i6 THE POPULAR SCIENCE MONTHLY. 



path between successive collisions of the molecules of the residual gas. 

 The extra velocity with which the negatively electrified molecules re- 

 bound from the excited pole keeps back the more slowly moving mole- 

 cules 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 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 diifering materially from those which it obeys at atmos- 

 l)heric 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 ordi- 

 nary gaseous state of matter, depending upon constant collisions, 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 rarefac- 

 tion the mean free path has become so long that the hits in a given 

 time in comparison to the misses may be disregarded, and the average 

 molecule is now allowed to obey its own motions or laws without in- 

 terference. The mean free path, in fact, is comparable to the dimen- 

 sions of the vessel, and Ave have no longer to deal with a continuous 

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

 hausted, but we must here contemplate the molecules individually. 

 In these highly exhausted vessels the molecules of the gaseous residue 

 are able to dart across the tube with comparatively few collisions, and 



radiating from the pole with enormous velocity, they assume proper- 

 ties so novel and so characteristic as to entirely justify the application 

 of the term borrowed from I'araday, that of radiant matter. 



