change affected the nominative and accusative neut. plur. of disyllabic words, such as bĕna, ĕa, in which we know from kindred languages that the -a was originally long.
§ 17. But in the compound word intereă ‘among those things, meanwhile,’ which had become an adverb, the original long -a remained. It was not affected by the phonetic change, because the word had four syllables; and as it had come to be felt merely as an adverb, no longer as a preposition and a pronoun, it was not affected by the analogical change which spread the short -a of the very common disyllabic words ĕă, bonă, mălă, mĕd, tŭă, sŭă through all the neuter plurals of pronouns (like illa), nouns (like dōna), and adjectives (like multa). Words like intereā and modo which became separated in consciousness from the sets of forms (in a Declension or Conjugation) to which they once belonged are said to have been “isolated” or “crystallised.” They are often a great help to our study, because in these we have true phonetic forms preserved of which otherwise we might know nothing.
§ 18. The examples just given show the power of Analogy both to restore and to innovate. In Ablatives like via the old quantity was put back on the pattern of words like mensā (§ 15); but in all the Neut. Plurals of the Second Declension the final -a, originally long, has been shortened to match that of the disyllables like ĕă (§ 17).
§ 19. Such modification by Analogy of what we may call the natural form which words have taken
