To touch on

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. [1913 Webster]

2. To fasten; to take effect; to make impression. [R.] [1913 Webster]

Strong waters pierce metals, and will touch upon gold, that will not touch upon silver. --Bacon. [1913 Webster]

3. To treat anything in discourse, especially in a slight or casual manner; -- often with on or upon. [1913 Webster]

If the antiquaries have touched upon it, they immediately quitted it. --Addison. [1913 Webster]

4. (Naut) To be brought, as a sail, so close to the wind that its weather leech shakes. [1913 Webster]

{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}, (a) to come or go to for a short time. [R.] [1913 Webster]

I made a little voyage round the lake, and touched on the several towns that lie on its coasts. --Addison. [1913 Webster] (b) to discuss briefly, as only a small part of a discourse. [PJC]