rāresco, ere, v. inch. n. [rarus], to grow thin, lose its density, to become rare, be rarefied (poet. and in post-Aug. prose). Lit.: fulgit item cum rarescunt quoque nubila caeli, Lucr. 6, 214; cf. id. 6, 513; Stat. S. 1, 2, 186: umor aquai ab aestu, Lucr. 6, 875; cf.: rarescit terra calore, becomes loose, id. 6, 841: corpus, id. 4, 865; 892: resolutaque tellus In liquidas rarescit aquas, Ov. M. 15, 246: quadrupedibus senectute lanae rarescunt, Plin. 11, 39, 94, § 231.

— Transf.: rarescunt alta colonis Maenala, i. e. become empty, depopulated, Stat. Th. 4, 284; cf.: moenia densae Romae, id. S. 4, 4, 14: rarescit multo laxatus vulnere miles, the ranks grow thin, Sil. 17, 423; 5, 382; and: rarescunt cuspide pugnae, Val. Fl. 6, 617: umbrae rarescentes, Stat. Th. 1, 343; 11, 74: ubi angusti rarescent claustra Pelori, i. e. shall open themselves, grow wider, i. q. laxabuntur, patebunt, * Verg. A. 3, 411; so, colles paulatim rarescunt, Tac. G. 30.

— Trop.: sonitus rarescit, becomes feeble, diminishes, dies away, Prop. 3, 15 (4, 14), 35: quod justitia rarescit, iniquitas increbrescit, Tert. Apol. 20: ita justitia rarescet, ita impietas et avaritia crebrescent, Lact. 7, 15, 8: Ibis efficit, ut rarescant mortiferae pestes absumptae, become rare, Amm. 22, 15, 25; 26, 3, 1. (In Ov. M. 15, 246, the read. varies between rarescit and rorescit.)