I was under the impression it wasn’t swearing unless it was on a Bible. (Otherwise it is affirmation).
This is the first I’ve heard that JQA swore on a law book.
Previously I heard that only Franklin Pierce had used the affirmation option.
http://www.inaugural.senate.gov/swearing-in/bibles
Hmm...the story about JQA appears to be true....no mention though about Pierce affirming.