rarefacio

rārē-făcĭo (written also separately rārē făcĭo ), fēci, factum, 3, v. a. [rarus], to make thin or rare, to rarefy (Lucret.):

rarefieri,

Lucr. 1, 648:

rarefacta,

id. 2, 1139; 3, 442:

rareque facit,

id. 6, 233; 870.