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him as “ entirely repudiating” Professor Reynolds’s explanation 
of the phenomena. It will be seen from the preceding page 
that this is hardly a fair representation of the remarkably im- 
personal and unbiassed attitude which Mr. Crookes maintained 
throughout the discussion. 
On the 30th of the same month Mr. Crookes again takes up 
the subject in a paper “ On the Movement of the Glass Case of a 
Radiometer.” He describes a large radiometer with ten arms, 
eight of brass and two formed of a long watchspring magnet. The 
directive power of the earth prevents rotation unless a candle be 
brought near ; but if once started it continues to revolve with 
the light some distance off. It was floated in a vessel of water, 
and four candles were placed round it so as to set the arms in 
rotation. A mark was placed on the glass envelope to show the 
amount of this. It turned about once an hour with the vanes. 
On bringing a powerful magnet near the moving arms they 
stopped, and the envelope began to revolve in the opposite 
direction to the arms, about once in two minutes. When the 
candles were removed and a magnet used to rotate the vanes, 
the case rotated in the same direction about once in three 
minutes. These experiments showed that internal friction, 
whether of point and socket or of vanes against residual air, is 
considerable. 
He concludes by specifically denying his antagonism to “some 
strongly urged explanations,” saying that he has already stated 
that he wishes to keep free from theories. 
The April number of the “ Philosophical Magazine ” contains a 
further explanation of Mr. Johnstone Stoney’s views as to the 
radiometer, wherein he distinguishes two stages: (1) the brief 
period of adjustment which arises when the light first falls on the 
blackened face of the disc ; and (2) the permanent state which 
supervenes when the period of adjustment is over. During the 
former stage, in a partially exhausted globe, a thin layer of air 
in contact with the disc has been warmed and rarefied; another 
of varying temperature and density is in contact with the glass. 
If the exhaustion be carried farther, these two layers will become 
thicker until there is not room between the disc and the glass 
for their full development. Then arises what he terms 
“Crookes’s pressure.” “ A procession of swift molecules at once 
commences, which is ever starting from the front or warmed 
side of the disc (the heater of the little engine) and jostling its 
way to the opposite hemisphere of cool glass (the cooler of the 
heat-engine). Another procession is as constantly starting from 
the cool hemisphere of glass, and working its way both up to 
the disc and to the regions behind the disc. Any member of the 
slow procession that comes into contact with the front of the 
disc is, in rebounding, converted into a member of the swift 
