investigation might be made, showing how much heat was given out per hour by the black cloth to the surrounding glass and water. Here thermal energy was communicated to the black cloth by waves of sunlight, and given out as thermometric heat to the glass and water around it. Thus, through the water, there was actually energy travelling inwards, in virtue of waves of light, and outwards through the same space in virtue of thermal conduction. This suggestion respecting radium might be regarded as utterly unacceptable, but, at all events, Lord Kelvin thought it would be conceded that experiments should be made comparing the thermal emission from radium wholly surrounded with thick lead with that found with the surroundings hitherto used. … Such were Lord Kelvin's ideas concerning radium.