an object such as a block of steel. It happens, therefore, that the condition of perfect radiation is attained by the appliances in everyday use; and, moreover, black-body radiations can always be secured by placing a tube, closed at one end, in the heated space, and receiving the radiations through the open end; for this again represents an enclosure at a constant temperature. Similarly, radiations from a solid in the interior of the tube of the electric furnace shown in fig. 29 will be of the same description, and we can therefore apply with accuracy any instrument based upon black-body radiations, knowing that the same may be readily realised in practice. The law connecting the energy radiated by a substance, under given conditions, with its temperature, was variously stated by different observers until Stefan, in 1879, deduced the true relation from certain experimental data obtained by Tyndall. Stefan concluded that the figures given by Tyndall indicated that the energy radiated by a given solid varied as the fourth power of its absolute temperature. Numerous experiments, under different conditions, showed that the fourth-power law did not apply to all kinds of surfaces or circumstances; but a strong confirmation of its truth when applied to black-body radiations was forthcoming in 1884, when Boltzmann showed, from thermodynamic considerations, that the quantity of energy radiated in a given time from a perfect radiator must vary as the fourth power of its absolute thermodynamic temperature. Certain assumptions made by Boltzmann in this investigation were subsequently justified by experiment; and numerous tests under black-body conditions have since amply verified the law. It is upon the Stefan-Boltzmann law that radiation pyrometers are based; the energy received by radiation from the heated substance, under black-body conditions, being measured by the instrument, and translated into corresponding temperatures on its scale. Expressed in symbols, the fourth-power law takes the form— E = K(T_{1}^4 - T_{2}^4),