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11
burned in the furnace of the locomotive. For the burning of the food
in the body or the coal in the furnace, air is used to supply oxygen.
When the fuel is oxidized, be it meat or wood, bread or coal, the latent
energy becomes active, or, in technical language, the potential energy
becomes kinetic; it is transformed into heat and power. As various
kinds of coal differ in the amount of heat given off per ton, so various
kinds of food and food ingredients give off different amounts of energy;
that is, have different values as fuel in the body.
HEAT OF COMBUSTION.
The processes of oxidation of material and transformation of energy
in the body are less simple than in the engine and less clearly understood.
Late research, however, has given us ways of measuring the
energy latent in coal, wood, and in food materials as well. This is
most generally done in the chemical laboratory by an apparatus called
the bomb calorimeter. The amount of heat given off in the oxidation
of a given quantity of any material is called its "heat of combustion,"
and is taken as a measure of its latent or potential energy. The unit
commonly used is the calorie, the amount of heat which would raise the
temperature of 1 kilogram of water 1 C., or, what is nearly the same
thing, 1 pound of water 4 F. Instead of this unit of heat a unit of
mechanical energy may be used-for instance, the foot-ton, which represents
the force required to raise 1 ton 1 foot. One calorie is equal
to very nearly 1.54 foot-tons; that is to say, 1 calorie of heat, when
transformed into mechanical power, would suffice to lift 1 ton 1.54 feet.
THE CONSERVATION OF ENERGY IN THE BODY.
The amounts of energy transformed in the body when food and its
own material are burned within it are measured with the respiration
calorimeter referred to on p. 13. It is well known that the food is not
completely oxidized in the body. These experiments have shown that
the material which is oxidized yields the same amount of energy as it
would if burned with oxygen outside the body, e. g., in the bomb
calorimeter. The experiments show also that when a man does no
muscular work (save, of course, the internal work of respiration, circulation,
etc.), all the energy leaves his body as heat; but when he does
muscular work, as in lifting weights or driving a bicycle, part of the
energy appears in the external work thus done, and the rest is given
off from the body as heat. The most interesting result of all is that
the energy given off from the body as heat when the man is at rest,
or as heat and mechanical work together when he is working, exactly
equals the latent energy of the material burned in the body. This is
. in accordance with the law of the conservation of energy. It thus
appears that the body actually obeys, as we should expect it to obey,
this great law which dominates the physical universe.
