Any hot substance radiates heat to as well as absorbs radiant heat from its surroundings at all times. Hence, there always exists a radiant heat exchange between a body and its surroundings. Therefore, the rise or fall of the temperature of a body depends on the exchange of heat between the body and its surroundings. This is called Prevost’s theory of heat exchange.
According to this theory:
1. When the rate of absorption of heat from the surroundings is greater than the rate of radiation of the body, the temperature of the body rises up.
2. When the rate of absorption of heat from the surroundings is lower than the rate of radiation of the body, the temperature of the body decreases.
3. When the rate of radiation equals the rate of absorption, the temperature of the body remains constant and the body remains at thermal equilibrium with its surroundings.