Qualitative evidence of the microscopic nature of gases is shown by an effect called Brownian motion.
This java applet shows Brownian motion for gas molecules.
(Gas molecules in a container continually collide with one another and with the walls of the container.

Please check out [url=http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=120.0]Collision 2D[/url]  for how to process collision between two particles.

Here is a related youtube movie