[263] Journ. of Morph. II, p. 49, 1889.
[264] Phil. Trans. CLI, p. 183, 1861; Researches, ed. Angus Smith, 1877, p. 553.
[265] Cf. Kelvin, On the Molecular Tactics of a Crystal, The Boyle Lecture, Oxford, 1893, Baltimore Lectures, 1904, pp. 612–642. Here Kelvin was mainly following Bravais’s (and Frankenheim’s) theory of “space-lattices,” but he had been largely anticipated by the crystallographers. For an account of the development of the subject in modern crystallography, by Sohncke, von Fedorow, Schönfliess, Barlow and others, see Tutton’s Crystallography, chap. ix, pp. 118–134, 1911.
[266] In a homogeneous crystalline arrangement, symmetry compels a locus of one property to be a plane or set of planes; the locus in this case being that of least surface potential energy.
[267] This is what Graham called the water of gelatination, on the analogy of water of crystallisation; Chem. and Phys. Researches, p. 597.
[268] Here, in a non-crystalline or random arrangement of particles, symmetry ensures that the potential energy shall be the same per unit area of all surfaces; and it follows from geometrical considerations that the total surface energy will be least if the surface be spherical.
[269] Lehmann, O., Flüssige Krystalle, sowie Plasticität von Krystallen im allgemeinen, etc., 264 pp. 39 pll., Leipsig, 1904. For a semi-popular, illustrated account, see Tutton’s Crystals (Int. Sci. Series), 1911.
[270] As Graham said of an allied phenomenon (the so-called blood-crystals of Funke), it “illustrates the maxim that in nature there are no abrupt transitions, and that distinctions of class are never absolute.”
[271] Cf. Przibram, H., Kristall-analogien zur Entwickelungsmechanik der Organismen, Arch. f. Entw. Mech. XXII, p. 207, 1906 (with copious bibliography); Lehmann, Scheinbar lebende Kristalle und Myelinformen, ibid. XXVI, p. 483, 1908.
[272] The idea of a “surface-tension” in liquids was first enunciated by Segner, De figuris superficierum fluidarum, in Comment. Soc. Roy. Göttingen, 1751, p. 301. Hooke, in the Micrographia (1665, Obs. VIII, etc.), had called attention to the globular or spherical form of the little morsels of steel struck off by a flint, and had shewn how to make a powder of such spherical grains, by heating fine filings to melting point. “This Phaenomenon” he said “proceeds from a propriety which belongs to all kinds of fluid Bodies more or less, and is caused by the Incongruity of the Ambient and included Fluid, which so acts and modulates each other, that they acquire, as neer as is possible, a spherical or globular form....”