We start with an explanation of the difference between “chemical atoms” and “physical atoms”. We discuss Josef Loschmidt's calculations that added to James Clerk Maxwell's earlier work on the kinetic molecular theory (KMT) of gases and how these results so avidly supported physical atoms. We move on to discuss the intense debates between Ernest Mach, the anti-atomist, and Ludwig Boltzmann, the atomist. We describe Mach's reasons for not believing in atoms, why he lost the debate, and how his bust now stands alone in the Vienna Rathauspark. We discuss Boltzmann's contributions to the KMT and his explanation of entropy – both of which are predicated on the existence of physical atoms – and his suicide in Trieste, partly attributed to his perception that he was losing the atomic debate. We visit the UniGraz@Museum in Graz and several sites in in Trieste and Vienna, including the Zentralfriedhof, where both Loschmidt and Boltzmann are buried. We discuss Wilhelm Ostwald's work in founding physical chemistry, his long-time belief that energy, not matter, was the chief component of the universe, and the evidence that finally turned him into an atomist. We end by describing his elaborate Energiehaus in Grossbothen, Germany.