delicate instrument would doubtless show traces of movement 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 was applied to it.
I continued exhausting. On next applying heat underneath, 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 rarefactions. With the gauge three millimetres 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; while 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 repelling the ball to its fullest extent.
To verify these unexpected results, air was gradually let into the apparatus, and observations were taken as the gauge sank. The same effects were produced in inverse order, the point of neutrality being when the gauge was about seven millimetres below a vacuum.
A piece of ice produced exactly the opposite effect to a hot body.
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 vacuum-tube instead of outside it as before; and the pith-balls of the former apparatus were replaced by brass balls. By careful manipulation and turning the tube round, I could place the equipoised brass ball either over, under, or at the side of the source of heat. With this apparatus I tried many experiments, to ascertain more about the behavior 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 one experiment, which is described in detail in my paper on this subject before the Royal Society, the pump was worked until the gauge had risen to within five millimetres 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 downward a distance of two millimetres. The pump continuing to work, the gauge rose until it was within one millimetre 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, however, was much less decided than before; and, in spite of previous experience, the inference was very strong that the attraction would gradually diminish until the vacuum was absolute, and that then, and not till then, the neutral point would be reached. Within one milli-
- "Philosophical Transactions," 1874, vol clxiv., p. 501.