240 EADIAT10N. [SKCT. xxv. 



fications of the same agent which produces the sensation of 

 light. Rays of heat dart in diverging straight lines from 

 flame, and from each point in the surfaces of hot bodies, 

 in the same manner as diverging rays of light proceed from 

 every point of the surfaces of such as are luminous. Ac- 

 cording to the experiments of Sir John Leslie, radiation 

 proceeds not only from the surfaces of substances, but also 

 from the particles at a minute depth below it. He found 

 that the emission is most abundant in a direction perpen- 

 dicular to the radiating surface, and that it is more rapid 

 from a rough than from a polished surface : radiation, how- 

 ever, can only take place in air and in vacuo ; it is altogether 

 imperceptible when the hot body is inclosed in a solid or 

 liquid. Heated substances, when exposed to the open air, 

 continue to radiate caloric till they become nearly of the 

 temperature of the surrounding medium. The radiation is 

 very rapid at first, but diminishes according to a known law 

 with the temperature of the heated body. It appears, also, 

 that the radiating power of a surface is inversely as its re- 

 flecting power ; and bodies that are most impermeable to 

 heat radiate least. 



Rays of heat, whether they proceed from the sun, from 

 flame, or other terrestrial sources, luminous or non-luminous, 

 are instantaneously transmitted through solid and liquid 

 substances, there being no appreciable difference in the 

 time they take to pass through layers of any nature or 

 thickness whatever. They pass also with the same facility 

 whether the media be agitated or at rest; and in these 

 respects the analogy between light and heat is perfect. 

 Radiant heat passes through the gases with the same facility 

 as light ; but a remarkable difference obtains in the trans- 

 mission of light and heat through most solid and liquid 

 substances, the same body being often perfectly permeable 

 to the luminous, and altogether impermeable to the calorific 

 rays. For example, thin and perfectly transparent plates 

 of alum and citric acid sensibly transmit all the rays of light ' 



