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thumb|right|[[Enoch Seeman's 1726 portrait of Newton.]]
During his residence in London, Isaac Newton had made the acquaintance of John Locke. Locke had taken a very great interest in the new theories of the Principia. He was one of a number of Newton's friends who began to be uneasy and dissatisfied at seeing the most eminent scientific man of his age left to depend upon the meagre remuneration of a college fellowship and a professorship.
1693
During the period 1692–1693 Newton is known to have suffered a breakdown of nervous functioning, or a supposed depression lasting for 18 months, as reported by Huygens.
During exhumation the hair from Newton's dead body was found to contain high levels of mercury, remains of desiccated hair were later found to contain four times the lead, arsenic and antimony and fifteen times mercury than in normal range samples. Two hairs contained mercury and separately lead at levels indicating chronic poisoning. Symptoms of mercury poisoning exhibited by Newton were apparently tremor, severe insomnia, delusions of persecution or paranoid ideas, problems with memory, mental confusion, and withdrawal or decline from personal friendships, significant in the period of time, the deterioration of his relations with his protégé Nicolas Fatio de Duillier.
Newton documented the first performed alchemy experiment during 1678, documented also by the scientist were similar experiments with arsenic, gold and lead. In a letter written to Samuel Pepys, Newton stated he was