The precise cause of His Majesty's death, which occurred on 6 Februrary, 1685, is uncertain. His autopsy revealed fluid in the brain. This led to the relatively recent conclusion that he suffered acute poisoning from proximity to mercury vapour in his laboratory.
What is known is that he suffered an apoplectic attack (now referred to as an "epileptic seizure") on the morning of 2 February. This date is considered the birthday of mistress Nell Gwyn, mother of his son Charles Beauclerk, Duke of St Albans--who at that time was nearly fifteen years old. Although the young duke spent much of his adolescence in Paris, where he was educated, he happened to be in England at that time and was present as his father lay dying at Whitehall Palace.
The king had slept badly and on waking on the morning of the 6th was "pale as ashes." At the time he was suffering from a sore on his legs, and the royal doctors had come to his rooms to change the dressing. Rather than joining them, Charles returned to his bedchamber. His speech was somewhat impaired. He took a glass of sherry and, still in his nightgown, prepared to be shaved.
Before the razor was applied, he let out what was later described as "the most dreadfulest shriek" and fell back unconscious into his barber's arm.
All the usual 17th century medical applications were attempted--bleeding, blisters--and perhaps it was the agony they produced that revived the king. He asked to see the Queen. The doctors continued torturing the patient with potions containing cantharides, vitriol, sal ammoniac, and one containing spirits of human skull.
Occasionally he would rally, but finally succumbed to another convulsion followed by a fever. Word of his decline was spreading beyond the palace walls.
Anti-Catholic sentiment was so great that many people assumed that the king had been poisoned. Sir Charles Lyttelton reported "his disease being, as is supposed, has fallen upon his lungs which makes him labour to breathe." Some modern authorities speculate that he was a victim of a kidney complaint, resulting in uraemia. Others propose that many hours he spent in his laboratory led to some form of chemical poisoning. The onset of his complaint was characterised by an epileptic seizure, which apparently affected brain function. The original post-mortem was destroyed when Whitehall Palace burned to the ground in 1697. A copy quoted in the British Medical Journal is inconclusive, as the autopsies of that era were less sophisticated than those of today.
Whatever felled the king, in his final hours he was surrounded by doctors, clerics, and family members. At that time a royal death, like a royal birth, was extremely public. Those in attendance included, at intervals, Queen Catherine of Braganza, various royal mistress (though Nell Gwyn was excluded from the sickroom), and the royal bastards--though not the Duke of Monmouth, who was living in exile at the Dutch court of his cousins, Prince William and Princess Mary of Orange.
King Charles famously remembered the Duchess of Portsmouth--the Frenchwoman, Louise de Kerouaille. He charged his brother (and successor) James, Duke of York, not to let "poor Nelly" starve and requested that he care for her son and ensure that his mother didn't spoil him.
It was to young Charles Beauclerk that the king gave a memento of his own father. While still lucid, he asked that a gold and carnelian ring be taken from his hand and given to his illegitimate son. Charles I wore the ring when he arrived at the scaffold with Bishop Juxon (later Archbishop of Canterbury.) Before his execution he presented it to the bishop, who later delivered it to Charles II. The ring is set with an intaglio depicting Charles I in the guise of a Roman Emperor, wearing armour, and the closed back is decorated with coloured enamel. This memento of two Stuart kings remains in the possession of the Beauclerk family.
The king's deathbed conversion to Catholicism was achieved by Father Huddleston, who had been with Charles at the battle of Worcester. When he entered the sickroom through a secret door, Charles supposedly greeted him with the words, "You who have saved my body are now come to save my soul." After making his confession, he received the sacrament, and Huddleston read the prayers for the dying.
At six o'clock in the morning of 6 February, the king asked that the curtains be drawn back so he could see the dawn breaking one last time. He also requested the winding of his clock. After that his speech failed him, and by ten o'clock he had fallen into a coma. He died between eleven-thirty and noon, aged fifty-five.
The King saw one more sunrise lighten the sky before he slept, never to wake again. By midday Charles Beauclerk, Duke of St. Albans, was no longer the son of a living monarch.