What is "aerial acid"

aerial \aerial\, Aerial \A*["e]"ri*al\, a. [L. a["e]rius. See Air.]

1. Of or pertaining to the air, or atmosphere; inhabiting or frequenting the air; produced by or found in the air; performed in the air; as, a["e]rial regions or currents; the a["e]rial maneuvers of a fighter plane. ``A["e]rial spirits.''
   --Milton. ``A["e]rial voyages.''
   --Darwin.

2. Consisting of air; resembling, or partaking of the nature of air. Hence: Unsubstantial; unreal.

3. Rising aloft in air; high; lofty; as, a["e]rial spires.

4. Growing, forming, living, or existing in the air, as opposed to growing or existing in earth or water, or underground; as, a["e]rial rootlets, a["e]rial plants; the aerial roots of a philodendron.
   --Gray.

5. Light as air; ethereal.

6. operating or operated overhead especially on elevated cables. aerial conveyers for transporting raw materials

7. operating or moving in the air. an aerial cable car; aerial combat

   Aerial acid, carbonic acid. [Obs.]
   --Ure.

   Aerial perspective. See Perspective.