Sentence:  If * * f is the change per unit volume in Gibbs function caused by the shear field at constant P and T , and |r is the density of the fluid , then the total potential energy of the <WSD>system</WSD> above the reference height is * * f . 
Meaning of the word marked with <WSD> tags:  (physical chemistry) a sample of matter in which substances in different phases are in equilibrium