The other above-named groups of organic compounds which are formed from the three elements carbon, hydrogen, and oxygen--(f) _carbohydrates_ and (g) _fats_--are very important to the food chemist. These will be considered in detail in Lesson IV. See pages 107-125. ORGANIC NITROGENOUS COMPOUNDS 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. LESSON IV CHEMISTRY OF FOODS [Sidenote: Four general classes of food] The chemistry of carbon compounds and the general composition of plant and of animal substances were discussed in Lesson III. We are now prepared to take up the chemistry of food. The chemistry of food substances will be considered under the common divisions of carbohydrates, fats, proteids, and mineral salts. (See "Classification of Organic Carbon Compounds," Lesson III, p. 89.) [Sidenote: Classes vs. groups of related compounds]