Image change in photographic negative

Can any of you photo types enlighten me?  The Louisiana State
Archives has several large format (8x10) negatives taken on the
scene of Huey Long's assassination.  On several of them there is a
shifting of the negative image to a positive one.  The result is a
negative in some areas and a positive transparency on other areas.
On one the shift occupies roughly one half of the entire negative.
Others are less affected by this phenomenon, and still others--from
the same collection--are not affected at all.

What, chemically speaking, is going on here?  Can any treatment stop
the process?

Doug Harrison
Conservator
Louisiana State Archives

                                  ***
                  Conservation DistList Instance 12:92
                   Distributed: Friday, May 28, 1999
                       Message Id: cdl-12-92-006
                                  ***