Thermodynamics is not directly a theory of the structure of matter, although its development paralleled and supported the development of the atomic theory of matter. The theory of heat and temperature was beginning to be placed on a firm footing late in the eighteenth century, at about the same time that chemical experiments were providing the first solid evidence for the existence of atoms. By the middle of the nineteenth century the evidence of chemistry could leave little doubt about the existence of atoms, and it was at this time that the microscopic and macroscopic threads of physical theory united to create what we now call thermodynamics. By the end of the nineteenth century there was monumental evidence for the existence of atoms, but all of it was indirect. Finally in 1905, Einstein rendered the bridge of thermodynamics visible. His application of thermodynamics to Brownian motion made clear that a grain of dust dancing in a liquid is reacting visibly to molecular impact. Before long other and even more direct manifestations of atoms and molecules were being observed.
When an object is set into motion (say a baseball being thrown), its structure is normally unaffected. If the same object is electrically charged, it is also unlikely to change its shape or size or appearance. At the macroscopic level, mechanical and electric phenomena may be studied without paying attention to the structure of matter. The same is not true of heat phenomena. If an object is heated, it will, at the very least, change its size slightly. It may also melt or boil or cook or catch fire or explode. How the object reacts will depend very much on its composition. Thermodynamics, being concerned with heat, cannot separate itself from the structure of matter. It must be concerned not only with the fact that matter consists of atoms and molecules, but with how these atoms or molecules are arranged—whether in solids, liquids, or gases—how they move, and how they interact with each other.