“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 {{w|temperature}} 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 withdrawn...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.”