145 
The figures for income and outgo* of energy require a word of 
explanation. A distinction is here ma*de between the total a income, 
which is represented by the potential energy of the food, and the net a 
income, which is the energy of the material actually oxidized in the 
body. This energy of net income is represented by the available 
energy of the nutrients of the food (i. e.. potential energy of total food 
less that of the urine and feces) minus the potential energy of the 
material gained or plus that of material lost by the body when 
the latter is not in nitrogen and carbon equilibrium. The total energy 
of outgo would be the kinetic energy given off from the body in heat 
and external muscular work plus the potential energy of the unoxi- 
dized materials in the urine and the feces. The net energy of outgo 
consists of the heat given off and the external muscular work done. 
The balance of income and outgo is best shown by the net rather than 
the total quantities. These ma}' be seen in the table given above 
(Table 151). The averages for the groups of experiments are for the 
number of days covered by the whole group — i. e., they are not aver- 
ages for individual experiments. 
It is to be remembered that the figures for net income of energy 
represent the heat of combustion of the material actually oxidized. 
This material consists mainly of the available portion of the food of 
which the amount and heat of combustion are found by direct deter- 
mination. To its heat of combustion is added that of the material 
lost, or from it is subtracted that of the material gained by the body. 
The amounts of material o-ained or lost are determined from the o-ain 
© © 
or loss of nitrogen and carbon, and their heats of combustion are cal- 
culated by the use of factors based upon direct determinations of the 
heats of combustion of similar substances. The fimires for net outo-o 
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are the results of direct experimental measurement. In other words, 
the net income of energy is mainly and the net outgo entirely the 
result of direct determinations. 
A common usage is followed in applying the term "potential 
energy** to the energy latent in the food and body material oxidized. 
Whether chemical energy would or would not be a more correct expres- 
sion no attempt is here made to decide. Ordinary usage is also fol- 
lowed in expressing potential energy in terms of heat— i. e.. as cal- 
ories — thus employing a unit of kinetic energy for the measure. This 
discrepancy is unavoidable, since we have neither the means for meas- 
uring potential energy as such, nor a unit for expressing such measure- 
ments if they were made. The use of heat of oxidation for the meas- 
ure is especially appropriate here, since the energy is liberated mainly 
by oxidation and appears chiefly or entirely as heat. 
a The terms "total" and "net" here applied to income and outgo of material and 
energy are used for present convenience, and may in future reports be replaced by 
more satisfactory expressions. 
13007— No. 109—02 10 
