Felix Mendelssohn published more than 100 songs during his lifetime, of which one in particular, On Wings of Song, is a signature work. Mendelssohn's song The Human Heart is Like a Mine – that's mine as in goldmine rather than as in minefield – stands at the other end of the spectrum of popularity, for the simple reason that most of us had no idea it existed. Mendelssohn wrote it privately in 1842 at the request of a Berlin friend; it was never published. The manuscript turned up in auctions in 1862 and 1872 but then disappeared. Now it has resurfaced in the United States and is being auctioned later in the month. Mendelssohn's songs, like the human heart, are a mine full of treasures (a few of them were actually composed by his sister Fanny). But the world's interest at knowing that a lost song has been found after 140 years has probably lifted this "new" one to an instant, if brief, second place of popularity among the composer's songs.