In 1850 he returned to the United States and became a professor at Brown University. He left in 1852 to take the place of the retiring Professor John Pitkin Norton at Sheffield Scientific School (then Yale Scientific School). He was the Professor of Analytical and Agricultural Chemistry from 1852 to 1856, and Professor of Organic Chemistry from 1856-1864. He remained at Yale until he had to resign for health reasons in 1864, two years before his death in New Haven. In 1872 the Kingsley Trust endowed at Yale a prize in his honor to be given annually.