30 THEORY OF HEAT. [CHAP. I. 



metallic body will be more quickly cooled if its external surface is 

 covered with a black coating such as will entirely tarnish its 

 metallic lustre. 



33. The rays of heat which escape from the surface of a body 

 pass freely through spaces void of air; they are propagated also 

 in atmospheric air: their directions are not disturbed by agitations 

 in the intervening air: they can be reflected by metal mirrors 

 and collected at their foci. Bodies at a high temperature, when 

 plunged into a liquid, heat directly only those parts of the mass 

 with which their surface is in contact. The molecules whose dis 

 tance from this surface is not extremely small, receive no direct 

 heat; it is not the same with aeriform fluids; in these the rays of 

 heat are borne with extreme rapidity to considerable distances, 

 whether it be that part of these rays traverses freely the layers of 

 air, or whether these layers transmit the rays suddenly without 

 altering their direction. 



34. When the heated body is placed in air which is main 

 tained at a sensibly constant temperature, the heat communicated 

 to the air makes the layer of the fluid nearest to the surface of the 

 body lighter; this layer rises more quickly the more intensely it is 

 heated, and is replaced by another mass of cool air. A current 

 is thus established in the air whose direction is vertical, and 

 whose velocity is greater as the temperature of the body is higher. 

 For this reason if the body cooled itself gradually the velocity of 

 the current would diminish with the temperature, and the law 

 of cooling would not be exactly the same as if the body were 

 exposed to a current of air at a constant velocity. 



35. When bodies are sufficiently heated to diffuse a vivid light, 

 part of their radiant heat mixed with that light can traverse trans 

 parent solids or liquids, and is subject to the force which produces 

 refraction. The quantity of heat which possesses this faculty 

 becomes less as the bodies are less inflamed ; it is, we may say, 

 insensiblefor very opaque bodies however highly theymaybe heated. 

 A thin transparent plate intercepts almost all the direct heat 

 which proceeds from an ardent mass of metal ; but it becomes 

 heated in proportion as the intercepted rays are accumulated in 



