"A most suitable body... seemed H. Hertz's rectilinear oscillator (dipole) whose laws of emission for a given frequency he had just... developed. If a number of such oscillators be distributed in an enclosure surrounded by reflecting walls, there would take place, in analogy with sources and resonators in... sound, an exchange of energy by means of the emission and reception of electro-magnetic waves, and... corresponding to Kirchoff's law should establish itself in the vacuum-enclosure."