o as they appear under the
microscope. (See practical work.)

*Sugars.*--There are several varieties of sugar, but the important ones
used as foods fall into one or the other of two classes, known as _double
sugars_ (disaccharides) and _single sugars_ (monosaccharides). To the
first class belong _cane sugar_, found in sugar cane and beets, _milk
sugar_, found in sweet milk, and _maltose_, a kind of sugar which is made
from starch by the action of malt. The important members of the second
class are _grape sugar_, or dextrose, and _fruit sugar_, or levulose, both
of which are found in fruits and in honey.

The most important of all sugars, so far as its use in the body is
concerned, is _dextrose_. To this form all the other sugars, and starch
also, are converted before they are finally used in the body. The close
chemical relation between the different carbohydrates makes such a
conversion easily possible.

*Fats.*--The fats used as foods belong to one or the other of two classes,
known as solid fats and oils. The solid fats are derived chiefly from
animals, and the oils are obtained mostly from plants. Butter, the fat of
meats, olive oil, and the oil of nuts are the fats of greatest importance
as foods. Fats, like the carbohydrates, are composed of carbon, hydrogen,
and oxygen. They are rather complex chemical compounds, though not so
complex as proteids. Since neither fats nor carbohydrates contain
nitrogen, they are frequently classed together as _non-nitrogenous_ foods.

*Purpose Served by Carbohydrates, Fats, and Albuminoids.*--These classes of
nutrients all serve the common purpose of supplying energy. By uniting
with oxygen at the cells, they supply heat and the other forms of bodily
force. This is perhaps their only purpose.(50) Proteids also serve this
purpose, but they are not so well adapted to supplying energy as are the
carbohydrates and the fats. In the first place they do not completely
oxidize and therefore do not supply so much energy; and, in the second
place, they form waste products that are removed with difficulty from the
body.

*Mineral Salts and their Uses.*--Mineral salts are found in small
quantities in all of the more common food materials, and, as a rule, find
their way into the body unnoticed. They supply the elements which are
found in the body in small quantities and serve a variety of purposes.(51)
Calcium phosphate and calcium carbonate are important constituents