REPULSION RESULTING- EROM RADIATION. 
513 
37. I now wished to see the effect of applying to the gravitating masses a hot body 
inside the exhausted balance-tube instead of outside, and accordingly constructed an 
apparatus shown in fig. 4. 
Eig. 4. 
a, b are two balls suspended on a straw beam ; c is a platinum spiral fastened to two 
stout copper wires cemented into holes drilled through a plate of glass, which can be 
cemented to the end of the tube ; d, d are the battery-wires ; e is the extremity of the 
tube, drawn out for attachment to the Sprengel pump. A single Grove’s cell served 
to heat the platinum spiral to redness. By careful management and turning the tube 
round the necessary degree, I could place the equipoised brass ball either over, under, 
or at the side of the source of heat. A contact-key enabled me to heat the spiral 
without removing the eye from the micrometer. With this apparatus I wished to learn 
more about the behaviour of the balance during the progress of the exhaustion, both 
below and above the point of no action, and also to ascertain the pressure corresponding 
with this critical point. 
In air of ordinary density the action of the hot spiral was one of attraction both 
above and below. Not wishing, however, to complicate the action by air-currents, I 
exhausted with the Sprengel pump until the gauge stood about 40 millims. below the 
barometer. I then tried the following experiments : — 
38. The brass ball was placed so that its position when in equilibrium was about 
1 millim. above the spiral, and the latter was rendered incandescent. The ball was 
immediately drawn down to the spiral, sometimes touching and then rebounding with 
considerable force. 
39. The brass ball was then arranged so that it was about 1 millim. below the 
spiral. On turning on the battery-current the ball rose to the hot platinum. This 
latter action might be due to air-currents ; but it is difficult to imagine that air-currents 
could drag the ball down to the hot spiral when the latter was beneath it (38). 
40. The ball was arranged so that the platinum spiral was opposite the end, but a 
little above, as shown in fig. 4 a. On igniting the spiral the movement was very slightly 
Fig. 4 a. Eig. 4 b. 
upwards. When the spiral was rather below the ball, as shown in fig. 4 b, the ball 
moved downwards when contact with the battery was made ; the tendency in each of 
