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microscopic behaviors of the very large numbers of its microscopic
constituents, such as molecules. Its laws are explained by statistical
mechanics, in terms of the microscopic constituents.
Thermodynamics applies to a wide variety of topics in science and

engineering.
Historically, thermodynamics developed out of a desire to increase the
efficiency and power output of early steam engines, particularly through

the work of French physicist Nicolas Léonard Sadi Carnot (1824) who
believed that the efficiency of heat engines was the key that could help
France win the Napoleonic Wars. Irish-born British physicist Lord Kelvin
was the first to formulate a concise definition of thermodynamics in 1854

"Thermodynamics is the subject of the relation of heat to forces acting
between contiguous parts of bodies, and the relation of heat to electrical
agency."

Initially, thermodynamics, as applied to heat engines, was concerned
with the thermal properties of their 'working materials' such as steam, in an
effort to increase the efficiency and power output of engines.

Thermodynamics later expanded to the study of energy transfers in
chemical processes, for example to the investigation, published in 1840, of
the heats of chemical reactions by Germain Hess, which was not originally

explicitly concerned with the relation between energy exchanges by heat
and work. The study of Chemical thermodynamics and the role of entropy
in chemical reactions evolved from this.
The plain term 'thermodynamics' refers to a macroscopic description

of bodies and processes. "Any reference to atomic constitution is foreign
to classical thermodynamics." The qualified term 'statistical
thermodynamics' refers to descriptions of bodies and processes in terms of

the atomic constitution of matter, mainly described by sets of items all
alike.
Thermodynamics arose from the study of two distinct kinds of
transfer of energy, as heat and as work, and the relation of those to the

system's macroscopic variables of volume, pressure and temperature.
Thermodynamic equilibrium is one of the most important concepts
for thermodynamics. The temperature of a thermodynamic system is well

defined, and is perhaps the most characteristic quantity of
thermodynamics. As the systems and processes of interest are taken further
from thermodynamic equilibrium, their exact thermodynamically study

becomes more difficult. Relatively simple approximate calculations,
however, using the variables of equilibrium thermodynamics, are of great

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