(a) _Upon Substances_
[Sidenote: Effect of air upon iron and wood]
Upon some substances oxygen acts at ordinary temperature. Iron becomes
covered with rust when exposed to air and moisture. Wood and other
vegetable and animal substances undergo slow decomposition when exposed
to the air. This is partly due to the action of oxygen at ordinary
temperature.
[Sidenote: Pure oxygen aids combustion]
A splinter of wood will burn brilliantly in a jar of pure oxygen, and
much more rapidly than in common air. Pure oxygen gas will cause many
substances to burn which will not burn in air. Iron can be burned in
pure oxygen, leaving only a reddish powder.
[Sidenote: Formation of iron-rust]
When iron rusts the carbon dioxid and water vapor combine chemically
with the iron, and form what is known as a basic hydroxid or carbonate
of iron. The process is somewhat complex. When iron burns in oxygen a
red powder is formed--ferric oxid, Fe₂O₃. Iron dissolves in water, or
moisture from the air containing carbonic acid, forming acid ferrous
carbonate--
Fe + 2H₂CO₃ = FeH₂(CO₃)₂ + H₂
Iron + Carbonic acid = Acid ferrous carbonate + Hydrogen
This acid ferrous carbonate, on drying or further oxidation, is
converted into _iron-rust_. If we represent _iron-rust_ by the formula
Fe₂O₃. 2Fe(OH)₃, the equation is as follows:
4FeH₂(CO₃)₂ + O₂ = Fe₂O₃. 2Fe(OH)₃ + H₂O + 8CO₂
Acid ferrous carbonate + Oxygen = Iron-rust + Water + Carbon dioxid
(b) _In Living Bodies_
The most interesting action of oxygen at ordinary temperature, however,
is that which takes place in our bodies and the bodies of all other
animals.
[Sidenote: Rate of blood circulation]
[Sidenote: Oxidation of waste matter]
By the constant action or beating of the heart all the blood in the body
is brought to the lungs every two or three minutes. The actual time
has not been determined in man. In large arteries the blood flows ten
times as fast as in very small ones. The usual time through a capillary
is one second. The time has been determined, however, in lower animals.
In a horse the blood travels one foot a second in the largest artery.
At present the accepted theory is that in the circuit the blood makes
throughout the body, it picks up the waste matter from tissue that has
been torn down by work or effort, and brings it to the lungs, where it
meets with the oxygen we breathe and is oxidized or burned.
If the body undergoes excessive effort or exercise, it tears down
an excessive amount of tissue, and there is created, therefore, an
excessive amount of waste or carbon dioxid. Nature very wisely provides
for this contingency by increasing the heart action, thereby sending
the blood through the body at greater velocity, forcing more blood to
the lungs, thus increasing the demand for oxygen, which is expressed by
deep and rapid breathing.
[Sidenote: Generation of heat and light]
When a substance burns it gives off heat, and generally light. The heat
is the result of chemical change or combination, and the light is the
result of heat. Whenever oxidation takes place, no matter in what form,
heat is produced.
[Sidenote: Amount of heat determined by amount of oxygen]
The amount of heat given off by the combination of a given amount of
oxygen with some other substance is always the same. If it takes place
at a very high temperature, as in explosives, the heat is all given off
at once, but if it takes place more slowly, the heat passes away, and we
may not observe it, but careful experiments prove that heat is always
present in oxidation, and the amount of heat is always measured by the
amount of oxygen.
[Sidenote: Law governing oxidation of given quantity of food]
That the combination of oxygen with other substances always produces a
certain amount of heat is a very important fact to the food scientist,
as this law enables him to determine in the laboratory the exact amount
of heat that is produced in the oxidation of a pound, or of any given
quantity of food; this food will also produce exactly the same amount of
heat if oxidized in the human body.
[Sidenote: Heat and motion]
We know that by means of heat we can produce motion. The steam-engine
is the best example of this law. We build a fire under the boiler; the
oxygen of the air unites with the carbon in the coal; the combustion
converts the water into steam; the steam is conveyed to a cylinder; the
pressure pushes a piston; the motion of the piston causes motion in the
engine, and the train or ship moves.
[Sidenote: Determination of body-heat and energy]
From such facts we know that not only the amount of heat, but the amount
of work or energy that food or fuel will yield can be determined with
reasonable accuracy. Many conditions obtain in the body, however, that
do not occur in the laboratory, hence we must study these conditions
before we can fully understand the natural laws that govern the
production of heat, and energy or work, by oxidation in the living body.