The great absorption of all matter for the radiation of the extreme ultra-violet, on both sides of 30o A.U., has already been emphasized. From this region onwards the penetrating power of the radiation increases as the wave-length becomes shorter. With X-rays any distinction between conductors and non-conductors ceases, for the absorption here is entirely an atomic affair, and de pends upon the binding of the inner electrons by the nucleus. Sharp absorption edges correspond to the frequencies at which the quantum energy of the radiation first suffices to eject an electron from a given inner group. (See X-RAYS, NATURE OF : Röntgen Rays.) For very hard X-rays, shorter than the K absorp tion edges of the elements considered, the absorption of a given element is proportional to the third power of the wave-length, while for a given wave-length and different elements it is propor tional to the fourth power of the atomic number of the element, and thus independent of the periodic physical and chemical prop erties of the absorbing substance.
Broadly speaking, then, in the Hertzian region the behaviour of the radiation incident on matter can be explained in terms of the macroscopic electric conductivity, without consideration of the mechanism by which the conduction takes place. In the ex treme infra-red the free-period of the crystal lattice makes itself felt, and the phenomenon of metallic reflection by crystals is observed in restricted regions, but outside these regions the laws of the Hertzian waves are obeyed. In the visible and adjacent regions the phenomena of absorption and dispersion are governed by damped oscillators of determined free-period, the nature of which is discussed under LIGHT. The chemical constitution and physical state of the body is of prime importance in these regions, and each substance has its regions of high absorption. For the absorption of the very short waves of the X-ray and 7-ray region only the inner regions of the atom play a part, and conductivity in particular, and chemical and physical state in general, cease to have any important influence.
The question of the absorption spectra of gases and vapours is considered in full under BAND SPECTRUM and SPECTROSCOPY.
of the distribution of energy in the spectrum of black body radia tion, which comprises, as far as experiment is concerned, the visible and infra-red, led to the Quantum Theory (q.v.), which has found significant application in such apparently remote sub jects as specific heat. The particular study of radiation termed Spectroscopy (q.v.) is at the basis of the modern theory of the atom. The theory of Maxwell and the consequent discovery of long wave radiations by Hertz have left their mark in every branch of theoretical physics. To attempt a full list would be to run over every branch of physical science.
The extension of the electromagnetic spectrum constitutes one of the great unifying principles of modern science, and is a part of the attempt to explain everything in terms of electric and magnetic forces which is characteristic of the age. Until very re cently there was, however, a sharp distinction made between radiation and matter. The interaction of the two, both in the way of the emission of radiation by matter and the action of matter on radiation falling on it, has proved to be a subject full of diffi culties and paradoxes, in spite of the continued assault of the best brains of science on the problems presented. One group of phenomena is sufficiently explained by a wave theory of radiation; another group seems to demand for radiation a corpuscular char acter. (See PHOTOELECTRICITY, COMPTON EFFECT.) During the last few years a promising path to a general solution seems to have been found in the theory of wave mechanics initiated by L. de Broglie and Schrodinger. On this theory the distinction between matter and radiation becomes very shadowy. An electron is a certain singularity in a group of waves, and while waves possess certain of the properties of particles, at the same time particles possess many wave-properties. The electromagnetic theory of light was the great achievement of the last quarter of the past century : the explanation of matter in terms of electric charges was the great achievement of the first quarter of the present cen tury. The next period bids fair to be one in which electric charges and radiation will be explained on a common basis, as different manifestations of something more fundamental, but at present less definite, foreshadowed by the wave function in the new wave mechanics.
The subject of wave-mechanics is discussed at more length in the article QUANTUM THEORY.