Qualitative evidence of the microscopic nature of gases is shown by an effect called Brownian motion.
All the particles are moving with different velocity and in random direction.
The only interaction between partciles is coiision.
There is one larger particles which will be collide by surrounding particles and the behavior is similar to a random walk pattern.
This simulation let you get a visual representation of Brownian motion.

/htdocs/ntnujava/ejsuser/2/users/ntnu/fkh/browianmotion_pkg/browianmotion.propertiesFull screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1352.2kB):double click downloaded file to run it. (98 times by 65 users) , Download EJS source (32 times by 23 users) View EJS source

Here is a related youtube movie