In the 18th and 19th centuries, the important advances were in cla rification of the concept of energy and in the discovery of the laws of thermodynamics. In the 19th century scientists began to understand the atomic nature of matter, and this led to further understanding of thermal properties of matter. To understand some of these discoveries, we need a few b asics. An object in motion can do considerable damage if it is suddenly brought to rest. An object moving twice as fast has four times as much energy.
Any material consists of atoms and molecules that are in continual, random motion.
In a gas, the molecules move freely, interacting with each other briefly during collisions. In solids the molecules are locked in a lattice, and vibrate about a relatively fixed position. As the temperature is increased, the motions increase. Temperature is actually a measure of the amount of energy possessed by each molecule. The heat engines built by engineers such as James Watt in the 18th century are devices that extract some of the thermal energy and convert it to other, more practical, forms of energy.