There are a couple of -in prefixes. One means negation or privation, as in impossible, illiterate, irregular. Another means "'into, in, within; on, upon; towards, against', sometimes expressing onward motion or continuance, sometimes intensive, sometimes transitive, and in other cases with little appreciable force" (OED), as in inflammable (and inflame), irradiate, imperil, ingenious. Both of these are borrowed from Latin and exhibit assimilation with the following sound.