COOLING, Velocity of, a body isolated in air or other gas, or in vacuo, and surrounded by bodies colder than itself, loses heat, by radiation in the last mentioned case, and in the former case partly by radiation and partly by convection. The rate at which its temperature decreases depends on a variety of circum stances,— on the nature of its surface, for ex ample. But, other things remaining the same, the velocity of cooling is proportional to the excess of the temperature of the body in ques tion above that of its surroundings. This is Newton's Law of Cooling.
The law of cooling has been verified by Dulong and Petit experimentally. A copper ball was suspended in the midst of a metallic chamber, round the outside of which water was kept flowing, in order to maintain a constant temperature. The copper ball was heated be fore being suspended in the chamber, and a thermometer was inserted in a hole in the ball, and so arranged that the stem, which was long, projected to the outside of the chamber and could be read there. The fall of the tempera
tore of the ball during equal intervals of time was noted, and it was found to become less and less as the temperature of the ball gradually approached that of the walls of the surround ing chamber, the law of decrease being New ton's as stated above.
The following example will illustrate practi cally the meaning of this law: Suppose the temperature of the ball to be 20° higher than that of the enclosure at the beginning of the experiment, and that during the first five minutes it loses 1°, that is, one-twentieth or 5 per cent of the excess of temperature. Dur ing the next five minutes it will lose 5 per cent of the excess that remains, that is, of 19°. It will therefore lose 0.95°, or the temperature of the ball will be 18.05° above that of the enclosure. At the end of the next five minutes the difference of temperatures of the ball and enclosure will be 17.15°, and so on.