Leslie's cube is a copper cube filled with boiling water. The sides are painted matt black, gloss black, white and unpainted. It is a closed system but energy crosses the system boundary as infra-read heat radiation. I used a thermopile and an old-fashioned spot beam galvanometer to detect the infra-red. The matt black side gave the highest reading today showing that it is the best surface colour for emitting infra-red.
