CHAPTER VIII. 
METHODS OF INVESTIGATION: 
Tue food is the sole known source of energy as well as of matter 
to the body of the warm-blooded animal, and the total income of 
potential energy, according to the principles Jaid down in the pre- 
ceding chapter, is represented by the heat of combustion of the 
food. 
A portion of this food, as we have seen in Part I, is metabolized 
in the body, while part of it escapes complete oxidation and is re- 
jected as undigested matter in the feces, as metabolic products in 
feces, urine, and perspiration, and as combustible intestinal gases. 
All these substances still contain more or less of their original store 
of potential energy and collectively constitute one main division of 
the outgo of energy. We may call it, for brevity, the outgo of 
potential energy. A portion of the food may also be applied to the 
production and storage of tissue (protein and fat) in the body, and 
this, from our present point of view, is to be classed with the 
outgo of potential energy. 
The potential energy of the remaining portion of the food, viz., 
that which is completely oxidized, may take various transitory 
forms in the organism, but ultimately it leaves it in one of two 
forms of kinetic energy, viz., as mechanical work or as heat. Here 
we have the second main division of the outgo of energy, viz., the 
outgo of kinetic energy. These relationsemay be briefly expressed 
in tabular form, as shown at the head of the opposite page. 
As in the corresponding chapter of Part I, it is proposed to con- 
sider here simply the general principles of the more important 
methods available for determining the income and outgo of poten- 
tial and kinetic energy, without entering into technical details. 
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