[ 501] 
XV. On Attraction and Repulsion resulting from Radiation *. 
Ry William Crookes, F.R.S. &c . 
Received August 12, — Read December 11, 1873. 
1. In a paper “ On the Atomic Weight of Thallium,” presented to the Royal Society 
June 18, 1872, after describing a balance with which I was enabled to perform weigh- 
ings of apparatus &c. in a vacuum, I noted a peculiarity m relation to the effect of heat 
in diminishing the apparent weight of bodies. I said, “ That a hot body should appear 
to be lighter than a cold one has been considered as arising from the film of air or 
aqueous vapour condensed upon or adhering to the surface of the colder body, or from 
the upward currents of air caused by the expansion of the atmosphere in the vicinity 
of the heated body. But neither hypothesis can be held when the variation of the 
force of gravitation occurs in a vacuum as perfect as the mercurial gauge will register, 
and under other conditions which I am now supplying, and which I purpose embodying 
in a paper to be submitted to the Royal Society during a subsequent session ” f. 
With the vacuum-balance mentioned above I carried out many experiments, but was 
unable to obtain results which were at all concordant; and it was soon found necessary 
to investigate the phenomena with smaller and less complicated apparatus. 
2. Most chemical manuals warn beginners against the errors occasioned by weighing 
substances while hot ; and, up to a moderately high degree of exhaustion, I was prepared 
to find a piece of glass apparatus, when hot, apparently lighter than the weights which 
should balance it were the whole system at the same temperature. But instead of the 
interfering causes diminishing as the rarefaction proceeded, they seemed rather to 
increase, or at all events to become irregular in their action, sometimes appearing to 
oppose, and at others to supplement the force of gravity. In such a vacuum as a good 
air-pump would produce, the actions of the ascending current of air and of the adhering 
film, it might be presumed, should cease to exert an influence ; and I could think of no 
other disturbing cause except the lengthening of the beam, owing to the heat radiated 
from the apparatus below it. An increase in the length of the beam should make a 
mass suspended at its extremity appear heavier; but wdiilst I frequently noticed an 
action which might be due to this cause, I occasionally obtained results which were so 
anomalous as to convince me that some cause which I had not hitherto recognized was 
at work (49), and to lead me to hope that perhaps I might succeed in tracing a con- 
nexion between heat and the force of gravity. 
* This paper was read before the Royal Society, and the abstract was published in the £ Proceedings ’ 
(vol. xxii. p. 37) under the title “ On the Action of Heat on Gravitating Masses.” 
t Philosophical Transactions, 1873, vol. clxiii. p. 287. 
MDCCCLXXIV. 3 X 
