Model concept: Gas is “pourable” (fluid) and flows in a way similar to a liquid.
The continuum theory and the summarization of the gas laws which follows
are based on experience and can explain all the processes in gases
near atmospheric pressure. Only after it became possible using ever better
vacuum pumps to dilute the air to the extent that the mean free path rose
far beyond the dimensions of the vessel were more far-reaching assumptions
necessary; these culminated in the kinetic gas theory. The kinetic gas
theory applies throughout the entire pressure range; the continuum theory
represents the (historically older) special case in the gas laws where atmospheric
conditions prevail