The two ultimate uses of all food are to supply the body with materials for growth or renewal, and with energy or the capacity for doing work. The energy received in a latent form, stored in the various chemical combinations of foods, is liberated as kinetic or active energy in two chief forms: first, as heat; second, as motion.. Force is the manifestation of energy. The force developed by a healthy adult man at ordinary labour averages 3,400 foot tons per diem, a foot ton being the amount of force required to raise a weight of one ton through the height of one foot. Of this, less than one fifth is expended in motion, and more than four fifths, or 2,840 foot tons, in heat, which maintains the body temperature at its normal average. A man weighing one hundred and fifty pounds - or nearly one thirteenth of a ton - obviously expends considerable energy in merely moving his own body about from place to place, aside from carrying any additional burden.

The original force developed in the various functions of animal life which result in heat production and motion is in part obtained from the radiant heat of the sun stored by plants in the latent form of certain chemical compounds - chiefly starches and sugars - which, on being consumed as food by animals, furnish energy.

A useful comparison may be made between the processes of nutrition and development of energy from food in the human body and the energy derived from a steam engine and boiler. In both cases the main source of energy is oxidation, and principally of carbon. In both cases the latent energy of the carbon liberated by oxidation processes is converted into heat and motion, forms of energy which bear a definite relation to one another. If a large part of the original latent energy is converted into heat, less will yield motion, and conversely. The proportion of these two forces to each other is in the case of the most perfectly constructed engine about one of motion to eight of heat; whereas m the human body it was calculated by Helmholtz that the motion obtainable from a given amount of food may stand in relation to the heat in the proportion of one to five. Hence, as regards the production of work through motion, the human body is a more perfectly constructed machine than the engine. Furthermore, after combustion of the carbon by the fires of the boiler a certain amount of waste matter or ash is produced. If this is allowed to accumulate, it obstructs the draught and interferes with active oxidation.

In the human body, in like manner, the fuel or food consumed produces ashes, such as urea and other forms of waste material, which, if not removed, accumulate in the system and embarrass or retard the normal oxidation processes. The body possesses the additional power of modifying and distributing the fuel food which it receives so as to develop its energy to the best advantage in different organs.

R. C. Carpenter, in 1898, made an exhaustive study of the energy developed by a bicycle-rider named Miller. He found that the energy developed by this man equalled 45 per cent of the total heat of combustion calculated for his food. Professor Carpenter says: "The best record of any heat engine is probably that of the Deisal motor," which develops 33.7 per cent of the heat energy of its fuel, and "the best record of a steam engine is that of the Nordberg pumping engine at Pittsburg," which develops 22.7 per cent of energy.

"With the exception of the Deisal motor the best record of any oil engine per delivered horse power is about 16.5 per cent efficiency. From this comparison it would seem that the human machine is decidedly superior to any heat engine which has been developed in form so as to be of any value for practical use".

Whether elementary substances are burned outside of the body or oxidised within the body, the resulting products are the same. There can be no loss of matter, and there can be no loss of energy. The matter is simply changed in form by molecular rearrangement, the energy is converted from one type into another. The following simple experiment will illustrate this point: In a large covered glass jar place an ounce of alcohol in a small metal vessel. Also place in the jar a little lime water in a tumbler, and a thermometer. On -igniting the alcohol and allowing it to burn away completely, a film of aqu^us vapour will accumulate on the surface of the jar, and a film of calcium carbonate will form on the surface of the lime water produced by the union of carbonic-acid gas with the lime water.

The thermometer will indicate a rise in temperature of the air in the jar. An ounce of alcohol consumed as food will be similarly converted into carbonic-acid gas and water, and in this process the body heat will be increased. No substance is a good food unless it fulfils two conditions - viz., easy assimilation and complete combustion. The proportion of any given food actually assimilated (i. e., not rejected in the feces) is called its "coefficient of digestibility".

Metabolism within the body is not alone controlled by muscular work, but by the nervous energy expended in its performance. For example, a day labourer, like an iron founder, may be stronger and do much more mechanical work than an oarsman or foot-ball player in time of contest, yet he expends very little nervous energy in his routine daily work, and requires less protein in his diet than the athlete. In other words, severe muscular work performed for a brief time under conditions of great mental excitement and nervous tension demands an excess of protein, whereas continued muscular effort without great fatigue or mental strain is maintained upon a liberal allowance of food, which may be varied in composition if it be easily digestible.

