- Chapter 1
Though the work of Joseph Black and others established clearly the distinction between heat and
temperature, the nature of heat remained an enigma for a long time. Whether heat was a indestructible
substance without mass, called the "caloric", that moved from substance to substance or whether it is a
form of microscopic motion was still under debate as late as the nineteenth century. After considerable
debate and experimentation it became clear that heat was a form of energy that could be transformed to
other forms and the caloric theory was abandoned -- though we still measure the amount of heat in
"calories", in addition to the SI units of Joules.
Temperature can be measured by noting the change of a physical property, such as the volume of a fluid
(such as mercury), the pressure of a gas or the electrical resistance of a wire, with degree of hotness. This
is an empirical definition of temperature. In this case, the uniformity of the unit of temperature depends
on the uniformity with which the measured property changes as the substance gets hotter. The familiar
Celsius scale, which was introduced in the eighteenth century by Anders Celsius (1701-1744), has largely
replaced the Fahrenheit scale, which was also introduced in the eighteenth century by Gabriel Fahrenheit
(1686–1736). As we shall see in the following chapters, the development of the Second Law of
thermodynamics during the middle of the nineteenth century gave rise to the concept of an absolute scale
of temperature that is independent of material properties. Thermodynamics is formulated in terms of the
absolute temperature. We shall denote this absolute temperature by T.
The Laws of Gases
In the rest of this section, we will present an overview of the laws of gases without going into much
details. We assume the reader is familiar with the laws of ideal gases.
One of the earliest quantitative laws describing the behavior of gases was due to Robert Boyle
(1627-1691), an Englishman and a contemporary of Isaac Newton(1642-1727). The same law was also
discovered by Edmé Mariotte (1620(?)-1684) in France. In 1660, Boyle published his conclusion in his "
New Experiments Physico-Mechanical Touching the Spring of the Air and its Effects": at a fixed
temperature, Τ, the volume, V, of a gas was inversely proportional to the pressure p, i.e.,
, f1(T) is some function of the temperature T.
(Though the temperature that Boyle knew and used was the empirical temperature, as we shall see in
chapter 3, it is appropriate to use the absolute temperature T in the formulation of the law of ideal gases.
To avoid excessive notation we shall use T whenever it is appropriate). Boyle also advocated the view