Which atomist did Wilhelm Ostwald debate about the reality of atoms?

Wilhelm Ostwald, a proponent of "energetics," famously engaged in philosophical debates regarding the reality of atoms with the Austrian physicist Ludwig Boltzmann.1

Driven by his belief in thermodynamics and a positivist rejection of theoretical concepts lacking strict empirical foundations, Ostwald rejected atomism for approximately 15 years. He maintained that matter was merely a manifestation of energy. However, he eventually acknowledged the validity of the atomic hypothesis in 1909 following the accumulation of sufficient experimental evidence.1