3.28 On the Passage of Electric Currents through Rarefied Air. 



rod and the plate of the air-pump. An insulated thin disc of 

 tin was now placed between the electrodes, at a distance of 5 or 

 6 centimetres from each of them ; the luminous appearance 

 within the receiver was completely changed. When the current 

 traversed the cylinder from top to bottom, a cone of red light 

 descended from the copper rod, and the upper surface of the disc 

 was covered with a blue light ; a second cone of red light de- 

 scended from the lower surface of the disc, and a second covering 

 of blue light surrounded the exhausting tube of the air-pump. 

 Thus the tin disc constituted a double pole, in exactly the same 

 manner as when a metallic disc is placed between the electrodes 

 iu a liquid electrolyte, its surfaces become contrary poles, and 

 the resistance in the circuit, instead of being diminished, is in- 

 creased. This fact being established, the copper rod was next 

 depressed so that its lower extremity approached the tin disc ; 

 the latter was then pierced as a sheet of paper would have been, 

 and the current passed unbroken through the tube which was 

 formed ; only one cone of red light, only one dark space, and 

 only one blue covering was then seen. M. Gaugain considers 

 this a decisive proof that electricity suffers resistance in its pass- 

 age from a solid to a gaseous, or from a gaseous to a solid body ; 

 " for it is clear," says he, " that the obstacle which compels the 

 current to pierce the tin disc does not proceed from the resist- 

 ance due to the metal itself, inasmuch as this resistance is much 

 more feeble than that of the air displaced by the tin." 



The conductibility of the partial vacuum obtained by means 

 of an air-pump being so small, it was natural to expect that the 

 more perfect one iu a bai-ometer tube would be still less con- 

 ductible. M. Gaugain proved this by connecting the poles of 

 Ruhmkorff's apparatus with the mercury, and with a platinum 

 wire fused into the top of an ordinary barometer tube. He 

 foiuid it impossible to obtain the least appearance of light in the 

 barometer chamber when the distance from the summit of the 

 mercurial column to the platinum wire exceeded 3 or 4 millims., 

 although he employed six of Bunsen's cells. 



This result is in opposition to one obtained by M. Masson, 

 and communicated to the Academy on the 7th of February, 1853; 

 but M. Gaugain accounts for this divergence satisfactorily. In 

 fact, the greatest possible care is necessary to expel the air and 

 moisture in the barometer tube; indeed, when all precautions 

 have been taken and a vacuum obtained which mil not allow the 

 induced currents to pass, the application of a spirit-lamp to the 

 top of the tube for a few moments is sufficient, although it may 

 have become cool afterwards, to make the vacuum conduct the 

 currents again. It appears that the tubes used by Masson were 

 heated by a lamp after the introduction of the mercury, and 

 hence his results. 



