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From The Collaborative International Dictionary of English v.0.48:
Alkali \Al"ka*li\ (?; 277), n.; pl. Alkalis or Alkalies. [F. alcali, ultimately fr. Ar. alqal[imac] ashes of the plant saltwort, fr. qalay to roast in a pan, fry.] 1. Soda ash; caustic soda, caustic potash, etc. [1913 Webster] 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. [1913 Webster] 3. Soluble mineral matter, other than common salt, contained in soils of natural waters. [Western U. S.] [Webster 1913 Suppl.] Fixed alkalies, potash and soda. Vegetable alkalies. Same as Alkaloids. Volatile alkali, ammonia, so called in distinction from the fixed alkalies. [1913 Webster]