268 ABSORPTION OF HEAT. SECT. XXVII. 



the strongest arguments in favour of the common nature of the 

 chemical, visible, and calorific rays. They are all capable of 

 reflection from polished surfaces, of refraction through diaphanous 

 substances, of polarization by reflection and by doubly refracting 

 crystals ; their velocity is prodigious ; they may be concentrated 

 and dispersed by convex and concave mirrors ; they pass with 

 equal facility through rock-salt and are capable of radiation ; and 

 they are subject to the same law of interference with those of 

 light : hence there can be no doubt that the whole assemblage of 

 rays visible and invisible which constitute a solar beam are pro- 

 pagated by the undulations of the ethereal medium, and conse- 

 quently as motions they come under the same laws of analysis. 



When radiant heat falls upon a surface, part of it is reflected 

 and part of it is absorbed ; consequently, the best reflectors 

 possess the least absorbing powers. The temperature of very 

 transparent fluids is not raised by the passage of the sun's rays, 

 because they do not absorb any of them ; and, as his heat is very 

 intense, transparent solids arrest a very small portion of it. The 

 absorption of the sun's rays is the cause both of the colour and 

 temperature of solid bodies. A black substance absorbs all the 

 rays of light, and reflects none ; and since it absorbs, at the same 

 time, all the calorific rays, it becomes sooner warm, and rises to 

 a higher temperature, than bodies of any other colour. Blue 

 bodies come next to black in their power of absorption. And, 

 since substances of a blue tint absorb all the other colours of the 

 spectrum, they absorb by far the greatest part of the calorific 

 rays, and reflect the blue where they are least abundant. Next 

 in order come the green, yellow, red, and, last of all, white bodies, 

 which reflect nearly all the rays both of light and heat. How- 

 ever, there are certain limpid and colourless media, which in 

 some cases intercept calorific radiations and become heated, while 

 in other cases they transmit them and undergo no change of 

 temperature. 



All substances may be considered to radiate heat, whatever 

 their temperature may be, though with different intensities, 

 according to their nature, the state of their surfaces, and the 

 temperature of the medium into which they are brought. But 

 every surface absorbs as well as radiates heat ; and the power of 

 absorption is always equal tp that of radiation ; for, under the 

 same circumstances, matter which becomes soon warm also cools 



