Definition of "ferrumination"

Now look out in the GRADUS for Purus, and you find as the first synonime, lacteus, for coloratus, and the first synonime is purpureus. I mention this by way of elucidating one of the most ordinary processes in the ferrumination of these Centos.

1817, S[amuel] T[aylor] Coleridge, Biographia Literaria; or Biographical Sketches of My Literary Life and Opinions, volumes (please specify |volume=I or II), London: Rest Fenner […]