IONS AND ELECTRONS J. H. van't Hoff first explained osmotic pressure, the molecular depression of the freezing-point and raising of the boiling-point of solutions by showing that in dilute solution substances behaved like a gas. This conception was widened by S. Arrhenius and W. Ostwald, who concluded that salts and strong acids in aqueous solution were dissociated into charged ions, and the electric con ductivity of the solution measured the number of such ions. The theory of electrolytic dissociation has given a consistent ex planation of a great number of observed facts concerning solu tions, and familiarized the idea of a mobile hydrogen atom bear ing a unit charge of positive electricity. After Crookes' pioneer ing work on discharges in high vacua—his "fourth state of matter"—the discovery of the Röntgen rays in 1895, and of the radioactivity of uranium in 1896 opened a new era. In 1897 J. J. Thomson showed the independent existence of "gaseous" electrons as electrified units with a mass very minute compared with the lightest atom known; and since the same electrons were produced from different elements, he inferred that all atoms might be built up of these minute electrons. The charge carried by a gaseous ion was proved to be equal to that borne by the hydrogen ion in electrolysis.
The separation of radium salts from uranium ores by the Curies in 1898 revealed that the uranium was undergoing a spontaneous transformation through a succession of atomic ex plosions giving out radiations of three kinds—the heavy a-par ticles, the 13-electrons and the 7-high-frequency waves. That the a-particles shot out from radium were atoms of helium was shown in Ramsay's laboratory in 1903.
Then followed Rutherford's achievement in counting the indi vidual helium atoms shot out in unit time from a disintegrating element, from which he could deduce the actual number of atoms and molecules of any gas in a defined volume. The speed of the rejected a-particle showed the intense internal energy of the disintegrating atom, and the use of this "projectile" to probe other atoms led to the conception of an atom as composed of a heavy but very small positive nucleus, round which the inner electrons revolved with enormous rapidity, while the orbits of the outer electrons, circular or eliptical, were far removed from the positive focus (see ATOM).