e was measured. In some cases they fasted,
and in others they received diets generally not far from sufficient to
maintain nitrogen, and usually carbon, equilibrium in the body. In these
experiments the amount of energy expended by the body as heat and as
external muscular work measured in terms of heat agreed on the average
very closely with the amount of heat that would be produced by the
oxidation of all the matter metabolized in the body. The variations for
individual days, and in the average for individual experiments as well,
were in some cases appreciable, amounting to as much as 6%, which is not
strange in view of the uncertainties in physiological experimenting; but
in the average of all the experiments the energy of the expenditure was
above 99.9% of the energy of the income,--an agreement within one part
in 1000. While these results do not absolutely prove the application of
the law of the conservation of energy in the human body, they certainly
approximate very closely to such demonstration. It is of course possible
that energy may have given off from the body in other forms than heat
and external muscular work. It is conceivable, for example, that
intellectual activity may involve the transformation of physical energy,
and that the energy involved may be eliminated in some form now unknown.
But if the body did give off energy which was not measured in these
experiments, the quantity must have been extremely small. It seems fair
to infer from the results obtained that the metabolism of energy in the
body occurred in conformity with the law of the conservation of energy.

3. _Composition of Food Materials._--The composition of food is
determined by chemical analyses, the results of which are conventionally
expressed in terms of the nutritive ingredients previously described. As
a result of an enormous amount of such investigation in recent years,
the kinds and proportions of nutrients in our common sorts of food are
well known. Average values for percentage composition of some ordinary
food materials are shown in Table I. (Table I. also includes figures for
fuel value.)

It will be observed that different kinds of food materials vary widely
in their proportions of nutrients. In general the animal foods contain
the most protein and fats, and vegetable foods are rich in
carbohydrates. The chief nutrient of lean meat and fish is protein; but
in medium fat meats the proportion of fat is as large as that of
prote