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heavy mass of metal, when brought near a delicately suspended light ball, attracts 
or repels it as follows: when the ball is in air of the ordinary density, if the mass 
is colder than the ball, it repels the ball; if it is hotter than the ball, it attracts 
the ball. But when the ball is in a vacuum, if the mass is colder than the ball it 
attracts, if it is hotter it repels the ball. And Mr. Crookes’ experiments showed 
him that whilst the action is in one direction in ordinary dense air, and in the oppo- 
site direction in a vacuum, there is an intermediate pressure at which heat produces 
no effect ; and there is an intermediate pressure at which the critical point is passed, 
that the sign then changes, and instead of attraction there is then faint but unmis- 
takeable repulsion ; and as exhaustion increases repulsion continues to increase 
likewise. After discussing the explanations which may be given of these pheno- 
mena, and showing that they cannot be due to air currents, Mr. Crookes refers to 
the evidences we have in nature of this repulsive action of heat and attractive 
action of cold. He says that in that portion of the sun’s radiation which we call 
heat, we have the radial repulsive force, possessing successive propagation, which 
is required to explain the phenomena of comets, and the shape and changes of the 
nebule, and to compare small things with great—to argue from pieces of straw up 
to heavenly bodies—it is not improbable that the attraction now shown to exist 
between a cold and a warm body will equally prevail, when for the temperature of 
ice is substituted the inconceivable cold of space; for a pith ball is substituted a 
celestial sphere, and for an artificial vacuum a stellar void. In the radiant mole- 
cular energy of cosmical masses, Mr. Crookes thinks may at last be found that 
“agent acting constantly according to certain laws,” which Newton held to be the 
cause of gravitation. 
In Mr. Crookes’ second paper, read before the Royal Society last year, after 
describing some improved apparatus of great sensibility, he discusses the question 
of the action of a cold body, such as ice, on the suspended balance. As cold is 
simply the absence of heat and not a mode of motion at all, it is not very obvious 
at first sight how it can produce the opposite effect of heat. However, Mr. Crookes 
explains the matter very satisfactorily by Provost’s ‘‘ Law of Exchanges,” and 
shows that attraction by a cold body is really repulsion by radiation following on 
the opposite side. Provost showed that all bodies not absolutely cold are always 
radiating heat, even when no cold body is there to receive it. A cold body has no 
power of acting on hot bodies at a distance, and causing them to begin to emit 
radiations, nor has a hot body any power of stopping the radiations from another 
hot body near it. Radiation, in fact, is always taking place, even when all the 
neighbouring bodies are at an equal temperature. In the case before us, one side 
of the vacuum chamber, and the air near it, are chilled by the ice, which, there- 
fore, checks radiation on to the balance from that side, while radiation is still going 
on from the other, and, acting on the opposite side of the balance, causes it to 
rotate the reverse way. 
Mr. Crookes then goes on to describe numerous experiments made for the 
purpose of ascertaining whether the attraction by heat, which is greater as the 
enclosed air around the balance approaches the normal density, increases still 
