254 ABSORPTION OF CALORIC. [SECT. xxv. 



higher temperature than bodies of any other colour. Blue 

 bodies come next to black in their power of absorption. Of 

 all the colours of the solar spectrum, the blue possesses least 

 of the heating power ; 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. However, 

 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 caloric, what- 

 ever their temperature may be, though with different inten- 

 sities, 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 

 caloric; and the power of absorption is always equal to that 

 of radiation ; for, under the same circumstances, matter which 

 becomes soon warm also cools rapidly. There is a constant 

 tendency to an equal diffusion of caloric, since every body 

 in nature is giving and receiving it at the same instant ; each 

 will be of uniform temperature when the quantities of caloric 

 given and received during the same time are equal that is, 

 when a perfect compensation takes place between each and 

 all the rest. Our sensations only measure comparative de- 

 grees of heat : when a body, such as ice, appears to be cold, 

 it imparts fewer calorific rays than it receives ; and when a 

 substance seems to be warm for example, a fire it gives 

 more caloric than it takes. The phenomena of dew and 

 hoar-frost are owing to this inequality of exchange; the 

 caloric radiated during the night by substances on the surface 

 of the earth, into a clear expanse of sky, is lost, and no return 

 is made from the blue vault, so that their temperature sinks 

 below that of the air, whence they abstract a part of that 



