262 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
rotation produced being simply due to the slightly superior absorption which the 
lampblack possesses over the solid surface of charcoal. 
240 . The experiments with clear microscopic glass, given in Table XII., were tried 
in order to test the theory advanced in my paper of Feb. 5 , 1876 ( 194 , 195 ), that 
the “ apparent viscosity of a vacuum”* was due to the absorption, and subsequent 
slow communication of heat from the substance on which the light fell to the adjacent 
gaseous molecules. That this effect does occur to an appreciable extent has been 
proved by the results obtained with aluminium and gold backed with lampblack ( 232 ), 
and with mica and pith backed with lampblack ( 237 ). Mr. Johnstone Stoney 
suggested that this might be tested in a different way by coating the inner side of the 
exhausted tube with lampblack opposite to the disk of glass. Upon allowing lig ht to 
fall across the vacuum on this blackened surface, the disk should move towards the 
light. The results show that such an action does not take place to an appreciable 
extent. Being fully convinced that the theory is correct, I have carefully examined 
the apparatus, and thought over the conditions under which the experiment was 
tried, in the hope of discovering the cause of the anomaly and thereby reconc ilin g 
this result with the numerous array of proofs ranged on the opposite side. I think 
a sufficient explanation is to be found in the behaviour of the glass disk when it is 
exposed to radiation behind a water screen. When the total radiation from the 
candle falls on the thin glass, both the light and heat are acting, and the movement 
of 6 '5 may be due to either of these or to both combined. By interposing a water 
screen I practically cut off all the invisible heat rays, and any action which now takes 
place must be due to the luminous rays. But behind water the repulsion is only 0 ' 5 , 
in comparison to lampblacked pith behind water = 100 , or not more than ( 0'5 - 4 - 12 =) 
0 ' 04 , in comparison to the repulsion of 6 ‘5 when the water screen is away. The 
luminous rays are therefore 162‘5 times less active on thin glass than are the heat 
rays, and may consequently be left out of consideration altogether ; they pass through 
* This must not be confounded with the viscosity of the residual gas, in the sense in which I speak of it 
in the preliminary notice published in the ‘ Proceedings of the Royal Society,’ No. 175, 1876. What I 
meant by the “apparent viscosity of a vacuum,” spoken of in par. 195, is rendered clear by the description 
of the experiment, which I will quote from par. 194 : — “ A candle held close to the screen, and the shutter 
momentarily opened and closed, sent the index with some violence to the extreme limit of the scale. It 
then slowly came back to zero, and there stopped The movement of the beam seemed as if it 
were held in check by a force acting the whole time of its movement, and not only for the time the light 
acted. The impression conveyed was that the beam was swinging in a viscous fluid I then gave 
the apparatus a sudden twist round, so as to cause the beam to knock against the side of the tube. This 
set it swinging through a large arc, and the oscillations kept up with perfect freedom for several minutes, 
declining in amplitude at each oscillation, till the beam ultimately came to zero. This perfectly free 
movement is in strong contrast to the constraint under which the beam moves when the initial blow is 
given by a ray of light instead of by a mechanical push.” 
In an earlier experiment of the same kind, described in Part II. of this research, I express the same 
idea thus : — “ (107.) This behaviour points to the return movement taking place under the influence of a 
force which remains active after the original radiation is cut off, and which is only gradually dissipated.” 
