One afternoon in 1624, the writer James Howell stumbled upon his colleagues engaged in a duel. He tried to intervene, as good friends do. With his left hand, he grabbed one sword’s hilt; with his right, the other’s blade. But the combatants were seeing red, “transported with fury against the other”, and pulled their weapons away. Howell’s left hand was cut to the bone; his right, slashed across its back. The wounds were deadly, and King James’ own surgeons proved useless. Howel went to visit his neighbor, Sir Kenelm Digby (1603–55), hoping to stave off gangrene.

Of the Sympathetick Powder (originally published in English in 1658 and based on a discourse delivered the year prior in French) opens with this likely contrived anecdote. Digby, a diplomat and natural philosopher, then proceeds to share his treatment. Sourcing Howell’s bloodied garter, he soaked it in a basin of vitriol or ferrous sulfate. The patient suddenly felt cool relief. Later, when Howell was resting, Digby decided to dry off the garter near the fireplace. Howell’s servant came running: the patient’s palms were burning. Digby submerged the cloth once again and a few days later “the wounds were cicatriced; and entirely healed”.

This is the power of the “powder of sympathy”, which Digby supposedly had learned about from a Carmelite in Florence who had traveled extensively through present-day China, India, and Iran. Claiming that he was now the only person in Europe to know the value of this powder (the Carmelite had conveniently returned to live out his life in Persia), Digby sought scientific priority in the court of King James. The powder acts through a property that Digby names “the remembrance of bodies”. Things connected or similar — that share an origin, weight, density, rarity, or figure — have the capacity to participate in sympathetic relations and even to act upon each other at a distance. This is why, during times of contagion, you must carry a toad, because the venomous creature attracts and absorbs “the poison of a plague sore”. Digby’s most curious suggestion is a version of the so-called weapons salve, a magical balm that perhaps first appeared in the Pseudo-Paracelsian book Archidoxis Magica (1570), whereby a wound can be healed by applying a salve to the offending blade rather than its victim.

While one could sneer at his science today, Digby was thinking in terms of atoms and early molecular physics — and looking for an explanation of observed phenomena that did not require any “recourse to the intervention of Demons, and Spirits”. Bodies shed particles as they move through the world. And, according to Digby, acting upon these particles acts upon their source. Practices based upon affinity, which historians of science sometimes call the doctrine of signatures or sympathetic magic, were rife in the early modern and ancient period. But whereas the weapons salve and other Paracelsian medicines were justified by arcane principles, Digby was, according to Elizabeth Hedrick, trying to explain previously “occult phenomena in more or less mechanistic terms”. Francis Bacon (who may have once sought to incorporate Digby’s findings into his own work) tells a curious story about such magical operations. While the teenage Bacon was traveling in Paris, an ambassador’s wife noticed that his hands were covered with hundreds of warts. She rubbed the spots with lard and then nailed it to a window, facing south. As the fat dwindled, Bacon’s warts receded. One wonders if the philosopher’s name may have also played a role in the magic’s success. . .

Digby’s powder offered another kind of cure to a problem vexing seventeenth-century Britain: the issue of longitude. Latitude was relatively easy to determine from observing the position of celestial bodies. Longitude, however, required knowing the solar time difference between your current location and that of a fixed reference point, and clocks were not yet precise enough to keep good time at sea. This lack of accurate positioning led to hundreds of shipwrecks and the loss of imperial capital, in response to which Parliament eventually established the Longitude Act of 1714, offering a monetary award to anyone who could solve the problem. Beating Parliament to the punch, an anonymous 1688 satirical pamphlet proposed an ingenious and cruel solution. Keep wounded dogs on ships at sea and rags soaked with their blood in London. Then every day at precisely noon, submerge the creatures’ rags in Digby’s sympathetic powder to soothe their pain. And thus the exact longitude of a ship can be calculated from the sudden calm of maimed hounds.

Digby’s ideas were popular enough to attract detractors. But it is hard to imagine he much cared: his treatise went through twenty-nine editions and was translated into five languages. It was only natural that Digby might attract some unsympathetic readers. His other works were less controversial, such as an early recipe book published posthumously by his servant under a deliciously immodest title: The Closet of the Eminently Learned Sir Kenelme Digbie Kt. Opened (1669), which includes a recipe for his famous powder alongside instructions for making Hotchpot and Humble Pyes.