ME. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
545 
strong repulsion. Now the effect of raising a platinum spiral to whiteness in the air 
would be to rarefy the air all round, and the suddenness of its ignition would cause the 
air to be driven from it, as a centre, on all sides. Hence I was prepared to find that 
the pendulum would be mechanically blown on one side by what might be likened to a 
miniature explosion of heated gas. But the action was always one of attraction, whilst, 
when there was no air at all present to be expanded and driven away by the hot 
platinum, the action was one of violent repulsion. A possible explanation of the attraction 
in air in this experiment may be given by assuming that the pendulum was driven 
inwards by the rush of cold air supplying the place of that rising upwards from the hot 
spiral ; but it is not likely that this action should so completely overcome the effect of 
expansive action ; and, moreover, it will only account for half the phenomenon (that in 
air), and leaves the still stronger action in a vacuum entirely unexplained. 
119. I have tried special experiments to put the air-current theory to a decisive test. 
Bulb-tubes (84) and torsion-apparatus (102) have been prepared, containing terminals of 
metal, ivory, glass, mica, or pith, in the form of thin flat surfaces. These surfaces have 
been placed at an angle with the plane passing through the index and suspending- 
thread, in such a manner that the action of heat would he to cause currents and 
drive them round like the vane of a windmill. I, however, found the action of heat 
in vacuo to be repulsion, and in air to be attraction ; and the latter was even strong 
enough to overcome the action of the air-currents, which could not fail to be developed 
under the circumstances of the experiment. 
120. The pendulum-apparatus has also been used to show that electricity is not the 
cause of the attraction and repulsion. On referring to the description (99), it is seen 
that the mass of magnesium forming the weight was in metallic contact with the 
platinum wire which supported it, and that the upper end of this platinum wire was 
fused into the glass tube, and passed thence to the outside. With this I have tried 
numerous experiments bearing on the action of electricity. I have connected the pro- 
jecting end of the platinum wire with “ earth,” with either pole of an induction-coil (the 
other pole being more or less insulated), with either pole of a voltaic battery, and with a 
delicate electroscope ; I have charged it with an electrophorus, and have submitted it 
to the most varied electrical conditions ; and still, on allowing radiation to fall on the 
suspended mass, I invariably obtain attraction in air and repulsion in a vacuum. The 
heat has been applied from the outside, so as to pass through the glass, and also inside 
by means of the ignited spiral ; and the results show no difference in kind, but only in 
degree, under electrical excitement. I often obtain troublesome electrical interference 
with the usual phenomena, but never of such a character as would lead me to imagine 
that the normal results were due to electricity. I also obtain the normal actions 
whether the apparatus has been standing insulated in the air*, or whether it has been 
completely immersed in water connected electrically with “earth” or surrounded with 
wet blotting-paper. 
mdccclxxiv. 4 c 
