Ammoniac \Am*mo"ni*ac\, Ammoniacal \Am`mo*ni"a*cal\, a. Of or pertaining to ammonia, or possessing its properties; as, an ammoniac salt; ammoniacal gas. {Ammoniacal engine}, an engine in which the vapor of ammonia is used as the motive force. {Sal ammoniac} [L. sal ammoniacus], the salt usually called {chloride of ammonium}, and formerly {muriate of ammonia}.