[Sidenote: Sources of nitrogen]
We have learned that the air is composed chiefly of oxygen and nitrogen.
These are not combined as oxygen and hydrogen are in water, but are
simply mixed together, four-fifths of the mixture being nitrogen.
Nitrogen is also found in combination in a large number of substances in
nature. It is found in the nitrates, as salt-peter or potassium nitrate,
KNO₃, and Chili salt-peter or sodium nitrate, NaNO₃. It is also found in
the form of ammonia, which is a compound of nitrogen and hydrogen of the
formula NH₃, and exists in that form in a limited quantity of the air.
In most foods, especially in those of animal origin, nitrogen occurs in
chemical combination.
[Sidenote: Properties of nitrogen]
Nitrogen is a colorless, tasteless, odorless gas which does not burn,
and does not combine readily with oxygen, or with any other element
except at a very high temperature, and except in the formation of living
plants, or in animal life. Just as nitrogen does not support combustion,
so also it does not support life. An animal would die confined in a tank
of nitrogen, not on account of any active poisonous properties in the
nitrogen, but for lack of oxygen.
[Sidenote: Compounds of nitrogen]
When a compound containing carbon, hydrogen and nitrogen is heated in
a closed vessel, so that the air is excluded, and so that it cannot
burn, the nitrogen passes out of the compound, not as nitrogen, but
in combination with hydrogen, which forms ammonia. Nearly all animal
substances contain carbon, hydrogen, oxygen, and nitrogen, and many of
them give off ammonia when heated as above described.
[Sidenote: Why ammonia is used in making artificial ice]
Ammonia is written by the chemist NH₃, or one part of nitrogen gas to
three parts of hydrogen. It is a colorless, transparent gas with a
very penetrating, characteristic odor. In concentrated form it causes
suffocation. It is but little more than half as heavy as air. It is
easily converted into liquid form by pressure and cold. When pressure
is removed from the liquefied ammonia, it passes back very rapidly
into gaseous form, and in so doing it absorbs heat. Investigators have
taken advantage of these facts and are employing liquid ammonia in the
manufacture of artificial ice.
[Sidenote: Importance of proportioning food]
While air is merely a mixture of oxygen and nitrogen, this does not
prove that these two elements cannot unite. In fact they do unite in
five different proportions so as to form five different substances.
These are given below to illustrate how different substances can be
formed from the same things, by merely combining them in different
proportions. This example is also given to impress upon the mind of the
practitioner the great importance of proportioning nutritive elements
in diet so that the patient will not be overfed on some elements while
underfed on others. It is absolutely essential, in order to know what
effect a substance will have in the laboratory, or in the body, to know
not only of what it is composed, but with what substances and in what
proportions it is combined.
Nitrous oxid .....N₂O
Nitric oxid ......NO or N₂O₂
Nitrogen trioxid .. N₂O₃
Nitrogen peroxid.. NO₂ or N₂O₄
Nitrogen pentoxid N₂O₅
To further illustrate the wonders of chemical combinations, we give the
properties of two of these oxygen and nitrogen compounds:
[Sidenote: Properties and uses of nitrous oxid]
Nitrous oxid, N₂O, is colorless, transparent, and has a slightly
sweetish taste. When inhaled it causes a kind of intoxication which
manifests itself in the form of hysterical laughing, hence it is
commonly called "laughing gas." Inhaled in larger quantities it causes
unconsciousness and insensibility to pain. It is, therefore, used in
many surgical operations, particularly by dentists in extracting teeth.
Nitrogen peroxid, NO₂, is a reddish-brown gas. It has an extremely
disagreeable odor and is very poisonous.
[Sidenote: Composition of nitric acid]
[Sidenote: Properties of nitric acid]
By oxidation the nitrogen of animal substances is converted into nitric
acid, HNO₃. Furthermore, the silent, continuous action of minute living
organisms in the cell is always tending to transform the waste-products
of animal life into compounds closely related to nitric acid. This
acid, as its chemical formula indicates, is formed by the combination
of the three elements we have just studied, namely, hydrogen, nitrogen,
and oxygen. Pure nitric acid is a colorless liquid. It gives off
colorless, irritating fumes, when exposed to the air. Strong nitric acid
acts violently upon many substances, particularly those of animal and
vegetable origin, decomposing them very rapidly. Nitric acid burns the
flesh, eats through clothing, disintegrates wood, and dissolves metals.
It is one of the most active of chemical substances.
The compounds of nitrogen that occur in food are very numerous and of
complex composition. They will be discussed in Lessons III and IV, pages
99 and 125 respectively.