In the early years of the eastern Tsin dynasty, (317-420 A.D.), alchemist Ko Hung wrote in his book Pao Pu Tzu that "Tan sha (cinnabar) can be turned into mercury, and back into cinnabar."
中國煉丹術，到後漢末年已經很發達，當時有六百篇的爐火術書，和託名黃帝所寫的「龍虎經」（也是一本討論煉丹的書）。