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XXII. On the repulsive Power of Heat. By the Rev. Baden Powell, M.A. F.R.S. 

 Savilian Professor of Geometry in the Utilversity of Oxford. 



Received April 10, — Read June 19, 1834. 



1 HE expansion of bodies by heat seems to imply a mutual repulsion of their par- 

 ticles ; and it is a question naturally suggested, whether such a power of repulsion 

 may not generally belong to heat, or be excited by it, between particles or masses of 

 matter, at sensible as well as insensible distances. 



But however obvious the suggestion of such an inquiry, it is not of a nature easy 

 to be pursued or decided. The subject has been partially investigated by Signor 

 LiBRi and by MM. Fresnel and Saigey ; but their researches do not seem to have 

 attracted much attention, and their results have even been regarded with consider- 

 able doubt. Very recently, however. Professor Forbes, of Edinburgh, has revived 

 the inquiry, by referring to the same principle to account for the singular phenomena 

 presented in certain vibrations of heated metallic bars, first noticed by Mr. Tre- 

 VELYAN, and since fully investigated by himself*. In a different form the subject had 

 occupied my attention before I was acquainted with Professor Forbes's investigations ; 

 but on reading his paper, a new interest attached to the inquiry, and in pursuing it, 

 I have obtained some results which appear to me decisive on a question, at once of 

 importance in the analogies of physical action, and which has been hitherto regarded 

 as involved in considerable uncertainty. 



Signor Libri, I believe in 1824, examined the influence of heat on capillary attrac- 

 tion, and found that a drop of water suspended on a wire, when the wire was heated 

 at one part, moved away from that part, both when the wire was horizontal and even 

 when inclined upwards from the heated part. This he inferred was due to repulsion 

 produced by the heat between the wire and the particles of the water. 



M. Fresnel-j- employed discs of foil and of mica fixed vertically at the extremities 

 of a delicately suspended magnetic needle in vacuo, placed so little out of the meri- 

 dian that it just produced a pressure of the disc against another fixed disc. On heating 

 either of the two with the sun's rays, concentrated by a lens, a sensible repulsion 

 was produced. He showed that the effect was not occasioned by any current of the 

 little air remaining, as it was not increased on the admission of more air ; that it 

 bore no relation to magnetic or electric conditions ; and did not increase, but gene- 



* Transactions of the Royal Society of Edinburgh, vol. xii. 

 + Annalea de Chimie, vol, xxix. pp. 57, 107. (1825). 

 MDCCCXXXIV. 3 R 



