Why is "Missouri" pronounced with -z-

[Mark A. Mandel]

Back in prehistory Gerald Cohen asked:
>>>>>
     I suppose this is very basic for the dialecticians in our group, but
I've often wondered about it: Why is "Missouri" (with a double "s")
pronounced with a "z" (Mizzouree or Mizzourah)?  Similarly, "possess" is
pronounced with a "z" (after the "o").
  <<<<<

Also

        scissors
        dessert
        dissolve

And sometimes, in similar though not identical exceptions,

        electri<c>ity
        equa<ti>on

A Happy New Year to all!

m a m