[Illustration: QUEEN NEFERTITI, MOTHER-IN-LAW OF TUTANKHAMEN This wonderful work of an unknown Egyptian sculptor represents the wife of Ahknaton, the "heretic" king of Egypt (originally Amenhotep or Amenophis IV). The original is now in the Berlin Museum.] Chemists had long recognized the fact that certain chemicals like preparations of zinc, fluorine, and phosphorus were phosphorescent. It was found early in the eighties that Welsbach gas mantles, when placed on a photographic plate and exposed in a dark room for two weeks, made a fine picture. Invisible rays in the mantle imprint its image. Röntgen, in 1895, discovered what are now known as the X-rays. This discovery was the result of experiments begun in 1859 by Plucker to ascertain the cause of fluorescence in light glass, and Sir William Crookes, between 1879 and 1885, carried out beautiful experiments on fluorescence. These were the immediate pioneers of the discovery of the cathode rays and the other great radio discoveries of recent years. Crookes, remembering Faraday's suggestions concerning a fourth state of matter, expressed the opinion, in 1885, that the matter constituting cathode rays is neither solid, gaseous, or liquid, but in a fourth state which transcends the gaseous condition. Perren found in 1895 that the rays carried electrically negative charges and Sir J. J. Thomson noticed that their velocities are appreciably less than the speed of light. Owing, however, to their great momentum, hardly anything can long endure their impacts. They fuse platinum and make diamonds buckle up into coke. Electrons, which constitute the cathode rays, were originally studied in Crookes vacuum tubes, though they are now found to pervade the universe. Larmor in 1897 proposed an electronic theory of magnetism. Henri Becquerel was the first to discover radioactivity. He made radiographs from uranium salts in 1896. M. and Madame Curie undertook the investigation of uranium and found that among the minerals occurring in pitchblende, or uranium ore, bismuth and barium showed radioactive properties, whereas when these metals are found in their ordinary ores they are not radioactive. This discovery led to the finding of two new metals, polonium and radium. Radium is now obtained by fractional distillation of solutions obtained from American and Australian pitchblende. Helium, one of the lightest substances known, was discovered in 1895 by Sir William Ramsay, and liquefied, at a temperature 3 degrees above absolute zero, or -270 degrees centigrade, by Onnes in 1908. Helium appears to be one of the ultimate products of the disintegration of all radioactive elements.