EADIATION OF HEAT. 



419 



: jelly formed an extremely thin film on the surface. Now, although the nature 

 '. of these two surfaces was precisely the same with respect to material and 

 > smoothness, they were found to radiate very differently ; the thinner film de- 

 i pressed the thermometer 38, while the thicker depressed it 54. The in- 

 creased radiation must in this case be attributed to the increased quantity of 

 radiating material. The increase of radiation was found to continue until the 

 coating amounted to the thickness of about 1000th part of an inch, after which 

 no further increase took place. It might, therefore, be inferred that, in the case 

 of the surface of jelly,such as that here submitted to experiment, the particles 

 radiate heat from a depth below the surface equal to the 1000th part of an inch. 

 A similar effect was found with other substances. In the case of metals, no 

 increase was observed when leaf metal of gold, silver, and copper, was used ; 

 but on using glass, enamelled with gold, a slight increase of radiating power 

 was produced, as compared with the ordinary radiating power. 



In these experiments the heat radiated undergoes three distinct physical ef- 

 fects : 1. The radiation from the surface of the canister; 2. The reflection 

 from the surface of the reflector ; 3. Absorption by the glass surface of the 

 focal ball, for without such absorption the air included could not be affected. 

 Now, of these three effects, we have hitherto examined but one, viz., the radi- 

 ating power. Let us consider what circumstances affect the power of reflect- 

 ing heat, and the power of absorbing it. 



The reflector used in the experiments already described was formed of pol- 

 ished tin. If, instead of this, a reflector of glass be used, it will be found that 

 the thermometer will be affected in a much less degree, whence we infer that 

 glass is a worse reflector than metal. If the surface of the reflector be coated 

 with lampblack, all reflection whatever is destroyed, and no effect is produced 

 on the thermometer. Thus it appears that, as different surfaces have different 

 radiating powers, so also they have different reflecting powers ; but to deter- 

 mine the reflecting power of different surfaces with great exactness, Leslie re- 

 ceived the rays proceeding from the reflector M, fig. 8, on a flat reflecting sur- 



Fig. 8. 



face, S, before they came to a focus ; and by the laws of reflection they were 

 reflected to another focus,/, as far before the reflecting surface S as the focus 

 /, to which they would have proceeded is behind it. The reflecting power of 

 the surface S will, therefore, be determined by the intensity of the heat in the 

 focus/', compared with the intensity which it would have had in the focus/, 

 had the rays been allowed to converge to that point. By experiments con- 

 ducted in this way, and exposing the surfaces of different substances to receive 



