The intensity of radiant heat is proportional to the temperature of the source. (a.) Near a differential thermometer, place a vessel of water 10 deg. warmer than the temperature of the room. Notice the effect upon the thermometer. Heat the water 10 deg. more and repeat the experiment at the same distance. Then heat the water 10 deg. still more and repeat the experiment again. The effects upon the thermometer will be as the numbers one, two and three.