The ‘h’, the pure aspiration, is an expulsion of ‘flatus’ through the position of the adjacent letter, whether vowel, semivowel, or nasal; in English it occurs only before a vowel, or before ‘w’ and ‘y’, in such words as ‘when’ and ‘hue’. It is, then, the common surd to the three classes of sonant sounds just mentioned.
- Whitney (1875), Pag. 67