'Fringe' recap: You can ring by Bellllllll, ring my Bell!
In solving the case, though, Walter's puzzled that the combination of heavy elements osmium and rhenium should be lighter than air. But he's got it: the physics of our universe are starting to go all wonky, thanks to the rift between universes he made twenty-five years ago.
And bookending the main plot is (along with Walter smoking up with Jorge Garcia at Massive Dynamic) is Walter's obsession with bringing William Bell back to life, because Walter's still worried that the missing pieces of his brain are preventing him from being as totally universe-saving as he can possibly be. So: soul magnets.
The bell that William, not coincidentally, left to Nina early this season. Walter science rings science the bell science to make William science appear science but it science appears not to work science. At least, not that Walter's aware. It doesn't make William Bell walk in the door, but elsewhere, after an episode of new couple Peter and Olivia being nauseatingly cute in the way couples are when they think they've invented relationships, Olivia's voice suddenly gets gravelly and more Leonard Nimoy-esque. Yes, Walter's soul is apparently in the Olivia's body. If nothing else that should make Peter more aware of the differences between Olivia and Fauxlivia. I imagine it will make things a little more difficult in the bedroom, unless he's a really big Star Trek fan.