264 



colour appears in the case of common German glass, a fine blue 

 if the glass contain lead. Hitherto I have tried in vain to get speci- 

 mens of a different and well- determined chemical composition, which, 

 without doubt, would offer new and curious cases of fluorescence*. 

 A beautiful fluorescence is obtained by surrounding suitable ex- 

 hausted tubes, through which the discharge is passing, with a solution 

 of eesculine, of fraxine, of sulphate of quinine, &c. If the tubes 

 include residual traces of hydrogen, scarcely any fluorescing light is 

 seen ; if they contain traces of nitrogen, on the other hand, the fluores- 

 cence is very intense. All depends upon the nature and the density 

 of the gas. In certain cases, a very faint electric light, scarcely seen 

 by the eye, produces a brilliant fluorescence, especially when the 

 light, if belonging to the negative wire, is concentrated by the magnet. 

 M. Geissler, in constructing his tubes, first observed (about eighteen 

 months ago) the fluorescence of gases, which even continue luminous 

 some seconds after the interruption of the current : further researches 

 will explain this curious phenomenon. 



VI. The spectra of the electric current in rarefied gases and 

 vapours. 



The light of the electric discharge through large tubes is rather 

 too much diffused to give a distinct spectrum. I made trial, therefore, 

 of sending the discharge through a capillary tube, and fully succeeded. 

 I got a brilliant luminous line within the tube, of which a beautiful 

 spectrum was obtained by replacing the distant illuminated slit, which 

 Fraunhofer used in his observations, by the self-luminous line. 

 Afterwards I employed Babinet's goniometer. The slit of this 

 instrument was illuminated by the current within the capillary tube, 

 which was placed before it at a distance of about 0*4 of an inch. 

 The aperture of the slit could be changed ; but where it is not 

 particularly mentioned, it was seen under a constant angle of 

 3'. After having interposed the prism of heavy flint glass, the 

 refracted image of the slit, in the general case, was found to be 

 divided into a less or greater number of differently coloured bands, 

 appearing each under the just-mentioned constant angle of three 

 minutes. Hence it follows that the analysed electric light is com- 



* From the beautiful experiments recently made by E. Becquerel, we may 

 deduce a satisfactory explanation of these phenomena. 



