MR. W. CROOKES ON REPULSION RESULTING EROM RADIATION. 
523 
87. With a large bulb, very well exhausted and containing a suspended bar of pith, 
a somewhat striking effect is produced when a lighted candle or other radiant source is 
brought about 2 inches from the globe. The pith bar commences to oscillate to and 
fro, the swing gradually increasing in amplitude until the dead centre is passed over, 
and then several complete revolutions are made. The torsion of the suspending fibre 
now offers resistance to the revolutions, and the index commences to turn in the oppo- 
site direction. This movement is kept up with great energy and regularity as long as 
the candle burns — producing, in fact, perpetual motion, provided only the radiation 
falling on the pith be perpetual *. If the candle is brought closer to the bulb, the 
rotation of the pith becomes more rapid; if it is moved further away the pith ceases 
to pass the dead centre, and at a still further distance the index sets equatorially. The 
explanation of the different movements of the pith index according to the distance the 
radiant body is off, is not difficult on the supposition that the movement is due to the 
direct impact of waves on the suspended body. 
88. It is not at first sight obvious how ice, or a cold substance, can produce the 
opposite effect to heat, cold being simply negative heat (33). The law of exchanges, how- 
ever, explains this perfectly. The pith index and the whole of the surrounding bodies 
are incessantly exchanging heat-rays ; and under ordinary circumstances the income and 
expenditure of heat are in equilibrium. A piece of ice brought near one end of the 
index cuts off the influx of heat to it from that side, and therefore allows an excess of 
heat to fall upon it from the opposite side. Attraction by a cold body is therefore seen 
to be only repulsion by the radiation from the opposite side of the room. 
Bearing the law of exchanges in mind, several apparent anomalies in the movements 
of these indices are cleared up ; and it is also easy to foresee what the movement of a body 
will be when free to move in space under the influence of varying amounts of radiation. 
The heat which all bodies radiate into space can have no influence in moving them, 
except there be something in the nature of a recoil in the act of emitting radiation. 
And even should there be such a recoil, if the body radiates heat equally all round, the 
recoil will be uniform, and will not move the body in one direction more than in another. 
I need therefore only consider the effect of the radiation received by a body. Here also 
the influx of radiation to a body free to move in space of a uniform temperature may be 
considered to be equal, and it will acquire the temperature of space without moving in 
any direction. 
89. The case is, however, different if two bodies, each free to move, are near each 
other in space, and if they differ in temperature either from each other or from the 
limiting walls of the space. I will give here four typical cases, with experiments 
sufficient to prove the reasoning to be correct. 
Case I. Two hot bodies, A and B, in space of a lower temperature than themselves. 
The body A receives heat uniformly from space, except where the body B intervenes ; 
and on this side A receives more heat, as B is hotter than the space behind it ; A will 
* This experiment -was exhibited for the first time at the Royal Society’s Soiree; April 22nd, 1874. 
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