Mercury(I) chloride
It is a white to pale yellow crystalline solid. It is toxic, but not as toxic as mercury(II) chloride because it does not dissolve in water. It turns back into mercury and mercury(II) chloride when reacted with ammonia or exposed to ultraviolet light. The black color that came from the reaction with ammonia gave its name "calomel" (beautiful black). It is a weak reducing agent. It is the most common mercury(I) compound.