The study of thermodynamics is separated into two branches: the classical and the statistical thermodynamics.
Classical thermodynamics was the original study of thermodynamics in early 1800s. It was concerned with thermodynamic states, and properties as energy, work, and heat, and with the two laws of thermodynamics. However, classical thermodynamics lacked an atomic interpretation of the processes. Classical thermodynamics derives from the research done by physicist Robert Boyle. He developed the concept that the pressure P of a given quantity of gas varies inversely to its volume V at constant temperature. In other words this equation was derived: PV = k, a constant. From here, the thermo-science began to develop with the construction of the first successful atmospheric steam engines.
The first and second laws of thermodynamics emerged simultaneously in the 1850s.
With the development of atomic and molecular theories in the late 19th century, thermodynamics was given a molecular interpretation, which the classical thermodynamics lacked. This field is called statistical thermodynamics, which can be thought of as a bridge between macroscopic and microscopic properties of systems. Statistical thermodynamics is focused around the macroscopic results. The statistical approach is to derive all macroscopic properties (temperature, volume, pressure, energy, entropy, etc.) from the properties of moving particles and the interactions between them in the given system. Statistical thermodynamics was found to be very accurate and successful; therefore it is widely used by scientists around the world.