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Dr. A. Schuster on the Spectra of Metalloids. [May 16, 



always involves the destruction of the other. The fourth spectrum is 

 that which is always seen at the negative pole in vacuum tubes filled, 

 with oxygen. 



The Continuous Spectrum. — The following facts prove the statement 

 that at the lowest temperature at which oxygen is luminous it shows a 

 continuous spectrum. 



1. The wide part of a Pliicker tube generally shines with a faint 

 yellow light. When looked at by means of a prism the spectrum is 

 perfectly continuous. 



2. If a spark of an ordinary Ruhmkorff coil is taken in oxygen at 

 atmospheric pressure, one of the line spectra generally appears, but 

 when the break is put out of adjustment so as to weaken the spark, 

 the lines disappear and are replaced by a continuous spectrum which 

 has its maximum of intensity in the greenish-yellow, and gradually 

 fades away towards both ends of the spectrum. 



3. Becquerel mentions an observation according to which the point 

 of the oxyhydrogen flame takes a yellow colour when an excess of 

 oxygen is present. The description of the somewhat characteristic 

 colour which Becquerel gives coincides exactly with the colour of the 

 spark in oxygen, when it shows the continuous spectrum. According 

 to Pliicker an excess of hydrogen shows the hydrogen lines, and it is 

 therefore reasonable to suppose that in Becquerel's experiment the 

 oxygen was sufficiently heated up to become luminous. 



The continuous spectrum must not be confounded with the con- 

 tinuous spectrum which under high pressure forms the background to 

 the line spectrum. 



The Elementary Line Spectrum. — This is the spectrum which is seen 

 when a strong spark passes through oxygen at the atmospheric pres- 

 sure. It can be seen at all pressures when a jar and air break are 

 introduced into the circuit. Several measurements exist of these 

 lines, but only those of Thalen are given in absolute measure. Thalen's 

 measurements, however, refer to the lines seen wheu a spark passes 

 through air. Under these circumstances the oxygen lines are weak 

 compared to the nitrogen lines, and some strong oxygen lines do not 

 appear at all in Thalen's list. I have made a careful measurement of 

 all the lines. Some of the weaker bands have a different appearance 

 and suggest, therefore, a doubt whether they are really due to oxygen ; 

 but they always appear with the same relative intensity, and I could 

 not obtain any evidence which would justify their exclusion from the 

 list of oxygen lines. 



The Compound Line Spectrum. — Pliicker in his first investigation of 

 oxygen says it consists of four lines, one in the red, two in the 

 green, and one in the blue. In his later drawing of the spectrum of 

 oxygen, he gives a great number of lines of which these four form a 

 part. Wullner says that the four lines in question are always the 



