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The term “rare” may be applied to a metal with two rather different meanings, either the occurrence of the metal is limited to relatively small quantities or its usefulness in metallic form is not commensurate with the difficulty of producing it in that form. Of the ninety known elements, about seventy are generally classed as metallic, and of this number by far the greater proportion may be designated as rare metals from the standpoint of quantity produced in metallic form.
Swanger, W. H.
Metallurgist, U. S. Bureau of Standards, Washington, D. C.