284 
MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
were acquired by the fly, if more candles than one were at work, or if sufficient 
molecular pressure were generated on the face of the mica vanes, there would be no 
difficulty in getting over the dead centre, and rotation would take place ; in default 
of these conditions the fly soon comes to rest. 
278. I will now describe the action of the candle on the aluminium radiometer. 
When a candle is brought near, the dark heat rays are stopped by the glass, and cause 
molecular pressure to react from it, as explained in the case of the mica radiometer 
(276). The luminous rays, which in the former case mostly passed through the vanes 
and bulb, are now arrested by the aluminium, some being reflected and some absorbed. 
I have been unable to find an account of experiments on the proportion of incident 
rays absorbed by aluminium, but considering that highly polished speculum metal 
absorbs 36 per cent, of the light falling on it, I am not far wrong in estimating 
that ordinary aluminium foil absorbs nearly 50 per cent, of the incident light. There- 
fore about half the light falling on the aluminium vanes is absorbed and converted 
into heat of temperature. Molecular pressure is thus produced on both surfaces of 
the plate, for the aluminium is so thin, and is so good a conductor of heat, that the two 
surfaces almost immediately become of the same temperature. In fig. 14 I have 
Fig. 14. 
endeavoured to represent part of the action which takes place when the candle shines 
on the aluminium radiometer. The molecular pressure generated on the hot surface of 
the glass opposite the candle is omitted, as it will be the same as with the mica 
radiometer shown in fig. 13, and its representation would only complicate the 
drawing. The light passing through the bulb falls on the aluminium plate, and, 
raising its temperature, causes pressure to be exerted on all sides. The pressure from 
the face away from the bulb is mostly dissipated, and may be disregarded, but the 
molecules rebounding from the face next the glass, cause increased molecular pressure 
on that side, and produce movement in the direction of the arrows, or positive 
rotation. As each vane passes the candle it takes up heat, and acquires extra driving 
energy. As it swings round, the opposite side of the glass acts as a cooler, and by the 
time the vane has completed the circle and has radiated away some of its extra heat, it 
is ready to recommence the cycle of transformation — light, heat, molecular pressure, 
motion. 
Unlike mica, which generates very little pressure on its surface, the aluminium 
