 Welcome back. Let us look at some aspects of the history of thermodynamics. If we go back in history, the early history of technology, engineering and science, you will notice that fire plays an important role. The prehistoric human being must have come across fire which occurred naturally. They must have discovered that it is useful or benevolent. It could also be dangerous and hence an important step was the discovery and the development of the ability to control fire, to initiate it when needed, control it so that it does not become dangerous and extinguish it when you do not need it anymore. And with this control of fire, the initial uses were perhaps in roasting, baking and cooking, what we generally may call heating, maybe drying. Later on as technology progressed and we could work with metals, vessels and things like that, it was realized that fire could be used to convert water into steam and steam could be used to produce work or deliver power. So we had heat on one side and work on the other side and there must have been a lot of confusion between heat and the work which is produced using heat. This confusion led initially to the caloric theory of heat. It was considered that heat was something which is distinct from work and the caloric was supposed to be a fluid, invisible massless fluid that could flow from one body to another. Conservation of energy was not known. The first law was not discovered, the first law was not proposed. Quantification of this led to what was called the mechanical equivalent of heat, which we today know in a school by knows is defined today as 4.1868 joule per calorie. Today we know it only as a convergent factor but in the earlier days this was known as the mechanical equivalent of heat and this number was something like a magic number. Using the caloric theory of heat, Carnot proposed a theorem on the efficiency of engines which later on got formulated into and absorbed into the second law of thermodynamics. We cannot say that Carnot proposed the second law of thermodynamics but the work of Carnot definitely led to parts of the second law of thermodynamics. So historically the thinking about the second law of thermodynamics perhaps started very early with Carnot. Then came Kahn-Rumford's experiment on cannonball. This led to the understanding that heat and work are two forms of the same thing which is energy. And then finally Joule's experiment led to the ideas of conservation of energy, the ideas of first law of thermodynamics and which finally led to the first law. Thank you.