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PHYSICS: E. H. HALL
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straints.' The optical properties of most substances show their electrons to be held rather rigidly, but many of the properties of metals, and especially of metals pronouncedly electro-positive in character, indicate a high degree of electronic freedom. It is therefore our belief that in the metals under consideration the difference between the heat capacity observed and that calculated from the regular curve, which fits the experimental curve at low temperatures, may be regarded as representing the actual heat capacity of their more loosely bound electrons. Whether these electrons are 'free' in the sense that each electron occupies a position symmetrical with respect to two or more atoms, or whether they remain attached to individual atoms, we should expect them to add to the heat capacity of the substance, provided that they are held by sufficiently weak constraints. We have thus an entirely new method of investigating the freedom of electrons in a metal, and it is to be hoped that when further quantitative data are available a comparison of the results obtained by this method with those obtained through a study of the photo-electric effect, or the Volta effect, will prove of interest.
'Lewis, J. Amer. Chem. Soc., Easton, Pa. 29, 1907, (1165); Zs. anorg. Chem., Hamburg, 55;2 1907, (200). Lewis and Adams, Physic. Rev., Ithaca, N. Y., (Ser. 2), 4, 1914, (331). 3 Langmuir, J. Amer. Chem. Soc., Easton, Pa., 38, 1916, (2236), calls attention to the possibility that the atom in a solid may not be vibrating in simple harmonic motion and hence that the potential energy may not be equal to the kinetic energy, as has been assumed. However, if this were the case the potential energy would in all probability be less than the kinetic energy and not greater. 4Dewar, London, Proc. R. Soc., A, 89, 158, 1913, (158). 6 Nerhst and Schwers, Berlin Sitz. Ber. Ak. Wis