510 
MR. W. CROOKES ON ATTRACTION AND 
true action of the heat was one of attraction, instantly overcome by ascending currents 
of air. A hot metal or glass rod and a tube of hot water applied beneath the pith 
ball at b produced the same effect as the flame; when applied above at a , they pro- 
duced a slight rising of the ball. The same effects take place when the hot body is 
applied to the other end of the balanced beam. In these cases air-currents are suffi- 
cient to explain the rising of the ball under the influence of heat. 
28. In order to apply the heat in a more regular manner, a thermometer was in- 
serted in a glass tube, having at its extremity a glass bulb about 1^ inch diameter ; 
it was filled with water and then sealed up (see fig. 3). This was arranged on a 
revolving stand, so that by means of a cord I could bring it to the desired position 
Eig. 3. 
without moving the eye from the micrometer. The water was kept heated to 70° C., 
the temperature of the laboratory being about 15° C. 
29. 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. 
30. The pump was again set to work ; and when the gauge was 147 millims. below the 
barometer, the experiment was tried again ; a similar 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. between the 
barometer and the gauge, neither the hot-water bulb, the hot rod, nor the spirit-flame 
caused the ball to move in an appreciable 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 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 were applied to it. 
31. I continued exhausting. On next applying heat, the result showed that I was 
