Source : Webster's Revised Unabridged Dictionary (1913)

Alkali \Al"ka*li\ (?; 277), n.; pl. {Alkalis} or {Alkalies}. [F.
   alcali, ultimately fr. Ar. alqal[=i] ashes of the plant
   saltwort, fr. qalay to roast in a pan, fry.]
   1. Soda ash; caustic soda, caustic potash, etc.

   2. (Chem.) One of a class of caustic bases, such as soda,
      potash, ammonia, and lithia, whose distinguishing
      peculiarities are solubility in alcohol and water, uniting
      with oils and fats to form soap, neutralizing and forming
      salts with acids, turning to brown several vegetable
      yellows, and changing reddened litmus to blue.

   {Fixed alkalies}, potash and soda.

   {Vegetable alkalies}. Same as {Alkaloids}.

   {Volatile alkali}, ammonia, so called in distinction from the
      fixed alkalies.