Selective Absorption.The extent of the absorption depends, in some substances, upon the temperature of the source of heat.
Demonstration. - Set a radiometer in front of a fishtail gas flame so near that it will rotate briskly. Place a pane of glass between them. Much of the radiation will be absorbed by the glass, some will be reflected, and the rotation will nearly stop. Set the radiometer in the sunshine. After it is in motion hold the glass plate between it and the sun. Does the rotation stop?
The fact that glass permits much of the radiation from the sun to pass through, but does not permit the passage of radiation from bodies of much lower temperature, is of great importance, since radiation from the sun enters our rooms through the windows and heats them, while the radiant heat from stoves and radiators cannot pass out.
The radiant heat that comes from the sun and passes through the glass readily is called luminous heat; because it comes from a source that also gives light. Radiant heat that comes from bodies of comparatively low temperature and is largely absorbed by the glass is called nonluminous heat.
Substances like rock salt, which permit radiant heat to pass through them readily, are called diathermanous, while those like glass and water, that absorb radiant heat, are called athermanous.
Demonstration. - Place a flat battery jar between a radiometer and the sun. Does the rate of rotation change? Fill the jar with water. Is there any greater change?
