Passage of Electricity through Gases. 187 



irrespectively of surrounding circumstances, essentially inde- 

 pendent of the distance between the electrodes. This result 

 has been substantially established by Herwig for the passage 

 of electricity through vapour of quicksilver between quick- 

 silver electrodes. The galvanic arc between electrodes of car- 

 bon or of metal ought also to offer an example of this kind of 

 discharge. In this case also no discontinuity can be observed 

 in the revolving mirror, and the conveyance of the electricity 

 occurs simultaneously with the conveyance of the particles .of 

 the electrode charged with it, which then, in fact, become va- 

 porized, as may be seen by the spectrum of the arc. In this, 

 as in the other gas-discharges, therefore, the supposed resist- 

 ance of the arc is only partially dependent upon its length ; 

 it diminishes in proportion to the quantity of electricity passed 

 through it in a unit of time, because the electrodes are thus 

 more heated, and consequently a greater vaporization and 

 conveyance of electricity are caused. 



The formation of the arc must also be preceded by a com- 

 paratively high tension of the electricity at the electrodes, 

 whether by their being brought nearer to one another or by 

 the passage of an electric discharge between them, whereby 

 the conveyance of matter from the electrode may be effected, 

 when a lower battery-tension suffices to maintain a continuous 

 current of electricity. 



Resistance in the ordinary " galvanic " sense, however, 

 would not be offered by a discharge of this kind. The quan- 

 tity of electricity conveyed by it would rather depend upon 

 very various circumstances — upon the medium between the 

 electrodes, and therefore also upon the facility with which parts 

 of the electrodes can be thrown off in solid particles or in the 

 form of gas, upon the nature of the electrodes, the strength of 

 the current, also upon the quantity and temperature of the par- 

 ticles conveyed from the electrodes to the space between them, 

 &c. These conditions are in many respects analogous to, 

 though more complicated than, those of the single discharges 

 from the electric machine. 



It may easily happen that between the electrodes, through 

 the connexion with the poles, a constant column of gas-dis- 

 charges should take place, at first discontinuously, possibly 

 with less participation in the matter of the electrodes. But 

 when by these means the stratum lying between the electrodes 

 is heated, rarefied, and eventually filled with the particles of 

 the electrodes, if the battery conveys the electricity in suffi- 

 cient quantity, a continuous discharge may take the place of the 

 discontinuous discharge in the thus altered intermediate space, 

 in which case, at the same time with the change in the manner 



