The meaning and origin of the expression: Men's evil manners live in brass; their virtues we write in water

Men's evil manners live in brass; their virtues we write in water

What's the origin of the phrase 'Men's evil manners live in brass; their virtues we write in water'?

From Shakespeare's Henry VIII, 1612:

GRIFFITH:
Noble madam,
Men's evil manners live in brass; their virtues
We write in water. May it please your highness
To hear me speak his good now?

The line was alluded to on Keats' tombstone - Here lies one whose name was writ in water.

See other phrases and sayings from Shakespeare.