1873.] Action of Heat on G) avitating Masses. 39 



phenomena; the pith ball rose steadily, and without that hesitation 

 which had been observed at lower rarefactions. 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 move- 

 ments 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 vacuum-tube 

 instead of outside it as before ; and the pith balls of the former apparatus 

 were replaced by brass balls. By careful 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 

 with respect to the incandescent spiral, and at different pressures^ the 

 general result is 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 rare- 

 fied air, surrounded the balance. The author continues : 



" 44. The pump was then worked until the gauge had risen to within 

 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 milliin. 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 move- 

 ment, however, was much less decided than before; and in spite of pre- 

 vious experience (33, 34) the inference was very strong that the attrac- 

 tion would gradually diminish until the vacuum was absolute, and that 

 then, and not till then, the neutral point would be reached. Within one 

 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 rarefac- 

 tion is close upon a vacuum commenced, and the falling mercury only 

 occasionally took down a bubble of air. On turning on the battery cur- 



