the Contact of Bodies having different Temperatures. 3 



a common temperature, and thus to stop the vibrations, instead 

 of exalting them, as supposed by Professor Faraday. Again, if 

 the phsenomenon be due to expansion, the greater the expan- 

 sion the greater ought to be the effect ; but the expansion de- 

 pends upon the quantity of heat transmitted from the hot rocker 

 to the cold block during their contact, and this again upon the 

 conductivity of the block ; so that instead of being a bad con- 

 ductor, the block, to produce the greatest effect, ought to be the 

 best conductor possible. The idea of an accumulation of heat at 

 the surface being more favourable to the action than a rapid 

 communication with the interior. Professor Forbes regards as an 

 " obvious oversight*/' 



Having thus, to all appearance, overturned the views previously 

 entertained, Professor Forbes proceeds to found a theory of his 

 own. His experiments have led him to the enunciation of cer- 

 tain "general laws," and these converge upon the still more 

 general conclusion, — " that there is a repulsive action exercised in 

 the transmission of heat from one body into another which has 

 a less power of conducting it." This repulsion Professor Forbes 

 considers to be "a new species of mechanical agency in heat;" 

 and he cites the remarkable experiments of Fresnel, on the mu- 

 tual repulsion of heated bodies in vacuOy as bearing directly upon 

 the subject. 



Such, apparently, was the unsettled state of the question when 

 my attention was drawn towards it last summer. The possibility 

 of the explanation offered by Professor Forbes, affording, as it 

 seemed to do, a chance of becoming more nearly acquainted with 

 the intimate nature of heat itself, was a strong stimulus to in- 

 quiry. I was not aware, until informed of it by my friend Pro- 

 fessor Magnus, that Seebeck had further examined the question, 

 and substantiated the conclusions arrived at by Faraday. On 

 reading Seebeck's interesting paper, I found that he had already 

 obtained many of the results which it was my intention to seek ; 

 nevertheless the portion of the subject which still remained un- 

 touched presented sufficient interest to induce me to prosecute 

 my original idea. 



I purpose in the present memoir to examine the experimental 

 basis of those laws which Professor Forbes regards as establishing 

 the existence of "a new mechanical agency in heat;" and as I 

 am anxious to place it within the power of every experimenter to 

 test the results to be communicated, I shall connect with each 

 series of experiments a sufficiently exact description of the instru- 

 ments made use of. 



The first general law enunciated by Professor Forbes is as 

 follows : — 



* Philosophical Magazine, Series 3, vol. iv. pp. 15 and 182. 

 B2 



