Many popular writers have divided foods into flesh-formers, heat-givers,
and bone-formers. Although attractive from its simplicity, this
classification will not bear criticism. Flesh-formers are also
heat-givers. Only a portion of the mineral matter goes to form bone.

Class I.--INORGANIC COMPOUNDS.
Sub-class 1. Water. 2. Mineral Matter or Salts.
Class II--ORGANIC COMPOUNDS.
1. Non-Nitrogeneous or Ternary Compounds. _a_ Carbohydrates.
_b_ Oils. _c_ Organic Acids.
2. Nitrogenous Compounds. _a_ Proteids. _b_ Osseids.
Class III.--NON-NUTRITIVES, FOOD ADJUNCTS AND DRUGS.
Essential Oils, Alkaloids, Extractives, Alcohol, &c.

These last are not strictly foods, if we keep to the definition already
given; but they are consumed with the true foods or nutrients, comprised
in the other two classes, and cannot well be excluded from consideration.

Water forms an essential part of all the tissues of the body. It is the
solvent and carrier of other substances.

Mineral Matter or Salts, is left as an ash when food is thoroughly
burnt. The most important salts are calcium phosphate, carbonate and
fluoride, sodium chloride, potassium phosphate and chloride, and compounds
of magnesium, iron and silicon.

Mineral matter is quite as necessary for plant as for animal life, and is
therefore present in all food, except in the case of some highly-prepared
ones, such as sugar, starch and oil. Children require a good proportion of
calcium phosphate for the growth of their bones, whilst adults require