The alloy of mercury is amalgam. An amalgam is a type of alloy that consists of mercury as one of the components. Amalgams are usually used in dentistry to fill cavities. When mercury is mixed with other metals like silver, tin, or copper, it forms a soft, pliable material that can be easily shaped to fill the cavity. Once the amalgam hardens, it becomes a durable and long-lasting filling. Therefore, the correct answer is Amalgam.