18 Prof. E. Edlund on the Electrical 



electrode is composed. When it was aluminium, r was at least 

 4 times less than when the same electrode was composed of 

 silver or of platinum, 2'5 times less than when the consti- 

 tuent material was zinc. Aluminium being, of all metals, 

 that which suffers least from the current, while the latter tears 

 silver and platinum without difficulty, Hittorf is inclined to 

 admit that here, as in the voltaic arc and the electric spark, at 

 the surface of the electrodes exists an electromotive force 

 acting in the opposite direction to the discharge. 



In order to decide if the diminution of what we designate 

 by r^l proceeds continuously with the decrease of the gas- 

 pressure, or if it ceases at a certain degree of rarefaction, 

 Hittorf carried on the latter till the pressure was, by calcula- 

 tion, only 0*003 millim. : but to this extreme limit i\l con- 

 tinued to diminish with the pressure. Detailed study of the 

 questions relative to the foregoing furnished to Hittorf the 

 result that there exists at the negative electrode an altogether 

 peculiar cause opposing the passage of the electric current 

 from the gas to the surface of the electrode. When the light 

 surrounding this electrode was explored with a magnet, it was 

 ascertained that the current here passed from the gas to the 

 electrode. 



In the subsequent researches which he has published, Hit- 

 torf substituted for the Ruhmkorff induction-apparatus a gal- 

 vanic battery of from 400 to 600 cells. These were composed 

 of carbon, amalgamated zinc, and a solution of potassium bi- 

 chromate and sulphuric acid. With the Holtz machine and 

 Ruhmkorff apparatus the current which passes through the 

 gas is constantly discontinuous- with the battery, on the con- 

 trary, the current was always continuous as soon as the resist- 

 ance of the rheostat inserted in the circuit did not exceed a 

 certain limit. But if the resistance was above that limit, the 

 current of the battery became discontinuous. It was therefore 

 easy to give the current those two forms at pleasure. Hittorf 

 calls attention to the very slight increase of temperature pro- 

 duced by the current in highly rarefied air. When the tube 

 was a little wider, a thin strip of paper enclosed in it did not 

 show a trace of carbonization when the current traversed it 

 producing an intense light. This is in accordance with the 

 result found by E. Wiedemann*, that a rarefied gas can be 

 rendered luminous by the passage of the current, although the 

 temperature of the gas may be much below 100° C. Thia has 

 received further confirmation from the observations of I : 

 bergf. But while the gas was so little heated by the current, 



* YVied. Ann. vi. p. 298 (1879). 



t Mem. de VAcad. des Ski. de St. Pe'terebourg, [7] xxvii. no. 1 (1879). 



