ME. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
525 
as from any other. B will therefore not move. The result will be that A will be appa- 
rently attracted towards B, whilst B will remain stationary. 
The force with which the bodies A and B in these four cases will be repelled, or 
apparently attracted, will vary with their distance from each other, being stronger when 
they are close and weaker when they are far apart. The diminution will not, however, 
follow the usual law of inverse squares, but a more complicated law. 
90. Experiment proves the above reasoning to be correct. A bulb-tube was prepared 
in the manner already described (84), but in it were suspended, by separate silk 
fibres, two glass stems, each having pith balls at its extremity. Fig. 3 shows the eleva- 
tion and plan of the apparatus. The torsion of the silk fibres was so arranged that the 
pith balls a b hung freely about a millimetre from the balls c d. The glass stems were 
looped in the middle, and bent so that they did not touch each other. After complete 
exhaustion the following experiments were tried. 
A beam of radiant heat was concentrated on to Fig. 3. 
the two balls a c. When applied momentarily and 
then removed the radiation simply drove the halls 
apart, and immediately allowed them to come together b 
again. When, however, the beam was allowed to play 
upon the balls for about half a minute they became 
warm and widely separated ; and upon now removing 
the beam of heat the balls did not fall together at 
once, but took several mfnutes to regain their original 
position. This experiment therefore proves Case I. 
The bulb and contents being of the ordinary tem- 
perature, a spirit-flame was rapidly passed round the 
bulb to warm it quickly on all sides. The balls were 
thus in the condition imagined in Case II., being in a 
space warmer than themselves. They immediately 
came together, a touching c, and d touching b. 
Many experiments were tried with the object of 
proving experimentally the propositions in Cases III, 
and IV. ; but with this apparatus it was found im- 
possible to warm one of the balls without at the same 
time producing repulsion of the ball by the beam of 
radiation concentrated upon it. There is, however, 
little doubt, from the experimental proof of Cases 
I. and II., that the reasoning is equally correct in the 
other cases. 
91. With a highly exhausted bulb and light pith index, which was found to be 
exceedingly sensitive to radiation, numerous experiments were tried to see if there was 
any difference in action between the fingers and a tube of water of the same tempe- 
