POTASSIUM, KUB.IDJ.U3I, CAESIUM, AND LITHIUM .571 



common salt be introduced into the flame of a gas-burner, a portion of 

 it is decomposed, first forming,. in all probability, with water, hydro- 

 chloric acid and sodium hydroxide, and the latter then becoming partially 

 decomposed by the hydrocarbons, giving metallic sodium, whose incan- 

 descent vapour emits light of a definite refrangibility. This conclusion 

 is arrived at from the following experiment : If hydrochloric acid gas 

 be introduced into a flame coloured by sodium it is observed that the 

 sodium spectrum disappears, owing to the fact that metallic sodium 

 cannot remain in the flame in the presence of an excess of hydrochloric 

 acid; The same thing takes place on the addition of sal-ammoniac, 

 which in the heat of the flame gives hydrochloric acid. If a porcelain 

 tube containing sodium chloride (or sodium hydroxide or carbonate), and 

 closed at both ends by glass plates, be so powerfully heated that the 

 salt volatilises, then the sodium spectrum is not observable ; but if the 



FIG. 75. Bright spectra of copper compounds. 



salt be replaced by sodium, then either the bright line or the absorp- 

 tion spectra is obtained, according to whether the light emitted by the 

 incandescent vapour be observed, or light passing through the 

 tube. Thus the above . spectrum is not- given by sodium chloride or 

 other sodium compound, but is proper to the metal sodium itself. This 

 is also the case with other analogous metals. The chlorides and other 

 halogen compounds of barium, calcium, copper, &c., give independent 

 spectra which differ from those of the metals. If barium chloride be 

 introduced into a flame, it gives a mixed spectrum belonging to metallic 

 barium and barium chloride. If besides barium chloride, hydrochloric 

 acid or sal-ammoniac be introduced into the flame, then the spectrum 

 of the. metal disappears, and that of the chloride remains, which differs 

 distinctly from the spectrum of barium fluoride, barium bromide, or 

 barium iodide. A certain common resemblance and certain common 



