8 Prof. Pliickei- on the Spectra in Rarefied Gases 



whatever be the nature of the metal forming the electrode. The 

 sui-rounding surface of glass is gradually blackened by the finely 

 divided metal; when the deposit becomes thicker, a beautiful 

 metallic mirror is formed. A thin deposit of platinum appears 

 blue in consequence of its fine state of division, and precisely 

 resembles in colour one of the shades of Faraday's finely-divided 

 gold*. I have seen a beautiful mirror of zinc at one end of a 

 wide glass tube, which extended as far as the zinc electrode pe- 

 netrated. This mirror was shaded off grey at the limits of its 

 opake borders. In this manner a new series of experiments might 

 he performed in reference to the optical proper-ties of metals in a 

 state of extreme division. 



1)0. In order to lessen the unpleasant blackening of the glass 

 which occurs under ordinary circumstances, instead of the thin 

 wires ordinarily taken, which become hot and sometimes glow 

 brightly when serving as negative electrodes, we may employ 

 thick wires, which do not undergo any sensible warming in the 

 current. In the case of thin wires being employed, the great 

 transference of the metal from the negative electrode appears to 

 arise in part from the greater amount of heat developed, and not 

 to be exclusively owing to the greater concentration of the elec- 

 tric current upon the smaller surface of the thinner electrode. 



M. Geissler sought in another manner to avoid the blackening 

 of the glass, namely, by surrounding the negative platinum elec- 

 trode by a narrow glass tube which projected a little distance 

 beyond the free end of the wire into the wider glass cylinder. 

 The inside surface only of the smaller tube became black in this 

 case, as far as the electrode reached. 



91. The following observation supports in a manner, and inde- 

 pendently, the opinion that in tubes of rarefied gases the metal 

 is not the bearer of the electrical discharge, and consequently 

 the cause of the pha?nomenon of light. Three different tubes, 

 of a form previously described (63), were respectively filled with 

 pure hydrogen, with arseniuretted and with antimoniuretted hy- 

 drogen, and then exhausted as far as possible. The stream of 

 light in the narrow jiait of each tube gave the characteristic 

 instantly-recognizable spectrum of hydrogen. The two last- 

 mentioned gases had obviously suffered decomposition by the 

 electric current. The metals arsenic and antimony separated by 

 this decomposition cannot be detected in the current of light. 



9.2. Before proceeding to the analysis of the light of the dif- 

 ferent gas-vacua, we must briefly consider the question whether 

 an absolute vacuum bars|the passage of the electric current, and, 

 by doing so, extinguishes the light. An absolute vacuum, like 

 a mathematical j)eiuhilum, is a fiction ; and the practical question 



* " Exi>i'iiniciital Htlations of Gold to Light," Phil. Trans. 1S57, ]>. 145. 



