what does the fireman in Seattle do to the Mechanical Hound?

The question is a little vague. Initially, Beatty sends the Mechanical Hound to Montag's house because Beatty suspects that Montag is hording books. This is done as a not so subtle warning to Montag that he needs to watch himself.

By the end of the book, Montag is being hunted by another mechanical hound. Montag plunges into the river to escape. Montag meets up some "book men" who give him something to ingest that changes his scent.