During the years 1814–1819, William Higgins, an Irish chemist who worked at the Dublin Society, claimed he had anticipated John Dalton in developing the atomic theory and insinuated that Dalton was a plagiarist. This essay focuses not on William Higgins, but on his uncle Bryan Higgins, a well-known chemist of his day, who had developed his own theories of caloric and chemical combination, similar in many respects to that of Dalton. New evidence is first introduced addressing Bryan's disappearance from the scientific community after 1803. In his later years, Bryan apparently suffered from a condition resulting in a decline in his mental health, which explains why he never lodged any priority claims of his own against Dalton, or defended those of his nephew. Dalton's mention of Bryan's name in Part II of A New System of Chemical Philosophy, his laboratory notebook entries, and a fresh look at his correspondence with chemist Thomas Charles Hope indicate that Dalton adopted a Higgins-like caloric model in 1803. Together these factors provide evidence to support the argument that Dalton learned of Bryan's theories via a meeting he had with William Allen on 10 July 1803. Existing evidence related to the origin of the atomic theory is worthy of re-examination in light of Dalton's possible prior knowledge of Bryan's work.