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In page Magnesium:

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In 1618, a farmer at Epsom in England attempted to give his cows water from a local well. The cows refused to drink because of the water's bitter taste, but the farmer noticed that the water seemed to heal scratches and rashes. The substance obtained by evaporating the water became known as Epsom salts and its fame spread.[3] It was eventually recognized as hydrated magnesium sulfate, MgSO4·7 H2O.[citation needed]