= Solution
The relevant comparison is a short-range, uniaxial <Ising> ferromagnet and the liquid–gas <critical point> of water in three dimensions. Both have one real scalar <order parameter>: <magnetization> in the ferromagnet and the appropriately shifted density in the fluid. The magnetic field and a suitable pressure/chemical-potential combination provide the corresponding <fields conjugate to an order parameter>.
The fluid has no exact microscopic spin-reversal <symmetry>. Nevertheless, at the liquid–gas <critical point> the first three derivatives of the local thermodynamic potential can be eliminated or tuned by shifting the density and mixing the thermodynamic control variables. The leading stable expansion is then quartic, with the same long-distance <renormalization-group fixed point> as the three-dimensional short-range <Ising model>. The microscopic asymmetry produces corrections and analytic mixing of observables, rather than a different leading <universality class>.
Thus \b[the scalar liquid–gas critical point and the short-range Ising ferromagnet share a universality class]. This does not assert that every ferromagnet does: an isotropic vector-spin magnet or one dominated by long-range dipolar interactions can have different critical behavior. Nor does it identify every phase transition of water with its liquid–gas <critical point>.
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