Image is from Theatrum Chemicum Britannicum first published in 1652, an extensively annotated compilation of English alchemical literature selected by Elias Ashmole. The book preserved and made available many works that had previously existed only in privately held manuscripts. Alchemists believed that they could not work without the help of god. The believed there was a hidden spirit hidden in the prima materia and this was interpretetted as the Holy Spirit. The dragon represents the prima materia being fertilized by the Holy Spirit (the avis Hermetis). 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.