Today is the birthday of Axel Fredrik Cronstedt, a Swedish chemist born in 1722 who is best known for his discovery of nickel and his mineral classification scheme. In 1751, Cronstedt described a new mineral he called kupfernickel, or "demon copper," after the copper color of nickel ore. Many experts at the time, however, believed he had confused nickel with cobalt, and his findings were not confirmed until the early 1900s. Seven years after identifying nickel, Cronstedt published his "Essay on the New Mineralogy," in which he described four classes of minerals--earths, bitumens, salts, and metals. The work classified minerals according to chemical composition, rather than by the physical characteristics of color and hardness.