The history of equations of state goes back to 1662 when Boyle concluded that for a given mass of confined gas within a closed system, the absolute pressure and volume are inversely proportional to each other, provided the temperature is kept constant.An improvement over the ideal gas constants of state based on elementary molecular arguments was suggested in 1873 by Van Der Waals who noted that gas molecules actually occupy more than the negligibly small volume presumed by the ideal gas model and also exert long-range attractive forces on each other.
In the van der Waals equation, constant a' is a measure for the attraction between the particles and constant b' is the average volume excluded from v by a particle.