1835.] between Bodies by the Action of Heat . 251 



belong to a class of phenomena hardly coming within the 

 specific range of any one of the great divisions of Science ; 

 or rather, belonging equally to several, are but little con- 

 sidered in any. In the " Records of General Science," 

 however, some account of them may perhaps find a place. 



At the meeting of the British Association at Edinburgh, 

 I gave a short statement of the experiments which I had 

 made : an account of them was also read before the Royal 

 Society, and is now printed in the Philosophical Transactions 

 for 1834. But a brief outline of their nature and object 

 may not be unacceptable to some readers, especially as it 

 will be essential to render intelligible the further observa- 

 tions I wish to add. 



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 of a nature not easy to be pursued or 

 decided. 



The subject has been partially investigated by Sig. Libri, 

 and by M . M . Fresnel and Saigey ; but their researches do 

 not appear to have been regarded as decisive, and have 

 ever been viewed with considerable doubt ; and they are 

 eertainly dependant upon experiments of the most extremely 

 delicate and difficult kind, and those of Fresnel confessedly 

 left in an incomplete state. 



Recently, the inquiry has been revived by Professor 

 Forbes of Edinburgh, who has referred to the same 

 principle to account for the singular phenomena of certain 

 vibrations of heated metallic bars, first noticed by Mr. 

 Trevelyan, and since fully investigated by himself in a 

 paper in the Edinb. Trans, vol. xii. 



In a different form the subject had occupied my attention 

 before I was acquainted with Professor Forbes's investiga- 

 tions ; but, on reading his paper, a new interest attached to 

 the inquiry, and in pursuing it, I conceive I have obtained 

 some results which appear decisive on a question at once 

 of importance in the analogies of physical action, and which 

 has hitherto been regarded as at least involved inconsiderable 

 uncertainty. 



The method I pursued was that of forming Newton's rings 



