Resistance of Vacuum. 19 



the negative electrode became intensely incandescent, though 

 the temperature of the positive electrode was much lower. 

 There must therefore be, at the negative electrode, some cause 

 opposing a considerable obstacle to the propagation of the cur- 

 rent. Moreover, on employing the battery as the source of elec- 

 tricity the same phenomena were verified as when the Holtz ma- 

 chine or the induction-apparatus was employed — namely, that 

 the tension necessary for the passage of the current diminished 

 when the gas was rarefied, until the rarefaction had reached a 

 certain limit, past which that tension rapidly augmented. In 

 a cylindrical tube of 1 centim. diameter, and with filiform 

 electrodes, the pressure of the air or hydrogen amounted to 

 0' 7-0*5 millim. when the tension necessary for the discharge 

 was at its minimum. With less than 200 cells it was possible, 

 at the rarefaction mentioned, to pass the current through those 

 gases. The increase or the decrease of the distance between 

 the electrodes had little or no influence upon the number of 

 cells necessary for the passage of the current. 



§6. 

 It has, then, been proved by direct observations that the 

 resistance, properly so called, in rarefied gas, or r 1; diminishes 

 constantly with the pressure of the gas till that pressure has 

 fallen to an insignificant fraction of a millimetre, and that 

 during that time r, which represents the resistance undergone 

 by the current on passing from the gas to the solid electrode, 

 constantly increases in value. The question may now be 

 stated thus : — Does the known fact that the current is incapable 

 of traversing the Torricellian vacuum depend on r 1} after con- 

 stantly decreasing simultaneously with the pressure of the 

 gas, suddenly acquiring a very high value when the last mole- 

 cules of gas are removed ? or is it more probable that the 

 resistance r upon the electrodes continues to increase ? Or, 

 in other terms, which of these two magnitudes is it that in- 

 creases when the highly-rarefied gas passes to absolute vacuum '? 

 Without any doubt the correct assumption is that r continues 

 to increase with the continuation of the rarefaction, and, in 

 doing so, receives so great an augmentation that the current 

 cannot circulate. Only in this manner, too, is it possible to 

 explain the fact that, without electrodes, a current can be called 

 forth, by induction, in a vacuum deprived of gas, while it is 

 impossible for a current to pass there between electrodes. 

 The observations above related lead therefore, in my opinion, to 

 the remarkable result that vacuum is a conductor of electricity. 

 This at the same time annihilates the difficulty of explaining 

 how one heavenly body can exert an electrical action upon 



C2 



