According to my archives of old Netnews postings on Christmas lights,
the _new_ bubble lights use methylene chloride, something you don't
want to breathe much of. My understanding -- and that's all it is --
is that older lights use alcohol; which type of alcohol I couldn't
say. In any case, inside the bubble tube is a vacuum except for the
vapors of the liquid, to encourage boiling at low temperatures.
There's quite a story -- I forget where I read it -- about how
Wurlitzer "borrowed" the bubbler concept from a salesman who
demonstrated a bubbling sign for advertising purposes.
Any jukebox restoration dealer should know what's in those long bubble
tubes. They're still being made.