244 W. Crookes—Radiant Matter. 
the lines of molecular pressure caused by the excitement 0 
the negative pole. The thickness of this dark space is the 
measure of the mean free path between successive 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 toward 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 
aseous residue—within the dark space is in an entirely differ- 
ent state to that of the residual gas in vessels at a lower 
degree of exhaustion. To quote the words of our last years 
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 ebeys 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 
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 disregard 
and the average molecule is now allowed to obey its ow? 
motions or laws without interference. The mean free path, 2 
fact, is comparable to the dimensions of the vessel, and wé 
have no longer to deal with a continuous portion of matter, 8 
would be the case were the tubes less highly exhausted, but we 
must here contemplate the molecules individually. In these 
