Sentence:  If * * f is the change per unit volume in Gibbs function caused by the <WSD>shear</WSD> field at constant P and T , and |r is the density of the fluid , then the total potential energy of the system above the reference height is * * f . 
Meaning of the word marked with <WSD> tags:  (physics) a deformation of an object in which parallel planes remain parallel but are shifted in a direction parallel to themselves