rarefy

verb (used with object), rar·e·fied, rar·e·fy·ing.

1. to make rare or rarer; make less dense: to rarefy a gas.
2. to make more refined, spiritual, or exalted.

verb (used without object), rar·e·fied, rar·e·fy·ing.

3. to become rare or less dense; become thinned: Moisture rarefies when heated.

verb -fies, -fying or -fied

1. to make or become rarer or less dense; thin out

v.

late 14c., from Old French rarefier (14c.) and directly from Medieval Latin rarificare, from Latin rarefacere “make rare,” from rarus “rare, thin” (see rare (adj.1)) + facere “to make” (see factitious). Related: Rarefied.