The Mystery of Words and Music Together

Some passages in opera seem to reflect a perfect blending of words and music,yet it seems to me very difficult to articulate just why such passages are so effective.We can comment on the beauty (e. g. evocativeness, sense of mystery, effective use of metaphor, clarification of the dramatic moment--the "parola lirica" of Verdi, etc.) of the words--that is possible. Where it becomes difficult is describing the music and just what it is that makes it so beautiful/effective. Harder yet is to account verbally for the blend of words and music and why it is so effective. I would be interested in hearing some thoughts on this. Private replies welcome if you prefer. Roberto il Canadese