• In thermodynamics as well as in other branches of molecular physics , the laws of phenomena have to a certain extent been anticipated, and their investigation facilitated, by the aid of hypotheses as to occult molecular structures and motions with which such phenomena are assumed to be connected. The hypothesis which has answered that purpose in the case of thermodynamics, is called that of "molecular vortices," or otherwise, the "centrifugal theory of elasticity.

    William John Macquorn Rankine (1869). “A Manual of the Steam Engine and Other Prime Movers by William John Macquorn Rankine”, p.32