The expansion of bodies by heat seems to imply a mutual repulsion of their particles; 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 considerable 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. Trevelyan, 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.