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Grew's experiments originated amid the long and bitter arguments over desalination machines outlined in the preceding chapter. But his investigations extended far more widely and deeply, to include ideas about the air, the body, and the maintenance of life and health. Having worked in Leyden under the celebrated Paracelsian Franciscus Sylvius, he was well acquainted with the arguments for chymical medicine, and he drew implicitly on these in broadening the ambit of his trials.8 When he moved to exploit Epsom's water for himself, he thus knew very well what he was doing. His technique was essentially another bid to mechanize the separation of salt and water. But unlike Fitzgerald and Walcot he focused on the salt, not the water; and his market was on land, not at sea.

Epsom's spa had become a favorite destination for Londoners since its discovery in about 163o. The water there had a good but gentle purgative property. Drinking it was supposed to help rid the body of impurities and restore one's humors to balance, thus alleviating a large number of conditions. Perhaps two thousand people, Grew estimated, had gone to the village to drink the water by the time he launched his own enterprise; and nobody knew how many more had bought bottles of it in the city from apothecaries. It was this metropolitan market- and in particular its vulnerability to fraud-that gave him his opportunity. A customer trying to buy Epsom water in London ran two major hazards. One was the tendency of such water to spoil if stored; it would "corrupt and stink" if kept for more than a few days. The other was that the authenticity of the water itself was hard to guarantee. Spring water varied in strength naturally as it emerged from the ground, but the bigger problem was a social one. Apothecaries were known to adulterate their products, either by dilution (to make a scarce supply last longer) or by the addition of new ingredients, or both. The mediation of the apothecary between spa and patient, Grew believed - and this was a standard physician's view- therefore created an intractable problem of credit. Grew realized that he could address that problem experimentally. By converting a social issue into a chymical one, he could also convert it into an opportunity to make a fortune.

Grew proposed to extract the "bitter purging salt" that was the active constituent of Epsom water. This salt could then be stored and distributed safely. He would monopolize its production by using a secret process carried out only by his own trusted operators. All a user need do would be to dissolve it in freshwater to reproduce the original effect. In the early 169os Grew thus established his own laboratory-not at Epsom itself, but at Acton, another village near London that boasted a spa producing water even better, as he had learned from discourse at the Royal Society than Epsom's. There he employed a trusted operator namedThomasTramel to produce the salt in quantity. He used newspaper advertisements to advertise that apothecaries could get the genuine article wholesale fromTramel at an address in St. Paul's Churchyard, the traditional booksellers' quarter. And he affirmed that his salt was even better than the original water. It contained no impurities, did not spoil over time, and could be easily transported and used. Above all, it was innocent of adulteration. "Some who sell these Waters," he warned, "when they find their Store begins to fail, willventure to adulterate them with common Water."The salt, by contrast, was "always alike." This property of always-alikeness was critical to his plan. It was an ideal of early modern medicaments that they should be so predictable, but one that could almost never be assured. Grew had seen a way to make the salt secure, and safe to use as part of a physician's bill.9 Thanks partly to such assurances, a market arose that endured long after Grew himself vanished from the scene. We still use his substance today, and we call it "Epsom salt."

To support this venture, Grewwrote his own treatise on