molecules are composed of atoms which are alike. When
the atoms comprising a molecule are all alike, the substance is called
an element, and is said to be a simple substance. Throughout the
length and breadth of this vast world of ours there are only about
eighty known elements. An element is the simplest substance
conceivable, because it has not been separated into anything simpler.
Water is a compound substance. It can be separated into oxygen and
hydrogen.

Gold, silver, and lead are examples of elements, and water, alcohol,
cider, sand, and marble are complex substances, or compounds, as we
are apt to call them. Everything, no matter what its size or shape or
character, is formed from the various combinations into molecules of a
few simple atoms, of which there exist about eighty known different
kinds. But few of the eighty known elements play an important part in
our everyday life. The elements in which we are most interested are
given in the following table, and the symbols by which they are known
are placed in columns to the right:

|Oxygen |O |Copper |Cu |Phosphorus |P |
|Hydrogen |H |Iodine |I |Potassium |K |
|Carbon |C |Iron |Fe |Silver |Ag |
|Aluminium Al |Lead |Pb |Sodium |Na | |
|Calcium |Ca |Nickel |Ni |Sulphur |S |
|Chlorine |Cl |Nitrogen |N |Tin |Sn |

We have seen in an earlier experiment that twice as much hydrogen as
oxygen can be obtained from water. Two atoms of the element hydrogen
unite with one atom of the element oxygen to make one molecule of
water. In symbols we express this H_2O. A group of symbols, such as
this, expressing a molecule of a compound is called a _formula_. NaCl
is the formula for sodium chloride, which is the chemical name of
common salt.

CHAPTER X

LIGHT

98. What Light Does for Us. Heat keeps us warm, cooks our food,
drives our engines, and in a thousand ways makes life comfortable and
pleasant, but what should we do without light? How many of us could be
happy even though warm and well fed if we were forced to live in the
dark where the sunbeams never flickered, where the shadows never stole
across the floor, and where the soft twilight could not tell us that
the day was done? Heat and light are the two most important physical
factors in life; we cannot say which is the more necessary, because in
the extreme cold or arctic regions man cannot l