"If the medium," so we read in Maxwell, "instead of being a perfect insulator, is a conductor, the disturbance" (viz. that which is produced by an incident beam of light) "will consist not only of electric displacements but of currents of conduction, in which electric energy is transformed into heat, so that the undulation is absorbed by the medium." After having stated in these words one of the most important consequences drawn from his theory, Maxwell goes on to calculate the coefficient of absorption as a function of the conductivity, and he proceeds: "Gold, silver and platinum are good conductors, and yet, when formed into very thin plates, they allow light to pass through them. From experiments which I have made on a piece of gold leaf, it appears that its transparency is very much greater than is consistent with our theory, unless we suppose that there is less loss of energy when the electromotive forces are reversed for every semi-vibration of light than when they act for sensible times, as in our ordinary experiments." Later researches have amply confirmed what Maxwell says here; obviously, bodies, both conductors and dielectrics, behave in general differently towards rapidly alternating electric forces and towards stationary ones. Yet, Hagen and Rubens have been able to show that when, instead of working with visible light, one uses infra-red rays of sufficiently great wave-length, the properties of metals will, in the limit, exactly conform to the theory, if we reckon with the ordinary conductivity. Hagen and Rubens did not measure the amount of radiation that is transmitted through a thin plate but the coefficient of reflexion of a thick mirror. For the case of normal incidence, this coefficient and therefore also the loss of energy, i.e. the quantity that is absorbed by the mirror, can easily be calculated as a function of the conductivity. For the residual rays of sylvin, whose wavelength is 12 μ, and for silver, copper, gold and platinum, the absorbed energy was found to be respectively 1·15, 1·6, 2·1 and 3·5 per cent, of the incident energy, whereas it ought to have been 1·3, 1·4, 1·6, and 3·5 per cent, according to the theoretical formula.