This coefficient can be found in terms of the previously described derivatives as, μ μ J = v ( α α T − − 1 ) c p so when μ μ J is positive the gas temperature decreases when it passes through a throttle, and if it is negative the temperature increases. Therefore the condition μ μ J = 0 defines a curve that separates the region of the T , p plane where μ μ J > 0 from the region where it is less than zero. This curve is called the inversion curve, and its equation is α α T − − 1 = 0 . Using the expression for α α derived previously for the van der Waals equation this is 2 a ( v − − b ) 2 − − R T v 2 b R T v 3 − − 2 a ( v − − b ) 2 = 0 or 2 a ( v − − b ) 2 − − R T v 2 b = 0 Note that for v ≫ ≫ b there will be cooling for 2 a > R T b or in terms of the critical temperature T < 27 T c / 4 . As Sommerfeld noted, "This is the case with air and with most other gases. Air can be cooled at will by repeated expansion and can finally be liquified. "