SECT. IY. SPECTRA Of GASES. 139 



the Thames, and he has lived to see his telegraphic 

 lines spread over the surface of the earth and the bottom 

 of the ocean. 



Mr. Wheatstone had perceived that the bright lines 

 on the spectra of the metals are different and more 

 complicated when taken in air than in vacuo, and Pro- 

 fessor Angstrom made the important remark that the 

 electric spark gives two superposed spectra, one due to 

 any metal that may be under examination, the other to 

 the incandescence of the air through which the spark 

 passes. Hence the importance of the spectrum analysis 

 of gaseous substances, especially of those which consti- 

 tute our atmosphere, a subject that has been ably and 

 successfully investigated by Professor Pliicker. For that 

 purpose he made use of the Geissler or vacuum tubes, I 

 similar to those he used in his .experiments on the 

 stratification of electric light. When electricity was 

 sent through a tube containing oxygen gas, the gas 

 combined so rapidly with the platinum of the negative 

 terminal of the battery that there was little time to 

 examine the spectrum. The electral light in the tube 

 was too red at first, but as the attenuated gas gradually 

 disappeared it changed through flesh-colour to green, 

 then through blue to reddish- violet, and at length there 

 was too little gas to convey the electricity. However, the * 

 oxygen spectrum has a remarkably bright red band at 

 its red extremity, two bright orange lines divided by a 

 black one in the orange, and some bright bands in the 

 green. 



The electric" light of attenuated hydrogen is red, and 

 almost the whole light in its spectrum is concentrated; 

 into six bright bands of nearly equal breadths. There" 1 

 is a dazzling red band near the red end of the spectrum, 

 which, however, does not coincide with the oxygen 

 band ; then comes a very beautiful yellow band, in which 

 the whole of the yellow rays seem to be concentrated, 



