1874.] | Physics. 275 
at a still nearer approach to a vacuum; but it seemed evident that when the 
last trace of air had been removed from the tube surrounding the balance 
(when the balance was suspended in empty space only), the pith-ball would 
remain motionless wherever the hot body were applied to it. 34. I continued 
exhausting. On next applying heat, the result showed that I was far from 
having discovered the law governing these phenomena; the pith-ball rose 
steadily, and without that hesitation which had been observed at lower rare- 
factions. With the gauge 3 millims. below the barometer, the ascension of the 
pith when a hot body was placed beneath it was equal to what it had been in 
air of ordinary density ; whilst with the gauge and barometer level its upward 
movements were not only sharper than they had been in air, but they took 
place under the influence of far less heat; the finger, for example, instantly 
sending the ball up to its fullest extent.” A piece of ice produced exactly the 
opposite effect to a hot body. Numerous experiments are next given to prove 
that the action is not due to electricity. The presence of air having so 
marked an influence on the action of heat, an apparatus was fitted up in 
which the source of heat (a platinum spiral rendered incandescent by 
electricity) was inside the balance-tube instead of outside it as before; and 
the pith-balls of the former apparatus were replaced by brass balls. By care- 
ful management, and turning the tube round, the author could place the 
equipoised brass pole either over, under, or at the side of the source of heat. 
With this apparatus it was intended to ascertain 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. After describing many experiments with the ball in various 
positions in respect to the incandescent spiral, and at different pressures, the 
general result appeared to be expressed by the statement that the tendency 
in each case was to bring the centre of gravity of the brass ball as near as 
possible to the source of heat, when air of ordinary density, or even highly 
rarefied, surrounded the balance. The author continues :—‘‘ 44. The pump 
was then worked until the gauge had risen to 5 millims. of the barometric 
height. On arranging the ball above the spiral (and making contact with the 
battery), the attraction was still strong, drawing the ball downwards a 
distance of 2 millims. The pump continuing to work, the gauge rose until it 
was within 1 millim. of the barometer. The attraction of the hot spiral for 
the ball was still evident, drawing it down when placed below it, and up when 
placed above it. The movement was, however, much less decided than 
before; and in spite of previous experience (33, 34) the inference was very 
strong that the attraGtion would gradually diminish until the vacuum was 
absolute, and that then, and not till then, the neutral point would be 
reached. Within x millimetre of a vacuum there appeared to be no room for 
a.change of sign. 45. The gauge rose until there was only half a millimetre 
between it and the barometer. The metallic hammering heard when the 
rarefaction is close upon a vacuum commenced, and the failing mercury only 
occasionally took down a bubble of air. On turning on the battery current, 
there was the faintest possible movement of the brass ball (towards the spiral) 
in the direction of attraction. 46. The working of the pump was continued. 
On next making conta& with the battery no movement could be detected. 
The red-hot spiral neither attra@ed nor repelled; I had arrived at the critical 
point. On looking at the gauge I saw it was level with the barometer. 
47. The pump was now kept at full work for an hour. The gauge did not 
Tise perceptibly, but the metallic hammer increased in sharpness, and I could 
see that a bubble or two of air had been carried down. On igniting the spiral, 
I saw that the critical point had been passed. The sign had changed, and the 
action was faint but unmistakable repulsion. The pump was still kept. going, 
and an observation was taken from time to time during several hours. The 
repulsion continued to increase. The tubes of the pump were now washed 
out with oil of vitriol,* and the working was continued for an hour. 48. The 
action of the incandescent spiral was now found to be energetically repellent, 
* This can be effeted without interfering with the exhaustion, 
