Before anyone could truly understand the cold, scientists first had to understand heat. But in the early 1800s, chemists assumed heat was made by a substance called caloric, which simply traveled from object to object and made things warm. No one thought heat was worth studying anymore since they already thought they had the answer.
Sadi Carnot (1796 - 1832) was a French physicist who changed all that, and made the study of heat a fashionable topic again. He was the son of Lazare Carnot, a prominent political figure and former war minister for Napoleon . Carnot attended the Ecole Polytechnique in Paris and was alive at a time when steam engines were first being used. He was alive at a time when steam engines were first being used, and he studied them to better understand exactly what properties heat – and steam – had. In a steam engine, the steam pushes the pistons to do work. The heat, in effect, can cause other things like motion to happen. Carnot realized that heat and work were directly connected, since the energy from the heat could be turned into energy that made things move.
Carnot also noticed that when heat energy was turned into motion, the steam got colder. Once people understood this connection between heat and work, scientists could use it to actively try to make a substance cold. By taking the heat energy of a gas – like water steam or nitrogen or hydrogen -- and converting it into some other kind of energy like motion, they could cool the gas down.
Carnot had given scientists the first tool they needed in the quest for trying to reach absolute zero.