during tke passage of Electric Discharge. 19 



of platinum is deposited, the less refrangible light disappears from 

 the spectrum (104) ; as the oxygen is gradually removed, the in- 

 terior of the tube approaches to a vacuum. 



116. The remarkable analogy in the chemical relations of 

 iodine, bromine, and chlorine, reappears also in their spectra*. 

 I shall not at present describe these spectra, because the manner 

 in which the tubes have hitherto been made did not admit of 

 complete exclusion of the air, and the spectra consequently were 

 not pure, but a superposition of two spectra. Moreover, during 

 the passage of the current, the above bodies combine with the 

 platinum of the negative electrode. The iodine spectrum remained 

 sufficiently long to admit of being copied. It was necessary to 

 employ four bromine tubes in succession, each of which sufficed 

 for the determination of one portion of the spectrum. The 

 entire spectrum was formed by putting the four portions together. 

 The spectrum of chlorine was of short duration, so that one could 

 see it distinctly but could not copy it. That which the three 

 spectra have in common, and by which they are distinguished, 

 as far as present observations extend, from all other gas-spectra, 

 consists in lines of light, which at first are constant, but after- 

 wards only flickering, and whose width is about the same as that 

 of the narrow Fraunhofer's black lines. In the iodine spectrum, 

 the position in the green of five such lines of light of great 

 brightness has been determined ; two of these lie close together. 

 The bromine spectrum showed a greater number of such lines, 

 extending over many of the central colour divisions, and accom- 

 panied by black lines precisely similar to those of Fraunhofer. 

 In the chlorine spectrum there appear to exist a greater number 

 of such narrow lines, both black and bright ones ; the position 

 of these, however, it has not been as yet possible to determine. 



117. The chemical results hitherto obtained may be summa- 

 rized briefly as follows : — 



I. Certain gases (oxygen, chlorine, bromine, and iodine va- 

 pours) combine more or less slowly with the platinum of the 

 negative electrode, and the resulting compounds are deposited 

 upon the neighbouring glass suiface. When the gases are pure, 

 we thei'eby gradually approach to a perfect vacuum. 



II. Gases which are composed of two simple gases (aqueous 

 vapour, ammonia, nitrous oxide, nitric oxide, nitrous acid), imme- 

 diately split up into their simple constituents, and then remain 

 unchanged if these latter (ammonia) do not combine with the 

 platinum. If one of the constituents is oxygen (in water and 



* The action of the magnet upon chlorine, bromine, and iodine vapour, 

 is characteristic of these bodies, as I have already noted (IG, 17, 69). I 

 shall return to this point subsequently. 



C2 



