116 CHEMICAL DISCOVERY AND INVENTION 



of the vessel, and the properties which constitute the ordinary 

 gaseous state of matter, which depend upon constant collisions, 

 are observed. But by great rarefaction the free path is made so 

 long that the hits in a given time may be disregarded in com- 

 parison to the misses, in which case the average molecule is 

 allowed to obey its own motions or laws without interference ; 

 and if the mean free path is comparable to the dimensions of the 

 vessel, the properties which constitute gaseity are reduced to a 

 minimum, and the matter becomes exalted to an ultra-gaseous 

 state, in which the very decided but hitherto masked properties 

 now under investigation come into play." Matter then, accord- 

 ing to Crookes, exists under the circumstances of a very high 

 vacuum in what he regards as a fourth state, which is neither 



c 



FIG. 48. PHOSPHOIIESCENCE PRODUCED BY CATHODE RAYS. 



solid, liquid, nor gaseous in the ordinary sense, but differs 

 altogether from the gaseous. This idea helps to explain the 

 facts that the radiation from the cathode appears to be material, 

 that it travels in straight lines, and when it strikes on glass it 

 produces phosphorescence. If a screen is interposed in the path 

 of the rays, no phosphorescence is produced within the area of 

 its shadow. If the cathode, instead of being flat, is made concave 

 the rays thrown off from it may be brought to a focus, and in 

 this focus any solid object is heated intensely, and, if fusible, 

 may be melted. 



If the degree of exhaustion in the tube is increased beyond a 

 certain point the discharge ultimately refuses to pass. Several 

 other facts also require to be noted. For example, it was found 

 by Lenard twenty years ago that the emanation from the 

 cathode when directed on to a very thin aluminium plate is 



