A spectro-photometric comparison of the emissivity of solid and liquid gold at high temperatures with that of a full radiator

It is well known that copper and gold emit greenish or bluish light at high temperatures. Kirchhoff's radiation law obviously suggests a connection between this selective emissivity and the selective reflectivity of these metals at ordinary temperatures, which gives rise to their colour. According to that law the emissivity E' of a surface at a temperature T is connected with its absorptivity A, by the relation E'/E = A, where E is the emissivity of a full radiator or “black body” at the same temperature T. This relation holds for each particular wave-length. The ratio E'/E may be conveniently called, and will be referred to hereafter in this paper as, the “relative emissivity” of the surface. The above law then states that the relative emissivity of a body is equal to its absorptivity for the same wave-length and temperature. For opaque bodies, such as metals, the reflectivity R is equal to 1 — A: and hence the relative emissivity E'/E = A = 1 — R. This relation holds strictly only if emission and reflection take place at the same temperature; but if, as stated by several investigators, the reflectivity of metals for visible rays does not vary with the temperature, a heated metallic surface should emit well those rays for which it is when cold a poor reflector, andvice versâ . This has actually been observed qualitatively for gold and copper by Schaum and Wüstenfeld in a recent photographic comparison of the emission spectra of these metals with that of an approximate “black body.” They also draw attention to the “green-glow” which first makes its appearance when these metals are heated, instead of the usual "red-glow."