For studying the effect of type of radiating surface on rapidity of radiation. Two metals cans, each about 7.5 cm in diameter by 14.5 cm high, one finished dull black and the other in bright tin, are finished. Each has a metal cover with center hole for insertion of a thermometer, and when both cans are filled with hot water at the same temperature, it will be noted that the water in the black can cools much faster than that in the bright one.