nsoluble nutrients into substances that are soluble in water.

*Exercises.*--1. How does oxidation at the cells make necessary the
introduction of new materials into the body?

2. What different purposes are served by the foods?

3. What is a nutrient? Name the important classes.

4. What are food materials? From what sources are they obtained?

5. Name the different kinds of proteids; the different kinds of
carbohydrates. Why are proteids called nitrogenous foods and fats and
carbohydrates non-nitrogenous foods?

6. Show why life cannot be carried on without proteids; without water.

7. What per cents of proteid, fat, and carbohydrate are found in wheat
flour, oatmeal, rice, butter, potatoes, round beef, eggs, and peanuts?

8. State the objection to a meal consisting of beef, eggs, beans, bread,
and butter; to one consisting of potatoes, rice, bread, and butter. Which
is the more objectionable of these meals and why?

9. State the general plan of digestion.

10. Show that digestion is not a simple process like that of dissolving
salt in water.

PRACTICAL WORK

*Elements supplied by the Foods.*--The following brief study will enable
the pupil to identify most of the elements present in the body and which
have, therefore, to be supplied by the foods.

_Carbon._--Examine pieces of charred wood, coke, or coal, and also the
"lead" in lead pencils. Show that the charred wood and the coal will burn.
Recall experiment (page 114) showing that carbon in burning forms carbon
dioxide.

_Hydrogen._--Fill a test tube one third full of strong hydrochloric acid
and drop into it several small scraps of zinc. The gas which is evolved is
hydrogen. When the hydrogen is coming off rapidly, bring a lighted
splinter to the mouth of the tube. The gas should burn. Hold a cold piece
of glass over the flame and observe the deposit of moisture. Hydrogen in
burning forms water. Extinguish the flame by covering the top of the tube
with a piece of cardboard. Now let the escaping gas collect in a tumbler
inverted over the tube. After holding the tumbler in this position for two
or three minutes, remove and, keeping inverted, thrust a lighted splinter
into it. (The gas should either burn or explode.) What does this
experiment show relative to the weight of hydrogen as compared with that
of air?

_Nitrogen._--Nitrogen forms about four fifths of the atmosphere, where,
like oxygen, it exists in a free state. It may be separated fro