The answer lies in radiation absorption and reflection. All bodies absorb and emit radiation. A perfect black body absorbs all radiation falling on it and emits radiation dependent on its temperature. This property of a black body caused a crisis in physics at the end of the nineteenth century and was solved by Max Planck who, in doing so, introduced the first quantum theory. ultraviolet catastrophe
When a black body is heated up it emits radiation according to Planck’s formula. As you know, a hot body appears first red, then yellow, then white as its temperature increases. Oddly enough the sun is an almost perfect black body. That sounds like a contradiction, but it isn’t, as the spectrum of radiation from the sun fits almost perfectly the spectrum of a black body at 5800K. black body
Image may be NSFW.
Clik here to view.
A white body reflects most radiation, and is also a poor emitter of radiation. Reflective surfaces, like most metals, act similarly.
In Rumford’s first experiment, with the two cylinders, the thermoscope is absorbing heat from the hot cylinder at about the same rate as it is emitting radiation to be absorbed by the cold cylinder, so its temperature remains constant. When the hot cylinder is blackened, its albedo is reduced and it emits much more radiation, while the cold cylinder will not absorb this heat, but is reflecting heat back to the thermoscope causing the thermoscope to heat up. When the cold cylinder is also blackened it now absorbs more heat from the thermoscope, a similar amount of heat as that emitted by the hot cylinder, so the temperature of the thermoscope stays at 50C.
