Rarefy (verb)

1. Make (something) less dense or more refined.
2. Reduce the density or thickness of (a gas or liquid).
3. Make (a thing) more rare or precious.

Origin:

Late middle english (in the sense "make thin or rare"): from latin rarificare, from rarius "rarer" + facere "make".

Examples:

1. The air was rarefied by the altitude.
2. The air was rarefied by the altitude.
3. Rarefy the air in the chamber.
4. The atmosphere was rarefied by the fire.
5. Rarefying the liquid by heating.