The discovery that the Pale Horse kills its victims with thallium

The discovery that the Pale Horse kills its victims with thallium is the decisive reversal in Mark Easterbrook’s investigation: what had seemed to be a supernatural service for causing death proves to be a practical poisoning scheme. A recurring sign—hair coming out in handfuls—links otherwise dissimilar illnesses, while the organization’s elaborate rituals and its clients’ separation from their targets help make murder look like natural disease.

The clue in the hair

The breakthrough begins with an account of Lady Hesketh-Dubois’s final illness. While discussing her, a woman recalls that Lady Hesketh-Dubois had been in good health until shortly before her decline, and that her thick white hair came out in handfuls on the pillow. The detail brings two apparently unrelated cases to mind: Mary Delafontaine, whose hair had also fallen out, and a girl Mark had seen in a Chelsea coffee place, where another girl pulled at her hair. The girl had said it did not hurt. Mark had first taken that response for bravery; now he understands it as evidence that the hair was already coming loose unusually easily.

Hair loss changes the meaning of these scattered observations. Rather than treating each death or illness as an isolated medical misfortune, Mark recognizes a repeated physical symptom that may point to a common cause. When he asks whether Katherine Corrigan’s hair is also coming out by the roots in handfuls, the answer strengthens his fear that she is suffering from the same process. He rejects fever as an adequate explanation and identifies Thallium salts as the likely poison. At that moment, the question is no longer whether the Pale Horse’s performance can kill at a distance, but whether the poison has been recognized in time to save Katherine Corrigan.

From symptom to poison

Mark’s conclusion draws on medical knowledge he remembers from an article read in New York and has since researched further. That article described factory workers who died one after another, with their deaths attributed to a striking range of conditions, including paratyphoid, apoplexy, alcoholic neuritis, bulbar paralysis, epilepsy, and gastro-enteritis. In another case, a woman poisoned seven people whose diagnoses included brain tumour, encephalitis, and lobar pneumonia. These histories show how varied the apparent illnesses can be, allowing a poisoning to escape notice behind diagnoses that seem unrelated.

The reported symptoms also vary in their onset. They may begin with digestive distress, an intoxicated state, or pain in the limbs; the latter can be mistaken for polyneuritis, rheumatic fever, or polio. One patient was placed in an iron lung, and skin pigmentation may also occur. Against that shifting catalogue, hair loss stands out as the sign that eventually recurs. Mark recalls that Thallium salts had once been used to remove hair, particularly in children with ringworm, before the danger became clear. The substance is also described as tasteless, soluble, and easy to obtain, and as being used mainly for rats. Its ordinary availability and unobtrusive character make it suitable for a crime designed not to look like one.

The medical explanation is subsequently verified: the suspected poison is confirmed as thallium. That confirmation replaces conjecture with a concrete cause of illness and death. It also gives Mark’s scattered recollections a new order: the unusual hair loss is not an incidental resemblance between cases, but a diagnostic clue that exposes a shared method.

The occult performance as cover

The revelation strips the Pale Horse’s public spectacle of its apparent power. Thyrza Grey’s talk of rare poisons, Sybil Stamfordis’s trance, Bella Webb’s rites, the white cockerels, brazier, pentagrams, and reversed crucifix form a display intended to impress clients, not to cause death by magic. Mark reasons that the show offers different kinds of reassurance to different audiences: occult claims attract the superstitious, while the supposed machine can appeal to people who distrust spirits but are susceptible to scientific-sounding explanations. He imagines that its bulbs and humming valves are theatrical apparatus, not a device capable of killing remotely; the available evidence establishes that the cause of death is poisoning, not any supernatural force.

That distinction is central to the scheme’s protection. Thyrza Grey can boast about occult powers without providing a practicable explanation of a murder for a court to test. The machine, too, can be dismissed as harmless stagecraft. By directing suspicion toward impossible powers, the performance diverts attention from an ordinary poison and the human actions that put it into use. The occult setting is therefore not the mechanism of death but a screen behind which a more mundane method can operate.

Distance, substitution, and concealment

Lejeune’s explanation of the operation shows how the poisoning is kept apart from the client. The client visits the Pale Horse and sees a performance by Thyrza Grey and her associates. Separately, women employed by consumer-research concerns canvass a neighbourhood with questionnaires about products residents use, including bread, cosmetics, tonics, sedatives, and remedies for indigestion. Because people are accustomed to answering surveys, the questions can quietly establish which ordinary product a prospective victim regularly takes.

The final substitution is made in person by someone posing as a worker with a plausible reason to enter the home. The visitor might appear to be a porter, a meter reader, a plumber, an electrician, or another tradesman, and may carry credentials that look appropriate. Under the cover of examining a meter, testing water pressure, or attending to pipes, the visitor replaces a familiar product with a similar preparation containing the poison. The target may continue using what appears to be an ordinary household article, with no obvious reason to suspect it.

Illness follows only after a delay, and a doctor has little reason at first to question a product the patient has used for years. The client, meanwhile, has stayed away from the intended victim: there is no apparent interested party with access to food or drink, and no purchase of poison that can be traced to the person who commissioned the death. Lejeune describes the actual substitution as an act performed by the scheme’s originator, whose identity is concealed from the other participants. The visitor leaves and is not seen in the neighbourhood again, leaving no continuing presence to connect the client, the survey, and the victim’s illness.

False certainty and investigative significance

The discovery also clarifies why confidence and evidence must be tested against the conditions in which they arose. Zachariah Osborne had given Lejeune an unusually detailed description of the man he said followed Father Gorman: a tall, very thin figure with sloping shoulders, long hair beneath a Homburg, a prominent nose, and a pronounced Adam’s apple. Lejeune initially considered that the account might come from an unusually observant witness rather than an imagined portrait. Osborne later explained that the fog had cleared in patches and that the man’s lighter had illuminated his profile, apparently accounting for the detail.

But Lejeune’s examination of the distance and visibility leads him to challenge what Osborne could have seen. In particular, he judges that the Adam’s apple could not have been distinguished across the street on a foggy night. Osborne’s false account draws investigative attention to him; Lejeune later explains that the lie may at first have been an attempt to make himself important, but it focused scrutiny on a man who wanted to appear more consequential than he was. This strand does not identify the poison, but it forms part of the investigation’s movement away from appearances and asserted certainty toward facts that can be examined.

Taken together, the medical clue and the reconstructed method reverse the apparent logic of the Pale Horse. The rituals suggest distant supernatural force; the evidence instead points to an accessible poison, familiar products, delayed symptoms, and a visitor whose connection to the victim is deliberately obscured. The discovery makes the deaths intelligible without making the scheme simple to trace: the poison supplies the cause, while the organization’s division of roles and its theatrical misdirection conceal how that cause was delivered.