The law of the Conservation of Energy is already known — viz., that the sum of all the energies of the universe, actual and potential, is unchangeable.

It is possible to express the laws of thermodynamics in the form of independent principles , deduced by induction from the facts of observation and experiment, without reference to any hypothesis as to the occult molecular operations with which the sensible phenomena may be conceived to be connected; and that course will be followed in the body of the present treatise. But, in giving a brief historical sketch of the progress of thermodynamics, the progress of the hypothesis of thermic molecular motions cannot be wholly separated from that of the purely inductive theory.

A hypothetical theory is necessary, as a preliminary step, to reduce the expression of the phenomena to simplicity and order before it is possible to make any progress in framing an abstractive theory.

The law of the Conservation of Energy is already known — viz., that the sum of all the energies of the universe, actual and potential, is unchangeable.

The words 'theory' and 'practice' are of Greek origin; they carry our thoughts back to the ancient philosophers by whom they were contrived, and by whom they were also contrasted and placed in opposition, as denoting two mutually conflicting and mutually inconsistent ideas. ... [this fallacy] based on a double system of natural laws retarded for centuries the development of physical science, notably mechanics.

This law (regarding the theoretical efficiency of heat engines by Mr. Joule), and the law of the maximum efficiency of heat engines, are particular cases of a general law which regulates all transformation of energy, and is the basis of the Science of Energetics.

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.