34 THEORY OF HEAT. [CHAP. I. 



If by modifying the state of the surface we increase the force 

 by which it reflects the incident rays, we increase at the same time 

 the power which it has of reflecting towards the interior of the 

 body rays which are tending to go out. The incident rays intro 

 duced into the mass, and the rays emitted through the surface, are 

 equally diminished in quantity. 



44. If within the enclosure above mentioned a number of 

 bodies were placed at the same time, separate from each other 

 and unequally heated, they would receive and transmit rays of heat 

 so that at each exchange their temperatures would continually 

 vary, and would all tend to become equal to the fixed temperature 

 of the enclosure. 



This effect is precisely the same as that which occurs when 

 heat is propagated within solid bodies ; for the molecules which 

 compose these bodies are separated by spaces void of air, and 

 have the property of receiving, accumulating and emitting heat. 

 Each of them sends out rays on all sides, and at the same time 

 receives other rays from the molecules which surround it. 



* 45. The heat given out by a point situated in the interior of 

 a solid mass can pass directly to an extremely small distance only; 

 it is, we may say, intercepted by the nearest particles ; these parti 

 cles only receive the heat directly and act on more distant points. 

 It is different with gaseous fluids ; the direct effects of radiation 

 become sensible in them at very considerable distances. 



46. Thus the heat which escapes in all directions from a part 

 of the surface of a solid, passes on in air to very distant points ; but 

 is emitted only by those molecules of the body which are extremely 

 near the surface. A point of a heated mass situated at a very 

 small distance from the plan^ superficies which separates the mass 

 from external space, sends to that space an infinity of rays, but 

 they do not all arrive there; they are diminished by all that quan 

 tity of heat which is arrested by the intermediate molecules of the 

 solid. The part of the ray actually dispersed into space becomes 

 less according as it traverses a longer path within the mass. Thus 

 the ray which escapes perpendicular to the surface has greater in 

 tensity than that which, departing from the same point, follows 



