Action of Heat on Gravitating Masses. 67 



seemed as if the true action of the heat was one of attraction, in- 

 stantly overcome by ascending currents of air 



" 31. In order to apply the heat in a more regular manner, a 

 thermometer was inserted in a glass tube, having at its extremity 

 a glass bulb about 1^ inch in diameter ; it was filled with water and 

 then sealed up. . . The water was kept heated to 70° C, the tem- 

 perature of the laboratory being about 15° C. 



" 32. The barometer being at 767 millims. and the gauge at zero, 

 the hot bulb was placed beneath the pith ball at b. The ball rose 

 rapidly ; as soon as equilibrium was restored, I placed the hot- 

 water bulb above the pith ball at a, when it rose again, more slowly, 

 however, than when the heat was applied beneath it. 



" 33. The pump was set to work ; and when the gauge was 147 

 millims. below the barometer, the experiment was tried again; the 

 same result, only more feeble, was obtained. The exhaustion was 

 continued, stopping the pump from time to time, to observe the 

 effect of heat, when it was seen that the effect of the hot body 

 regularly diminished as the rarefaction increased, until when the 

 gauge was about 12 millims. below the barometer the action of 

 the hot body was scarcely noticeable. At 10 millims. below it was 

 still less ; whilst when there was only a difference of 7 millims. be- 

 tween the barometer and the gauge, neither the hot-water bulb, 

 the hot rod, nor the spirit-flame caused the ball to move in an ap- 

 preciable degree. The inference was almost irresistible that the 

 rising of the pith was only due to currents of air, and that at this 

 near approach to a vacuum the residual air was too highly rarefied 

 to have power in its rising to overcome the inertia of the straw 

 beam and the pith balls. A more delicate instrument would doubt- 

 less 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 re- 

 moved 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 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 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 bv electricity) was inside the 



F2 



