Melville transforms the shaggy minutiae of life and its
myriad characters (whether Hawthorne, Malcolm, a
besieged wife or a shipmate) into an alembic of wishes,
conflicts and disappointments that, taken together, reflect
him, a mysterious, roiling, poignant writer alive, painfully
alive, in every phrase he wrote." Brenda Wineapple, Melville
at Sea, The Nation (New York), May 20, 2002.

I think the passage quoted using alembic is not well written. An alembic was a flask with a neck
that bent a bit more than ninety degree, used to separate components of a liquid mixture according to their boiling points. Turning characters into an alembic full of abstractions seem a goofy metaphor to me.