cathode rays were (see below). There were two theories: Crookes believed they were 'radiant matter'; that is, electrically charged atoms, while German scientists Hertz and Goldstein believed they were 'aether vibrations'; some new form of electromagnetic waves. The debate was resolved in 1897 when J. J. Thomson measured the mass of cathode rays, showing they were made of particles, but were around 1800 times lighter than the lightest atom, hydrogen. Therefore, they were not atoms, but a new particle, the first subatomic particle to be discovered, which was later named the electron. It was quickly realized that these particles were also responsible for electric currents in wires, and carried the negative charge in the atom. The colorful glowing tubes were also popular in public lectures to demonstrate the mysteries of the new science of electricity. Decorative tubes were made with fluorescent minerals, or butterfly figures painted with fluorescent paint, sealed inside. When power was applied, the fluorescent materials lit up with many glowing colors. In 1895, Wilhelm Röntgen discovered X-rays emanating from Crookes tubes. The many uses for X-rays were immediately apparent, the first practical application for Crookes tubes. Medical manufacturers began to produce specialized Crookes tubes to generate X-rays, the first X-ray tubes. Crookes tubes were unreliable and temperamental. Both the energy and the quantity of cathode rays produced depended on the pressure of residual gas in the tube. Over time the gas was absorbed by the walls of the tube, reducing the pressure. This reduced the amount of cathode rays produced and caused the voltage across the tube to increase, creating more energetic cathode rays. In Crookes X-ray tubes this phenomenon was called "hardening" because the higher voltage produced "harder", more penetrating X-rays; a tube with a higher vacuum was called a "hard" tube, while one with lower vacuum was a "soft" tube. Eventually the pressure got so low the tube stopped working entirely. To prevent this, in heavily used tubes such as X-ray tubes various "softener" devices were incorporated that released a small amount of gas, restoring the tube's function. The electronic vacuum tubes invented later around 1904 superseded the Crookes tube. These operate at a still lower pressure, around 10−9 atm (10−4 Pa), at which there are so few gas molecules that they do not conduct by ionization. Instead, they use a more reliable and controllable source of electrons, a heated filament or hot cathode which