Figure 1.4 depicts the variation of the pressure of a sample of gas as the volume is
changed. Each of the curves in the graph corresponds to a single temperature and
hence is called an isotherm. According to Boyle’s law, the isotherms of gases are
hyperbolas (a curve obtained by plotting y against x with xy = constant). An alternative
depiction, a plot of pressure against 1/volume, is shown in Fig. 1.5. The linear
variation of volume with temperature summarized by Charles’s law is illustrated
in Fig. 1.6. The lines in this illustration are examples of isobars, or lines showing the
variation of properties at constant pressure. Figure 1.7 illustrates the linear variation
of pressure with temperature. The lines in this diagram are isochores, or lines showing
the variation of properties at constant volume