On Radiant Matter. 19 



platinum terminal sealed into the glass. At the top of the 

 radiometer bulb a second terminal is sealed in. The radio- 

 meter therefore can be connected with an induction-coil, the 

 movable fly being made the negative pole. 



For these mechanical effects the exhaustion need not be 

 so high as when phosphorescence is produced. The best 

 pressure for this electrical radiometer is a little beyond 

 that at which the dark space round the negative pole ex- 

 tends to the sides of the glass bulb. When the pressure 

 is only a few millims. of mercury, on passing the induc- 

 tion current 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 

 separate the violet halo from the metal. At a pressure 

 of half a millim. this dark space extends to the glass, and 

 rotation commences. On continuing the exhaustion 

 the dark space further widens out and appears to flatten 

 itself against the glass, when the rotation becomes very 

 rapid. 



FIG. 1-5. 



Here is another piece of apparatus (Fig. 13) which illus r 

 trates the mechanical force of the Radiant Matter from the 

 negative pole. A stem (a) carries a needle-point in which 



c 2 



