340 The Mechamcal Action of Light. (July, 
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 in- 
strument 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 sus- 
pended in empty space only—the pith ball would remain 
motionless, wherever the hot body were applied to it. 
Icontinuedexhausting. On nextapplying heat underneath, 
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 baro- 
meter, 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 up- 
ward 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 effecéts were produced in inverse order, the 
point of neutrality being when the gauge was about 7 millims. 
below a vacuum. 
A piece of ice produced exactly the opposite effect to a 
hot body. 
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 by 
electricity) was inside the vacuum-tube instead of outside it 
as before; and the pith balls of the former apparatus were 
replaced by brass balls. By careful manipulation and turning 
the tube round, I could place the equipoised brass ball either 
