
108 STORRS AGRICULTURAL EXPERIMENT STATION. 
with those given off in the solid, liquid and gaseous excretions 
from the body, it is possible to strike a balance between the 
total income and total outgo of matter in the body and to de- 
termine whether it has increased or diminished its store of 
material. In this way a gain or loss of even a small fraction 
of an ounce of body fat or protein during a period of one or 
several days can be detected and measured. 
The above indicates the method of investigating the metabo- 
lism of matter in the body. It consists practically in measuring 
the income and outgo of matter and striking the balance be- 
tween the two. At the same time it is desirable to study the 
metabolism of energy. This is done likewise by determining 
the balance of income and outgo. ‘The measurements made in 
these investigations are in terms of heat, since other forms of 
energy may be transformed into heat. To this end it is neces- 
sary to know how much energy is taken into the body in food 
and drink, how much is given off unused in the solid and 
liquid excreta, and how much is transformed in the body and 
given off in the forms of heat and external muscular work. 
So far as we know the only energy received by the body is 
‘the potential energy of the food, and the only forms in which 
it leaves the body are (1) partly in the potential energy of the 
unoxidized residues of food and body material which are elimi- 
nated in the solid and liquid excreta, but (2) chiefly in the 
kinetic energy resulting from the oxidation of material i ‘in the 
body. 
The only forms in which kinetic energy is known to leave 
the human body are heat and external muscular work. Some 
animals, as the electrical eel, can give off small amounts of 
electrical energy; others, as the firefly, emit minute quantities 
as light; but they are exceptional. It has been surmised that 
‘mental work and nervous tension may represent forms of phys- 
ical energy analogous to light, heat and other forms now known, 
just as the X-ray represents a form of energy unknown untila 
few years ago. But there is no experimental proof for this 
theory and it is opposed by the fact, explained beyond, that 
the energy given off from the body in the forms of heat and, 
external muscular work is found to equal the potential energy 
of the materials oxidized in the body. 

