There is only one thing in the whole world from which our Mercury can be obtained.  It is like gold in essence, but different in substance, and if you change its elements you will have what you seek. Join heaven to earth in the fire of love, and you will see in the middle of the firmament the bird of Hermes. Do not confound the natures, but separate and re-combine them, and you will reign in honour all your life.

Spagyrics, by Manfred M Junius. has a very good description of how to seperate the Mercury in  Plants