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.