Potassium cyanide

Potassium cyanide is the swift, near-instantaneous poison that the murderer on Indian Island uses to dispatch two of the ten victims, and its presence—and the impossibility of accounting for it as an accidental substance—becomes one of the earliest and most damning pieces of evidence that the deaths are not suicides or accidents but deliberate killings. The poison acts by asphyxiation, producing a characteristic purple, convulsed face in its victims, and it leaves no distinctive smell of bitter almonds when it is the potassium salt rather than the pure acid. Its very nature as a substance that no ordinary person carries about, and that requires deliberate procurement and administration, forces the surviving guests to confront the reality that a calculating murderer is among them.

Properties and Lethal Action

Potassium cyanide kills with extraordinary speed. When Dr. Armstrong examines Anthony Marston's body after the young man collapses at the dinner table, he notes that the death was caused by asphyxiation and that the symptoms point to one of the cyanides. He finds no distinctive smell of Prussic acid, which leads him to conclude that the poison was probably potassium cyanide, a substance that acts "pretty well instantaneously." The victim's face is contorted and purple, and the glass from which he drank contains the fatal residue. Later, when Emily Brent is found dead in the dining-room chair, Armstrong again identifies cyanide as the cause—this time injected by a hypodermic syringe—and notes that she must have died almost immediately by asphyxiation, with the same suffused, blue-lipped appearance.

The Problem of Its Origin

The presence of potassium cyanide on the island creates an immediate logical puzzle. As Philip Lombard points out during the discussion of Marston's death, nobody in their senses carries potassium cyanide about with them; it is not the kind of substance one takes in a waistcoat pocket. The only innocent explanation Armstrong can offer—that someone might carry it for destroying a wasps' nest—does not fit Anthony Marston, a wealthy young man who has come to the island for a holiday and who shows no sign of intending to kill himself. The impossibility of accounting for the poison as an accidental or suicidal substance is what first convinces Lombard and Armstrong that Marston was murdered, and that the murderer must be someone else among the party.

The Murderer's Use of the Poison

In his confession, Lawrence John Wargrave explains exactly how he obtained and deployed the potassium cyanide. He notes that the substance is easily obtained by householders for putting down wasps, and that he had some in his possession. He slipped it into Marston's almost empty glass during the tense period after the gramophone accusation, when everyone was distracted and moving about the room. For Emily Brent's death, he used a more elaborate method: he first drugged her coffee with chloral hydrate at breakfast, then, when she was left alone in the dining-room and nearly unconscious, he injected a strong solution of cyanide into her. The bumblebee that Vera Claythorne noticed at the window was a piece of theatrical staging—the murderer's playful adherence to the nursery rhyme's line about a bee stinging one of the little Indian boys—but the actual killing was done by the hypodermic syringe loaded with cyanide.

Narrative and Thematic Significance

Potassium cyanide functions in the novel as the material embodiment of the murderer's method: clean, swift, and impossible to trace back to its source once the killer has covered his tracks. It is the poison that cannot be explained away, the substance that shatters any remaining pretense that the deaths on Indian Island are natural or accidental. Its presence forces the characters to acknowledge that they are trapped with a deliberate killer, and its use—first in a drink, later by injection—demonstrates the murderer's flexibility and his willingness to adapt his methods to the circumstances. The poison also serves as a marker of the murderer's sophistication: he has planned ahead, obtained the substance discreetly, and administered it in ways that leave no forensic trail that could lead back to him. In the end, the potassium cyanide is one of the tools that allows Lawrence John Wargrave to execute his grand design of extra-legal justice, killing those whom the law cannot touch, before he himself dies by a bullet rather than by his own poison.