220 ABSORPTION OF CALORIC. SECT. XXV. 



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 : none of 

 these rays add sensibly to the weight of matter; 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 ; the chemical rays are subject to the same 

 law of interference with those of light ; and although 

 the interference of the calorific rays has not yet been 

 proved directly, the indirect evidence places it beyond a 

 doubt. As the action of matter in so many cases is the 

 same on the whole assemblage of rays, visible and 

 invisible, which constitute a solar beam, it is more than 

 probable that the obscure as well as the luminous part is 

 propagated by the undulations of an imponderable ether, 

 and consequently comes 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 color 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 color. Blue 

 bodies come next to black in their power of absorption. 

 Of all the colors of the solar spectrum, the blue pos- 

 sesses least of the heating power ; and since substances 

 of a blue tint absorb all the other colors 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. However, there are certain limpid 

 and colorless media, which in some cases intercept 

 calorific radiations and become heated, while in other 



