As to the peculiar effect of c final in certain particles to “lengthen” the vowel before it, this c is doubtless the remnant of the intensive enclitic ce, and the so-called ‘length’ is not in the vowel, but in the more forcible utterance of the c. It is true that Priscian says:
[Keil. v. II. p. 34.] Notandum, quod ante hanc solam mutam finalem inveniuntur longae vocales, ut hōc, hāc, sīc, hīc adverbium.
And Probus speaks of c as often prolonging the vowel before it. But Victorinus, more philosophically, attributes the length to the “double” sound of the consonant:
[Mar. Vict. I. v. 46.] Consideranda ergo est in his duntaxat pronominibus natura c litterae, quae crassum quodammodo et quasi geminum sonum reddat, hic et hoc.
And he adds that you do not get that more emphatic sound in, for instance, the conjunction nec.
Si autem nec conjunctionem aspiciamus, licet eadem littera finitam, diversum tamen sonabit.
And again:
Ut dixi, in pronominibus c littera sonum efficit crassiorem.
Pompeius, commenting upon certain vices of speech, says that some persons bring out the final c in certain words too heavily, pronouncing sic ludit as sic cludit; while others, on the contrary, touch it so lightly that when the following word begins with c you hear but a single c:
[Keil. v. V. p. 394.] Item litteram c quidam in quibusdam dictionibus non latine ecferunt, sed ita crasse, ut non discernas quid dicant: ut puta siquis dicat sic ludit, ita hoc loquitur ut putes eum in secunda parte orationis cludere dixisse, non ludere: et item si contra dicat illud contrarium putabis. Alii contra ita subtiliter hoc ecferunt, ut cum duo c habeant, desinentis prioris partis orationis et incipientis alterius, sic loquantur quasi uno c utrumque explicent, ut dicunt multi sic custodit.