ON THE NATURE OF HEAT. 2 IT 



2. Motion of caloric* 



It is observed, that the best conductors of heat receive Motion of 

 and deliver it most easily and rapidly; those bodies, wbich. " 

 absorb heat with most avidity, are also such as radiate most 

 copiousl3\ The first part of these operations nniight be 

 ascribed to the attraction exerted between the particles of 

 the body and o! caloric ; but the second pheuonnena are 

 directly adverse to such an explanation. Considering calo- 

 ric as matter, and subject to attraction like other matter, 

 the circumstances above related appear to require explana- 

 tion. 



In the radiation of heat many phenomena occur, which Radiation of 

 have not been satisfactorily explained: and first, the re- ^^^^^ 

 markable difference between solar and culinary heat does Difference be- 

 not appear to be by any means understood*. The first is J^^JJ ^!&fir^ 

 transmissible through and refrangible by glass, and other 

 transparent media; the second is in a large proportion inter* 

 cepted by every solid body : the first is reflected in circum- 

 stances, in which the second is absorbed t : culinary heat, 

 unlike the solar ray^ suffers a considerable aberration in its 

 reflection : and lastly, the absorption of culinary heat is not 

 affected by the colour of the absorbing surface, in the same 

 manner as that of solar heat J. What can be the cause — 

 what the rationale of these differences ? 



It is however in the radiation of cold, I conceive, that we Radiation of 

 have the most forcible and direct objection to the hypothesis ^°"^* 

 of material caloric, and the most certain indication of the 

 real nature of this principle. It is scarcely necessary to 

 say, that no unexceptionable explanation of this phenome- 

 non has been proposed. According to Prevost's supposition, 

 the effect of radiation from a cold surface ought i« reality 

 to be that of heating, and not of cooling the opposed ther- 

 mometer ; this will be rendered evident by the assistance of 

 a diagram. 



PI. VI, fig. 4, ah, cd, are two concave mirrors. TacI^ 

 Tb dl are rays of heat issuing from the thermometer, 

 X c a T, IdbTi i^re rays also of heat, issuing fronj the iee ; 



• Nicholson's Journal, vQl.yiii, p, 297. 



t LesUe^s Inq. p. 83, et seq. % lljid, p, 97. 



for. 



