not the rule, however, but 

 rather exceptional, for most bodies emit light of various 

 kinds, possessing different degrees of refrangibility ; thus 

 the light from glowing strontium vapor analyzed by a 

 prism gives a spectrum made up of several yellow and red 

 rays, together with one blue one. 



400. Use of the Slit. The appearance of a continu- 

 ous spectrum, obtained from light of any source, depends 

 much upon the size and shape of the opening through 

 which the light passes before passing through the prism. 

 If a round opening be used, a series of disks will be seen 

 overlapping each other, as shown in Fig. 105 (p. 284). 

 If an opening having parallel sides be employed, the dif- 

 ferent colors will shade off into each other imperceptibly, 



