The idea expressed in DALTON'S atomic hypothesis (1802), and universally held during the nineteenth century, that the material world is made up of a certain limited number of elements unalterable in quantity, subject in themselves to no change or development, and inconvertible one into another, is quite alien to the views of the alchemists. The alchemists conceived the universe to be a unity; they believed that all material bodies had been developed from one seed; their elements are merely different forms of one matter and, therefore, convertible one into another. They were thoroughgoing evolutionists with regard to the things of the material world, and their theory concerning the evolution of the metals was, I believe, the direct outcome of a metallurgical application of the mystical doctrine of the soul's development and regeneration. The metals, they taught, all spring from the same seed in Nature's womb, but are not all equally matured and perfect; for, as they say, although Nature always intends to produce only gold, various impurities impede the process. In the metals the alchemists saw symbols of man in the various stages of his spiritual development. Gold, the most beautiful as well as the most untarnishable metal, keeping its beauty permanently, unaffected by sulphur, most acids, and fire--indeed, purified by such treatment,--gold, to the alchemist, was the symbol of regenerate man, and therefore he called it "a noble metal". Silver was also termed "noble"; but it was regarded as less mature than gold, for, although it is undoubtedly beautiful and withstands the action of fire, it is corroded by nitric acid and is blackened by sulphur; it was, therefore, considered to be analogous to the regenerate man at a lower stage of his development. Possibly we shall not be far wrong in using SWEDENBORG'S terms, "celestial" to describe the man of gold, "spiritual" to designate him of silver. Lead, on the other hand, the alchemists regarded as a very immature and impure metal: heavy and dull, corroded by sulphur and nitric acid, and converted into a calx by the action of fire,--lead, to the alchemists, was a symbol of man in a sinful and unregenerate condition.