  The nucleation of the first order phase transition of superfluid 3He-B from
superfluid 3He-A is quite remarkable since it requires a seed of the order of a
micron. We have studied this nucleation for 3He confined to a very dilute
silica aerogel. This dirty superfluid behaves in a manner similar to previous
reports for the pure superfluid. But we have discovered a novel magnetically
driven nucleation switch acting on the pure superfluid B-phase. Lastly, we find
the surprising result that the proximity effect between the pure and dirty
superfluids at their interface is insufficient to nucleate the B-phase in
either superfluid.
