Heat rays have the same nature as light rays... The invisible heat ray — Gustav Kirchhoff

"Let a body which satisfies these conditions be surrounded by an enclosure, having the same temperature [and kept constant], through which no heat rays can penetrate... The body sends out heat rays and is encountered by... heat rays... in part... from the enclosure, in part... thrown back... by reflection from it, absorbing a part of them. Its temperature must thus remain the same, unless heat is withdrawn from it or communicated to it as follows on the principle from which Carnots law results. For this reason the vis viva of the rays, which it sends out in a certain time, must equal the vis viva of the rays which it absorbs in the same time."

"[R]adiation in empty space will be investigated..⇒ the [associated] black bodies must have a refracted index which differs infinitely little from 1."

"Of the heat rays... sent... to a body by its surroundings a part are absorbed, the others are... varied by reflection and . The rays refracted and reflected... pass off... with those sent out by it, without... mutual disturbance..."

"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 withdrawn...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."

"[I]n certain cases an exception to this rule may occur... [when] absorption and the radiation produce other changes in the body.., for example in bodies... chemically changed by light... [etc.]"

"A ... must have the same as the medium... then there will be no reflection at its surface, and all incident rays... wholly absorbed."