414 Molecular Physics in High Vacua. [June, 
Three years ago I had the honour of bringing before you 
the results of some researches on the Radiometer. Let me 
now take up the subject where I then left off. I have 
here two radiometers which have been rotating before you 
under the influence of a strong light shining upon them. 
The explanation of the movement of the radiometer is 
this, — the light, or the total bundle of rays included in the 
term “ light,” falling upon the blackened side of the vanes, 
becomes absorbed, and thereby raises the temperature of the 
black side : this causes extra excitement of the air mole- 
cules which come in contact with it, and pressure is pro- 
duced, causing the fly of the radiometer to turn round. 
Fig. 1. 
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 induc 5 tion-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 ideal layer of molecular pressure in the radiometer in- 
creases and diminishes. 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. If the vacuum is insufficient to permit the 
