140 SPECTRA OF RAREFIED GASES. PAET i. 



followed by a grey interval which, separates the yellow 

 from three bright lines in the green, the first of which 

 is yellowish green, the last a beautiful greenish blue ; a 

 black and a dark space separates the latter from the 

 violet in which there is a bright line. The electric 

 light in a tube containing highly rarefied aqueous vapour 

 is red, the vapour is resolved into its simple elements by 

 the electricity, the oxygen combines with the platinum of 

 the negative or heat pole, and the spectrum is that of 

 pure hydrogen with the three most prominent bands only. 



The nitrogen spectrum is brilliant with all the seven 

 colours ; there are no broad dark spaces like those which 

 divide the bright bands in the hydrogen spectrum, but 

 it is crossed by numerous very fine black and grey lines. 

 Fifteen of the latter stripe the red and orange ; the green 

 is separated from the yellow by a black narrow band ; 

 it is terminated by two bright blue lines, and very fine 

 dark lines cross it and the rest of the spectrum. The 

 tube light is yellowish red. 



The spectrum of highly rarefied atmospheric air is 

 chiefly that of nitrogen, for the oxygen combines with 

 the platinum of the negative terminal, and is in too 

 small a quantity to transmit the electricity through the 

 tube. 



The rarefied vapours of chlorine, bromine, and iodine 

 are so rapidly combined with the platinum of the nega- 

 tive terminal, that it is difficult to determine their 

 spectra ; but they have peculiarities in common, which 

 distinguish them from all other spectra. The bright 

 lines that cross them are first at rest, but soon become 

 flickering. In the iodine spectrum, five of those lines of 

 flickering light of great beauty are in the green, two of 

 them close together. The bromine spectrum shows 

 a greater number, which extend across the colours 

 of its middle part, accompanied by dark lines ; and in 

 the chlorine spectrum there are many lines, both of 



