The alchemists generally likened the work to be performed by their agent to the killing of a living thing. They constantly use the allegory of death, followed by resurrection, in describing the steps whereby the Essence was to be obtained, and the processes whereby the baser metals were to be partially purified. They speak of the mortification of metals, the dissolution and putrefaction of substances, as preliminaries to the appearance of the true life of the things whose outward properties have been destroyed. For instance, Paracelsus says: "Destruction perfects that which is good; for the good cannot appear on account of that which conceals it." The same alchemist speaks of rusting as the mortification of metals; he says: "The mortification of metals is the removal of their bodily structure.... The mortification of woods is their being turned into charcoal or ashes." Paracelsus distinguishes natural from artificial mortification, "Whatever nature consumes," he says, "man cannot restore. But whatever man destroys man can restore, and break again when restored." Things which had been mortified by man's device were considered by Paracelsus not to be really dead. He gives this extraordinary illustration of his meaning: "You see this is the case with lions, which are all born dead, and are first vitalised by the horrible noise of their parents, just as a sleeping person is awakened by a shout." The mortification of metals is represented in alchemical books by various images and allegories. Fig. I. is reduced from a cut in a 16th century work, _The Book of Lambspring, a noble ancient Philosopher, concerning the Philosophical Stone_. [Illustration: Here the father devours the son; The soul and spirit flow forth from the body. FIG. I.] The image used to set forth the mortification of metals is a king swallowing his son. Figs. II. and III. are reduced from Basil Valentine's _Twelve Keys_. Both of these figures represent the process of mortification by images connected with death and burial. [Illustration: FIG. II.] In his explanation (?) of these figures, Basil Valentine says:--