It's more than likely the female ejaculate, not semen.

Considering that, it is what I'm thinking as well.

I can't figure out what else it could be, so it's the likely culprit.

There is no clear analogue in the Earthly sciences, and I'm sure the Cabal have made sure of that...

So what's left is in Anatomy.

For clarification, Quicksilver/Mercury to the alchemist is not the same as the silvery stuff often found in a glass thermometer... The text is encoded, and things have 7 meanings (at least) in order to confuse those who would seek the prize without truly understanding the process.

In the first place, you must procure our Mercury; common Mercury will produce no effect if you operate on it till doomsday.

Thus you see that in our Art we have two Sulphurs and two Mercuries (i.e., Sulphur, and Mercury of Mercury, and Sulphur and Mercury of gold), but their only difference consists in degrees of perfection and maturity. Now, the perfect body of gold is reduced to its (two) first principles by means of our Divine water which does not wet the hands (viz., Mercury and Sulphur). This operation for a time gives the ascendancy to the female agent; but this being unnatural, the male agent soon reasserts itself, and by means of its heat dries up the moisture of the female agent, and—through calcination—converts it all into a most subtle and viscous powder, which powder is then changed by dissolution into a water, in which the spirits of the solvent and the thing dissolved, the male and the female principles, are mingled.