EXPERIMENTS ON METABOLISM OF ENERGY. rele] 
of the food (i. e., 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 unoxidized 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 may 
be seen in Table 31. The averages for the groups of experi- 
ments are for the number of days covered by the whole group, 
1. e., they are not averages for individual experiments. 
It is to be remembered that the figures for net income of 
energy represent the heats of combustion of the material 
actually oxidized. This material consists mainly of the avail- 
able portions of the food of which the amounts and heats of 
combustion are found by direct determination. ‘To the heat of 
combustion is added that of material lost, or from it is sub- 
tracted that of the material gained by the body. The amounts 
of material gained or lost are determined from the gain 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 figures 
for net outgo are the results of direct experimental measure- 
ment. 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 oxi- 
dized. Whether chemical energy would or would not be a 
more correct expression no attempt is here made to decide. 
Ordinary usage is also followed in expressing potential energy 
in terms of heat, i. e., as calories, thus employing a unit of 
kinetic energy for the measure. ‘This discrepancy is unavoid- 
able, since there is a lack both of means for measuring po- 
tential energy as such and of a unit for expressing such 
measurements. ‘The use of heat of oxidation for the measure 
is especially appropriate here, since the energy is liberated 
mainly by oxidation and appears chiefly or entirely as heat. 
