rārē-făcĭo — Lewis & Short
rārē-făcĭo (written also separately rārē făcĭo), fēci, factum, 3, v. a.rarus,
I to make thin or rare, to rarefy (Lucret.):
rarefieri,Lucr. 1, 648:
rarefacta,id. 2, 1139; 3, 442:
rareque facit,id. 6, 233; 870.