Leng's Arcanum was a chemical formula created by 19th century scientist Enoch Leng. Its purpose, development, abandonment, and attempted re-creation are central to the events surrounding the Surgeon Copycat Killings in The Cabinet of Curiosities.
In the late 19th century, New York scientist Enoch Leng perfected a formula that allowed him to extend human life indefinitely. He developed the formula after years of study and tested it on himself and his young orphan ward, Constance Greene. The serum, taken regularly, allowed Leng to live over 160 years, and caused Constance to barely age from the late 1880s until 2002.
The exact composition of Leng's formula is not known (Pendergast burned the only known recipe shortly after its discovery) but it involved chemicals found in the human body, including opiates. Pendergast noted that "it involves fairly straightforward biochemistry...any reasonably talented chemistry graduate student could perform [it] in a well-equipped laboratory. But there's a trick involved...which makes it unlikely that it will be independently rediscovered-at least not in the foreseeable future."
In the early years of his experiments, Leng got the necessary chemicals by killing people and excising the bundle of nerves at the base of the spine, called the cauda equina. At first his victims were visitors to J.C. Shottum's Cabinet of Natural Productions and Curiosities, a business located on the ground floor of the building where he had a laboratory.
After the owner of the cabinet discovered the bodies, Leng murdered him and burnt the building to the ground. He moved his laboratory to an abandoned waterworks on Doyers Street and brought orphaned children from the local workhouses back there to kill them. By 1885, Leng had perfected the arcanum and achieved effective immortality. From that point, he killed infrequently, only when he ran out of the necessary chemicals to make another batch of serum. Also around this time he adopted Constance Greene, possibly out of guilt after learning that her older sister and caretaker, Mary Greene, had been one of his early victims. Constance herself became a test subject for the arcanum.
Perfection and Purpose
Leng saw his serum as a means to an end. He predicted he would need several lifetimes to achieve his ultimate goal: ridding earth of the human race Leng had decided that humanity was a cancer on the earth and needed to be eliminated before it destroyed the world entirely. He built a laboratory inside his sprawling mansion on Riverside Drive and began to experiment with poisons and other means of causing mass death. Meanwhile, he further refined his arcanum until he could synthesize the chemicals and no longer needed to kill for them.
In March 1954, after hearing of the Castle Bravo nuclear weapons test, Leng decided that his work was unnecessary. He was convinced that humanity would soon extinguish itself without his help. He stopped taking the arcanum, although he continued to produce it for Constance.
The effectiveness of the arcanum as taken by Constance Greene seems to slow the aging process by around 90%. Constance began taking it at the age of twelve; over 130 years later, she appears to be a young woman in her twenties, suggesting she has aged approximately one year for every ten years that have passed.
Revival and Involvement of Pendergast
Some time before 2002, real estate developer and New York Museum donor Anthony Fairhaven learned of the arcanum. Fairhaven was obsessed with extending his life after watching his brother die of progeria as a boy. He tracked down Leng and tortured him in an attempt to learn the formula. Leng died before sharing the secret, but Fairhaven found Leng's notebooks in the basement ruins of the Cabinet of Curiosities and learned enough of the process to begin killing people and extracting the cauda equina from their bodies. The killings that followed were known as the Surgeon Copycat Killings.
Pendergast, Nora Kelly, and Bill Smithback followed Fairhaven to the mansion on Riverside Drive. They found Leng's body and fought with Fairhaven himself. Fairhaven died after coming into contact with poisons in Leng's laboratory and Pendergast buried him in the basement.
After six weeks of searching, Pendergast found the formula somewhere in the Riverside Drive mansion. He read it, then gathered Kelly and Smithback and took them to Gates of Heaven Cemetery where Mary Greene and Leng's other victims were buried. Standing over Mary Greene's grave, he produced a lighter and burnt the formula, ignoring Smithback's protest and saying, "Nothing created out of such suffering can be allowed to exist."
Pendergast noted that ironically, Leng had had the solution to his ultimate goal in hand all along. By releasing the formula to extend human life on the world, he would have caused societal changes that may well have brought down humanity.
The Deleted Epilogue
An unpublished epilogue to The Cabinet of Curiosities depicted a convalescent Pendergast paying a late-night visit to a New York chemist supply shop with an urgent request for a "pretty sophisticated" list of chemicals. Despite arriving minutes prior to the shop's closing time, Pendergast offers to pay triple the cost to have the order fulfilled immediately rather than waiting even until the following morning. Having presumably procured the ingredients of Leng's arcanum, he disappears into the winter night, having appropriated even the shopkeeper's own record of the order, citing "proprietary information."
According to Preston and Child, "We were very, very, conflicted about whether to include this epilogue in the book. Having done so would have had ramifications for all future Preston-Child books in which a certain character appears." They would later state unequivocally that Pendergast had not taken the arcanum.