ON RADIANT MATTER. 



15 



able 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 ordi- 

 nary 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, 



3Iean Free Path — Radiant Matter. — I have long believed that a 

 well-known appearance observed 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 

 molecules before they enter into collision, the passage of electricity 

 shows that the " dark s^jace " has shrunk to small dimensions. \Ye 

 naturally infer that the dark space is the mean free path of the mole- 

 cules of the residual gas, an inference confirmed by experiment, 



I will endeavor to render this " dark space " visible to all present. 

 Here is a tube (Fig. 1), having a pole in the center in the form of a 



metal disk, and other poles at each end. The center pole is made neg- 

 ative, and the two end poles connected together are made the jDositive 

 terminal. The dark space will be in the center. When the exhaus- 

 tion is not very great, the dark space extends only a little on each side 

 of the negative pole in the center. When the exhaustion is good, as 

 in the tube before you, 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 illuminating the 

 lines of molecular pressure caused by the excitement of the negative 

 pole. The thickness of this dark space is the measure of the mean free 



