36 SCIENTIFIC AGRICULTURE. 



in all directions like the rays of light. This effect is not 

 produced by the gradual conduction of caloric by the ain 

 because the same effect takes place in a vacuum, and in a 

 direction opposite to the wind. When rays of heat fall upon 

 any body, they are, like the rays of light, either absorbed, 

 reflected, or transmitted. Highly .polished substances reflect 

 the heat, while rough surfaces absorb it The angles of 

 incidence and reflection are equal in radiant caloric, as well as 

 in light and sound. The color of bodies has an important 

 influence on their radiating power: dark colors radiate better 

 than light ones. 



The transmission of heat through the air takes place 

 without any obstruction, as is the case with light; but with 

 respect to other transparent media it is different " If a para- 

 bolic or concave mirror be taken, and its axis directed towards 

 the sun, the rays of both heat and light will be reflected to 

 the focus, which will exhibit a temperature sufficiently high to 

 fuse a piece of metal, or fire a combustible body. If a plate 

 of glass be now placed between the mirror and the sun, the 

 effect will be but little diminished. Now let the same experi- 

 ment be made with the heat and light of a common fire ; both 

 will be concentrated by reflection as before, but on inter- 

 posing the glass the heating effect of the focus will be reduced 

 to almost nothing, while the light will not" have undergone 

 perceptible diminution. 



" Thus the rays of heat coming from the sun traverse glass 

 with great facility, which is not the case with those emanating 

 from an ordinary red hot body." Rays of heat are not 

 transmitted equally through different bodies of equal trans- 

 parency: for example, of 100 rays falling on a crystal of rock 

 salt, 8 were intercepted: of the same number, glass inter- 

 cepted 61, and alum 91. Color also varies the power of 

 bodies to transmit heating rays. Black and opake bodies stop 

 the rays completely. Rays of heat from different sources 



