SES OF FOOD. All kinds of food substances may be divided into four
classes. _Proteids, Fats, Amyloids_, and _Minerals_. Proteids are
composed of the four elements, carbon, hydrogen, oxygen, and nitrogen,
sometimes combined with sulphur and phosphorus. In this class are
included the _gluten_ of flour; the _albumen_, or white of eggs; and the
_serum_ of the blood; the _fibrin_ of the blood; _syntonin_, the chief
constituent of muscle and flesh, and _casein_, one of the chief
constituents of cheese, and many other similar, but less frequent
substances.

Fats are composed of carbon, hydrogen, and oxygen only, and contain more
hydrogen than would be required to form water if united with the oxygen
which they contain. All vegetable and animal oils and fatty matters are
included in this class.

Amyloids consist of substances which are also composed of carbon,
oxygen, and hydrogen only; but they contain just enough hydrogen to
produce water when combined with their oxygen, or two parts of hydrogen
to one of oxygen. This division includes _sugar, starch, dextrine_, and
_gum_. The above three classes of food-stuffs are only obtained through
the activity of living organisms, vegetable or animal, and have been,
therefore, appropriately termed by Prof. Huxley, _vital food-stuffs._

The mineral food-stuffs may, as we have seen, be procured from either
the living or the non-living world. They include water and various
earthy, metallic, and alkaline salts.

VARIETY OF FOOD NECESSARY. No substance can serve permanently for food
except it contains a certain quantity of proteid matter in the shape of
albumen, fibrin, casein, etc., and, on the other hand, any substance
containing proteid matter in a shape in which it can be readily
assimilated, may serve as a permanent vital food-stuff. Every substance,
which is to serve as a permanent food, must contain a sufficient
quantity, ready-made, of this most important and complex constituent of
the body. In addition, it must also contain a sufficient quantity of the
mineral ingredients which enter into the composition of the body. Its
power of supporting life and maintaining the weight and composition of
the body remains unaltered, whether it contains fats or amyloids or not.
The secretion of urea, and, consequently, the loss of nitrogen, goes on
continually, and the body, therefore, must necessarily waste unless the
supply of proteid matter is constantly renewed, since this is the only
class