Board Thread:Questions and Answers/@comment-11935184-20160907061010/@comment-4802891-20160907062827

Mercury (quicksilver) is one the three primary components of any material in Alchemy alongside sulfur and salt. Quicksilver represents the spirit (differing from the soul and body).

I'm not sure what's the link between magical energy and the spirit in the Nasuverse, but this might be the relation you are searching for. That's just a guess.