Josiah Willard Gibbs (1839 - 1903) has been reckoned as one of the greatest American scientists of the 19th century. He provided a sound thermodynamic foundation to much of Physical Chemistry. "Gibbs' Phase Rule" is well known to all in the field today.
Yale educated, he was awarded the first Doctor of Engineering in the U.S., and was appointed Professor of Mathematical Physics at Yale in 1871. In 1873 he published his first major works, "Graphical Methods in the Thermodynamics of Fluids," and "A Method of Geometrical Representation of the Thermodynamic Properties of Substances by Means of Surfaces". In 1876 came his most famous paper: "On the Equilibrium of Heterogeneous Substances."
You can find more details about him at the Wikipedia page