  In order to provide an experimental access to the statistical theory of Lee
and Yang (1952) the density function g(theta) of zeros on the Lee-Yang circle
has been determined for the first time by analyzing isothermal magnetization
data, m(H), of the Ising ferromagnet FeCl2 in axial magnetic fields, H, at
temperatures 34 < T < 99K. The validity of our approach is demonstrated by the
perfect agreement of magnetic specific heat data as calculated from g(theta)
and m(H) via Maxwells relation, respectively. Moreover, the correct in-plane
exchange constant of FeCl2 emerges from the series expansion of m(H) involved
in the analysis.

Statistical Mechanics