About two years ago, I sent DW an email asking if he could explain Rosicrutian alchemy to me, and define "philosophical mercury", and this was his response:

I can't pretend to know all the allegorical meanings of the Chemical Wedding of Christian Rosencreutz. I've always understood it to be a metaphorical story about remaking one's self through a series of philosophical examinations/methods. I think that is one of the funniest things about the misinterpretations of alchemy as a literal pseudo science. Mercury (according to my books) represents the "volatile intellect" which is the polar opposite of sulphur which represents the soul.
http://www.levity.com/alchemy/p-philmr.html
I like this quote from a "famous rosicrucian" Julianus de Campis, "our material is of the spirit not of the body".
Just like in Taoism, and Sufism, the thing I love about alchemy is how every single fucking thing is allegory, how everything is a means of talking about something else. It is funny, because everyone I've ever encountered who is deep into this shit uses it as a means of differentiating themselves from the great unwashed masses through hidden knowledge that they've built up...to differentiate them from everyone else. My grandfather was a big fan of the Faire Queene which I've always been scared of reading because I feel like you'd have to have a book five times as big as it to explain it to you.
I'd recommend Yates' book on "the Rosicrucian Enlightenment" which has a chapter on the Wedding, she is much more level headed than all these internet alchemists.
http://books.google.com/books?id=MTvjkifGp04C&printsec=frontcover&dq=frances+yates+rosicrucian+enlightenment&source=bl&ots=nbeHkyMkKh&sig=hjHB7rmTWHtwApQLovCFpiJfTi4&hl=en&ei=puypS4fhIYi4swOrssz3BQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CBQQ6AEwAg#v=onepage&q=&f=false