686 THE POPULAR SCIENCE MONTHLY. 



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 ap- 

 plied to it. 



I continued exhausting. On next applying heat underneath, the 

 result showed that I was far from having discovered the law govern- 

 ing 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 

 efiects 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 ha<l 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 attrac- 

 tion 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 

 expei-ience, 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," 18'74, vol clxiv., p. 601. 



