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Literature Abstracts

 1516.    HULL, A. W.  Crystal Structure of Iron. Phys. Rev., vol. 9, 1917, pp. 84-87; Chem. Abs., vol. 12, 1918, p. 2064.

        In the X-ray analysis of Fe a special procedure is necessary on account of the difficulty of obtaining large crystals.  (The large crystals of pure ferrite, reaching a length at times of several inches, which are met with in blast-furnace “bears,” are not considered.)  Single crystals about 6 mm. square and 2 mm. thick were isolated from a sample of Si steel.  The value of d100=1.43X10-5 cm. requires 2 atoms for a cube of side 2d100.  A lattice having atoms at cube corners and cube centers satisfies this condition and gives the observed spacing for other planes.  Pure Fe was then investigated in the form of a fine powder.  A narrow beam of rays from a W target passed through the powder and formed on the plate a kind of generalized Laue photograph in which every possible plane in the crystal structure and an equal opportunity of reflecting and reflected all wave lengths present.  What was actually observed was the position of the K lines, which with the tube running at 110,000 volts, stood out very clearly.  Their reflection in different planes appeared as concentric, nearly circular lines whose distance from the center should be inversely proportional approximately to the spacing of the planes.  On the assumption that the atoms were arranged on a centered cubic lattice, and that the scattering electrons in each atom were concentrated at its center, the calculated spacings agree remarkably well with the observed values but fail to account for the following facts:  (1) That the intensity of the lines falls off continuously with increasing distance from the center; (2) that the 1st-order (100) reflection is much too weak for its position; and (3) that the 2d-order (100) reflection is entirely lacking.  In calculating intensities it is assumed (1) that the rays, when they reach the crystal, consist of long trains of waves, to which the electromagnetic theory is applicable; (2) that the scattering is due entirely to the electrons in the atoms, each of which scatters independently and equally; (3) that the number of electrons in an atom is equal to its atomic number; and (4) that the electrons have fixed positions in the atom, about which they move in very small orbits.