The tail biting dragon represents the prima materia shown discarding the nigredo which then needs to be skimmed off. The prima materia is the original material from which the universe is created. The idea that all things in the world are made, ultimately, from the same substance is the basis for the alchemical theory of transmutation, and the infamous belief that you can turn lead into gold. Nigredo, or blackness, in alchemy means putrefaction or decomposition. The alchemists believed that as a first step in the pathway to the philosopher's stone all alchemical ingredients had to be cleansed and cooked extensively to a uniform black matter. 16th century.