 Although these discoveries are often presented as having been explained by the atomic or molecular hypothesis, skepticism about the status of atomism persisted throughout the 19th century. Late 19th century skeptics such as Ernst Mach, Georg Helm, Wilhelm Ostwald, and Pierre Duhem did not see atomism as an adequate explanation of these phenomena, nor did they believe that there was sufficient evidence to accept the existence of atoms. Instead, they advocated non-atomistic theories of chemical change grounded in thermodynamics (on Helm and Ostwald, see the introduction to Deltete 2000).