356 SCIENTIFIC AMUSEMENTS. 



chilled on account of the lesser amount of radiation from their 

 surfaces. 



Expt. 382. Absorption of Radiant Heat. In general, surfaces 

 that radiate heat best absorb radiant energy in the form of rays of 

 comparatively low refrangibility the best. Thus two canisters, 

 such as those used in the preceding experiment, filled with the 

 same quantity of cold water and placed side by side at equal 

 distances from a good fire, will absorb heat and cause the water to 

 become warmed at very different rates according to the nature of 

 the outer coating ; a polished bright smooth surface will absorb 

 heat least freely, and the contained water will take a much longer 

 time to rise through a given range of temperature, than will be 

 the case with the lampblacked canister; and similarly in other 

 instances. 



It does not necessarily follow that a body which absorbs radiant 

 energy of one degree of refrangibility more readily than another 

 body will show the same difference with regard to radiant energy 

 of another degree of refrangibility. Thus a copper vessel contain- 

 ing boiling water radiates forth rays of very low refrangibility, 

 which are absorbed almost equally freely by a surface coated with 

 powdered white lead, and by another covered with lampblack ; 

 but if a copper surface at a temperature of 400 be used as the 

 source of radiant heat instead of boiling water (temp. = 100, Expt. 

 28), the lampblack absorbs better than the white lead in the pro- 

 portion of 9 to 8 ; whilst, if the radiation be from white hot 

 platinum, the lampblack absorbs nearly double as much as white 

 lead does. 



Just as many bodies, when exposed to white light, scatter light 

 of one refrangibility more than any other kind, and therefore 

 appear to be coloured (Chapter XXII.), so bodies exposed to rays 

 of lower refrangibility often scatter or diffuse certain of these heat 

 rays more freely than others. Hence such bodies possess, as it 

 were, a sort of heat colour ; the other rays incident upon the body 

 being more freely absorbed than the heat rays which are thus pre- 

 ferentially diffused. 



Just also as transparent substances always absorb more or less 

 of the visible form of radiant energy when this traverses their 

 substance, the emergent light being white if all kinds are equally 

 absorbed, and coloured if certain kinds are preferentially absorbed, 

 so are the heat rays of different refrangibility analogously affected ; 

 thus different gases (e.</., nitrogen, oxygen, sulphur dioxide, 

 ammonia, &c.) absorb radiant heat with very different degrees of 

 facility; ammonia, for example, absorbs about 1200, and sulphur 

 dioxide about 700, times as much as either oxygen, nitrogen, or 



