GENERAL ACCOUNT OF RADIATION. 31 



Reflection of radiation from a Locatelli lamp 



Silver . . 97 per cent, of incident light 



Gold 95 



Brass 93 



Speculum metal 86 



Tin . 85 



Steel 83 



Diffusion. When radiation falls on a surface it may be in part 

 diffused or irregularly reflected. The diffusion depends on the polish of 

 the surface. 



A method of studying diffusion will be understood by the aid of 

 Fig. 136, which represents the apparatus used by Melloni to determine 

 the variation of diffusion of radiation received from different sources. 

 The incident beam falls on a thin plate of copper a b, coated with lamp- 

 black. Two thermopiles c, d are arranged symmetrically in front and 



Incident Beam 



VlQ. 136. Diffusion. 



behind the plate. The front one c registers emission plus diffusion, 

 while d registers emission alone. The heat received by the former was 

 about 1 '18 of that received by the latter. On coating the front of a & with 

 white lead the difference was very much greater, the diffusion by the 

 white lead being much larger. 



Knoblauch found that lampblack diffused about 10 per cent, of the 

 incident radiation whatever the source from which it was received. 



General Results. Even though no reliance is to be placed on the 

 exactness of the numbers obtained for the different qualities of absorp- 

 tion, emission, and reflection, a comparison of their magnitudes at once 

 leads to an important conclusion. For instance, lampblack has no regular 

 reflection, low diffusive power, and the highest emissive and absorptive 

 power. Indian ink has emissive power about 88 per cent, of that of 

 lampblack, and absorptive power 85 per cent. Metals reflect over 75 

 percent, of the incident radiation. Their emissive and absorptive powers 

 are less than 20 per cent, of those of lampblack. These may serve as 

 typical cases, and we conclude that high reflectors, which must therefore 

 have low absorbing powers, have also low emitting powers, while low reflec- 

 tors, which, if opaque, have high absorbing powers, have also high emitting 

 powers. In other words, Radiating and absorbing powers vary together. 

 High radiators are high absorbers. Low radiators are low absorbers. 



