For accessibility and verification purposes, we require both information about the source and a transcript of the relevant portion of the screenshot. Since I think every post hasn’t done this so far, I’m just going to do it myself for these first ones instead of being a nag:

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• Transcript: “1.1 Introduction: Thermodynamics nad Statistical Mechanics of the Perfect Gas. Ludwig Boltzmann, who spent much of his life studying statistical mechanics, died in 1906, by his own hand. Paul Ehrenfest, carrying on the work, died similarly in 1933. Now it is our turn to study statistical mechanics.”