Page:Popular Science Monthly Volume 7.djvu/705

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685
THE MECHANICAL ACTION OF LIGHT.

considerably above its original position. It seemed as if the 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 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 sufficient 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 without moving the eye from the micrometer. The water was kept heated to 70° C, the temperature of the laboratory being about 15° C.

PSM V07 D705 Barometer.jpg
Fig. 2.

The 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 a, 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 difference of seven millimetres 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