98 ELEMENTS OF SCIENCE 



one-fifth of the radiating power of lamp-black, which is 

 almost equalled by writing paper ; while polished gold, 

 silver, or copper has but little more than half that of 

 mercury. But the rate of radiation has more to do with 

 the state of the surface of a body than with the nature 

 of the material whereof it is composed. Bright surfaces 

 radiate least, but their power will be almost doubled if 

 their surface be covered with lamp-black. 



Practically the rate of diminution which distance 

 occasions is more or less increased by heat being absorbed. 



(4) Absorption. We have already spoken of heat 

 appearing to become absorbed when it is rendered latent 

 or potential ; but it is said to be truly " absorbed " when 

 heat passes from the radiating to the conducting 

 condition. As radiant heat traverses a body, some of it 

 warms the body it traverses, and becomes conductable, 

 but the amount of this differs greatly in different bodies. 

 None, not even air, are what we may term absolutely 

 transparent to heat, or diathermous, but a certain quantity 

 of heat will often pass completely through bodies. This 

 is notably the case with the atmosphere, which allows so 

 great a quantity of the sun's rays to traverse it without 

 warming it, that almost the entire quantity comes to the 

 surface of the globe, which, being thus warmed, gives to 

 the air, by convection, that heat which it failed to receive 

 from the heat radiated through it. Bodies absorb heat at 

 precisely the same rate as they radiate it. There appears 

 to be some ground for supposing that rays of heat 

 differ amongst each other by some other quality besides 

 intensity; since some rays are more absorbable than 

 others, and so become filtered out first, while heat is 

 passing through some more or less diathermous medium. 

 The absorbability of heat differs then with respect to 

 different media, as we have just seen. 



