Isaac Newton's later life

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.

During the period 1692–1693 Newton is known to have suffered a breakdown of nervous functioning, or a supposed depression (Spivak & Epstein) lasting for 18 months, as reported by Huygens. He suffered insomnia and poor digestion, in his letters to friends showing signs of irrationality.

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 (Spargo & Pounds 1979). 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 Nicholas Fatio de Duillier.

Newton documented the first performed alchemy experiment during 1678, having first obtained furnaces and chemicals in 1669. Experiments with metal included analysis of taste of which there are 108 documented, including mercury:

strong, sourish, ungrateful,

documented also by the scientist were similar experiments with arsenic, gold and lead.

Newton recorded in his notebook of experimenting with chemicals during June 1693. The limited evidence for symptomatic mental illness of Newton during this period stem from correspondences (c.f. The Royal Society) revealing melancholia, desire for withdrawal from relations including his good friends, insomnia, apathy, loss of appetite, delusion of persecution, possibly failures in memory (amnesia), and bipolar. In a letter written to Samuel Pepys, Newton stated he was

extremely troubled by the embroilment I am in, have neither ate nor slept well in the last twelve months, nor have my former consistency of mind. 13th September 1693

On the 30th of that month Newton reported he had been seized by a distemper

that kept him awake for above five nights altogether

For the last half of Newton's adult life, 30 years, he was warden of the Royal Mint as well as Master of the Mint.

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