ENIGMA

I am composed of twenty-one letters.

My 3-6-21-19-14-8-1 is sincere.

My 12-17-7-18-20-5 is a mineral.

My 9-2-3-10-4-17-11-1 is a bird.

My 16-13-20-19-15 is to mingle.

My whole is the name of a well-known song.

William Harris.

I am the first, and one of seven,

I live betwixt the seas and heaven: