\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.

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