REPULSION RESULTING EROM RADIATION. 
515 
46. 1 now heated the sulphuric acid, in the U-tube desiccator attached to the pump, 
until it boiled. A little aqueous vapour was driven off ; the sharpness of the hammer- 
ing of the mercury changed to a duller sound ; and on trying an experiment once more 
with the ignited spiral, the action was seen to have returned to one of attraction. 
On leaving the pump to itself for an hour or two, the sulphuric acid, as it cooled, 
again took up the aqueous vapour ; and as the absorption proceeded I could trace the 
action of the hot spiral from attraction, through the point of neutrality, up to decided 
repulsion. 
47. The critical point, in the case of brass balls, is much higher than with pith balls. 
In the case of pith, applying heat outside, I obtained neutrality when the gauge was 
about 7 millims. below the barometer, and decided repulsion when there was still a 
difference of 2 millims. With, the apparatus just described, however, using brass balls 
and an internal source of heat, the critical point is very near the Sprengel vacuum. 
By close observation I can just distinguish that the gauge is a fraction of a millimetre 
below the barometer when the hot spiral ceases to exert an attractive action on the 
brass ball; but my unassisted eye is not able to detect a difference on the gauge 
between a Sprengel vacuum in which the ignited spiral is neutral to, and one in which 
it strongly repels, the brass ball. 
48. I have experimental grounds for believing that the position of the critical point 
varies with the density of the mass on which radiation falls, with the relation of 
its mass to its surface, and with the temperature of the apparatus (62, 63, 64, 65, 
67, 75). The temperature of the body used to attract or repel the brass or pith ball 
also affects the critical point, but it cannot, I think, modify it much ; for with the 
apparatus last described no difference in kind of movement, but only in degree, was 
noticed, whether the spiral were ignited to full redness, or whether it were merely 
warmed by a momentary contact of the wires. Further experiments are in progress 
which may throw light on this point. 
49. The very high amount of rarefaction needed before this neutral point is reached, 
and the change of direction of movement on applying a hot body to one arm of the 
balance, caused by a difference of exhaustion of a few millimetres on one side or the 
other of the point of neutrality, are, I think, a sufficient explanation of the anomalous 
results which were met with in my earlier experiments (2, 19, 20, 21, 22). 
50. Although these results were sufficient to show that air-currents could not be the 
cause of the movements of the balance, I was anxious to decide this point once for all, 
by a form of experiment which, whilst it would settle the question indisputably, would 
be likely at the same time to afford information of much interest. Having found that 
the balance last experimented with had its neutral point close to an ordinary Sprengel 
vacuum, that the repulsive action only came on by still further pushing the exhaustion, 
and that, as I further exhausted, the repulsion got stronger, it was of interest to see 
what would take place in a vacuum so nearly perfect that it would not carry a current 
from a Ruhmkorff’s coil. 
