The Collaborative International Dictionary
Rarefy \Rar"e*fy\ (r[a^]r"[-e]*f[imac]; 277), v. t. [imp. & p. p. Rarefied (r[a^]r"[-e]*f[imac]d); p. pr. & vb. n. Rarefying (r[a^]r"[-e]*f[imac]`[i^]ng).] [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.
Wiktionary
vb. (present participle of rarefy English)
Usage examples of "rarefying".
Hooke put Daniel to work mending his Condensing Engine, which was a piston-and-cylinder arrangement for compressing or rarefying air.
He was paying attention to part of a rarefying engine: one of many such piston-and-cylinder devices he and Boyle used to study the expansion of gases.
He handed it to Hooke, who poured it into the cylinder of the Rarefying Engine.
A head of one hundred and eighty degrees is sufficient to diminish the weight of the air it contains to the extent of one-half, by rarefying it.