This is a 1978 letter to James Serrin that contains preliminary notes on the Second Law of Thermodynamics, in particular about how a thermodynamic temperature scale, satisfying the Clausius Inequality for all processes, emerges as a consequence of a standard topogical vector space theorem on separation of disjoint convex sets by a hyperplane. These notes are posted here in connection with a far more recent article on connections between the Second Law and the Hahn-Banach Theorem.