PARKES, E. A., Proceedings Royal Society, xv, 339; xvi, 44. Footnote 26: SMITH, EDWARD, Philosophical Transactions, 1859, 709. Footnote 27: Authorities differ as to the amount of energy converted by the steam-engine. (See Note 16.) Compare MARSHALL, _op. cit._, p. 918. “Whilst, therefore, in an engine one-twentieth part only of the fuel consumed is utilized as mechanical power, one-fifth of the food absorbed by man is so appropriated.” Footnote 28: HEIDENHAIN, Mechanische Leistung Wärmeentwickelung und Stoffumsatz bei der Muskelthätigkeit, Breslau, 1864. See also HAUGHTON, SAMUEL, On the Relation of Food to work, published in “Medicine in Modern Times,” London, 1869, Macmillan & Co. Footnote 29: HEIDENHAIN, _op. cit._ Also by FICK, Untersuchungen über Muskel-arbeit, Basel, 1867. Compare also “Nature,” i, 159, Dec. 9, 1869. Footnote 30: DU BOIS-REYMOND, EMIL, On the time required for the transmission of volition and sensation through the nerves, Proc. Roy. Inst. Also in Appendix to Bence Jones’s Croonian lectures. Footnote 31: MARSHALL, _op. cit._, p. 227. Footnote 32: MELLONI, Ann. Ch. Phys., xlviii, 198. See also NOBILI, Bibl. Univ., xliv, 225, 1830; lvii, 1, 1834. Footnote 33: The apparatus employed is illustrated and fully described in Brown-Sequard’s Archives de Physiologie, i, 498, June, 1868. By it the 1-4000th of a degree Centigrade may be indicated. Footnote 34: LOMBARD, J. S., New York Medical Journal, v, 198, June, 1867. [A part of these facts were communicated to me directly by their discoverer.] Footnote 35: WOOD, L. H., On the influence of Mental activity on the Excretion of Phosphoric acid by the Kidneys. Proceedings Connecticut Medical Society for 1869, p. 197. Footnote 36: On this question of vital force, see LIEBIG, Animal Chemistry. “The increase of mass in a plant is determined by the occurrence of a decomposition which takes place in certain parts of the plant under the influence of light and heat.” “The modern science of Physiology has left the track of Aristotle. To the eternal advantage of science, and to the benefit of mankind it no longer invents a _horror vacui_, a _quinta essentia_, in order to furnish credulous hearers with solutions and explanations of phenomena, whose true connection with others, whose ultimate cause is still unknown.” “All the parts of the animal body are produced from a peculiar fluid circulating in its organism, by virtue of an influence residing in every cell, in every organ, or part of an organ.”