The relative importance of the different food fuels should be considered. This is well summarised by Charles E. Woodruff:

"For instance, cut off the supply of oxygen, and death ensues in from one to ten minutes. If water is withheld, preventing the transportation of the fuel and oxygen to various parts of the body, death follows in about two to seven days or more, according to climate, exposure, and exercise. If the fuel itself is taken away, death follows in from seven to forty days or more, according to the amount of exposure that would abstract heat and the amount of work that would use up the energy already stored up in the body. If materials for the repair of tissues be excluded, death follows in a variable time, dependent upon the importance of the tissue that is being starved - a time varying from a week if all nitrogen is excluded, to several months if the vegetable acids are excluded, or even to several years " - if certain more obscure substances are withheld.

It still remains extremely difficult in the case of all foods to trace their final uses in the body and determine with any approach to accuracy what proportions of each furnish respectively energy, repair of tissue, and heat, for there are no more complex chemical processes known than those of tissue metabolism. In other words, it is necessary to determine whether the actual physiological value of food in the body as producing energy for muscular activity and body heat corresponds with its calculated or calorimetric value, and to determine standards of " fuel value".

In order to study the quantity of energy which may be derived from different varieties of food, a man or an animal may be placed in an apparatus known as a calorimeter. There are numerous types 8 of such apparatus, and Professor Atwater collected, in 1897, data of 3,661 calorimeter experiments, 2,299 of which have been made by various observers upon man. In the great majority of these experiments the nitrogen balance was determined, and the carbon balance was computed by deduction, but in a few the latter was determined directly. The most elaborate and ingenious apparatus of this sort is the "respiration calorimeter" constructed at Wesleyan University by Profs. W. O. Atwater and E. B. Rosa. It consists of a chamber seven feet long, six feet four inches high, and four feet wide, in which a man may remain day and night, being supplied with fresh air and food. The chamber is practically a many-walled, air-tight box having air spaces between the walls which are so constructed as to maintain a uniform temperature within and prevent all external temperature changes from affecting the interior. There are two inner metal walls composed respectively of copper and zinc, and three outer wooden walls reinforced by thick builders' paper.

Between these five walls, which completely surround the box, over top and bottom as well as sides, are four air spaces several inches in thickness, two containing dead air and two circulating air, kept in motion by electric fans and warmed or cooled according to need, so as to maintain a constant temperature within. A glass window constructed with successive layers like the walls serves to admit the man under observation, after which it is hermetically soldered. A small air lock is used to admit food and to pass out excrement for analysis. The chamber contains a folding bed, table, and chair, a pair of scales, and a stationary bicycle which operates a small dynamo and electric light. The heat from the light is measured, together with that dissipated from the man's body within the chamber, and this gives a measure of muscular work converted into the energy of heat. The heat is conveyed away from the chamber by means of a current of cold water passing through copper pipes, and is measured by electric thermometers. Circulating air is supplied to the interior of the chamber, and samples are withdrawn for analysis as it enters and leaves the chamber. In this manner is measured the quantity of C02 and water eliminated through the lungs and skin.

The volume and temperature of the air is carefully regulated. The subject of experimentation is put upon a measured diet for four days before entering the calorimeter in order to establish nitrogenous equilibrium and record observations upon the food excrement, amount of work performed, etc. He then enters the calorimeter, where he remains four days and five nights. In the "test experiments" the subject makes as little muscular exertion as pos--sible, but in the "work experiments" he operates the stationary bicyck^or eight hours a day. The delicacy of the apparatus is shown by the fact that in the thermo-electric measurement system 304 pairs of metallic junctions are distributed throughout the inner wall air space, and the heat generated by such slight movements as turning in bed or rising from a chair at once produces a deflection of the thermal galvanometer. The main value of the experiments thus far conducted in this calorimeter consists in the actual demonstration that the law of the conservation of energy operates within the body in precisely the same manner that it does outside. In man it was found that the measured energy of the food consumed by the subject within the calorimeter was within 99 per cent of the calculated or theoretical energy.