The grammarians speak of the obscure sound of I and U, short and unaccented in the middle of a word; so that in a number of words I and U were written indifferently, even by classic writers, as optimus or optumus, maximus or maxumus. This is but a simple and natural thing. The same obscurity occurs often in English, as, for instance, in words ending in able or ible. How easy, for instance, to confuse the sound and spelling in such words as detestable and digestible.