This program simulates the behavior of gas molecules. Molecules are modeled as perfectly elastic particles with no internal energy except that which is due to their motion. Collisions between molecules are elastic. Unlike Gas in a Box, the Free Gas model has no boundaries. Molecules are free to "wrap" the screen; that is, if they leave one edge they reappear on the other edge. Particles are colored according to speed -- blue for slow, green for medium, and red for high speeds.
For detailed instructions on using the program, take a look at the info window from the Free Gas model.
Click on one of the pictures to see a quicktime movie of the model: