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MR. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
the movement of the radiometer, and generally of the repulsion resulting from radiation, 
the maximum effect being at a pressure of about 50 millionths of an atmosphere. 
According to the dynamical theory of gases, the repulsion is due to the internal move- 
ments of the molecules of the residual gas. When the mean length of path between 
successive collisions of the molecules is small compared with the dimensions of the 
vessel, the molecules rebounding from the heated surface, and therefore moving with 
an extra velocity, help to keep back the more slowly moving molecules which are 
advancing towards the heated surface; it thus happens that though the individual 
kicks against the heated surface are increased in strength in consequence of the heating, 
yet the number of molecules struck is diminished in the same proportion, so that there 
is equilibrium on the two sides of the disk, even though the temperatures of the faces 
are unequal. But when the exhaustion is carried to so high a point that the molecules 
are sufficiently few and the mean length of path between their successive collisions is 
comparable with the dimensions of the vessel, the swiftly moving, rebounding molecules 
spend their force, in part or in whole, on the sides of the vessel, and the onward 
crowding, more slowly moving molecules are not kept back as before, so that the 
number which strike the warmer face approaches to, and in the limit equals, the 
number which strike the back, cooler face ; and as the individual impacts are stronger 
on the warmer than on the cooler face, pressure is produced, causing the warmer face 
to retreat. 
This theory explains very clearly how it was that I obtained such strong actions in 
my earlier experiments when using white pith as the material to be repelled, and 
employing the finger as a source of heat, and how it happened that I did not discover 
for some time that dark heat and the luminous rays were essentially different in their 
actions on black and white surfaces. The explanation of this is as follows : — Bays of 
high intensity (light) pass through the wall of the glass vessel without warming it ; 
they then, falling on the white surface, are simply reflected off again ; but falling on the 
black surface they are absorbed, and, raising its temperature, produce the molecular 
disturbance which causes motion. Bays of low intensity (dark heat) do not, however, 
pass through the glass to any great extent, but are absorbed and raise its temperature. 
This warmed spot of glass now becomes the repelling body, through the intervention of 
the molecules rebounding from it with a greater velocity than that at which they struck 
it. The molecular pressure, therefore, in this case streams from the inner surface of 
the warm spot of glass on which the heat-rays have fallen, and repels whatever happens 
to be in front of it, quite irrespective of the colour of its surface. 
