Among the properties of matter revealed by electricity none are more striking than those displayed in tubes containing highly rarified gases. The study of their phenomena has led to discoveries which bring us within view of an ultimate explanation of properties, an understanding of how matter is atomically built. All this began simply enough as Plucker, in 1859, sent an electric discharge through a tube fairly well exhausted, producing singular bands of color. Geissler, afterward using tubes more exhausted, produced bands of still higher variegation. In 1875 Professor William Crookes devised the all but vacuous tube which bears his name, through which he sent electric pulses from a cathode pole, revealing what he called “radiant matter,” as borne in a beam of cathode rays, as much more tenuous than ordinary gases as these are more rare than liquids. In 1894 Professor Philipp Lenard observed that cathode rays passed through a thin plate of aluminium, much as daylight takes its way through a film of translucent marble. Next year came the epoch-making discovery of Professor Conrad Wilhelm Röntgen that cathode rays consist in part of X-rays which readily pass through human flesh, so as to cast shadows of bones upon a photographic plate. Cathode rays make air a fairly good conductor of electricity, while ordinary air is non-conducting in an extreme degree. This singular power is also possessed by the ultra-violet rays of sunshine, as readily shown by an electroscope. In 1897 Professor Joseph J. Thomson, of Cambridge University, demonstrated that cathode rays are made up of corpuscles, or electrons, about one-thousandth part the size of a hydrogen atom, and bearing a charge of negative electricity. Such electrons form a small part of every chemical atom, the remainder of which is, of course, positively electrified. All electrons are alike, however various the “elements” whence they are derived; as the most minute masses known to science they may be among the primal units of all matter.