I don’t know what the Belgian detective Hercule Poirot would make of noisy, crowded, grimy Ludhiana, but it was in that city 27 years ago, over the course of an unforgettable summer afternoon, that I became acquainted with his skills. The occasion was a fortnight-long family reunion involving aunts and cousins, to which my main contribution was corrupting the younger children with Friday the 13th videocassettes. Then one day, while the others settled in to their siesta, I settled down with my very first Agatha Christie novel Murder in Retrospect (original title Five Little Pigs, as I learnt much later).

All these years later this still counts among my most intense reading experiences: by the time I reached the last page I was spooked, afraid to turn my head even though it was daytime and street sounds were drifting through the curtained windows (not to mention the snores from adjacent rooms). The image that most haunted me was the one of an artist painting his muse in the final hours of his life, unaware, at least at first, that he was dying; that a slow-moving poison was working its way up to his heart. At what point did awareness come, if it came at all, for poor lovelorn Amyas Crale – a few minutes before the end, when paralysis had set in, or only in the final seconds?

Vivid though my memories of Murder in Retrospect were, I didn’t remember, until I read Kathryn Harkup’s A is for Arsenic, that the poison in Christie’s novel was coniine, extracted from hemlock – the same plant that was also famously used in Socrates’s death in 399 BC. Nor did I know that Christie’s choice of killing method was a carefully thought-out one, based on the nature of the story and the time-frames involved.

The right writer

If, as the cliché has it, you should write the book that only you and no one else can write, then A is for Arsenic, subtitled “The Poisons of Agatha Christie”, might be described as a near-perfect marriage of author and subject. Harkup, her profile tells us, is a chemist, an Agatha Christie fanatic, and a “freelance science communicator, specialising in the quirkier side of science”. This put her in a very good position to author a work that – whimsically and thrillingly – combines popular science and trivia with a form of literary commentary.

She looks closely at fourteen of the poisons used in Christie’s novels and short stories, from well-known ones like cyanide, strychnine and phosphorus to relatively obscure ones like monkshood (a saintly name for Aconitum variegatum) and ricin (lethal in tiny doses, less than 1 mg being enough to kill an adult human). While discussing the part they play in the stories, she also casts her net wider by providing contextual information about the poisons – their history, effects, symptoms, antidotes, molecular makeups, real-life cases involving their use.

This means the book has many balls in the air: to enjoy it in its entirety, to read each page with enthusiasm – as opposed to glossing over a few pages every now and again – you have to be an Agatha Christie buff while also having some aptitude for technical explanations of biological and chemical processes. Though a keen reader of popular science, I couldn’t always keep up with some of the stuff about chemicals and alkazoids. Sentences like “Myasthenia gravis patients produce antibodies circulating in the body that block acetylcholine receptors” and “Ricin is a toxalbumin, a poisonous protein formed from two chains, A and B, which are linked by a single bond between two atoms of sulfur” are unlikely to produce a throb of excitement unless you were paying closer attention than I was during chemistry classes in school.

What helps, though, is that Harkup never dwells on the heavy stuff for long; she intersperses it with juicy trivia or speculation. How intriguing it is to hear that digitalis – an extract from the foxglove plant – might have played a part in the halo-like sheen associated with the paintings in Vincent Van Gogh’s “yellow period”. Or that the use of arsenic-based dyes in wallpaper – common in the nineteenth century – may have contributed to Napoleon’s death. Or that rats in lab tests had to be physically stopped from pressing levers to enjoy nicotine’s effects, because it was keeping them from eating or sleeping.

There are also droll stories such as the one about a defence lawyer doomed to be known as “Apple-pips Kelly” for the rest of his career because he committed the gaffe of suggesting that a murder victim had died after accidentally ingesting cyanide from apple pips. (Thousands of such pips would have had to be eaten – and well-chewed too – for this to happen.)

A poison pen

Agatha Christie, with her training as an apothecary’s assistant during the First World War, was careful with the facts while detailing poison use – even in novels where much detail wasn’t required – but Harkup mentions the rare occasions where she got something wrong: for example, in the short story The House of Lurking Death, a victim is bumped off with a dose of ricin in a cocktail; but in reality, the high alcohol content would have led the protein to unfold, losing much of its potency. Even here, though, Christie’s mistake can be pardoned given that when she wrote the story not very much was known about this poison, and there wasn’t even a test available to identify its presence.

Harkup keeps her own book as spoiler-free as possible, but her plot discussions sometimes let slip a little too much information. Occasionally a spoiler alert is provided: while discussing Sad Cypress (“O is for Opium”), Harkup warns us that she has to disclose the murderer’s identity, because “explaining how a poisoner can kill only one person at a lunch where three people ate the same food, without revealing the murderer, would result in some very convoluted descriptions”. Which is fine: you can easily skip the last three pages of that chapter and return to them after reading Sad Cypress.

But things get a little trickier elsewhere, as in the chapter “D is for Digitalis”, which deals with another of my favourite Christie novels, Appointment with Death: there is no spoiler warning here since Harkup doesn’t actually name the murderer, but the last two paragraphs of the chapter give the reader crucial information that helps in eliminating a large number of suspects.

Which means you have to be careful while reading this book. But that’s a small price to pay for such an engaging ride. Being a true-crime buff (in addition to a Christie fan), I particularly enjoyed the references to poisonings of the past: such as the 1850 murder of Gustave Fougnies by his brother-in-law and sister (this story shows that committing and covering up a murder, even when you’re using a non-bloody method like poisoning, and even when you do it in the privacy of your large chateau, can be a messy and risky business); or the labours of psychotic Graham Young who killed his stepmother – thallium was his weapon of choice – as a 15-year-old, and after his release from psychiatric hospital dedicated himself to poisoning one person for each year that he had been incarcerated.

These nasty anecdotes are reminders that with all her attention to detail and meticulous plotting, the Queen of Crime was – at least in the gruesomeness stakes – often upstaged by real life. That apart, Harkup’s book will almost certainly deepen your appreciation for the spadework and thought that went into the breeziest Agatha Christie thriller.

A is for Arsenic: The Poisons of Agatha Christie, Kathryn Harkup, Bloomsbury.