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hin, in his own words to the Royal Society, ‘‘to detect, and then to render very 
sensible, an action exhibited by heat on gravitating bodies, which is not due to air 
currents, or to any other known form of force.” 
One of Mr. Crookes’ first forms of apparatus consisted of a balance formed 
of a straw beam with a pith ball at each end of it, the whole being enclosed in a 
glass tube filled with air at the ordinary density. On passing the flame of a spirit 
lamp across the tube, under one of the pith balls, the ball descended slightly, and 
then immediately rose to considerably above its original position. The inference 
from this was that the true action of heat is one of attraction, instantly overcome 
by ascending currents of air. He then went on to ascertain whether the density of 
the enclosed air had any effect one way or the other upon the phenomenon. The 
apparatus being connected by fusion with the Sprengel air pump (the barometer 
being at 767 millemetres and the guage at zero), the pump was set to work 3; and 
when the guage was at 147 millms. below the barometer, the experiment was tried 
again, with a small globe of hot water at a definite temperature instead of the 
spirit lamp. The same result, only more feeble, was obtained. The exhaustion 
was continued until, when the guage was at 12 millms. below the barometer, the 
action of the hot body was scarcely noticeable. When there was a difference of 
only 7 millms. between the guage and the barometer, neither the hot water globe, 
nor the spirit flame caused the pith ball to move in any appreciable degree. Mr, 
Crookes naturally inferred from this that the rising of the pith ball was really due 
to currents of air, and that at this near approach to a vacuum the remaining air 
was too highly rarefied to be able to overcome the inertia of the straw beam and 
the pith ball ; that there would still be some traces of a movement at a still nearer 
approach to a vacuum ; but it seemed evident that when the last trace of air was 
removed from the tube round the balance, and the balance suspended in empty 
space only, the pith balls would remain absolutely motionless when the spirit 
flame was brought near them. However, he continued exhausting, and when he 
next applied heat he found he was far from having discovered the law of the pheno- 
mena,—the pith ball rose steadily instead of falling first, and much more readily 
than before. When the guage was at 3 millms. below the barometer, the ascension 
of the pith ball, when a hot body was placed beneath it, was equal to what it had 
been in air of the ordinary density ; and, when the guage and the barometer were 
level, the upward movement was not only sharper than it had been in air, but it 
required much less heat to produce the same effect ; even the warm finger applied 
to the globe instantly sending up the ball to its fullest extent. A piece of ice was 
found to produce exactly the opposite effect of a hot body. 
Mr. Crookes’ next step was to ascertain whether electricity in some form 
had any share in producing these phenomena, and after an exhaustive series of ex- 
periments he was able to demonstrate that it had none whatever. It would weary 
you if I were to mention half the experiments which Mr. Crookes tried in the 
course of the next few months ; it will be sufficient to say that having experimen- 
ted for some months with an apparatus similar to that which Cavendish used in his 
celebrated investigations on gravitation, he obtained the following results, A 
