The Book of Pericles is a book found by Luigi in the Study of the mansion. A passage from this book describes Elemental Ghosts. It notes that one must find Elemental Medals first before attempting to vacuum these ghosts.
"There are spirits known as Elemental Ghosts. They live in the elements of fire, water, and ice that make up much of our world. To handle these Elements, one must first obtain an Element Medal. Perhaps the Elemental Ghosts are only visible to the bearer of such a medal."