THE CONTACT OF BODIES HAVING DIFFERENT TEMPERATURES. 
3 
to the enunciation of certain “ 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 mutual repulsion 
of heated bodies in vacuo, 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 Pro- 
fessor 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 inquiry. 
I was not aware, until informed of it by my friend Professor 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 ob- 
tained many of the results which it was my intention to seek ; nevertheless the 
portion of the subject which still remained untouched 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 experi- 
menter to test the results to be communicated, I shall connect with each series of 
experiments a sufficiently exact description of the instruments made use of. 
The first general law enunciated by Professor Forbes is as follows : — 
“ The vibrations never take place between substances of the same nature." 
Let us see whether this law will bear the test of experiment. 
I. Iron Rocker. 
Fig. 1 represents a sketch of the rocker; the length AC is 5T inches; the width 
AB T85, and the length of the stem EF is 12 inches. Fig. 2 is a transverse section 
Fig. 1. 
■— J fA 
of the rocker, showing the groove underneath ; the depth SN is 0’5 of an inch. 
b 2 
