If to the three elements carbon, hydrogen, and oxygen, the element
_nitrogen_ is added, it still further increases the number of possible
compounds that may be formed upon the base of the wonderful _carbon
atom_. With this additional nitrogen factor, a new and a distinct
quality is obtained.
[Sidenote: The elements that make life possible]
The chief characteristic of the element nitrogen is the ease with which
its compounds change their chemical form. To quote the chemist, "the
compounds of nitrogen are very unstable." Nearly all explosives are
nitrogenous compounds. When this element, nitrogen, is combined with
the wonderful variety of compounds formed by carbon, we have not only a
great many intimately related yet distinct substances, but compounds
which readily change from one form to another. These are the distinctive
qualities or conditions necessary, from a chemical standpoint, to make
the processes of life possible. _Protoplasm, which is the basis of all
life, is formed by an intimate mixture of a number of complex chemical
compounds, the chief elements of which are carbon, hydrogen, oxygen, and
nitrogen._
[Sidenote: Importance of nitrogenous compounds]
The organic compounds containing nitrogen are very numerous and very
interesting. As all tissues and substances of the animal body contain
nitrogen as a necessary element, we can see why this group of compounds
is of great importance to the student of food science.
Some of the nitrogenous compounds which are _not_ available as nutritive
substances, and many of which are poisonous or harmful to animal life,
will be considered in Lesson IX, under "Alkaloids and Narcotics." (See
Vol. II, p. 349.) The principal nutritive substances, and proteids or
compounds containing _available food nitrogen_, will be considered in
Lesson IV.