What is "spirits of hartshorn"

Ammonia \Am*mo"ni*a\, n. [From sal ammoniac, which was first obtaining near the temple of Jupiter Ammon, by burning camel's dung. See Ammoniac.] (Chem.) A gaseous compound of hydrogen and nitrogen, NH3, with a pungent smell and taste: -- often called volatile alkali, and spirits of hartshorn.

n. (context obsolete English) A mixture of ammonia and water.