Sentence:  If * * f is the change per unit volume in Gibbs function caused by the shear <WSD>field</WSD> 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:  the space around a radiating body within which its electromagnetic oscillations can exert force on another similar body not in contact with it