Passage of Electricity through Gases. Ill 



With reference to this question we must recur to the 

 experiments in which one electrode is put to earth while 

 the electricity of the machine is transmitted to the other 

 through an air-spark. In this case the air-charge must begin 

 at once, on the passage of the air-spark, with a certain ac- 

 cumulation of electricity, and without any previous distribution 

 of static electricity upon the sides of the glass ; in conformity 

 with this we find here no distinct charge of the sides of the 

 tube. 



When the discharge proceeding from the charged electrode 

 passes through the tube, the two electricities of the electrode 

 which is put to earth are also divided, and from the latter 

 also the discharge begins, though only a short time after the 

 other. Both discharges (which, though the movement pro- 

 ceeds from a communication between the molecules, still pro- 

 duce the impression of currents of air) meet towards the elec- 

 trode which is put to earth. Being thus attracted to one an- 

 other, they contract ; and since the discharge proceeding 

 from the positive electrode begins with greater velocity than 

 the negative, it penetrates more or less into the latter and 

 presents to it a convex surface ; and since at the meeting-point 

 the opposing electric molecules are much accelerated, the dis- 

 charge there appears brighter. 



As the dark space appears near the negative electrode, even 

 when the electricity is conveyed to it through an air-spark 

 and when the positive electrode is put to earth, it cannot be 

 the meeting-point of the positive and negative discharge, or 

 the place where the free electricity of the two are neutralized, 

 since this would rather be warmer and brighter, as mentioned 

 above. As, however, the dark space, according to De la 

 Rive's direct experiments, appears colder than the othei; part 

 of the discharge, and a true current of electricity cannot cer- 

 tainly be proved to exist in it by means of derived currents 

 (by which new electrodes are brought into the current), it 

 therefore follows that the passage of electricity through the 

 dark space must take place in a manner different from that of 

 the rest of the discharge. Its appearance must depend upon 

 the magnitude of the charge on the electrodes necessary for 

 the commencement of a discharge, because under circum- 

 stances otherwise similar it appears, to a certain extent, only 

 when the positive electrode is put to earth and the air-spark is 

 introduced between the machine and the negative electrode, 

 and not in a reversed arrangement. 



According to the experiments mentioned at the beginning 

 of this paper, the extent of the dark space increases with the 

 length of air-spark, it diminishes when between it and the 

 Phil Mag. S. 5. Vol. 3. No. 17. March 1877. N 



