44 
DR. A. SCHUSTER ON THE SPECTRA OF METALLOIDS. 
altogether, and I think I may say I have observed the spectrum of oxygen under as 
many different conditions as it is in our power to produce, at least for all pressures less 
than one atmosphere. When the light is weak, fig. 2 is a convenient form of tube ; 
e cl is the capillary part, and the collimator of the spectroscope is placed parallel to 
it, so as to receive the light from the whole length of the tube. These tubes were 
first used by M. van Monckhoven. 
Fig. 2. 
The following was the usual way of preparing tubes for experiment :—A plug of 
clean asbestos was introduced into the tube c, separating the hard-glass tube a from 
the Pll'CKEr’s tube. This plug prevents any solid substance from being bodily carried 
into the Plucker/s tube. As the asbestos had been subjected beforehand to a 
considerable temperature, no gas could be given out by it; but to prevent any 
possibility of error, some tubes were filled without the asbestos, and showed not the 
least difference which could be traced to the asbestos. Permanganate of potash, which 
had been heated in a test tube until the crystals were disintegrated and fell into 
powder, was next introduced into the tube a, which was then drawn out at the 
blow-pipe. The tube b was then fused to the sphere of the ball and socket-joint. The 
joint was rendered air-tight by means of sulphuric acid, and the tube was ready for 
exhaustion. There was now no danger of any carbon compounds, except such as were 
unavoidably attached to the glass tube ; and I found, indeed, that the tube had 
sometimes to be filled and exhausted ten times before the spectrum of pure oxygen 
appeared. It is not quite easy to get rid of all traces of moisture, but by continued 
exhaustion it is not impossible. 
One of the difficulties in the investigation of the spectrum of oxygen consists in the 
oxidation of the electrodes. It is necessary to work while the-tube is attached to the 
pump, so that it can always be refilled. If this is not done, the oxygen combines with 
the electrodes, and a vacuum is soon established which does not conduct electricity. 
