Around 1656, he performed a splenectomy on a dog, successfully keeping the animal alive afterwards for more than 2 years. This challenged the prevailing humoralist theory of the body, and attracted the attention of physicians and scientists in London, including William Harvey and Robert Boyle. During the great plague of 1665 he lived in London, and made a special study of the symptoms, even dissecting the body of a plague victim. In 1665 he published "Loimologia: a Consolatory Advice, and some brief Observations concerning the present Pest", in which he reflected on the conduct of those members of the Royal College of Physicians who left the city during the plague. He accused them of running away and "leaving this great city destitute of their help, when it most needed it". This pamphlet drew a furious reply from John Heydon entitled "Psonthonphanchia, or a Quintuple Rosiecrucian Scourge for the due Correction of that Pseudo-chymist and Scurrilous Emperick, Geo. Thomson" (London, 1665).