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to indicate that cyanide had been used. In fact, it’s not that cyanide smells of bitter almonds; bitter almonds smell of cyanide.
    Methods for distilling cyanide from natural sources have been known for thousands of years. One of the first references to poisoning dates from an ancient papyrus that refers to alethal punishment for speaking the name of God: ‘repeat not the name I.O.A. [i.e. ‘Jehovah’], under the penalty of the peach tree’; 35 indeed, in Agatha Christie’s Hallowe’en Party the murderer commits suicide by drinking a liquid smelling of peaches. Laurel leaves are another natural source of cyanide that has been used since the time of the Romans, giving ‘Caesar’s Laurel Crown’ in William Blake’s poem a particularly sinister symbolism.
    Natural sources of cyanide have also been used in more recent times. In 1845 apple pips were used as a defence in the trial of John Tawell, who was accused of murdering his mistress Sarah Hart with cyanide. 36 Tawell was known to have purchased prussic acid from a pharmacy some time before Hart died. Prussic acid was known to be a dangerous substance but Tawell claimed the prussic acid was for ’external application’. At the time there was a vogue for using solutions of prussic acid as a skin lotion. One commercial product, ‘Dr Eliotsom’s Lotion of Prussic Acid’, was recommended for moistening the skin before and after shaving. Lethal amounts of prussic acid can be absorbed through unbroken skin; shaving cuts and abrasions could only have eased the absorption. Thankfully the fashiondid not last. Tawell’s purchase was added to a bottle of beer, which Sarah drank. A neighbour saw Tawell leave the house, and hearing Sarah’s cries went to see if she was all right. She found Sarah writhing on the floor in agony, and frothing at the mouth. She died before the doctor arrived.
    The police were alerted and raced after Tawell, but they couldn’t stop him before he boarded a train for London. A telegraph message was sent to London describing him and instructing the police there to arrest him. It was the first time the telegraph system had been used in this way, and the case generated huge publicity because of it.
    At Tawell’s trial, the defence barrister, Sir Fitzroy Kelly, proposed that the cyanide that had killed Sarah had come from the pips of the apples she was so fond of eating. The lethal dose of apple pips is about 200g; Sarah would have had to eat thousands of apples to ingest enough pips, and they would also have had to be well chewed to release the poison. The jury were unimpressed, and he was found guilty. Tawell was executed for his crime and his barrister was known as ‘Apple-pips’ Kelly for the rest of his career.
    Bitter almonds and apricot kernels contain considerably more cyanide-containing amygdalin than apple pips; just a few apricot kernels can be deadly, but there is another plant that contains even more cyanide. Cassava is probably the most dangerous cyanide-containing plant regularly consumed by humans. It forms an important part of the diet of millions of people living in the tropics. Bitter cassava can contain 1g of cyanide in every kilogram of root, in the form of two compounds, linamarin and lotaustralin, which share a close similarity with amygdalin (all three are in the same class of compounds, the cyanogenic 37 glucosides). Sweet cassava is a different variety that contains considerably less cyanide, but people tend to grow the bitter variety as it is more resistant to pests and, quite possibly, to thieves. Eating a few mouthfuls ofraw bitter cassava is unlikely to kill you straight away, but it will cause severe illness. Raw cassava must be processed to remove the cyanide by grinding the vegetable into flour and soaking it in water for anything between five hours and three days (variations depend on cassava variety and local tradition). The soaking process allows the enzyme luminase, which is also present in the vegetable, to