ASSIOI, RtT.IDH'M, CAESIUM, AND LITHIUM 563 



and ronmin uiifli.-ui^'d. A high temperature forms one of those con- 

 ditions under which compounds most easily decompose ; and therefore 

 it' sodium or a similar element were a compound, then in all probability 

 it would be decomposed into component parts at the high temper- 

 ature of the sun. This may already be concluded from the fact that 

 in ordinary spectroscopic experiments the spectra obtained often 

 belong to the metals and not to the compounds taken ; this depends 

 011 the decomposition of these compounds in the heat of the flame. If 

 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 is then partially 

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

 descent vapour emits light of a definite ref rangibility. This conclusion 

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

 l>e 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 

 salt be replaced by sodium, then both the bright line and the absorp- 

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

 incandescent vapour be observed, or only that which passes through the 

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

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

 is the same 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 

 lines are observed in the spectra of two different compounds of one and 

 the same element obtained in the above-described manner, and also in. 

 the spectrum of the metal, but they all have their peculiarities. The 



as chemical learning, then, perhaps, the present ideas will undergo a deep change and 

 approach perfection. As yet spectroscopic science is still, for want of laws, at the epoch 

 of the accumulation of facts and not of their possession. 



002 



