On Radiant Matter. 5 



into collision ; or, in other words, the longer its mean free 

 path, the more the physical properties of the gas or air are 

 modified. Thus, at a certain point, the phenomena of the 

 radiometer become possible, and on pushing the rarefaction 

 still further, i.e., decreasing the number of molecules in a 

 given space and lengthening their mean free path, the expe- 

 rimental results are obtainable to which I am now about to 

 call your attention. So distinct are these phenomena from 

 anything which occurs in air or gas at the ordinary tension, 

 that we are led to assume that we are here brought face to 

 face with Matter in a Fourth state or condition, a condition 

 as far removed from the state of gas as a gas is from a liquid. 



Mean Free Path. Radiant Matter. 



I have long believed that a well-known appearance ob- 

 served in vacuum tubes is closely related to the phenomena 

 of the mean free path of the molecules. When the negative 

 pole is examined while the discharge from an induction-coil 

 is passing through an exhausted tube, a dark space is seen 

 to surround it. This dark space is found to increase 

 and diminish as the vacuum is varied, in the same way that 

 the mean free path of the molecules lengthens and contracts. 

 As the one is perceived by the mind's eye to get greater, so 

 the other is seen by the bodily eye to increase in size ; and if 

 the vacuum is insufficient to permit much play of the mole- 

 cules before they enter into collision, the passage of electri- 

 city shows that the "dark space" has shrunk to small 

 dimensions. We naturally infer that the dark space is 

 the mean free path of the molecules of the residual gas, 

 an inference confirmed by experiment. 



I will endeavour to render this " dark space " visible to 

 all present. Here is a tube, (Fig. i), having a pole in the 



FIG. 



