Bodies having the same temperature can nevertheless contain different quantities of heat.
For a long time it was generally thought that heat was a substance which penetrated bodies. Only after the beginning of the 19th century did the idea gain acceptance that what was then called heat was the random motion of molecules. Thereupon it became possible to associate heat with the law of conservation of mechanical energy. This is one of the Laws of Thermodynamics . A great impulse to fundamental physics resulted from later developments in the study of heat.