Nickel is not scarce in the Earth's crust; its content is greater than that of common metals like lead and tin, but significantly less than iron. Moreover, the melting points of nickel and iron are comparable, which is why nickel was discovered later than iron. In 1751, Swedish chemist Cronstedt obtained nickel by heating grains of red arsenic nickel ore, which had undergone surface weathering, together with charcoal. In 1952, there were reports suggesting that nickel exists in animals, and later it was proposed that nickel is an essential trace element for mammals. In 1973, someone first proposed that nickel is an essential trace element. After 1975, studies on the nutrition and metabolism of nickel were conducted.