1819.] Prof. Leslie on Heat and Climate. 12 



fluids in particular will admit of being heated to a very high de- 

 gree, which corresponds with experiment: Every substance 

 most probably contains the due mixture of the coloured particles 

 of light, but the attraction of the more refrangible seems to de- 

 viate sooner from the mutual repulsion. It is thus that, in the 

 progress of heating, bodies in general assume successively the 

 tints of red, orange, yellow, and white ; the emission consisting 

 at first of the most refrangible rays, and afterwards including 

 gradually a mixture of the others. 



If two portions of a body be heated unequally, the luminous 

 particles on the confines will flow from the redundant fluid by 

 the operation of two causes ; the excess of the mutual repulsion 

 on the one hand, and the excess of the attraction of the cold 

 matter, arising from the greater density on the other. But the 

 celerity with which the equilibrium is restored must depend on 

 a variety of peculiar circumstances ; such as the difference of 

 temperature, the extent of the body, and the figure, arrange- 

 ment, and interval of its integrant particles. In fluids the dif- 

 fusion of heat is in general quicker and more uniform ; because 

 the irregular density of the mass occasions an intestine motion. 

 If a part of a body be compressed, the contained particles of 

 light will approach each other, and likewise the rest of the 

 matter. But the attraction, as we have seen, increases at a 

 slower rate than the repulsion ; wherefore some of the light will 

 pass into the other part of the body, which is in the natural state. 

 This principle affords a satisfactory explanation of the heat 

 extricated by percussion and friction. Thus, when I strike 

 two stones against each other, a certain condensation is 

 produced in both at the points of collision, and heat flows 

 into the interior mass. But, recoiling like a spring, they 

 presently acquire dilatation equal to their former compression ; 

 the parts struck are now disposed to imbibe more heat than 

 at first, and this is supplied from all the contiguous matter, 

 especially from the air. By repeated percussion, the stones 

 in this manner receive continual accessions of heat, which, 

 though separately small, amount by their number to a very 

 considerable quantity. In the case of friction, the pressure 

 is successively applied to different points of the surface, so 

 that the sources of heat are multiplied. The same explana- 

 tion may be extended to all hard bodies. Even fluids may ac- 

 quire some heat if their surface be violently agitated ; and hence 

 the foundation of the ancient remark, that the sea is sensibly 

 warmer after the fever of a storm.* 



But there are many substances capable of receiving a per- 



• This explication of the heat occasioned by friction is subtle, and could not 

 easily be brought to the test of experiment. If the sea be generally warmer after a 

 storm, this must be owing to the heat then communicated to the superficial water 

 from the atmosphere, which has regained its temperature, after the depression, 

 during raiu.— A. 



