The fact that radiant heat can be transmitted through a vacuum may be shown by the following experiment: In the
top of a spherical glass vessel, Fig. 2,
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a thermometer is fixed in such a manner that its bulb occupies the center of the
vessel. The tube at the lower end of the vessel is then attached to a special kind of pump, known as an air pump, and all the air is drawn out of the vessel by the pump. A vacuum is thus produced inside the glass sphere. If a cloth dipped in hot water is now wrapped around the vessel, the mercury in the thermometer will be seen to rise at once. Since glass is a poor conductor of heat, it is evident that the immediate rise of the mercury was not due to heat conducted along the glass itself, but instead to the passage of radiant heat through the vacuum in the spherical vessel.
23. Intensity of Radiation. — The intensity of radiant heat received by a body varies inversely as the square of its distance from the source of heat. This is easily seen by a
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study of Fig. 3, in which a and b are two plates placed at distances of 1 foot and 2 feet, respectively, from a candle flame, which is the source of heat. The square plate b has
