THE MECHANICAL ACTION OF LIGHT. 685 



considerably above its original position. It seemed as if the true 

 action of the heat was one of attraction, instantly overcome by ascend- 

 ing 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 produced a slight rising of the 

 ball. The same effects take place when the hot body is a])plied 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. 



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 

 one and a half inch diameter; it was filled with water and then 

 sealed up {see Fig. 2). This was arranged on a revolving stand, so 

 that by means of a cord I could bring it to the desired position with- 

 out moving the eye from the micrometer. The water was kept heated 

 to 70 C, the temperature of the laboratory being about 15 C. 



Fig. 2. 



Tlie barometer being at 767 millimetres and the gauge at zero, the 

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

 The source of heat was then removed, and as soon as equilibrium was 

 restored I placed the hot-water bulb above the pith-ball at , when it 

 rose again more slowly, however, than when the heat was applied 

 beneath it. 



The pump was then set to work; and when the gauge was 147 

 millimetres 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 twelve millimetres below the barometer, the action of the hot 

 body was scarcely noticeable. At ten millimetres below it was still 

 less ; while when there was only a difterence of seven millimetres be- 

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

 hot rod, nor the si^irit-flame, caused the ball to move in an appreciable 

 degree. 



The inference was almost ii-resistible 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 



