Not all of Newton's scientific beliefs have stood the test of time quite as well as gravity. For instance, he also poured hours and hours of his time into alchemy. In fact, he was so interested in alchemy that it is now believed that alchemy was his main focus, and real science was more of a pastime for him. Unlike mathematics and the laws of physics, Newton wasn't even trying to take alchemy in new and interesting directions, opting instead to pursue theories that had long since been disregarded by people who still believed alchemy was possible. His time as an alchemist was mainly devoted to creating the philosopher's stone, which he believed could transmute other metals into gold and grant human beings immortality.
Apple-induced head injuries are kind of a double-edged sword for scientists.
This might sound familiar if you read the first Harry Potter book, where Harry pursues the same stone as Newton. Of course, in the book, Harry has to find the stone to keep the evil noseless warlock who lives on the back of his teacher's head from gaining too much power. Thanks to the alchemical experiments he performed on himself, Newton had so much lead and mercury in his system at the time of his death that he might have been pursuing the stone for the same reason as Harry.
"Behold, I control the RAINBOW. I am your God now, leprechauns!"