RADIATION OP HEAT. 



421 



perfectly transparent, while others, such as paper, horn, &c., are imperfectly 

 so. Analogy leads us to inquire whether bodies are also pervious to heat. 



In the preceding experiments, rays of heat passed through the atmosphere, 

 which is therefore, transparent to heat. It appears from the experiments of 

 Leslie and others, which have been since instituted, that all gases are pervi- 

 ous to the rays of heat, and equally so ; for the radiation of a given surface is 

 the same in whatever gas it takes place. 



Gases, therefore, as they have perfect or nearly perfect transparencies for 

 the rays of light, have the same quality in reference to the rays of heat. A hot 

 body placed behind a solid or a liquid is found, however, not to radiate sensibly 

 through them. But the most direct method of determining the transparency of 

 bodies for the rays of heat, is to interpose a screen between the radiating body 

 and the reflector, in the experiment already described, and to observe the 

 effect produced on the thermometer by this circumstance. Leslie's investiga- 

 tion respecting the property of transparency to heat of different bodies, form a 

 very remarkable part of that philosopher's discoveries. 



Different substances are pervious by heat in different degrees. A screen 

 of thin deal board, placed between the canister, c, and the focal ball,/, figure 

 7, produced a diminution in the effect on the thermometer, but did not destroy 

 that effect altogether. The heat transmitted through the board varied with its 

 thickness, slowly diminishing as its thickness increased. The radiation of 

 the surface of the lampblack, which, while unobstructed, produced an effect of 

 100 on the thermometer, produced an effect of 20 when a deal board the 

 eighth of an inch thick was interposed. It produced an effect of 15 when 

 the thickness was three eighths of an inch, and an effect of 9 when the board 

 was an inch thick. A plane of glass interposed reduced the effect of the radi- 

 ation by the surface of lampblack from 100 to 20. 



The distance of the screen from the canister was also found to produce a 

 considerable effect on its transparency. When placed near the canister, a con- 

 siderable quantity of heat was transmitted ; but if the distance was increased, 

 the quantity of heat transmitted diminished. A pane of glass at the distance 

 of two inches reduced the effect of radiation from 100 to 20. As its dis- 

 tance from the radiating surface was slowly increased, the effect on the ther- 

 mometer was gradually diminished ; and at the distance of one foot from the 

 radiating surface all effect of radiation was destroyed. 



It appeared that the metals, even when reduced to an extreme degree of 

 tenuity, were absolutely opaque to heat. A screen of tinfoil absolutely inter- 

 cepted all radiation. The thinnest gold leaves, 300,000 of which, piled one 

 upon another would not measure an inch, also absolutely stopped the rays of 

 heat. White paper is partially opaque. 



It appears, generally, that the bodies which intercept heat most effectually 

 are those which radiate heat worst, and vice versa. This, indeed, might easily 

 have been anticipated from what has been already proved of reflection. The 

 screens which are the best reflectors are the worst radiators, and must evi- 

 dently be also most powerful in intercepting heat ; for if they reflect much they 

 can transmit but little. Some other effects, which Leslie observed in his ex- 

 periments with screens, may also be accounted for by the same circumstance. 

 He took two panes of glass and coated one side of each with tinfoil. He then 

 placed their uncovered sides in close contact, so as to form one double pane, 

 both surfaces of which were covered with tinfoil. When this was interposed 

 as a screen before the radiating surface, all effect on the thermometer was de- 

 stroyed, and all the radiant heat intercepted. This is easily accounted for by 

 the perfect power of reflection which the coating of tinfoil possesses. The heat 

 incident on the surface of tinfoil is nearly all reflected ; and, consequently, no 



