In this article van der Waals used the Helmholtz Potential Minimum Principle to establish the conditions of stability. This principle states that in a system in diathermal contact with a heat reservoir T = T R , D F = 0 and D 2 F > 0 , namely at equilibrium the Helmholtz potential is a minimimum. This leads to the requirement ∂ ∂ 2 f / ∂ ∂ v 2 | T = − − ∂ ∂ p / ∂ ∂ v | T > 0 , which is the previous stability condition for the pressure, but in addition requires that the function, f = F N A / N , is convex at all v that describe stable states.