\Hy"dro*gen\, n. [Hydro-, 1 + -gen: cf. F.
hydrog[`e]ne. So called because water is generated by its
combustion. See {Hydra}.] (Chem.)
A gaseous element, colorless, tasteless, and odorless, the
lightest known substance, being fourteen and a half times
lighter than air (hence its use in filling balloons), and
over eleven thousand times lighter than water. It is very
abundant, being an ingredient of water and of many other
substances, especially those of animal or vegetable origin.
It may by produced in many ways, but is chiefly obtained by
the action of acids (as sulphuric) on metals, as zinc, iron,
etc. It is very inflammable, and is an ingredient of coal gas
and water gas. It is standard of chemical equivalents or
combining weights, and also of valence, being the typical
monad. Symbol H. Atomic weight 1.
Note: Although a gas, hydrogen is chemically similar to the
      metals in its nature, having the properties of a weak
      base. It is, in all acids, the base which is replaced
      by metals and basic radicals to form salts. Like all
      other gases, it is condensed by great cold and pressure
      to a liquid which freezes and solidifies by its own
      evaporation. It is absorbed in large quantities by
      certain metals (esp. palladium), forming alloy-like
      compounds; hence, in view of quasi-metallic nature, it
      is sometimes called {hydrogenium}. It is the typical
      reducing agent, as opposed to oxidizers, as oxygen,
      chlorine, etc.
{Bicarbureted hydrogen}, an old name for ethylene.
{Carbureted hydrogen gas}. See under {Carbureted}.
{Hydrogen dioxide}, a thick, colorless liquid, {H2O2},
   resembling water, but having a bitter, sour taste,
   produced by the action of acids on barium peroxide. It
   decomposes into water and oxygen, and is manufactured in
   large quantities for an oxidizing and bleaching agent.
   Called also {oxygenated water}.
{Hydrogen oxide}, a chemical name for water, H?O.
{Hydrogen sulphide}, a colorless inflammable gas, {H2S},
   having the characteristic odor of bad eggs, and found in
   many mineral springs. It is produced by the action of
   acids on metallic sulphides, and is an important chemical
   reagent. Called also {sulphureted hydrogen}.