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Allotropy of Lead

Until recently no allotropic form of lead had been recognised. It has been observed, however, by Cohen and Helderman that when pure lead is immersed in a solution of lead acetate containing nitric acid the metal undergoes a structural change, accompanied by a change of density. These facts are believed to indicate that ordinary lead consists of a metastable mixture of several allotropic modifications of the metal. Heller also found that pure lead becomes brittle after immersion in an acidified solution of lead acetate or nitrate for several days, and called the new form of metal "grey lead." Creighton has obtained a similar result by passing an electric current through nitric acid of density 1.42, with sheet lead as cathode, and platinum foil as anode; after eight hours the cathode became grey, soft, and powdery. Lastly, Cohen and Helderman observed that pure lead suspended in a 40 per cent, solution of lead acetate, containing 100 c.c. of 1.16 nitric acid per litre, showed ridges due to contraction; the changed lead is believed to be composed of several allotropic forms. Janecke has confirmed the existence of allotropic forms of lead by the study of pressure-temperature curves of the metal.

Lambert and Cullis also attribute to allotropy certain phenomena connected with the action of water and oxygen on pure lead.

Lead is now definitely recognised as a disintegration product of radium; and radium-D has been obtained in visible quantity, and proved to be chemically identical with lead.

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