By the Han period, Taoism took on connotations of magic as well. The Taoist religion aimed to enhance vitality and life by living in accord with nature's shifting balance. Legends arose that Taoist masters learned to extend their lives indefinitely, to fly through the clouds, to become invisible. In the late Han period, a fragmentary anonymous work called, A Chart of the Magic Art of Being Invisible appears, apparently a compendium of the techniques of the Taoist adepts (see Birrell 41). Many of these beliefs originated in literal readings of the Tao-te Ching and the Chuang-Tzu--especially in passages that were probably meant to be read allegorically. In any case, hordes of alchemists and magicians streamed into the Han courts where they attempted to refine crass material substances and make men immortal. From the imagery in Taoist poetry, they created a complex symbolism based on red. The holy color red represented the alchemical furnace and its beautiful, red-robed patron goddess. Another common symbol was the Manchurian crane, a symbol of longevity with the red spot of divinity on its crown. Cinnabar, that red compound of mercury and sulfur, was thought to have magical potency because it could be turned into a silvery liquid and then back into a solid. They valued gold--one of the few substances known to be indestructible (Schafer 62). They experimented with jade, which traditionally had preservative powers against decay.