The Vieno is merely a unit especially convenient in measuring the energy in food. In order that this energy may be drawn upon or liberated in the body, it is necessary for the food to pass through the process of metabolism, as heretofore described. THE MEASURE OF HUMAN ENERGY Food may be considered as a store-house of latent or potential energy. [Sidenote: Intake and outgo of energy accurately determined] Because of the law, the conservation of energy, which shows that no energy in the universe can be lost, it is possible to study, with great accuracy, the energy produced in, and given off by, the human body. The method by which energy is measured in accurate scientific experiments is by means of a device called the respiratory calorimeter. [Sidenote: How energy is measured] [Sidenote: Energy yielded from one gram each of proteids, carbohydrates and fats] This device is a small room, the walls of which are impervious to the transmission of both heat and air. In this room a man or an animal may be kept for a period of several days. The air breathed, the food eaten, the body-heat given off, the waste-products excreted, and the mechanical work done, are all measured with the greatest scientific accuracy. Many interesting results have been obtained from the investigations conducted with this wonderful scientific device. These experiments will not be given in detail in this work, but it might be remarked that experiments within the respiratory calorimeter have proved absolutely that the law of "the conservation of energy" works in the human body in the same manner as in the scientist's laboratory. Moreover, such experiments have confirmed the results of the oxidation of various foods in the laboratory, and have given us data from which to compute the stored energy in various food substances. It has thus been found that the amount of energy yielded to the body from one gram of proteid is 4.1 calories, and from one gram of carbohydrates 4.1 calories, while one gram of fat oxidized in the body yields 9.3 calories, which is more than twice that yielded by the proteids and the carbohydrates. [Sidenote: Simple method of finding number of calories in any food] Since it has been proved that the laws established in the laboratory also apply to the human body, it is not necessary to conduct expensive experiments upon the human subject in order to ascertain the amount of energy in some new food. The food may be analyzed chemically, and the energy computed according to the above figures, or a sample of the food may be burned with an oxidizing agent in the laboratory, and the heat measured. This latter process consists simply of oxidizing a gram of the food in a closed steel cylinder which is immersed in a known amount of water at a known temperature. The increase in the temperature of the water, multiplied by the weight of the water in grams, gives the number of calories contained in the substance tested. METABOLISM OF CARBOHYDRATES