[16] Widmanstätten, 1807. Leydolt (1855, Wien. Akad. Ber. 15, 59, T. 9, 10. Baumhauer, Pogg. Ann. 138, 563 (1869); 140, 271; 142, 324; 145, 460; 150, 619.) For an account of these investigations, see Mallard, _Traité de Crystallographie_ (Paris, 1884), Tome II, chapitre xvi. [17] More exactly .9525, being 3/4 × cot 38° 13′; see p. 53. [18] J. and P. Curie and C. Friedel, _Comptes Rendus_, 1882, 1883, 1886, 1892. [19] Allgemeine Theorie der piëzo- und pyroelectrischen Erscheinungen an Krystallen. W. Voigt, Königl. Gesellschaft der Wissenschaften zu Göttingen, August 2, 1890. [20] Wiedemann, _Annalen_, 1892, xlv, p. 923. [21] Generally miscalled ‘rotational.’ [22] See foot-note (2) to p. 54 above. [23] See foot-note (3) to p. 54 above. [24] See foot-note to § 22 above. THE END Oxford PRINTED AT THE CLARENDON PRESS BY HORACE HART, PRINTER TO THE UNIVERSITY End of Project Gutenberg's The Molecular Tactics of a Crystal, by Lord Kelvin