6 On Radiant Matter. 



centre in the form of a metal disk, and other poles at each 

 end. The centre pole is made negative, and the two end 

 poles connected together are made the positive terminal. 

 The dark space will he in the centre. When the exhaustion 

 is not very great the dark space extends only a little 

 on each side of the negative pole in the centre. When the 

 exhaustion is good, as in the tube before yon, and I turn 

 on the coil, the dark space is seen to extend for about an 

 inch on each side of the pole. 



Here, then, we see the induction spark actually illumin- 

 ating the lines of molecular pressure caused by the excite- 

 ment of the negative pole. The thickness *of this dark 

 space is the measure of the mean free path between suc- 

 cessive collisions of the molecules of the residual gas. 

 The extra velocity with which the negatively electrified 

 molecules rebound from the excited pole keeps back the 

 more slowly moving molecules which are advancing to- 

 wards that pole. A conflict occurs at the boundary of the 

 dark space, where the luminous margin bears witness to the 

 energy of the discharge. 



Therefore the residual gas or, as I prefer to call it, the 

 gaseous residue within the dark space is in an entirely 

 different state to that of the residual gas in vessels at a 

 lower degree of exhaustion. To quote the words of our last 

 year's President, in his Address at Dublin : 



" In the exhausted column we have a vehicle for electricity not 

 constant like an ordinary conductor, but itself modified by the passage 

 of the discharge, and perhaps subject to laws differing materially from 

 those which it obeys at atmospheric pressure." 



In the vessels with the lower degree of exhaustion, 

 the length of the mean free path of the molecules is 

 exceedingly small as compared with the dimensions of 

 the bulb, and the properties belonging to the ordinary 

 gaseous state of matter, depending upon constant col- 

 lisions, can be observed. But in the phenomena now 

 about to be examined, so high is the exhaustion carried 

 that the dark space around the negative pole has widened 

 out till it entirely fills the tube. By great rarefaction 

 the mean free path has become so long that the hits in 

 a given time in comparison to the misses may be disre- 

 garded, and the average molecule is now allowed to obey 

 its own motions or laws without interference. The mean 

 free path, in fact, is comparable to the dimensions of the 

 vessel, and we have no longer to deal with a continuous portion 

 of matter, as would be the case were the tubes less highly 



