Resistance of Vacuum. 9 



atmospheric air at 07, for carbonic acid at 0*8, and for nitrogen 

 at 1 millim. Employing tubes and electrodes of equal size 

 in all his experiments, Morren had not, like Gaugain, the op- 

 portunity of observing how the maximum of intensity of the 

 current depended on the width of the tube, as well as on the 

 distance and the size of the electrodes. 



Schultz*, in his researches, employed as source of electricity 

 the Holtz machine instead of the RuhmkorfTP apparatus. He 

 also used Faraday's method for the determination of the elec- 

 tric tension necessary upon the electrodes for the production 

 of a discharge at different densities of the enclosed gas. The 

 following were the results of Schultz's researches: — For the 

 passage, through rarefied air, of electricity between electrodes 

 at a determined distance from one another, the tension neces- 

 sary diminishes with the reduction of the density of the gas, 

 until the rarefaction has reached a certain limit at which the 

 before-mentioned tension acquires its minimum value ; if the 

 rarefaction is carried beyond that limit, the electric tension 

 must again increase for a discharge to be possible. The den- 

 sity of the gas at which the tension necessary for the discharge 

 presents its lowest value depends on the width of the tube and 

 the condition of the electrodes. These two conclusions are in 

 conformity with those established by Gaugain and Morren. 

 But Schultz made another important observation, of special 

 interest for our subject : he found that, if the pressure of the 

 gas was greater than that at which the minimum of tension was 

 presented, the tension necessary for the discharge increased, 

 all other circumstances being equal, with the distance between 

 the electrodes; when, on the contrary, the pressure of the gas 

 was inferior, the tension was independent of the distance be- 

 tween the electrodes. This agrees with the observation made 

 by De la Rive, according to which, cceteris paribus, the tension 

 is proportional to the distance between the electrodes f. In 

 the experiments made by De la Rive, however, the density of 

 the gases examined (hydrogen and nitrogen) was higher than 

 that at which they possessed their least resistance ; he made 

 no experiment at a lower density than that at which the resist- 

 ance was at its minimum. 



v G. Wiedemann and Riihlmann executed in conjunction, and 

 afterwards the former alone, some researches on the course of 

 electricity in rarefied gases % . The source of electricity em- 



* Pogg. Ann. cxxxv. p. 249 (1868). 



t Comptes Rendus, lvi. p. 669 ; Arch. d. Sci. Phys. et Nat. (2) xvii. 

 p t 53 : Mem. de la Soc. de Phys. et d'JKist. Nat. de Geneve, xvii. p. 69 

 (1863). 



X Pogg. Ann. cxlv. pp. 235, 364, and clviii. pp. 35, 252. 



