Alchemists considered the all things to be made up of four basic elements: earth, air, fire and water. These are the alchemy symbols for earth, air, fire and water.
The ancient Greek philosopher Plato further defined the four elements by adding two qualities to each element. There were four possible qualities: hot, cold, wet, and dry.
Earth has the qualities dry and cold.
Air is wet and hot.
Fire is both hot and dry.
Water is wet and cold.
These elements would appear in discussions of chemical science well into the 18th Century when elements were still referred to as airs for gasses and earths for minerals and metals. Now we know these ancient elements are really substances that are made up of elements rather than being elements themselves.
Colored versions of these symbols can be found in Earth, Air, Fire and Water Element Symbols. Colored versions with stylized backgrounds can be found in Four Ancient Element Symbols.