The molecular theory of magnetism presumes that every molecule of iron is a tiny magnet, having a north and south pole. In a piece of unmagnetised iron, these tiny magnets are all lying so that they neutralise one another. When they are turned round so that their north poles are all lying in one direction, then the iron is said to be magnetised. The electron theory of magnetism does not do away with the older molecular theory just referred to. The electron theory goes a step farther, and tells us that these molecules are magnets because of a steady motion of electrons around the atoms of iron. It was discovered in 1825 that when an electric current was sent through an insulated wire wound around a piece of soft iron, the iron became a magnet; when the current was stopped the magnetism disappeared. Such magnets are called _electro-magnets_. If a piece of hard steel is treated in the same way it becomes a _permanent magnet_. It was this intimate connection between electricity and magnetism, or, in other words, the invention of these electro-magnets, which brought us electric bells, telegraphs, telephones, dynamos, and electric motors. It should be noted that while iron is attracted by either pole of a magnet, there is such a thing as magnetic repulsion. This, however, takes place only between two magnets, and then only between like poles. * * * * * Some German physicists made a number of electrical experiments with vacuum tubes. When Sir William Crookes (England) was experimenting with similar vacuum tubes he suggested that matter was in a "radiant" state during the electric discharge within the tubes. In 1880, H. A. Lorentz, of Amsterdam, declared that light was due to the motion of small particles revolving around the atoms of matter. Professor Zeeman, of Holland, produced experimental proof of Lorentz's theory. He showed that the revolving "particles" were influenced by a powerful magnetic field, in the manner explained in the electron's story. This discovery was made in 1896, or sixteen years after Lorentz's declaration. It was Dr. Johnstone Stoney, of Dublin University (Ireland), who christened these particles "electrons." The X-rays were observed for the first time by Professor Roentgen, of Germany, in 1895. The screens used for viewing the luminous effects produced by the X-rays are coated with very fine crystals of _barium platinocyanide_. These screens were in use for another purpose previous to the discovery of X-rays. We know now that _chemical affinity_ is merely electrical attraction between the atoms of matter.