Arsenic, As, is a nonmetallic element that is a silver-gray, brittle, crystalline solid Mercury, Hg , is a liquid metallic element that is silvery in color and heavy. It is insoluble in water and has a specific gravity of 13.59, which is heavier than water. Mercury is highly toxic by skin absorption and inhalation of fumes or vapor. The TLV is 0.05 mg/m^3 of air, and the IDLH is 10 mg/m^3. All inorganic compounds of mercury are toxic by ingestion, inhalation, and absorption. Most organic compounds of mercury are also highly toxic. The target organs affected are the central nervous system, kidneys , skin, and eyes. Mercury is incompatible with acetylene and ammonia. The four-digit UN identification number is 2809. The uses of mercury are in electrical appliances, instruments, mercury vapor lamps, mirror coating, and as a neutron absorber in nuclear power plants.