It was a German mathematician and philosopher, Gottfried Leibniz (1646-1716), who in 1695 first suggested that heat and kinetic energy, or the energy of motion, had some connection. In 1780 Lavoisier and another French scientist, Pierre Laplace (1749-1827), came to the conclusion that heat given to a body was in some way absorbed by the particles of the body as kinetic energy.
A German scientist, Julius Mayer (1814-1878), extended the idea to other forms of energy transfer and gave the world its first understanding of the law of conservation of energy about the middle of the 19th century. At about the same time, James Joule (1818-1889), an English physicist, began a series of measurements which confirmed that different forms of energy were equivalent to each other. Appropriately his name is now used as the modern unit in which all energy measurements are made- the joule.
By combining the billiard ball model of the atom with the concepts of energy, scientists can obtain a much clearer picture of the behavior of solids, liquids and gases. The particles present in a solid are continually vibrating at room temperature and when more energy is supplied, such as when an electric fire is switched on, or a frying pan put on a gas stove, the molecular vibrations increase as the temperature rises.
If the energy is removed, as in a refrigerator, the vibrations die down, and if this process is continued they almost come to a stop. At this very low temperature, the particles have almost no kinetic energy and this indicates the lowest possible temperature that can be achieved, called absolute zero.
Physical apparatus has been built which can reach to within a millionth of a degree of absolute zero, but the last trace of kinetic energy has proved very difficult indeed to remove.
To convert solids into liquids and then gases, the particles of the solid must be moved further apart, and the energy absorbed during these changes of state is called latent heat.
If it were possible to watch the movements of gas particles in a transparent box, they would present a brilliant display of activity, more energetic than the busiest hive of bees and certainly less organized. Even at room temperature, air particles can attain speeds about a hundred times faster than an average man can run. Particles dash in all directions, some more energetic than others. They collide with each other, change direction continually and whenever they reach the walls of the box, bounce back into the center of the gas to re-join the fun. Every impact against the wall of the box contributes towards the total pressure of the gas.
Heating the gas gives the particles more energy and, as might be expected, this results in even greater chaos as the particles get more excited and fling themselves at the walls and each other with ever increasing speed. The pressure of the gas rises until the box can stand the strain no longer and it finally explodes. Harnessing these pressures was the ambition of the early engineers who were attempting to control and organize the random motions of particles to make the first steam engines.