THE 18th CENTURY
It was not until the mid 1700's that the first real progress was made towards developing a consistent theory using the model of a gas being composed of small particles. This theory was known as the kinetic theory of heat.
The first person to really develop this theory and use it to predict experiments was Daniel Bernoulli (1700-1782). In his work on Hydrodynamics, which is basically a theory of fluids, he described air as an elastic fluid with particles which are "moving extraordinarily fast in various directions."
Using this model he derived two laws which had been experimentally determined by Boyle and Charles a few decades ago. These laws were known as Boyle's Law 9 AND Charles' Law 10 and will be described below.
Although this theory predicted experimentally verified laws, there was still not enough evidence to prove the existence of atoms. Furthermore, Bernoulli never mentioned anything about the gas being composed of atoms; he simply assumed that you could model a gas AS IF it was composed of small particles.
SO WHO CARES
The importance behind these experiments is that they provide relationships between macroscopic (i.e. measurable quantities). These experiments allowed the formation of the IDEAL GAS LAW, which simply expresses both equations in the above two experiments into one single equation,
In this equation P is the pressure of the gas, V is the volume, T is the temperature and C is some constant which allows us to relate all of them.
This equation is one of the cornerstones of classical thermodynamics which we will discuss shortly.