  In 1912, Otto Sackur and Hugo Tetrode independently put forward an equation
for the absolute entropy of a monoatomic ideal gas and published it in "Annalen
der Physik." The grand achievement in the derivation of this equation was the
discretization of phase space for massive particles, expressed as \delta q
\delta p = h, where q and p are conjugate variables and h is Planck's constant.
Due to the dependence of the absolute entropy on Planck's constant, Sackur and
Tetrode were able to devise a test of their equation by applying it to the
monoatomic vapor of mercury; from the satisfactory numerical comparison of h
obtained from thermodynamic data on mercury with Planck's value from black-body
radiation, they inferred the correctness of their equation. In this review we
highlight this almost forgotten episode of physics, discuss the arguments
leading to the derivation of the Sackur--Tetrode equation and outline the
method how this equation was tested with thermodynamic data.
