264 REPORT— 1880. 



capillary part above the continuous spectrum. One of these lines is in the 

 red, two are in the green, and one is in the blue. The discharge still 

 passes as a narrow spark throughout the length of the tube. In the 

 wide part the spectrum remains continuous, and it extends more towards 

 the red and blue than in the capillary part. It seems as if the four lines 

 had taken away part of the energy of the continuous spectrum. As 

 the pressure diminishes, these lines increase considerably in strength, the 

 spark spreads out in the wide part of the tube, and the intensity of the 

 continuous spectrum is, therefore, considerably diminished, Avhile it still 

 forms a prominent part in the spectrum of the capillary part. When the 

 pressure is small the continuous spectrum decreases in intensity. At 

 the same time the negative glow, with its own characteristic spectrum, 

 gradually extends through the negative half of the tube into the capil- 

 lary part. The continuous spectrum has now entirely disappeared ; the 

 bands of the negative pole and the four lines stand out on a perfectly 

 black background. It is under these conditions that the change from the 

 compound line-spectrum to the elementary line-spectrum is best studied. 

 The mere insertion of theLeyden jar, I find, makes hardly any difference; 

 the jar does not seem to be charged at all. If, in addition to the jar, we 

 insert a movable air-break, which can be opened or closed at will, while 

 we look through the spectroscope, we shall be able to see alternately two 

 perfectly distinct spectra. If the air-break is closed, the four lines of the 

 compound spectrum only are seen ; if the air-break is opened these four 

 lines will disappear entirely, and the elementary line-spectrum will come 

 out. We have here as complete a transformation as we have from the 

 band to the line spectrum of nitrogen, taking place under exactly the 

 same circumstances. We have, therefore, the same right to consider the 

 two-line spectra of oxygen as two distinct spectra as we have in the case 

 of the two spectra of nitrogen.' ' 



Chemical Origin of Spectra. — There can be no doubfc that all the above 

 spectra really belong to oxygen. They appear iu whatever way the oxygen 

 has been prepared. They are seen with electrodes of aluminium, platinum, 

 silver, brass, and iridium. The glass also could not have introduced any 

 appreciable impurity, for all the spectra were observed in a large glass 

 receiver in which no part of the spark was within two and-a-half inches 

 from the glass. 



It has been observed already that great caution is necessary to exclude 

 all carbon impurities, and the reader is warned that several descriptions of 

 the carbonic oxide spectrum as a supposed oxygen spectrum have even 

 recently appeared.^ 



ly. Carbon. 



Swan: 'Phil. Trans. Ed.' xxi. p. 411 (1857). 



Pliicker : ' Pogg. Ann.' cv. p. 77 (1858) ; cvii. p. 533 (1859). 



V. d. Willigen : ' Pogg. Ann.' cvii. p. 473 (1859). 



Attfield: 'Phil. Trans.' clii. p. 221 (1862) ; * Phil. Mag.' xlix. p. 106 



(1875). 

 Dibbits : ' De Spectraal Analyse' (1863) ; 'Pogg. Ann.' cxxii. p. 497 



(1864). 

 Morren: 'Ann. Chim. Phys.' iv. p. 305 (1865). 

 Pliicker and Hittorf : ' Phil. Trans.' civ. p. 1 (1866). 



• Fhil, Tram. clxx. p. 51. ' Paalzow, Wied. Ann. vii. p. 130. 



