A colorless, odorless, tasteless, slightly water-soluble, highly flammable gaseous element, the lightest of all gases and the most abundant element in the universe. The hot flame produced by a mixtureFLAMMABLE! of oxygen and hydrogen is used in welding, and in melting quartz and glass. Such a flame is also called an oxyhydrogen torch. Tritium, one of hydrogen's three isotopes, is used in luminous paint. Tank size available: 191 cubic feet. Atomic symbol H, atomic number 1.Also see acetylene, argon, carbon dioxide, helium, and nitrogen.