"Such cases should be excluded on the assumption that neither by means of the rays which it radiates or absorbs, nor by... other influences... does the body... change, if its is kept constant by the addition or the subtraction of heat. Under these conditions... the... heat... transferred to a body in a given time to prevent cooling... in consequence of its radiation, is equivalent to the vis viva of the emitted rays; and the amount of heat... withdrawn... to counterbalance the heating from absorption of radiations, is equivalent to the vis viva of the absorbed rays."