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 CHAPTER III.

ALIMENTARY ENERGETICS.

Various Problems of Alimentation. § 1. Food the source of Energy and Matter. The two forms of Energy afforded by Food—Vital Energy, Thermal Energy. Food the source of Heat. The rôle of Heat.—§ 2. Measure of the output of Energy—by the Calometric Method—by the Chemical Method.—§ 3. The regular type of Food, Biothermogenic, and the irregular type, Thermogenic.—§ 4. Food considered as the source of Heat. The Law of Surfaces. The limits of Isodynamics.—§ 5. Plastic rôle of Food. Preponderance of Nitrogenous Foods.

AMONG the problems on which energetics has thrown a vivid light we have mentioned alimentation, muscular contraction, and, more general still, the intermittence of vital functional activity. We shall begin with the study of alimentation.

The Different Problems of Alimentation.—What is a food? In what does alimentation consist? The dictionary of the Académie will give us our first answer. It tells us that the word food is applied to "every kind of matter, whatever may be its nature, which habitually serves or may serve for nutrition." This is very well put, but here again we must know what nutrition is, and that is not a simple matter; in fact, it practically means whatever is usually placed on the table in a civilized and polished society. But