through Rarefied Gases. 417 



uniform; they are broadest towards the middle of the violet. 

 The broadest of these bands is on the one side of a bright 

 shining violet ; on the other side it is completely black. At 

 this part the spectrum presents in the telescope the appearance 

 of a strongly illuminated fluted column. 



72. For the present I shall confine myself to the description 

 of these two spectra. The most difficult question which arises 

 on the discharge of electricity through rarefied gases, is the che- 

 mical nature of the ponderable substance which gives rise to so 

 infinitely varied phaenomena of light. This question can only be 

 safely discussed in connexion with the prismatic analysis of the 

 light which is produced, — the more so as by this means every 

 sudden or gradual chemical change in the substance is recognized. 



73. In confirmation of this assertion I shall adduce the follow- 

 ing example. It appeared to me to be eminently desirable to 

 determine the spectrum of oxygen gas, and to draw it, like the 

 other spectra, in colours. M. Geissler, with his known skill, 

 prepared the tubes necessary for this purpose, employing thereby 

 chlorate of potash for the preparation of the oxygen. The pale 

 flesh-coloured light in the narrow part of the tube, gave a spec- 

 trum characterized by a remarkably bright band at the end of 

 the red, and by two beautiful orange-coloured bands, which were 

 separated by a narrower and perfectly black one. In the green 

 there also appeared bright bands ; the violet was very dark. 



I and my pupil, M. Liek, who purposed making a drawing of 

 the spectrum for me, on looking at it, the one after the other, 

 could not agree as to its appearance. The cause of this I soon 

 discovered to consist in the continual change of the spectrum. 

 The violet became more intense, black streaks appearing in it ; 

 the bright line to which the red was originally confined, became 

 paler and paler. Bright red bands appeared over a wider space, 

 alternating with dark ones. In short, the entire spectrum had 

 undergone an alteration. The light in the narrow tube became 

 continually greener, increasing in brightness. I increased the 

 induction current, by adding two to the three Grove's elements 

 used in exciting the Ruhmkorff's apparatus, allowing it to traverse 

 the tube continually in the same direction. The spectrum re- 

 mained for a time so constant, that it might easily be drawn ; 

 it then commenced to diminish rapidly in intensity, the light in 

 the narrow tube becoming gradually quite violet. The discharge 

 through the tube began to be discontinuous, till at last only 

 single separate discharges occurred ; finally the current entirehj 

 ceased to traverse the tube. 



I consider the original spectrum to be the spectrum of the 

 oxygen gas, which evidently loses gradually its free condition. 

 If we do not consider the possible formation of ozone, the second 



Phil. May. S. 4. Vol. IG. No. 101). Dec. 1858. 3 E 



