There is a footnote (45-35) in (Milman's 1845) Gibbon that looks like this:
See Brenkman, Dissert. Ima de Republicâ Amalphitanâ, pp.1-42, ad calcem Hist. Pandect. Florent.
I have not been able to decipher the Ima. I've found a couple of books that detail many of the Latin superscript abbreviations, but I haven't been able to find ma (or Ima) in those references. I've also found the source document (Historia pandectarum Florentinorum), and the De Republica Amalphitana at the end of it, but I see nothing in the title, etc. that would fit Ima, either.
Technically, I don't even know that it's Latin, but given that it's referring to a Latin dissertation in a Latin book, I'm assuming it is. I just haven't been able to track it down.