10 



Prof. Pliicker on the Spectra in Rarefied Gases 



Fia. 1. 



shading off with a 



Geissler's tabes, suffer changes which are 

 at ouce visible on account of the simulta- 

 neous alteration in colour. These changes 

 are often sudden, especially when the cur- 

 rent is concentrated by the magnet ; fre- 

 quently, however, they take place quite 

 gradually. Such changes are accompanied 

 by a decomposition of the gas, or by the 

 gas or its constituents entering into com- 

 bination with the electrodes, (It must be 

 borne in mind that we have here to do 

 with immeasurably small traces of gas.) I 

 may here relate a curious example of this 

 kind. M. Geissler had observed that tubes 

 which contained traces of sulphurous acid, 

 suffered a remarkable alteration by means 

 of the electric current. I had previously 

 observed how a tube containing a probably 

 less perfect vacuum of the same acid sud- 

 denly changed during the passage of the 

 current under the influence of the magnet 

 (16). In this case the change was slow 

 and continuous. The tube of which I am 

 now speaking was 400 millims. long and 25 

 wide. It showed at first a beautiful strati- 

 fication in the violet light, accompanied by 

 the customary phaenomeua. As Ruhm- 

 korff's apparatus was being discharged for 

 several minutes through the tube, the violet 

 light gradually became decolorized, and the 

 whole appearance was changed. After a 

 long time a constant appearance was esta- 

 blished, the tube then behaving exactly as 

 the most beautiful ones of Mr. Gassiot 

 (118). The sulphurous acid having entirely 

 disappeai'ed, a Torricellian va- 

 cuum, so to speak, had been form- 

 ed. The light about the negative 

 pole was separated by a broad dark 

 space from the broad white ne- 

 bulous clouds of light. These 

 latter extended to the positive 

 electrode, each one having a bright 

 white boundary towards the ne- 

 gative electrode, and gradually 

 rey tint towards the positive one. On 



