REPULSION RESULTING FROM RADIATION. 
511 
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 repelling the ball to its fullest extent. 
To verify these unexpected results, air was gradually let into the apparatus, and 
observations were taken as the gauge sank. The same effects were produced in inverse 
order, the point of neutrality being when the gauge was about 7 millims. below a 
vacuum. 
32. When the balance was in air of ordinary density, and the hot body was placed 
above the pith ball in the position a (see fig. 2), it will be remembered that the action 
was to cause the ball to rise ; the rising was, however, less decided than when the heat 
was applied below (27, 29). On re-exhausting the balance-tube and taking a series of 
observations, placing the hot bulb above the pith ball in the position a (fig. 2) instead 
of below it, the ascending tendency of the pith got less and less. Several millimetres 
below the previously ascertained point of neutrality the hot bulb at a ceased to exert 
an action, and when the neutral point was exceeded by some millimetres I could still 
detect no movement. However, at 2 millims. below a vacuum, I detected a tendency of 
the pith to sink when the experiment was tried ; and in a good Sprengel vacuum there 
was an unmistakable repulsion exerted between the two bodies, the pith sinking- 
in obedience to the radiation from above almost as strongly as it rose when the heat 
was applied beneath. 
33. I now wished to ascertain the effect of cold on the balanced pith balls, and for 
this purpose a lump of ice was employed. 
The experiments were first tried with the balance-tube full of air, cold being applied 
either above or below the pith ball ; a lump of ice generally produced an upward move- 
ment of the pith, but it was very faint, and sometimes the motion appeared to be in the 
opposite direction. It was evident that the true action of a cold body, whatever it might 
be, was here masked by currents of air ; and I therefore exhausted the apparatus and 
tried the effect of ice at the previously ascertained neutral point, viz. at about 7 millims. 
below the vacuum. It was absolutely inert. I then carried the exhaustion to the fullest 
extent, testing the balance with ice during its progress. As the gauge approached 
nearer and nearer to the height of the barometer, the ice commenced to attract the 
pith ; and at last, when the gauge and barometer were level, the attraction of the ice, 
whether applied above or below, was very marked, being exactly opposite but equal to 
the action of the bulb of hot water. 
34. In trying some of these experiments in a Sprengel vacuum an action was noticed 
which led me to think that some of these movements might be due to electricity. When 
a hot glass rod is held motionless against the lower side of the exhausted tube, the repul- 
