In 1893, Acheson invented his process for making carborundum, a compound of carbon and silicon, made in the electric furnace, and used for abrasive purposes; and in the same year Willson made carbide of calcium from carbon and quick-lime, also in the electric furnace. In 1895, Linde invented his process of liquefying air, and the first installation of great electric locomotives was effected: this was in the Baltimore and Ohio tunnel. In the same year, Röntgen made the epochal discovery of what he called by the significant name "X-rays," a name that still clings to them. They were discovered by Röntgen in the course of his researches with cathode rays. His discovery was in effect that electric rays emanated from the part of the tube struck by the cathode rays. They were not cathode rays, though produced by them, and had the amazing property of penetrating certain insulating substances, such as ebonite, paper, etc., while not penetrating metals, except through short distances. Unlike the cathode rays, they were not deflected by magnets; and neither did they seem to be reflected or refracted similarly. Their most important property was that of acting photographically on sensitized plates, even when in closed slides, and wrapped carefully in black paper. The greatest usefulness of the X-rays thus far made has been in photographing internal parts of the human body; for the rays pass through certain parts less readily than through other parts; through bones for instance, less readily than through soft parts. Fractures or displacements of bones can therefore be readily detected. So also can the formation of pus in cavities, and the appearance of abnormal products of many kinds. To this discovery we must give a rank as high as almost any other that we have noted in this book, though we cannot tell, of course, how long it will hold it. With mechanical and scientific inventions, as with books and poems and inventions of other kinds, the question of permanence of value or of usefulness cannot be decided until after many years. One of the curious properties of X-rays is that of rendering the air through which they pass a conductor of electricity. So far as the author is aware, no invention of practical usefulness has yet been made, based upon this property.