Mercury is a heavy, silvery-white metal. It is a relatively poor conductor of heat, but a fair conductor of electricity. Mercury readily forms alloys with other metals, called amalgams. An electrical discharge will cause mercury to combine with the noble gases argon, krypton, neon, and xenon. Mercury and its compounds are highly poisonous. Mercury is readily absorbed across unbroken skin or though the respiratory or gatroinstentinal tract. It acts as a cumulative poison. Mercury is very volatile in air. When room temperature air (20°C) is saturated with mercury vapor, the concentration greatly exceeds the toxic limit. The concentration, and thus the danger, increases at higher temperatures.