rar·e·fy also rar·i·fy 
(rârə-fī′)
(rârə-fī′)v. rar·e·fied, rar·e·fy·ing, rar·e·fies also rar·i·fied or rar·i·fy·ing or rar·i·fies
v.tr.
1. To make thin or less dense, as air.
2. To purify or refine: "The [medieval] Italians ... rarefy the image of the lady, treating her as a quasi-celestial figure" (Marcia L. Colish).
v.intr.
To become thin or less dense.
[Middle English rarefien, from Old French rarefier, from Medieval Latin rārificāre, alteration of Latin rārēfacere : rārus, rare + facere, to make; see dhē- in the Appendix of Indo-European roots.]
rar′e·fia·ble adj.