The purpose of this paper is to report on the first comprehensive expe
rimental test of the scaling hypothesis in two-dimensional physics. Th
is hypothesis predicts that the equation of state near a phase transit
ion of a system in thermodynamic equilibrium obeys a simple scaling la
w. Our experimental data, obtained on a truly two-dimensional magnetic
system consisting of a subnanometer thick Fe films grown on top of a
non-magnetic surface, explicitly display scaling. The experimental evi
dence suggests that this system is an almost perfect realization of a
2d Ising model.