Like the great work of alchemy itself, Mercurius is a playful, paradoxical, mysterious and deeply illuminating book. It is both a profound treatise on alchemy in novel form and an intriguing novel about the quest for the Philosopher’s Stone which allegedly transmutes base metal into gold, prolongs life and even brings about immortality. In lesser hands, the two aspects might not coagulate, but in this case the marriage proves a blissful union. Patrick Harpur has triumphed in this ambitious, imaginative and entertaining work.

In the introduction, the author claims that a mysterious woman, possibly a former girlfriend, leaves two bags of papers on his doorstep. He organises them into two narratives, one chapter of each following the other. They recount the story of Eileen, a woman in her thirties who suddenly decides to leave her boyfriend and job in publishing and rents a damp and dilapidated vicarage in an inward-looking village in the West Country. She gradually discovers that the previous tenant was called John Smith, a lonely and sensitive vicar engaged in alchemical work a generation earlier in the 1950s. In the burnt-out cellar, she finds in the side of a well a mysterious piece of stone and a manuscript which records the vicar’s alchemical experiments, his philosophical speculations and his tortured emotional and sexual life.

She starts writing, first in the form of letters to her previous boyfriend and then more directly for herself, in an attempt to interpret the meaning of alchemy, to understand the tragic fate of the vicar, and to come to terms with her own troubled self and past. We are drawn irretrievably into their worlds, wondering whether they will ever achieve the longed-for ‘Marriage of Heaven and Earth’ in their troubled lives.

Apart from the device of presenting alternate chapters written by Smith and Eileen, the novel is divided up into eight sections: calcination, solution, separation, conjunction, putrefaction, congelation, sublimation and projection. They not only mark the principal stages in the alchemical process but mirror the transformations which take place in the lives of the two central characters. At the same time, there is a rich array of mysterious secondary characters whose lives are intimately connected. Who is her landlady, the eccentric Mrs Zetterberg, who lives in the manor with the severely disfigured Pluto? What troubles the reclusive potter who is trying to replicate the blue glass of Chartres? What is the relationship between the young Nora and the vicar, and what becomes of her? Why does his dwindling congregation turn against him with such disastrous consequences? Indeed, for the most part the villagers remain a constant and menacing presence, ready to condemn and destroy those who dare step out of everyday moral and intellectual confines in the forbidden search for hidden truth and sacred wisdom.

The gripping plot, the well-drawn characters and the central mystery would make an exciting and page-turning novel on their own. But Harpur offers not only a profound psychological study of people going through a crisis but also an erudite and well-informed account of the concepts and practices of alchemy. Indeed, for those unfamiliar with the complex subject nearly sixty pages of explanatory notes are added. The result is one of the best and most insightful studies of alchemy that I have ever read.

In order to understand Smith’s experiments and writings and his quest for the Philosopher’s Stone, Eileen undertakes her own alchemical studies. I was not entirely convinced by her anthropological approach, which argues that all humans divide the world into opposites and transform Nature into culture through fire. More persuasive is her Jungian interpretation, which sees the figures in alchemy as archetypes, the Philosopher’s Stone as a symbol of psychic wholeness, and the stages of alchemy as mirroring the process of self-realisation.

But ultimately the novel goes beyond both to recognise that alchemy is the art and science of body, mind, spirit and above all soul, a subtle amalgam of ideas and practices which can reveal ultimate truths about the universe and our place within it. With its roots in the Hermetic tradition, which first emerged in Egypt, it recognises the intimate correspondence between heaven and Earth (‘as above, so below’), the microcosm and the macrocosm (‘as within, so without’) and unity in diversity (‘all is one’). It is a worldview which is partially reflected in modern physics and ecology but goes beyond both to insist that spiritual inner work is as important as outer experimentation. Sensitive and alert seekers might find all they need in this magisterial work.

Peter Marshall is the author of many books, including The Philosopher’s Stone: A Quest for the Secrets of Alchemy (Pan) and Riding the Wind: A New Philosophy for a New Era (Continuum).