Actually, you can change the volume for the above applet.
Just drag near the right edge and move it left/right.

However, I have created [url=]another applet[/url] due to your request.
I hope the following is what you ask for. Please let me know if you still want more.
User can change Pressure(P), Velocity(v), Temperature (T), number of particle (N) and width of the system.
However, I did not add Volume as an adjustable parameter, because we are simulated an equilibrium state.
The volume of the system is not determined by the gas system itself. And I think we should teach student about this.
(Otherwise, we need to decide which one should be fixed (P or T) when volume is changed).

[color=red]The applet has been modified to include [b]fixed volume[/b] option. 2009/01/19[/color]