Radiation from Hot and Cold Bodies
Two parabolic reflectors are aligned facing each other, with a thermopile at the focal point of one of the reflectors and a heated sphere at the other. A projection galvanometer's deflection shows the transmission of heat by radiation. Replace the hot sphere with a sphere dipped in liquid nitrogen and the galvanometer will deflect in the opposite direction (showing that the thermopile itself is radiating and heat flows to the cold sphere).
Only hold the hot sphere in the gas flame for 10 to 15 seconds.