This paper reveals that Josiah Willard Gibbs (1839-1903) attempted to explain certain chemical properties of matter, such as osmotic pressure of "diaphragms" (semi-permeable membranes) and electromotive force of chemical cells, from his theory of statistical mechanics, i.e., Elementary Principles in Statistical Mechanics (1902; EPSM). To begin with, I examine his thermodynamical theory developed in "On the Equilibrium of Heterogeneous Substances" (1876/78; EHS). Later, I attempt to support the above claim by analyzing his theory of statistical mechanics. In EHS and his later discussions on ...