Magic in Bartimaeus - Essence

The basic form of the magical entities consists of a substance named essence, a flowing shapeless matter, described as a smokey liquid, while in the other place it is described as a mixture between gas and liquid. Bartimaeus himself (in the 3rd book, Ptolemy's Gate, during a confrontation with the Djinn Faquarl, involving a silver pot of fish soup and a wall), describes Essence as an oily fluid mass, not unlike dishwater. It is responsible for their personality, power, and abilities. While on earth, they have to create a physical body for themselves, which may be of any form they please although they have a set appearance on the 7th plane that never changes. Maintaining this form pains them, and their essences weaken if summoned for a prolonged period. Their essences are also extremely vulnerable to silver and iron. These metals are often used to subdue spirits, such as when Bartimaeus chases Honorius (The Golem's Eye) and when Mandrake tries to threaten Bartimaeus with a silver-tipped spear (Ptolemy's Gate). It is to be noted that the Resistance often used silver throwing discs against spirits. The spirits are also vulnerable against rowan powder, rosemary, garlic, and a few other substances.