ABSORPTION AND EMISSION. 59 



copper and manganese. The light of lithium vapour consists 

 of only two rays, one in the red and the other in the orange. 

 The vapour of potassium likewise exhibits two rays, one in 

 the red, near to A, and the other in the violet. 



For the more refractory metals, the spark developed by 

 the Buhmkorff coil affords a convenient mode of ignition. 

 The metal to be examined may be employed as the elec- 

 trodes. It is, however, more convenient to allow the spark 

 to pass between two pieces of charcoal, one of which is 

 hollowed, and contains in the cavity a fragment of the metal 

 to be examined. 



(76) In the case of the more permanent gases, the spark 

 from the coil is allowed to pass through hermetically-sealed 

 glass tubes filled with the gases to be examined, the current 

 being directed by small wires of platinum at the two extre- 

 mities of the tube. The flame of the vapour of hydrogen is 

 thus found, when analyzed by the prism, to contain three 

 definite rays only. 



It thus appears that the lights of the incandescent gases 

 are not only distinguished from those of solids and liquids by 

 the discontinuity of their spectra, but, moreover, each gas or 

 vapour has its own characteristic spectrum whereby it is dis- 

 tinguished from all others. It further appears that when 

 two or more substances are present in the volatilized vapour, 

 the spectrum contains the characteristic lines of both, These 

 facts are the foundation of spectral analysis. The physicist, 

 measuring the refrangibility of the rays of such a spectrum, . 

 is enabled to detect by their means the presence of the sub- 

 stances whose vapours are so combined and volatilized ; and 

 he has even been led to the discovery of new elementary 

 substances hitherto unknown to chemistry. Thus Bunsen 

 and Hoscoe discovered the new metals rubidium and ccesium, 

 and Crooke discovered another alkaline metal, which he has 



