1879.] 



on Molecular Physics in Iliyh Vacua. 



141 



Fjg. 1. 



a second terminal is sealed iu. The radiometer can therefore be 

 connected with an induction-coil, the movable fly being made the 

 negative pole. 



As soon as the jiressure is reduced to a few millims. of mercury, 

 a halo of velvety violet light forms on the metallic side of the vanes, 

 the mica side remaining dark. As the pressure diminishes, a dark 

 space is seen to sejiarate the violet halo from the metal. At a 

 pressure of half a millim. this dark space ex- 

 tends to the glass, and positive rotation com- 

 mences. On continuing the exhaustion the 

 dark space further widens out and appears to 

 flatten itself against the glass, when the rota- 

 tion becomes very rapid. 



You perceive a dark space behind each vane 

 and moving round with it. In the first experi- 

 ment, radiation from the lime-light falling on 

 the metallic sides of the vanes, produced a 

 layer of molecular pressure which drove the 

 fly round ; so here the induction-current has 

 produced molecular excitement at the surface 

 of the vanes forming the negative pole, extend- 

 ing up to the side of the glass. 



When the negative pole is in rapid rotation 

 it is not easy to see this dark space, so I have 

 arranged a tube in which the dark space will 

 be visible to all present. The tube, as you 

 will see by the diagram (Fig. 2), has a pole in 

 the centre in the form of a metal disk, and 

 other poles at each end. The centre pole is 

 made negative, and the two end poles connected 

 together are made the jDositive terminal. The 

 dark space will be in the centre. When the 

 exhaustion is not very great the dark space ex- 

 tends only a little distance on each side of the negative pole in the 

 centre. When the exhaustion is very good, as it is in the tube before 

 you, and I turn on the coil, the dark space is seen to extend for about 

 2 inches 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 sj)ace — nearly 2 inches — is the 

 measui'e 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 towards 

 that pole. The conflict occurs at the boundary of the dark space, 

 where the luminous margin bears witness to the energy of the dis- 

 charge. 



I will endeavour to throw on the screen an illustration of this 



