Balfour Stewart's law asserts the identity of the emissive and absorptive powers of a body at any one temperature in all respects, wave-num ber and polarization. This principle is the basis of spectrum analysis. See SPECTROSCOPY.
It • is important, therefore, for theoretical reasons to realize as nearly as possible a 'black body' and to study its radiation at different temperatures. This has been done within recent years, notably by Paschen and by Lummer and Pringsheim. It can be shown that the radiation inside a hollow solid, whose walls are at a uni form temperature, is identical with that which a 'black body' at that temperature would emit (with certain simple limitations) : and so, if a narrow slit is made in the walls of this hollow solid, the radiation which escapes can be studied. To absorb and measure the radiation, it is necessary to coat with some 'black' sub stance a sensitive thermometer or instrument for measuring changes in temperature; for this purpose a bolometer or radio-micrometer or radiometer is covered with lampblack. which is almost 'black' for most wares. In this way the radiation of a 'black' body characteristic of definite temperatures has been studied; and it is possible to express the results in various formula?. One of these is called Stefan's law;
it states that the total radiation of a 'black body' whose surface is S sq. cm., and whose absolute temperature is T (i.e. 273 t° C), can be expressed as proportional to ST'. Other laws connect the temperature and the radiation of definite wave-lengths. As the temperature is increased, so is the radiation. If two bodies at different temperatures are put together, the one at the higher temperature emits more energy than it absorbs, while the reverse is true of the other body. In • the end their temperature should become equal.
For a full discussion of the radiation of a `black body,' reference should be made to the papers by Lummer and Wien, Reports of Inter national Congress of Physics, vol. ii. (Paris, 1900), and to recent articles by Paschen in nnalen der Physik. The original memoirs by Kirchhoff, Stewart, and others are given in Brace, "Radiation and Absorption," Scientific Memoir Series. vol. xv. (New York, 1901). See HEAT and LIGHT.