That having been said, we can begin to judge the reliability of the equation by comparing
the isotherms it predicts with the experimental isotherms in Fig. 1.15. Some
calculated isotherms are shown in Fig. 1.19 and Fig. 1.20. Apart from the oscillations
below the critical temperature, they do resemble experimental isotherms quite well.
The oscillations, the van der Waals loops, are unrealistic because they suggest that
under some conditions an increase of pressure results in an increase of volume.
Therefore they are replaced by horizontal lines drawn so the loops define equal areas
above and below the lines: this procedure is called the Maxwell construction (3). The
van der Waals coefficients, such as those in Table 1.6, are found by fitting the calculated
curves to the experimental curves.