The Taoist immortality project is physical and it is meant literally. Waidan, outer alchemy, worked with actual furnaces and actual minerals — cinnabar, which is mercury sulphide, was central because it appears to die and return when heated — and it killed people, including several Tang emperors who took elixirs prepared by court alchemists. Neidan, inner alchemy, which becomes dominant from the Tang and Song, keeps the entire vocabulary and relocates the operation: the furnace is the lower dantian, the ingredients are the practitioner's own jing, qi and shen, and the process is described in the same coded language of lead and mercury, dragon and tiger, firing times and cycles. The stated goal ranges from radically extended life to the formation of an immortal embryo that survives the body. Scholars of Chinese science, following Joseph Needham, have noted that waidan's failures produced an enormous body of real chemical and pharmacological knowledge, including gunpowder.