The Periodic Table People have wondered what the material world was actually made of. The search for the elements was begun in ancient times. Elements are defined as substances which cannot be taken apart and broken into other, simpler substances. Substances made of a combination of elements are called compounds. It was difficult to find out which substances were elements.
Water is plentiful, but it is made up of two gases, oxygen and hydrogen.
The air is made up of a mixture of different gases, but that is not obvious. As the science of chemistry developed, scientists discovered a number of ways to take compounds apart. However, it was difficult to see how their knowledge could be systematized. In the 19th century John Dalton, a British schoolmaster, showed that the elements always occurred in consistent proportions in specific compounds. This made it possible to make sense of the idea that matter was made of atoms.