What is "to touch upon"

Touch \Touch\, v. i.

1. To be in contact; to be in a state of junction, so that no space is between; as, two spheres touch only at points.
   --Johnson.

2. To fasten; to take effect; to make impression. [R.]

   Strong waters pierce metals, and will touch upon gold, that will not touch upon silver.
   --Bacon.

3. To treat anything in discourse, especially in a slight or casual manner; -- often with on or upon.

   If the antiquaries have touched upon it, they immediately quitted it.
   --Addison.

4. (Naut) To be brought, as a sail, so close to the wind that its weather leech shakes. To touch and go (Naut.), to touch bottom lightly and without damage, as a vessel in motion. To touch at, to come or go to, without tarrying; as, the ship touched at Lisbon. To touch on or To touch upon,

   1. to come or go to for a short time. [R.]

      I made a little voyage round the lake, and touched on the several towns that lie on its coasts.
      --Addison.

   2. to discuss briefly, as only a small part of a discourse.