But when it relates to the soldier, subjected to strange and unhealthy influences, the explanations involve much study, care, and research. In the natural condition of man it is easy to determine how much food will support life and sustain physical exertion. The dietaries of the public institutions of different countries, the experiments of physiologists, and the records of history give the data with sufficient clearness. As to the amount of food required daily to repair the waste and wants of the human organism, much depends upon the degree of muscular exertion and nervous excitation, as well as the temperature of the season. In the alimentation of armies scientific principles must not be disregarded. Food must be considered as force; it must contain, not only material, but power. The strength of men, says Baron Liebig, is in direct ratio to the plastic matter in their food. In determining the absolute quantities of nutrient substances required by the system, Lehman observes that there are three magnitudes especially to be considered. The first is the quantity requisite to prevent the animal from sinking by starvation. The second is that which affords the right supply of nourishment for the perfect accomplishment of the functions, and the last is that which indicates the amount of nutrient matter which may, under the most favorable circumstances, be subjected to metamorphosis in the blood. No one of the four classes, the carbohydrates, the fats, the albuminous matters, and the salts, will answer the purpose alone, but all must be employed together, and this invariable proportion according to the local, and, therefore, variable waste of the system. These considerations indicate how complicated the problem is. III. Life is an action; the principle of life, whatever may be its nature, is eminently and visibly a principle of excitation, of impulsion, a motive power. "It is taking a false idea of life," says Cuvier, "to consider it as a simple link which binds the elements of the living body together, since, on the contrary, it is a power which moves and sustains them unceasingly." These elements do not for an instant preserve the same relation and connection; or, in other words, the living body does not for an instant keep the same state and composition. "This law," adds Flourens, "does not affect alone the muscles, viscera, and tissues, but there is a continual mutation of all the parts composing the bone." These views have been substantiated by the extended experiments of Chossat, of Von Bibra, and a host of experimentalists, showing how positive and decided are the changes in the material composition of the body, and especially the constitution of even the bone from the influence of food. IV.