[Footnote 259: Murphy, "Habit and Intelligence," vol. i. p. 22.] [Footnote 260: Professor Charles Brooke, in _Nature_, vol. vi. p. 125.] [Footnote 261: "Familiar Lectures on Scientific Subjects," pp. 469-472.] [Footnote 262: "Natural Philosophy," vol. i. pp. 190, 191.] [Footnote 263: Ibid. p. 194.] [Footnote 264: _North British Review_, vol. xl. pp. 182, 183.] [Footnote 265: Helmholtz, "Correlation and Conservation of Forces," p. 245.] [Footnote 266: This is the hypothesis of Helmholtz, Mayer, and Thomson.] [Footnote 267: Tyndall, "Fragments of Science," p. 31; Murphy, "Habit and Intelligence," vol. i. p. 23.] [Footnote 268: Murphy, "Habit and Intelligence," vol. i. p. 43.] [Footnote 269: Professor Norton, "On Molecular Physics;" _American Journal of Science and Arts_, vol. iii, 3d Series, pp. 329-331.] [Footnote 270: Tyndall, "Fragments of Science," p. 76.] [Footnote 271: "Principia," Def. viii. p. 8.] [Footnote 272: "Does every grain of salt and pepper in a million salt-cellars and pepper-casters individually and separately pull and actually move the sun and fixed stars?"--De Morgan.] [Footnote 273: _North British Review_, vol. xlviii. March, 1868, p. 125.] [Footnote 274: Third Letter to Bentley.] [Footnote 275: _Nature_, vol. iii. p. 51; vol. ii. p. 422.] [Footnote 276: _Nature_, vol. i. p. 551.] [Footnote 277: Delivered at the Royal Institution, and reported in _Nature_, vol. vii. Nos. 174, 175.] [Footnote 278: _North British Review_, vol. xlviii. March, 1868; "Correlation and Conservation of Forces," p. 368; _Amer. Jour. of Science and Arts_, vol. xlix. p. 24.] [Footnote 279: _North British Review_, vol. xlviii. p. 127; _Nature_, vol. vii. p. 343.] [Footnote 280: "Familiar Lectures on Science," p. 90.] [Footnote 281: Picton, "Mystery of Matter," p. 49.] [Footnote 282: Herschel, "Familiar Lectures on Science," p. 467.] [Footnote 283: Sir William Thomson, "Papers on Electrostatics and Magnetism," p. 419.] [Footnote 284: _North British Review_, vol. xlviii. p. 127.] [Footnote 285: We do not by any means assert that two substances can not occupy the same point in space at the same moment in time. We accept the Hegelian maxim that "two substances may occupy the same point in space at the same time _provided their qualities are essentially different_." If the qualities of the ether are essentially different from gross matter, then to call ether "matter" is to confound and mislead the mind. May not ether be a "tertium quid" between matter and mind?] [Footnote 286: Prof. Clerk Maxwell, in _Nature_, vol. ii. p. 421.] [Footnote 287: Sir William Thomson, in _Nature_, vol. iv. p. 266.] [Footnote 288: Sir W. Thomson, in _Nature_, vol. i. p. 551.] [Footnote 289: _Nature_, vol. ii. p. 421.] [Footnote 290: _Philosophical Magazine_, 1868.] [Footnote 291: Thomson and Tait, "Natural Philosophy," vol. i. p. 164.] [Footnote 292: _Nature_, vol. viii. p. 280; also Challis, "Principles of Mathematics and Physics," pp. 685-687.]