HELMHOLTZ, H. L. F., says: “The best expansive engines give back as mechanical work only eighteen per cent. of the heat generated by the fuel.” Interaction of Natural Forces, in Correlation and Conservation of Forces, p. 227. Footnote 17: THOMSEN, JULIUS, Poggendorff’s Annalen, cxxv, 348. Also in abstract in Am. J. Sci., II, xli, 396, May, 1866. Footnote 18: American Journal of Science, II, xli, 214, March, 1866. Footnote 19: In this calculation the annual evaporation from the ocean is assumed to be about 9 feet. (See Dr. BUIST, quoted in Maury’s Phys. Geography of the Sea, New York, 1861, p. 11.) Calling the water-area of our globe 150,000,000 square miles, the total evaporation in tons per minute, would be that here given. Inasmuch as 30,000 pounds raised one-foot high is a horse-power, the number of horse-powers necessary to raise this quantity of water 3½ miles in one minute is 2,757,000,000,000. This amount of energy is precisely that set free again when this water falls as rain. Footnote 20: Compare ODLING, WM., Lectures on Animal Chemistry, London, 1866. “In broad antagonism to the doctrines which only a few years back were regarded as indisputable, we now find that the chemist, like the plant, is capable of producing from carbonic acid and water a whole host of organic bodies, and we see no reason to question his ultimate ability to reproduce all animal and vegetable principles whatsoever.” (p. 52.) “Already hundreds of organic principles have been built up from their constituent elements, and there is now no reason to doubt our capability of producing all organic principles whatsoever in a similar manner.” (p. 58.) Dr. Odling is the successor of Faraday as Fullerian Professor of Chemistry in the Royal Institution of Great Britain. Footnote 21: MARSHALL, JOHN, Outlines of Physiology, American edition, 1868, p. 916. Footnote 22: FRANKLAND, EDWARD, On the Source of Muscular Power, Proc. Roy. Inst., June 8, 1866; Am. J. Sci., II, xlii, 393, Nov. 1866. Footnote 23: LIEBIG, JUSTUS VON, Die organische Chemie in ihrer Anwendung auf Physiologie und Pathologie, Braunschweig, 1842. Also in his Animal Chemistry, edition of 1852 (Am. ed., p. 26), where he says “Every motion increases the amount of organized tissue which undergoes metamorphosis.” Footnote 24: Compare DRAPER, JOHN WM. Human Physiology. PLAYFAIR, LYON, On the Food of Man in relation to his useful work, Edinburgh, 1865. Proc. Roy. Inst., Apr. 28, 1865. RANKE, Tetanus eine Physiologische Studie, Leipzig, 1865. ODLING, _op. cit._ Footnote 25: VOIT, E., Untersuchungen über den Einfluss des Kochsalzes, des Kaffees, und der Muskelbewegungen auf den Stoffwechsel, Munich, 1860. SMITH, E., Philosophical Transactions, 1861, 747. FICK, A., and WISLICENUS, J., Phil. Mag., IV, xxxi, 485. FRANKLAND, E., _loc. cit._ NOYES, T. R., American Journal Medical Sciences, Oct. 1867.