462 PHYSIOLOGICAL PHYSICS. [Chap xxxv. 



light rays will be permitted to pass ; a cell of bisul- 

 phide of carbon will intercept neither. 



The words "transparent" and "opaque" might, 

 then, be applied to bodies according to their conduct 

 towards heat rays, as well as according to their con- 

 duct towards light rays. To avoid confusion, how- 

 ever, two other words are used, diathermanous and 

 athermanous, the former being applied to substances 

 like rock-salt, which permit the passage of radiant 

 heat, the latter being applied to those which inter- 

 cept the heat. To put it in another way, one body 

 transmits heat, the other absorbs it. Now the body 

 which transmits heat will not become elevated in 

 temperature. The particles of the body have no sym- 

 pathy, so to speak, with the particular vibrations 

 which are the cause of heat ; they are not in tune 

 with them, and offer no response to their movements. 

 On the other hand, the particles of the athermanous 

 body vibrate in harmony with the heat vibrations ; 

 they thus intercept the vibrations, and they them- 

 selves are set into harmonious oscillation. Bodies, 

 then, that absorb heat, are elevated in temperature by 

 the heat they absorb. Their own particles being 

 thrown into oscillation, they will themselves become 

 a source of heat and will radiate it outwards, just in 

 proportion to the degree with which they absorb it. 

 This offers an explanation of the fact noticed at the 

 beginning of these paragraphs on radiation, that heat 

 rays pass through air without raising its tempera- 

 ture ; air is found to have no absorptive power towards 

 heat rays. Tyndall has shown, by some remarkable 

 experiments, that while air is diathermanous, olefiant 

 gas is peculiarly opaque to heat rays, even a small 

 amount of it effectually intercepting them. 



The close connection between absorption and emis- 

 sion of heat has received experimental demonstration. 

 Glass absorbs heat ; rock-salt, generally speaking, does 



