cānus, gray, from *casnus (cf. cas-cus, very old). See for other cases of this lengthening, [170, 5], quīnī, for *quincnī; [170, 6], īgnōscō, for *in-gnōscō.
[122]. Induced lengthening. Before certain groups of consonants short vowels have a tendency to become long: as,
([a.]) The prefixes in- and con- before s or f lengthened their vowels in classical Latin (Cic. O. 159): as, īnsānus, mad; īnfēlīx, unhappy; cōnsuēvit, he grew used to; cōnfēcit, he accomplished. Elsewhere also the vowel before ns and nf appears to have been lengthened: as, fōns, fountain; pēnsus, weighty (Gell. 9, 6); forēnsis, forensic; cēnsor, censor; mēnsa, table; mēnsis, mouth; Valēns; Clēmēns; the o of īnsons, guiltless, however, is marked as short by the grammarian Probus.
([b.]) A similar lengthening of the vowel before nc followed by t or s appears: as, ūnctus, anointed, from unguō (Gell. 9, 6); iūnctus, joined, from iungō ([954]), coniūnx, spouse, genit. coniugis ([472]); quīnctus, fifth, whence quīntus ([170, 4]) and quīnque, five, derive their ī; sānctus, hallowed.
([c.]) Spellings like sIgnvm, sign (well supported in inscriptions), and dIgne, worthily (less well supported) show that i was at times lengthened before gn. The grammarian Priscian demands this lengthening for all vowels preceding the ending -gnus, -gna, -gnum.
([d.]) A lengthened vowel before r followed by a consonant is also certain for some words like ōrdō, order; fōrma, shape.
([e.]) Some speakers appear to have lengthened the vowel of prepositions like con-, sub-, ob-, in the compounds of iaciō ([104, c]); as ōbicit. This practice, which is disapproved by Gellius ([4], [17]), probably arose from the transfer by analogy of the quantity of the first syllable in forms like conieciant ([940]) to that of the shortened form. In the same way the occasional spelling CÓNIV́NX, spouse, for coniūnx, may owe its long ō to the analogy of cōiunx, CÓIVGI ([170, 6]).
([f.]) Many verb stems ending in -g have a long vowel in the past participle before the suffix -to-: as, tēctus, covered, from tego ([916]); tāctus, touched, from tangō ([925]); pāctus, fixed, from pangō ([925]); fīctus, moulded, from fingō ([954]); pīctus, painted, from pingō. The evidence for ā in maximus is very scanty: one case of A with the apex ([29, 3]) in a faulty inscription.
([g.]) Of the induced lengthenings enumerated above, only those given in (a.) (b.) (f.) seem to have been universal in classical Latin. The rest appear to have been local peculiarities, which, while making inroads upon the literary language, never gained full recognition.
[123]. (1.) Analogical lengthening. In noun stems in -o the stem vowel is lengthened in the genitive plural -ōrum ([449], [462]), by analogy to the stems in -ā ([435]): as, servōrum, of slaves, like mēnsārum, of tables. For other cases see [122], e.