Seventeenth-century 'treasure' found in Royal Society archives: the Ludus helmontii and the stone disease.

Our archival researches at the Royal Society reveal that a small envelope attached to a 1675 letter from an Antwerp apothecary, A. Boutens, contained a sample of the 'Ludus' prepared as a remedy for the 'stone disease' then sweeping through Europe, which was first announced in J. B. van Helmont's De lithiasi (1644). After examining the fascination with the medical use of the Ludus (which required the 'alkahest' for its preparation) and the tenacious efforts to procure it, we trace the fortunae of two other ludi in England, brought to and offered by Francis Mercurius van Helmont during his English sojourn. Both eventually found their way to the geologist John Woodward, one of them through Sir Isaac Newton. Finally we show how the allure of the Ludus helmontii vanished, with transformations in mineral analysis and reclassifications from Woodward to John Hill.