Rarefy

(Synop., § 130), v. t. [imp. & p. p. RAREFIED; p. pr. & vb. n. RAREFYING.] [Fr. raréfier, It. rarefare, rarificare, Sp. & Pr. rarificar, Lat. rarefacere, from rarus, thin, rare, and facere, to make.] To make rare, thin, porous, or less dense; to expand or enlarge without adding any new portion of matter to; — opposed to condense.

v. i. To become thin and porous.