Make it clear that the 'Air Density' slider only applies to the air inside the box

[matthew-blackman]

This was not clear to me at first and made the behavior of the 'Air Density' slider very confusing. In the java sim, the UI for 'Air Density' has buttons for 'Remove air from box', and 'Add air to box', making this more clear. I think a low-cost solution here would be to change the label of the 'Air Density' slider to 'Air Density in Box' or 'Density of Air in Box'.

[matthew-blackman]

The updated slider title makes this much more clear:

Closing.