Source : Webster's Revised Unabridged Dictionary (1913)

Rarefy \Rar"e*fy\, v. t. [imp. & p. p. {Rarefied}; p. pr. & vb.
   n. {Rarefying}.] [F. rar['e]fier; L. rarus rare + -ficare (in
   comp.) to make; cf. L. rarefacere. See {-fy}.]
   To make rare, thin, porous, or less dense; to expand or
   enlarge without adding any new portion of matter to; --
   opposed to condense.

Rarefy \Rar"e*fy\, v. i.
   To become less dense; to become thin and porous. ``Earth
   rarefies to dew.'' --Dryden.

Source : WordNet®

rarefy
     v 1: lessen the density or solidity of; "The bones are rarefied"
     2: make more subtle or refined [syn: {sublimate}, {subtilize}]
     3: weaken the consistency of (a chemical substance) [syn: {attenuate}]
     [also: {rarefied}]