Acid \Ac"id\, n.
   1. A sour substance.
      [1913 Webster]

   2. (Chem.) One of a class of compounds, generally but not
      always distinguished by their sour taste, solubility in
      water, and reddening of vegetable blue or violet colors.
      They are also characterized by the power of destroying the
      distinctive properties of alkalies or bases, combining
      with them to form salts, at the same time losing their own
      peculiar properties. They all contain hydrogen, united
      with a more negative element or radical, either alone, or
      more generally with oxygen, and take their names from this
      negative element or radical. Those which contain no oxygen
      are sometimes called hydracids in distinction from the
      others which are called oxygen acids or oxacids.
      [1913 Webster]

   Note: In certain cases, sulphur, selenium, or tellurium may
         take the place of oxygen, and the corresponding
         compounds are called respectively sulphur acids or
         sulphacids, selenium acids, or tellurium acids.
         When the hydrogen of an acid is replaced by a positive
         element or radical, a salt is formed, and hence acids
         are sometimes named as salts of hydrogen; as hydrogen
         nitrate for nitric acid, hydrogen sulphate for
         sulphuric acid, etc. In the old chemistry the name acid
         was applied to the oxides of the negative or
         nonmetallic elements, now sometimes called anhydrides.
         [1913 Webster]