[273] Science of Mechanics, 1902, p. 395; see also Mach’s article Ueber die physikalische Bedeutung der Gesetze der Symmetrie, Lotos, XXI, pp. 139–147, 1871.
[274] Similarly, Sir David Brewster and others made powerful lenses by simply dropping small drops of Canada balsam, castor oil, or other strongly refractive liquids, on to a glass plate: On New Philosophical Instruments (Description of a new Fluid Microscope), Edinburgh, 1813, p. 413.
[275] Beiträge z. Physiologie d. Protoplasma, Pflüger’s Archiv, II, p. 307, 1869.
[276] Poggend. Annalen, XCIV, pp. 447–459, 1855. Cf. Strethill Wright, Phil. Mag. Feb. 1860.
[277] Haycraft and Carlier pointed out (Proc. R.S.E. XV, pp. 220–224, 1888) that the amoeboid movements of a white blood-corpuscle are only manifested when the corpuscle is in contact with some solid substance: while floating freely in the plasma or serum of the blood, these corpuscles are spherical, that is to say they are at rest and in equilibrium. The same fact has recently been recorded anew by Ledingham (On Phagocytosis from an adsorptive point of view, Journ. of Hygiene, XII, p. 324, 1912). On the emission of pseudopodia as brought about by changes in surface tension, see also (int. al.) Jensen, Ueber den Geotropismus niederer Organismen, Pflüger’s Archiv, LIII, 1893. Jensen remarks that in Orbitolites, the pseudopodia issuing through the pores of the shell first float freely, then as they grow longer bend over till they touch the ground, whereupon they begin to display amoeboid and streaming motions. Verworn indicates (Allg. Physiol. 1895, p. 429), and Davenport says (Experim. Morphology, II, p. 376) that “this persistent clinging to the substratum is a ‘thigmotropic’ reaction, and one which belongs clearly to the category of ‘response.’ ” (Cf. Pütter, Thigmotaxis bei Protisten, A. f. Physiol. 1900, Suppl. p. 247.) But it is not clear to my mind that to account for this simple phenomenon we need invoke other factors than gravity and surface-action.
[278] Cf. Pauli, Allgemeine physikalische Chemie d. Zellen u. Gewebe, in Asher-Spiro’s Ergebnisse der Physiologie, 1912; Przibram, Vitalität, 1913, p. 6.
[279] The surface-tension theory of protoplasmic movement has been denied by many. Cf. (e.g.), Jennings, H. S., Contributions to the Study of the Behaviour of the Lower Organisms, Carnegie Inst. 1904, pp. 130–230; Dellinger, O. P., Locomotion of Amoebae, etc. Journ. Exp. Zool. III, pp. 337–357, 1906; also various papers by Max Heidenhain, in Anatom. Hefte (Merkel und Bonnet), etc.
[280] These various movements of a liquid surface, and other still more striking movements such as those of a piece of camphor floating on water, were at one time ascribed by certain physicists to a peculiar force, sui generis, the force épipolique of Dutrochet: until van der Mensbrugghe shewed that differences of surface tension were enough to account for this whole series of phenomena (Sur la tension superficielle des liquides considérée au point de vue de certains mouvements observés à leur surface, Mém. Cour. Acad. de Belgique, XXXIV, 1869; cf. Plateau, p. 283).
[281] Cf. infra, p. 306.
[282] Cf. p. 32.