I take from Atwater's tables the following estimate of fuel values for the energy furnished to the body by about fifteen grains (about one gram) or one pound each of the classes of nutrients.

Proteids

Fuel value, 4 calories per gram, or 1820 calories per pound

Fats

Fuel value, 8.9 calories per gram, or 4040 calories per pound.

Carbohydrates

Fuel value, 4 calories per gram, or 1820 calories per pound.

A calorie is a unit of heat, being the amount required to warm one gram of water one degree centigrade. The dynamical equivalent of 1 calorie is 42,000,000 ergs.

An erg is a unit of work, being the work done in pushing one gram of mass through one centimetre of space against a force of one dyne. It is also a unit of energy, since energy is measured by work.

Dyne

A unit representing the force which, acting for one second on a mass of one gram, gives it a velocity of one centimetre per second.

The functions and nutritive value of foods are learned by placing men and different animals in specially-constructed apartments, where they are kept for from three to twelve days. During this time analyses are made of all material which enters the body in the food and drink and of that which leaves it in the breath and excreta. A record is also kept of the energy given off as heat and muscular work. The difference between the energy of the food taken and that of the excreta, and the energy given off from the body as heat and muscular work, is the balance of energy; and if correctly estimated, should equal the energy of the body material gained or lost. The energy developed is like that developed by burning coal in a steam boiler. For further information on this subject, see Bulletin No. 142 United States Department of Agriculture, by Dr. Atwater, from which much of this matter has been taken. I append a table for general reference of the average composition of American food products, taken from this same bulletin. This table is of great value in the study of dietetics.