Another train of reasoning, starting from a different point, re inforces this result. The phenomena of the interference of beams of light in certain circumstances to produce darkness or colour, indicate that light is some form of wave motion, and, to carry these waves, a hypothetical luminiferous aether was invented.
The theoretical work of J. Clerk Maxwell (1831-1879) and the experiments of H. R. Hertz (1857-1894) showed that the proper ties and velocity of propagation of light and of electromagnetic waves were identical and that their other properties differed only in degree. Thus light became an electromagnetic phenomenon.
But light is started by some form of atomic vibration, and to start an electromagnetic wave requires a moving electric charge. Elec tric charges must exist within the atom, and we are led again to the theory of electrons by the road opened up by H. A. Lorentz and Joseph Larmor. Such a theory suggests the occasional instability of the atom, and the phenomena of radioactivity, shown in a re markable form by the substance radium, discovered by M. and Mme. Curie, have been explained satisfactorily by the theory of Rutherford and Soddy, who regard the energy liberated as due to the disintegration of the atom. The evolutionary view of nature, established in the biological and sociological sciences, is thus extended to physical science, not only in the development of planets and suns, but even in the chemical atoms, hitherto be lieved indestructible and eternal. Many complex radioactive changes have been investigated in detail by Sir Ernest Rutherford and his pupils, and several new elements have been discovered.
Relations between the atomic weights of the elements and their properties were traced by Mendeleeff (1834-1907) and others, but in 1913 and 1914 a new light was thrown on the problem by H. G. J. Moseley. As diffraction gratings, surfaces ruled with multitudes of parallel scratches, give spectra with ordinary light, so the much finer atomic layers of crystals produce spectra when the even more minute waves of X rays are reflected from them. The structure of crystals has been investigated thus by Sir William and W. L. Bragg. Bombarding an element with cathode rays and using a crystal as grating, Moseley obtained X ray spectra, and discovered that the square roots of the frequencies of vibration of the characteristic X rays increase by equal steps as we ascend the list of elements in order of increasing atomic weight. Each ele
ment can thus be given an atomic number, ranging from 1 for hydrogen to 92 for uranium, and save for three gaps, elements cor responding to all intervening integral numbers are known. These numbers have an important physical meaning in the modern theory of the atom.
Meanwhile the atomic weights were shown to be whole numbers by the researches of Aston, who, by deflecting positive electric rays in strong magnetic and electric fields, measured accurately the mass of flying atoms. The chief exception is hydrogen, with an atomic weight of 1.0o8. Helium consists of four hydrogen nuclei, but its atomic weight is exactly 4. Thus, when an atom of helium is built up from hydrogen, there is a destruction of mass and an equivalent liberation of energy.
It proved to be an impossibility to explain either sharp line spectra or the localization of energy in continuous spectra by the vibration of electrons in orbits in accordance with Newton's dynamics. To overcome these difficulties, Planck devised a Quan tum Theory, according to which energy is emitted in definite units or quanta. Led chiefly by the facts of radio-activity, Rutherford had come to view an atom as essentially a positive nucleus with negative electrons round it, and this view was adapted to the quantum theory by Bohr. Hydrogen has a nucleus of one positive unit or proton, with one electron circling round it, according to Bohr in one of four definite orbits. By supposing that radiation is emitted only when the electron leaps instantaneously from one stable orbit to another, Bohr explained many of the phenomena of the hydrogen spectrum. More complex atoms are imagined to be made of two or more positive protons bound together by a smaller number of negative electrons in a nucleus, the rest of the electrons outside the nucleus being more loosely connected. Moseley's atomic number gives the number of these outer electrons.