Describe the tonality of Purcell's 'Music for a while' and how this reflects the mood of the piece.

The tonic key of the piece is A minor, which reflects the sombre mood of the piece and sombre meaning of the lyrics. This is first confirmed by the perfect cadence in bars 3-4 and again in bars 9-10. At bars 13–14 the piece starts to modulate to E minor, with an addition of D#s in the harpsichord part and this is then solidified by a perfect cadence in E minor in bar 15. From this, a major tonality is then established as the text focuses on the Fury Alecto, reaching the relative major of E minor (G major) at bar 18. The piece then modulates to C major (the relative major of the tonic key - A minor) in bar 22, at the beginning of the 'B' section, before then continuing into a tierce de Picardie in C major at bar 23, which is used to highlight the word 'snakes' in the lyrics. There is a brief modulation back to E minor, the dominant key of A minor in bar 28, before the piece returns to A minor for section 'A1'.
On top of all this, Purcell uses a ground bass, a common feature in Baroque vocal music, which consists of the use of chromatic notes to give the bass part melodic interest, alongside other features of melancholy songs such as minor tonality and a slow, stately tempo.