xx.] DR. SCHUSTER ON OXYGEN. 287 



Dr. Schuster gives the following description of the appearance 

 of a vacuum-tube filled with pure oxygen, as it undergoes 

 gradual exhaustion, in order to give an idea of the way in which 

 the spectra of oxygen gradually diffuse into each other : 



At first the spark has a yellow colour, and the spectrum is 

 perfectly continuous. Almost immediately, however, four lines are 

 seen in the capillary part above the continuous spectrum. One of 

 these lines is in the red, two in the green, and one 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, while 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 capillary part. The continuous spectrum has now entirely 

 disappeared, and the bands of the negative pole and the four lines 

 stand out on a perfectly black background. It is under these con- 

 ditions that the change from the compound line-spectrum to the 

 elementary line-spectrum is best studied. The mere insertion of a 

 Leyden jar makes hardly any difference ; the jar does not seem to be 

 charged at all. If, in addition to the jar, we insert a moveable air- 

 brake 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-brake is closed, the four lines of the 

 compound-spectra only are seen, if the air-brake 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. 1 



The following extract from Dr. Schuster's paper will show 



1 Phil. Trans, clxx. 51. 



