Conservation of Force (Energy)

In 1841 Mayer, now backin Heilbronn, began a paper that summarized his point
of view in the broadest terms. He wrote that “all bodies are subject to change . . .
[which] cannot happen without a cause . . . [that] we call force,” that “we can
derive all phenomena from a basic force,” and that “forces, like matter, are invariable.”
His intention, he said, was to write physics as a science concerned
with “the nature of the existence of force.” The program of this physics paralleled
that of chemistry. Chemists dealt with the properties of matter, and relied on the
principle that mass is conserved. Physicists should similarly study forces and
adopt a principle of conservation of force. Both chemistry and physics were
based on the principle that the “quantity of [their] entities is invariable and only
the quality of these entities is variable.”
Mayer’s use of the term force requires some explanation. It was common for
nineteenth-century physicists to give the force concept a dual meaning. They
used it at times in the Newtonian sense, to denote a push or pull, but just as
often the usage implied that force was synonymous with the modern term energy.
The modern definition of the word “energy”—the capacity to do work—was not
introduced until the 1850s, by William Thomson. In the above quotations, and
throughout most of Mayer’s writings, it is appropriate to assume the second usage,
and to read “energy” for “force.” With that simple but significant change,
Mayer’s thesis becomes an assertion of the principle of the conservation of
energy.