In the 17th century the physical nature of the air surrounding the earth was es- tablished. This was a necessary prerequisite for the formulation of the gas laws. The invention of the mercuri barometer by Evangelista Torricelli (1608-47) and the fact that Robert Boyle (1627-91) introduced the pressure P as a new physical variable where im- portant steps. Then Boyle{Mariotte's law PV = const. for constant temperature, where V is the volume, was formulated.

Daniel Bernoulli (1700-82) who had been born in Groningen in the Netherlands and moved to Basle in Switzerland gave in 1738 in a treatise on hydrodynamics a derivation of the gas laws from a "`billiard ball"' model. He assumed that the gas consists of a very large number of small particles in rapid motion. He already identied heat with kinetic energy (living force). He then derived Boyle{Mariotte's law for the gas{pressure at constant temperature on a movable piston from the impact of the gas molecules onto the piston just in the same way as it is still done today in elementary text books. He also used the principle of conservation of mechanical energy and concluded that, if the temperature changes, the pressure will also change so that it is proportional to the square of the velocities of the gas{particles and thus, for constant volume, will rise with rising temperature. At that time mechanical energy was still called vis viva in Latin or living force in English. Bernoulli was, however, about a century ahead of his time with his kinetic theory of gases. His model was almost forgotten. At that time only his Swiss copatriots J.A. De Luc (1727-1817) and George-Louis Le Sage (1724-1803) in Geneva and M.V. Lomonossov (1711-65) in Russia mentioned it. As Stephen Brush points out in his book on the kinetic theory "The man who persuades the world to adopt a new idea has accomplished as much as the man who conceived that idea." The reason why Bernoulli's kinetic theory received so little attention was that most scientists at that time believed in the so{called caloric theory of heat.

In the caloric theory heat was a substance called "caloric". Caloric was considered to be a uid composed of particles which repel each other. Like the earth is surrounded by its atmosphere each matter particle was thought to be surrounded by an atmosphere of caloric whose density increases with temperature. Thus at small distances matter particles repel each other due to the repulsion of their caloric atmospheres. In those days one did not know that matter is held together by electrical forces but one thought that the attractive forces between matter particles are of gravitational origin just as the forces between the sun and the planets. At a certain distance between matter particles there would be equilibrium between the caloric repulsion and the gravitational attraction. As the temperature rises more caloric is added to each matter particle and consequently the caloric repulsion increases shifting the equilibrium point outwards. In this way one could explain the thermal expansion of matter including gases. It should be mentioned that such reputed scientists as Pierre Simon, Marquis de Laplace (1749-1827) gave a very sophisticated derivation of the gas laws within the caloric theory.

At the end of the 18th century various arguments against the caloric theory ap- peared. For instance: Does caloric have weight? In 1798 Benjamin Thompson (1753- 1814) could show that the expected additional weight when a body was heated could not be detected. Another argument against the caloric theory was raised by Rumford Humphry Davy (1778-1829). He remarked that an indefinite amount of heat can be produced from matter by mechanical work, for instance by friction. If caloric was a substance only a limited amount should be available in matter. But these arguments did not really convince the fans of the caloric theory.