Isothermal expansion curve for α-quartz at different temperatures. The integral over the cold compression curve (blue) is equal to E_vap. The minimum of the curve represents a point of instability. As we increase the temperature, we eventually reach the critical isotherm in which the minimum disappears. The critical point represents the point on the critical isotherm, in which the derivative goes to zero. Below the critical temperature the sinusoidal part of the curve is replaced by a constant vapor pressure using Maxwell construction.