The above simulation is created with EJS. Please
1. download [url=http://www.phy.ntnu.edu.tw/demolab/download/ejs.zip]EJS[/url] and unzip it to c:\
2. Click c:\Ejs4.2\EjsConsole.jar to test the installation (Your computer need to support java :assume java run time has been installed)
3. Download jar file from the link just below the simulation.
4. Double click downloaded jar file to tun it.
5. RIght click in the simulation and select "open EJS model" to open source code in EJS.
6. You can modify variables/ relations/GUI element and properties  in the simulation and click run button to compile it.

If you need the source code for the simulation at the first post, just download idealGas.java
However, it was written more than 10 years ago with JDK1.0.2.