Charlton T. Lewis, A Latin Dictionary (1879)
“ Finally, by dropping the b of ab and lengthening the a, ab was changed into â, which form, together with ab, predominated through all periods of the Latin language, and took its place before all consonants in the later years of Cicero, and after him almost exclusively.—By dropping the b without lengthening the a, ab occurs in the form ă- in the two compounds ă-bîto and ă-pĕrio, quod videas.—On the other hand, instead of reducing ap to a and ă, a strengthened collateral form, aps, was made by adding to ap the letter s (also used in particles, as in ex, mox, vix) . ”
