Learning about quinine in London, Castro Sarmento developed a medicine known as Água de Inglaterra, which became popular in Portugal where malaria was still widespread in the southern part of the country. In 1731 he elaborated a plan for a botanical garden in Coimbra. Castro Sarmento corresponded with many scholars, among others with Prof. João Mendes Sachetti Barbosa of Lisbon, who reported to him the terrible earthquake that destroyed the capital of Portugal in 1755, and with the Jesuit B. Soares, who communicated to him his astronomical observations made in the Portuguese colony of Brazil. He was a strong proponent of Newtonianism and made efforts to integrate it with Jewish theology. He published "Theorica verdadeira das marés, conforme à philosophia do incomparavel cavalhero Isaac Newton" (Treatise on the true theory of tides, according to the philosophy of the incomparable gentleman Isaac Newton), the first book in Portuguese to advocate Newton ideas (London, 1737).