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LETTERS TO THE EDITOR dilution used, 0.0000577Nw, gave an equivalent conductivity of 372.4 mhos, compared with Mr. Dye's 360.5 for his most dilute solution, 0.001089N~. The graph also shows the Onsager slope, calculated from the latest value for myristyl ion, 371.6 (2). It will be seen that the conductivity in this dilute region rises above its infinite dilution value, "a phenomenon which seems to admit of no other interpretation than the formation of (probably small) micelles" (3). This conclusion was also reached in the paper to which this experimental work is an extension. It would probably be found true for all colloidal electrolytes if special care were given to the study of the most dilute solutions.

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The relations between the electronic conductivity of certain spinels and the arrangement of the cations in the crystal structure (see preceding paper) are studied. Several arguments favor the assumption that Fe3O4 contains both divalent and trivalent iron in the 16-fold position. The transition point in the neighborhood of liquid-air temperature is probably associated with an increased degree of order at low temperature in the distribution of the 8 electrons between the 16 Fe-lattice points per unit cell. The considerably increased conductivity below the transition points shows tetragonal anisotropy when the crystal is cooled in a magnetic field. The possible distributions of the electrons in the crystal at low temperature are discussed. In more complicated spinels, containing other metal atoms as well as iron in both the divalent and the trivalent state, the electronic interchange is more or less inhibited by the foreign metal atoms. The higher values of their resistance in comparison to that of Fe3O4 can be roughly described by an increased activation energy. The investigation of a number of substances with different arrangements of the cations shows that the activation energy (and therefore the electrical resistance) is lowest for those cases in which the electrons can travel, as in Fe3O4, along the Fe of the 16-fold position.

By means of a detailed x-ray study of a large number of oxides having a spinel structure, it has been established whether they crystallize in the ``normal'' spinel-type structure, XY2O4, or in the ``inversed'' type of structure, Y(XY)O4, discovered by Barth and Posnjak. Certain rules can be derived which are also applicable to more complicated spinels or to solid solutions of different spinel oxides. Madelung constants have been calculated for different spinel types in order to investigate their stability on the basis of an ionic concept of the chemical bond (Born's lattice theory). The experimental results are only partially explainable on this basis. All exceptions to the electrostatic principles can be understood by assuming that Fe3+, Ga3+, In3+, Zn2+, and Cd2+ favor a configuration having the coordination number 4, because of a considerable covalent contribution to the chemical bond for that configuration.

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