Resistance of Vacuum. 5 



more and more absorbed by the potass. At last, when all the 

 carbonic acid had disappeared, the electricity could no longer 

 pass through the vacuum between the platinum wires. If a 

 galvanometer was then introduced into the circuit, it did not 

 show the slightest deflection. 



Gassiot here calls attention to a fact which appears to us to 

 possess a certain importance: — When the gas was so strongly 

 rarefied that the direct discharge ceased to take place, it was 

 nevertheless possible to render the tube luminous by influence. 

 It was sufficient for this purpose to put one of the poles of the 

 induction-coil into communication with one of the electrodes 

 of the tube and connect the other with a strip of tinfoil fixed 

 to the outside of the tube, or, again, to put the two poles each 

 into communication with a special strip of tinfoil fixed to the 

 outside of the tube. Then the current coming from the induc- 

 tion-apparatus could not develop itself, and acted only by 

 induction upon the vacuum. Although the latter was suffi- 

 ciently complete to prevent the direct current from passing 

 from the induction-apparatus, it was nevertheless possible to 

 produce a current in it by induction. 



Gaugain did not succeed in making the current of a Ruhm- 

 korff induction-apparatus traverse a perfect Torricellian va- 

 cuum*; but at the same time he makes another observation 

 which appears to us to be of great importance. If a strip of 

 tinfoil be inserted between the electrodes of a less complete 

 vacuum perpendicularly to the path of the current, the electric 

 light shows, by its colour, that the side of the foil turned 

 towards the positive electrode forms a negative pole, and that 

 turned towards the negative electrode a positive pole. If one 

 of the electrodes be then brought sufficiently near to the tin- 

 foil, the current pierces the latter with a fine hole, through 

 which it afterwards takes its path exclusively, which puts an 

 end to the polarity of the foil. This fact proves, according to 

 Gaugain, that the tinfoil does not act here solely as a simple 

 conductor of electricity; for, if that were the case, the greater 

 part of the current would pass through the tinfoil itself, which 

 must rightly be regarded as a better conductor than the 

 rarefied gas which fills the aperture. Gaugain draws from 

 this the conclusion that there must exist at the surface between 

 the metal and the gas a special resistance (we would add, or a 

 contrary electromotive force) opposing the passage of the cur- 

 rent through the metal. 



Pliicker f has also made an observation which throws a 

 bright light upon the point which now occupies us. In some 



* Comptes Bendus, xli. p. 152. 



t Pogg. Ann. cv. p. 70 ; cf. vol. civ. p. 629. 



