286 THE CHEMISTRY OF THE SQN. [CHAP. 



electric circuit. It consists of a great number of lines, especially in 

 the more refrangible part of the spectrum. It has been called 

 elementary line-spectrum to distinguish it from the other line- 

 spectrum, because, according to one hypothesis which has been 

 suggested to explain the variability of spectra, the molecule which 

 gives this spectrum is in a simpler or more elementary state than 

 that which gives the other so-called compound line-spectrum. We 

 may, however, adopt the nomenclature independently of any hypo- 

 thesis that may have suggested it. 



II. The Compound Line-Spectrum. This spectrum appears at 

 lower temperatures than the first. It consists of four lines : one in 

 the red, two in the green, and one in the blue. With the exception 

 of the blue line all the lines in this spectrum widen very easily, 

 and, with an increase of pressure, more easily even than the hydrogen 

 lines. They do not widen out equally on both sides, but more 

 towards the red than towards the violet. This fact is especially 

 noticeable in the more refrangible of the two green lines. The blue 

 always remains sharp. 



III. The Continuous Spectrum of Oxygen. This spectrum appears 

 at the lowest temperature at which oxygen is luminous. The wide 

 part of a Pliicker tube, filled with pure oxygen, generally shines 

 with a faint yellow light, which gives a continuous spectrum. Even 

 at atmospheric pressure this continuous spectrum can be obtained 

 by putting the contact-breaker of the induction coil out of adjust- 

 ment, so that the spark is weakened. According to Becquerel an 

 excess of oxygen in the oxyhydrogen flame produces a yellow 

 colour, which is very likely due to this continuous spectrum. 

 The continuous background which often accompanies the elementary 

 line-spectrum must not be confounded with this spectrum. 



IY. The Spectrum of the Negative Glow. This spectrum, which 

 was first accurately described by Wiillner, is always seen in the 

 glow surrounding the negative electrode in oxygen. It consists of 

 five bands : three in the red and two in the green. The least 

 refrangible of the red bands is so weak that it easily escapes ob- 

 servation ; the two other red bands are rather near together, and may 

 be taken for a single band if the dispersion applied is small. The 

 two green bands, which appear of the same brightness throughout, 

 with pretty sharply -defined edges, are resolved into a series of lines 

 when looked at with high optical powers. 



