III.] The action of the mineral agent in perfecting substances is often likened by the alchemists to the conjoining of the male and the female, followed by the production of offspring.

They insist on the need of a union of two things, in order to produce something more perfect than either.

The agent, they say, must work upon something; alone it is nothing.
The methods whereby the agent is itself perfected, and the processes wherein the agent effects the perfecting of the less perfect things, were divided into stages by the alchemists.

They generally spoke of these stages as _Gates_, and enumerated ten or sometimes twelve of them.

As examples of the alchemical description of these gates, I give some extracts from _A Brief Guide to the Celestial Ruby_.
The first gate is _Calcination_, which is "the drying up of the humours"; by this process the substance "is concocted into a black powder which is yet unctuous, and retains its radical humour." When gold passes through this gate, "We observe in it two natures, the fixed and the volatile, which we liken to two serpents." The fixed nature is likened to a serpent without wings; the volatile, to a serpent with wings: calcination unites these two into one.