BADIAN. 639 RADIATION. RADIAN. See Circle. BADIANT STAR, Order of the. An order of Zanzibar, founded by Sultan Bargash ben Said in 1875. It has two classes, of which the first is given only to sovereigns, the second forming an order of merit with four divisions. The decoration is a red cross with five arms edged with white on a green wreath. The cir- cular medallion bears the Sultan's name. The cross is suspended by a wreath from a red ribbon edged with white. RADIATA (Lat. nom. pi., having rays). The lowest of Cuvier's four great divisions of the animal kingdom. It derived its name from the organs of sense and motion being disposed as rays round a centre, and included ( 1 ) the Echinodermata, (2) the Entozoa (or intestinal worms), (3) the AcalephiE (or sea-nettles), (4) the Polypi, and (.5) the Infusoria. See Classification- of AjfisiALS. RADIATION (Lat. radiatio, a shining, from radiate, to shine, from radius, ray). The name given to the quantity of energy ca'rried by ether- waves or sometimes simply the waves themselves. Since wave-motion involves both the vibration and the displacement of the medium carrying the waves, there is always both kinetic "and potential energy associated with the advance of a train of waves. A source which is emitting trains of waves is losing energy, and a body which absorbs them gains energy. Ether-waves may be divided into two classes: irregular pulses, analogous to the disturbances produced in the svirface of a pond by dropping into it at irregular intervals a number of stones ; and regular trains of waves, analogous to the aerial waves produced by a vibrating tuning fork. It is believed that the radiation called "RiJntgen rays,' or 'X-rays' (q.v.), is of the nature of the former; while the waves emitted by a flame, the sun, etc., are of the latter kind. They should have quite distinct properties, from theoretical considerations. Inegular pulses, if very abrupt, should not be refracted, diflfracted, or polarized; their 'rays' should pass in straight lines. Reg- ular periodic trains of waves should obey the ordinary laws of light (q.v.). Periodic trains of ether-waves are emitted by all portions of matter, owing to the vibra- tions of the 'atoms;' they are also produced by electric oscillations. The lengths of these waves can be measured by suitable means — by diffraction gratings (q.v.) and by calculation, in the case of those due to the latter cause. Those waves produced by ordinary matter, which have been actually observed, vary in length from 0.000015 cm" to 0.006 cm.; and waves produced by electrical oscillations have been obtained whose lengths were as small as 0.4 cm., although in general they are much longer. When these waves are incident upon portions of matter, they may be partially re- flected if the body is large compared with the wave-length ; they may be transmitted or pass around it; or they may produce vibrations in the body by resonance, and so be absorbed themselves. Thus long ether-waves produced by electrical oscillations may produce electric oscillations in suitable electrical conductors. (See Electricity; Wireless Telegraphy.) The shorter ether-waves emitted by material bodies in their natural condition may be ab- sorbed by other material bodies. In this process of absorption the amplitude of the waves dies down, and their energy is gained by the body which absorbs them. "This absorbed energy is spent in various ways, depending upon the length of the absorbed waves and the nature of the absorbing body. To measure the energy carried by any train of waves it is neces- sary to have the absorbing body such that it absorbs all the energj' — that is, neither reflects nor transmits the waves — and consumes it in producing rise of temperature. In the case of extremely short waves, chemical action may be produced ; the limits of wave-length so far as now known which will do this are from about 0.00007 cm. to 0.000015 cm. Waves whose wave-lengths lie between about 0.00007 cm. and 0.0000.35 cm. affect the sense of sight of most human beings. In fluorescent bodies the energy due to absorption is spent in emitting other ether-waves. The radiation from any body is as a rule characteristic of its temperature and the nature of its surface principally : there are, however, many eases when this is not true. Such excep- tions are fluorescing and phosphorescing bodies, gases through which an electrical dis- charge is taking place, bodies such as the salts xised in a Welsbach mantle, which are rendered luminescent at quite low temperatures. The foUoiving statements do not uppltj to these ex- ceptional cases. Since absorption is due to resonance, it is natural to expect some connec- tion between the radiation emitted from a body and its absorptive properties. It was shown, first by Balfour Stewart and later by ICirchhoff, tliat the 'emissive power' of any body is iden- tical with its 'absorptive power' at the same temperature. The absorptive power of a body for a train of waves of a definite wave-number is defined as being the fraction of the incident radiation of that wave-number which the body absorbs. A body which absorbs all waves ab- solutely is called a 'black body.' The emissive power of a body for a train of waves of a definite wave-number is defined as the ratio of the energy radiated by 1 sq. cm. of the surface of the body in the form of waves of the specified wave-number to that emitted by I sq. cm. of a 'black body' imder the same conditions of temperature and in the same form. This law can be expressed in symbols: let E be the energy emitted by 1 sq. cm. of a 'black body' at a given temperature in the form of waves of a definite wave-number; let a be the ab- sorptive power of any body for waves of this kind; then e, the energy emitted by 1 sq. cm. of this body for these waves, is given by the for- mula e = fiE. 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
