90 M. E. Wiedemann's Investigations on 



Spectra of Mixed Gases. 



Dishargcs in Mixtures of the Vapours of Mercury and Sodium 

 with other Gases. 



In a Geissler tube filled with hydrogen a little mercury was 

 hermetically enclosed and the tube heated in an air-bath, 

 while at the same time the current of an induction-apparatus 

 passed 'through it. Whilst at the ordinary temperature the 

 hydrogen-spectrum was obtained, with the heating the lines 

 of mercury were added ; these became brighter and brighter 

 as the temperature rose ; and at the same time the hydrogen- 

 lines disappeared both in the wider portions of the tube and at 

 the electrodes *. 



In order to accurately investigate and with certainty estab- 

 lish this phenomenon multifarious experiments were made. 

 The tubes employed were either of the usual form of a Geissler, 

 consisting of a central capillary part and two wider parts at 

 the extremities, or they were prepared from a wide tube by 

 drawing out and thus narrowing the middle portion before 

 the blowpipe-flame, or else they consisted of a tube of uniform 

 width (about 10 millims. diameter) and 50 millims. length. 

 Finally, the shape of the electrodes (constantly of platinum) 

 was varied: either they were both spherical (diameter 3 

 millims.), or one spherical and the other pointed, or both 

 pointed. 



For the filling and exhaustion of the tubes, two side pieces 

 were joined on by fusing, of which the one was hermetically 

 closed, at about 1 centim. from the wall of the tube, immediately 

 after drying, while the other was connected with the air-pump. 

 This also, as soon as the intended pressure was generated in 

 the tube, was melted off, leaving a length of from 2 to 3 centims. 

 If further measurements were to be instituted at a different 

 pressure from that at first employed, the point was broken off, 

 the tube exhausted, and again hermetically sealed, and so on. 

 With a little care the tube could thus be used successively 

 three or four times. 



* This method of producing the spectrum of mercury may be of value 

 in optical investigations in which it is necessary to have homogeneous 

 light of a determined wave-length. While in the red the lines of lithium 

 and hydrogen (of which the blue-green and violet lines can be easily in- 

 tercepted), in the yellow the sodium-line, in the blue-green and violet 

 likewise hydrogen-lines are at disposal, homogeneous green light is only 

 to be obtained with difficulty, on account of the great volatility of thal- 

 lium. The mercury-spectrum is distinguished by a very beautiful green 

 line, from which the other, brighter lines (a double yellow and a violet) 

 are sufficiently distant to be covered by a diaphragm or eliminated by 

 suitably coloured glasses. 



