1879*] Molecular Physics in High Vacua. 415 
radiometer to turn, the passage of electricity shows that the 
“ dark space ” has shrunk to small dimensions. It is a 
natural inference that the dark space is the mean free 
path of the molecules of the residual gas. 
The radiometer which has just been turning under the 
influence of the lime-light is not of the ordinary kind. 
Fig. 1 will explain its construction. 
It is similar to an ordinary radiometer with aluminium 
disks for vanes, each disk coated on one side with a film 
of mica. The fly is supported by a hard steel instead of 
glass cup, and the needle point on which it works is con- 
nected by means of a wire with a platinum terminal sealed 
into the glass. At the top of the radiometer bulb a second 
terminal is sealed in. The radiometer can therefore be con- 
nected with an induCtion-coil, the movable fly being made 
the negative pole. 
As soon as the pressure is reduced to a few millims. 
of mercury, a halo of velvety violet light forms on the 
Fig. 2. 
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 
positive 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. 
You perceive a dark space behind each vane and moving 
round with it. In the first experiment, radiation from 
the lime-light falling on the metallic sides of the vanes, 
produced a layer of molecular pressure which drove the fly 
