-1859] WRITINGS OF JOSEPH HENRY. 143 



The heat from this source possesses peculiar properties. 

 First, it is readily absorbed by all bodies in proportion to 

 some peculiarity of the texture of their surface, but is wholly 

 independent of the color ; or in other words this kind of heat, 

 unlike light, is absorbed by light-colored substances as well 

 as dark, and this fact would be in accordance with the hy- 

 pothesis assumed, which supposes these two emanations to 

 consist of waves of different lengths, and perhaps of slightly 

 different form. Secondly, this kind of heat is incapable of 

 passing by direct radiation through many media which are 

 freely traversed by light, such as glass, alum, and many 

 other transparent substances, while it is freely transmitted 

 through polished plates of rock-salt, and partially through 

 many other bodies, some of which are impervious to light. 

 The former class of bodies is called athermanous, the latter 

 diathermanous. 



Let us now suppose the radiating body to be one which 

 can be increased in temperature until it becomes red-hot. At 

 a certain stage of incandescence, other rays than those de- 

 scribed as capable of exciting heat begin to be given off along 

 with the former, which are distinguished by different prop- 

 perties. First, they tend to be absorbed by all bodies in pro- 

 portion to the darkness of their color, and approximate in 

 this respect to the property of light. Secondly, they possess 

 a property of transmissibility without diminution, through 

 all transparent substances, through colorless media, and in 

 various proportions through colored media, according to the 

 nature of the latter. 



While bodies heated below redness give off exclusively 

 rays of the first class, (though approaching in character those 

 of the second as the temperature is increased,) incandescent 

 bodies simultaneously give off both species. 



As the intensity of heating still further increases, rays of 

 less and less length are given off, until they arrive at the 

 limit of the perceptibility of the sense of vision, and only 

 render their existence manifest by chemical and phosphoro- 

 genic effects. 



The following table exhibits some of the results which 



