194 THEORY OF RADIATION. 



attract and conduct it with a force and velocity 

 like lightning. 



Whatever may be the influence of colour on 

 the radiating power of bodies, it is trifling com- 

 pared with the relative proportions of sethereal 

 and ponderable matter of which they are com- 

 posed. 



It has been long known that metallic vessels, 

 such as tea-urns, retain their temperature much 

 longer than vessels of stone, earthenware, wood, 

 &c. which have less affinity for calorie than 

 metals ; and that the same metallic vessels retain 

 their temperature longer when highly polished 

 than when rough. The latter effect would seem 

 to be owing to the closer proximity of the par- 

 ticles composing a smoother surface ; for it has 

 been found, that when wood is reduced to saw- 

 dust, it radiates more rapidly than in the solid 

 state ; that locks of wool, cotton, flos-silk, &c. 

 radiate better than the same materials when 

 spun and woven, by which their fibres are brought 

 closer together. So when the particles of a 

 metallic surface are brought near to each other, 

 they exert a stronger affinity for caloric than 

 when more distant, and thus prevent it from 

 flying off, for the same reason that the cohesion 

 and conducting power of metals are augmented 

 by condensation as in wire drawing. Is it not 

 owing to the same cause, that electricity escapes 

 more rapidly from rough surfaces which consist 



