where "T" is the absolute temperature, "p" is the pressure, d"S" is an infinitesimal change in entropy and d"V" is an infinitesimal change of volume. The last term is the sum, over all the chemical species in a chemical reaction, of the chemical potential, "μ"i, of the "i"th species, multiplied by the infinitesimal change in the number of moles, d"N"i of that species. By taking the Legendre transform of this expression, he defined the concepts of enthalpy, "H" and Gibbs free energy, "G".