12 
INTRODUCTION 
wheat, potatoes, and corn is a compound of three elements; 
namely, hydrogen, carbon, and oxygen. Elements cannot 
be divided into simpler substances. Compounds are made 
up of elements in chemical combination. There are thou¬ 
sands of kinds of compounds. Calcium carbonate is a 
compound that makes up a large part of limestone rock. 
Water is a compound made of hydrogen and oxygen. Wood 
is made up of compounds, one of which is cellulose which is 
very similar to starch. There are fewer than ninety ele¬ 
ments known at present, but new ones are being discovered 
from time to time. 
Oxygen, carbon, nitrogen, hydrogen, calcium, and iron 
are the elements that make up the principal parts of plant 
structures and animal bodies. 
Oxygen. — This is a gas. It is uncombined in the air, 
making up one fifth of its volume. It readily combines with 
many other elements. The darkening of unpainted barns 
and fences is an example of the union of oxygen with other 
elements. Opening the draft of a stove or furnace allows 
more oxygen to enter and increases the rate of the com¬ 
bination of oxygen with the coal or wood. This raises the 
temperature. The process of oxygen combining with other 
elements, either slowly as in the rusting of iron or rapidly as 
in the furnace, is called oxidation. Slow oxidation, like the 
rusting of iron, shows little heat at any one time, while 
rapid oxidation produces a greater amount of heat in a given 
time. The amount of heat depends on the rapidity of 
oxidation. 
Oxygen is the most important element in the air. Plants 
and animals must use it to live. Respiration has been 
spoken of as a fundamental life process. Without oxygen 
there could be no respiration and consequently no plant and 
animal life as we know it. 
Carbon. — This element, mixed with oxygen, is the most 
important to plant and animal life. It combines with 
