418 On the Electrical Discharge through Rarefied Gases. 



spectrum is probably to be attributed to the traces of some other 

 gas not entirely driven out of the tube, making their effects felt. 

 Decisive experiments have still to be tried concerning this point. 

 After the complete disappearance of the free oxygen gas, the tube 

 contains too little gas to admit the passage of the stream, in 

 complete accordance with the older experiments of Mr. Faraday, 

 and the later ones of M. Masson : the interior of the tube behaves 

 as a complete and non-conducting vacuum. 



Before the entire extinction of the current in the tube, I in 

 vain endeavoured, by inverting the poles, to restore the continuity 

 of the discharge. By such inversion, the bulb about the nega- 

 tive pole became filled with the already mentioned enigmatical 

 beautiful green light (35) ; but this light was also extinguished 

 as soon as the current ceased to pass through the tube. 



The current had passed through the tube altogether for about 

 1^ hour. 



74. I shall here break off, because, apart from the chemical 

 question, investigations of the spectra of different gases cannot 

 have the requisite degree of accuracy. These spectra are essen- 

 tially different from those belonging to the electrical arch of light 

 in the air, and from metals glowing or burning in it. I doubt 

 whether the particles carried off from the electrodes exert any 

 influence upon the spectra above described (51) : I think rather 

 that these spectra belong entirely to the rarefied gases. On the 

 other hand, the electric arch of light in the air is never free 

 from matter, which is carried over (carbon or metal), whose in- 

 candescence gives rise to new bright lines in the spectrum, pecu- 

 liar to each substance*. 



75. In connexion with the chemical question, I propose re- 

 curring to the question of the spectra. The subject is one be- 

 longing, if I may use the expression, to Micro-chemistry. Con- 

 ditions occur in it which differ from those under which chemical 

 actions usually take place. It is only on the successful solution 

 of these questions, that many not unimportant points for the 

 molecular theory will be satisfactorily solved, such as — 



How may the spectrum of a mixed gas be derived fi'om the 

 spectra of its constituents? 



How are the spectra of a compound gas related to one another 

 before and after its chemical decomposition by the current ? 



IIow does the chemical combination which the gas effects with 

 the electrode influence the spectrum ? 



Do isomeric gases give rise to similar spectra ? 



Bonn, March 30th, 1858. 



* Compai'C the interesting investigation of M. Masson on electrical 

 photometry. 



