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The Collaborative International Dictionary of English v.0.48 (gcide)

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]