Although the phenomena of order‐disorder transition and the phenomena of condensation have a marked difference, there is a great similarity in their statistical mechanical treatment. Considering a hypothetical gas in which the molecules can occupy only lattice points of a crystal, and treating the ``wrong'' molecules as the gas molecules and the ``right'' molecules as the empty points, the partition function of a binary mixture is easily computed by applying Mayer's method. It is then pointed out that the ordered state is the analog of a condensing system.
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