\<>.w/: ((i.v// .u/'oA'./AT .U'l.i.wis IX rinshs ri im 



Ufti'H tlu-ri- is .1 luiniiuuis (-yliiitlrit.il sluMtli i-iic.isin^; the wire, and 

 from the lioundary of tlie sheatli outwards to the outer cylinder the 

 ^as is dark. It is customary to assume that tlie dark region, like the 

 (itluT dark spaces we have considered, is traversed by a procession of 

 inns of one sign, positive or negative as the case may Iw, moving at a 

 speed pro(Mirtional to the field and controlled by their own space- 

 charge according to the ecjuatioii in cylindrical cnorilinates corre- 

 sponding to {\7); and the ex|)erinicnls suppcjrt this assumption to a 

 certain extent. 



I must use my last paragraph to erase the impression — inevitably 

 to Ik.' given by an accoimt so short as this, in which the understood 

 phenomena must be stressed and the mysterious ones passed over — 

 that the flow of electricity through gases iss to be set down in minds 

 anti lx)oks as a perfected science, organized, interpreted and fin- 

 ished. Quite the contrary! there are as many obscure and mysterious 

 things in this tield of physics as there are in any other which has been 

 explored with as much diligence. Its remarkable feature is not that 

 most or many of the phenomena in it have been perfectly explained; 

 but rather, that for those few which have been explained, the ex- 

 planations are ven,- simple and elegant; they are based on a few funda- 

 mental assumptions about atoms and electrons which are not difficult 

 to adopt, for they arc not merely plausible but actually demonstrable. 

 Perhaps as time goes on all the phenomena will be explained from 

 these same assumptions. There will be experimenters who modify 

 the apparatus and the circumstances of past experiments so that all 

 of the avoidable complications are avoided and the phenomena are 

 simplified into lucid illustrations of the fundamental principles; and 

 there will be theorists, who take the complicated phenomena as they 

 are delivered over to us, and extend the power of mathematical analy- 

 sis until it overcomes them. They may find it necessary to make other 

 and further assumptions, beyond those we have introduced; at present 

 it is commonly felt that ours may be sufficient. Whether posterity will 

 agree with us in this, must be left for posterity to decide. 



