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.

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. 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.

The elements that make life possible.

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.)

Importance of nitrogenous compounds

The principal nutritive substances, and proteids or compounds containing available food nitrogen, will be considered in Lesson IV (Chemistry Of Foods).