REPULSION RESULTING FROM RADIATION. 
505 
test of the interval between the disks. Professor Powell found that the tints invari- 
ably descended in the scale when heat was applied, showing that the interval between 
the disks increased, and proving the existence of a repulsive power exerted between 
heated surfaces at small though sensible distances — the warping , or change of figure, 
if any, in the glasses by heat being readily seen to be such as ought to cause the rings 
to enlarge at the first instant. From experiments made by the contact of a lens with 
different substances, Professor Powell inferred that whatever tends to increase the 
rapidity of communication of heat, tends to increase the observed effect. The effect is 
increased when water, instead of air, is introduced between two lenses. 
12. In 1838 Professor Powell* gave some additional notes on the same subject, but 
no new form of experiment was tried. 
13. Dr. Joule, F.R.S.f, gave an account in 1863 of a new and extremely sensitive 
thermometer. It was based upon the disturbing effect of currents of air upon finely 
suspended magnetic needles. By diminishing the directive force of the needle, the 
instrument was made sufficiently delicate to move to the heat radiated from a small pan 
containing a pint of water heated to 30°, placed 3 yards off, and also to give evidence 
of the heat of the moon. I have little doubt that these movements were not so much 
due to currents of air as to the mechanical effects of radiation described in this 
paper. 
14. It is right that I should mention here that in September 1871 I received a letter 
from Mr. J. Reynolds, mentioning that he had constructed a little instrument which 
would turn to the hand, to a fire, or to any source of heat. It consisted of a thin slip 
of deal suspended by a filament of spider’s web, and enclosed in a thin glass flask. 
This little instrument was more sensitive than any I had then constructed, as the 
spider’s web was much freer than cocoon-silk from torsion, and Mr. Reynolds kindly 
allowed me to experiment with it. 
15. I cannot do better, in bringing this historical summary to a conclusion, than 
draw attention to a passage written in 1868 by Professor Guthrie, F.R.S. J, in which 
he distinctly points out a probable relation between heat and gravity. He says : — “ If 
the setherial vibrations which are supposed to constitute radiant heat resemble the 
aerial vibrations which constitute radiant sound, the heat which all bodies possess, and 
which they are all supposed to radiate in exchange, will cause all bodies to be urged 
towards one another.” 
16. Were it such a relation between heat and gravity of which I had been getting 
glimpses, it was evident that a much more delicate apparatus would be necessary to 
render it distinct, and I accordingly commenced a series of experiments with the view 
of ascertaining what form of apparatus would be most sensitive to the action sought. 
The first requisite was to get rid of the error arising from the expansion of the 
beam by heat ; and since, in working with hot bodies, the metallic masses used as 
* Phil. Mag. vol. xii. April 1838. f Chemical News, vol. vii. p. 150. 
± Proceedings of the Royal Society, vol. six. p. 35. 
