From the book When We Cease to Understand the World. The book, a fictionalized retelling of a series of scientific and mathematical discoveries, was published last month by New York Review Books. Translated from the Spanish.

The effects of cyanide are so swift that there is but one historical account of its flavor, left behind in the early twenty-first century by an Indian goldsmith, thirty-two years old, who managed to write three lines after swallowing it: “Doctors, potassium cyanide. I have tasted it. It burns the tongue and tastes acrid.” The note was found in the hotel room he had rented for the purpose of taking his own life. The liquid form of the poison, known in Germany as Blausäure, or blue acid, is highly volatile: it boils at twenty-six degrees Celsius and gives off a slight aroma of almonds, which not everyone can distinguish, as doing so requires a gene absent in 40 percent of humans. This evolutionary caprice makes it likely that a significant number of the Jews murdered with Zyklon B in Auschwitz-Birkenau, Majdanek, and Mauthausen did not even notice the scent of cyanide filling the gas chambers, while others died smelling the same fragrance that would later be inhaled by the men who had organized their extermination as they bit down on their suicide capsules.

Decades before, a precursor to the poison employed by the Nazis in their concentration camps had been sprayed on California oranges as a pesticide, and Zyklon B was used to delouse the trains in which tens of thousands of Mexican immigrants hid when entering the United States. The wood of the train cars was stained a beautiful blue, the same color that can be seen even today on certain bricks at Auschwitz; both hearken to cyanide’s origins as a byproduct isolated in 1782 from the first modern synthetic pigment, Prussian blue.

As soon as it appeared, Prussian blue caused a sensation in European art. Thanks to its lower price, in just a few years it all but replaced the color that painters had used since the Renaissance to depict the robes of the angels and the Virgin’s mantle—ultramarine, which was obtained by grinding lapis lazuli brought up from caves in Afghanistan’s Kokcha River valley. Crushed into a fine powder, this mineral yielded a lavish indigo that proved impossible to emulate by chemical means until the eighteenth century, when the Swiss pigmenter Johann Jacob Diesbach discovered Prussian blue. He did so by accident: his aim had been to mimic the ruby red made by crushing millions of female cochineals, small parasitic insects that grow on nopal cacti in Mexico and in Central and South America, creatures so fragile that they require even greater care than silkworms. Their scarlet blood was one of the greatest treasures the conquistadores stole from the American peoples, and it allowed the Spanish crown to establish a monopoly on carmine that would last for centuries. Diesbach tried to put an end to it by pouring potash over a distillation of animal parts, but instead of producing the furious carmine of the cochineals, the concoction yielded a blue of such beauty that Diesbach thought he had discovered hsbd-iryt, the original color of the sky—the legendary blue used by the Egyptians to adorn the skin of their gods. Passed down through the centuries and closely guarded by the priests of Egypt as part of their divine covenant, its formula was stolen by a Greek thief and lost forever after the fall of the Roman Empire. Diesbach dubbed his new color “Prussian blue” to establish a connection between his chance discovery and the empire that he believed would surpass the glory of the ancients.

In 1782, Carl Wilhelm Scheele stirred a pot of Prussian blue with a spoon coated in traces of sulfuric acid and created cyanide, the most potent poison of the modern era. He named this new compound “Prussic acid,” and was immediately aware of the enormous potential of its hyperreactivity. What he could not imagine was that two hundred years after his death, well into the twenty-first century, its industrial, medical, and chemical applications would be such that, each month, a sufficient quantity would be manufactured to poison every person on the planet. The Swede worked with such love and extraordinary rigor that he went so far as to smell and even taste the new substances he had conjured in his laboratory. He was wise enough not to do so with Prussic acid, which would have killed him in seconds; still, his bad habit cost him his life at age forty-three. He died with a ravaged liver, his body covered head to toe in blisters. These were the same symptoms suffered by thousands of European children whose toys were painted with an arsenic-based pigment that Scheele also manufactured: an emerald green so dazzling and seductive it became Napoleon’s favorite color.