therefore, the palatal and labial vowels, if brought in immediate contact with a following a, relapse naturally into their corresponding semi-vowels, y and v, and do never form the base of diphthongs. The vowel i+a, or u+a, if pronounced in quick succession, become ya and va, but they will never coalesce into one vocal sound, because the intonation of the a lies behind that of i; the vocal flatus has to be inverted, and this inversion amounts in fact to a consonantal stoppage sufficient to change the vowels i and u into the semi-vowels y and v.
The four Bases of Diphthongs.
According to our definition of a diphthong, their basis can only be guttural; but as the short guttural a is liable to three modifications, long a, ĕ, and ŏ, we get really a four-fold basis for diphthong sounds. Each of the four gutturals being liable to a palatal or labial modification, we may on physiological grounds expect eight different compound vowels.
This can best be represented by a diagram:
