What gas would be suited best to form a density gradient in a long tube? [closed]








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I want to set up an aero-optics experiment using a sealed ~5 m length of pipe with a gas inside. The gas must form a density gradient, and must be transparent enough to view a target at the other end of the pipe.


Barometers at the top and bottom of the pipe will measure density.


The purpose of the experiment is to replicate the density structure of earth's atmosphere to establish what affect density gradients alone have on light passing through. 


Question: What gas would be best to use for this experiment?


Would a moderate amount of boiling water in the bottom of the pipe do the job? Are there better gasses that could be used? My thinking is once the gas is added and the pipe is shaken up to distribute the gas around, then as it settles back down it would form some kind of gradient for a short while.


[![Basic outline of experiment](https://i.stack.imgur.com/cqA9o.png)](https://i.stack.imgur.com/cqA9o.png)


