[27] _Hibbert Journal_, 1909, pp. 626-28. [28] _Hibbert Journal_, 1909, p. 629. [29] _Hibbert Journal_, 1909, p. 632. [30] Schubert, “Mathematical Essays and Recreations,” pp. 70, 71. [31] See Van t’ Hoff, _La Chimie dans l’espace_, and Schubert, “Mathematical Essays and Recreations,” pp. 88-89. [32] Schubert, “Mathematical Essays and Recreations,” pp. 88-89. See also Mach, “Conservation of Energy” (trans. Open Court Publishing Co.). Printed by BALLANTYNE, HANSON & CO. Edinburgh & London Transcriber’s Note: Words and phrases in italics are surrounded by underscores, _like this_. Footnotes were renumbered sequentially and were moved to the end of the chapter. Words may have inconsistent hyphenation in the text. One misspelled word was corrected.