The Collaborative International Dictionary of English v.0.48:

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.
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   2. To fasten; to take effect; to make impression. [R.]
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            Strong waters pierce metals, and will touch upon
            gold, that will not touch upon silver. --Bacon.
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   3. To treat anything in discourse, especially in a slight or
      casual manner; -- often with on or upon.
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            If the antiquaries have touched upon it, they
            immediately
            quitted it.                           --Addison.
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   4. (Naut) To be brought, as a sail, so close to the wind that
      its weather leech shakes.
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   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.]
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                I made a little voyage round the lake, and
                touched on the several towns that lie on its
                coasts.                           --Addison.
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      (b) to discuss briefly, as only a small part of a
          discourse.
          [PJC]