Men interested in chemistry in the Middle Ages were called alchemists and most of them were concerned with the fruitless search for a secret compound which could turn base metal into gold. But by the 17th century the first true chemists were discovering some of the real reactions that take place between chemical substances and what happened to them when they were combined.
In 1808, John Dalton (1766-1844), an English chemist, brought together the idea of the chemical combination of elements with the atomic theory of matter. Much of the evidence for his theories was based on earlier work by such scientists as Robert Boyle (1627-1691) and Antoine Lavoisier (1743-1794). In 1789, Lavoisier had listed thirty-three 'elements' and although only twenty-eight of these were later shown to be acceptable, they were recognized as the basic building blocks of matter.
Dalton's theories about chemical substances slowly took shape. Matter was formed of elements, such as iron, copper, carbon and oxygen (modern science now recognizes ninety-two basic elements in their natural state and another eleven which have been made by man) and every element had its own set of special physical and chemical properties.
The smallest particle of such an element that could exist on its own was an atom. At this time it was thought that the simplest possible particle of anything was a single atom, although compounds of several elements would contain more than one atom from each element, stuck together by some kind of attractive force.
And the atom itself? This still remained, in scientists' minds, as a mysterious, unsplittable, miniature billiard ball. Iron atoms were in some way different from copper atoms, but all iron atoms - no matter where they came from - would be identical to each other.
Dalton's atomic theory explained much, but was still incomplete. It could not predict how many atoms of each of the combining elements were contained within a single particle of a compound.
It was left to Amedeo Avogadro (1776-1856), an Italian chemist, to introduce the idea of molecules. These, he suggested in 1811, were composed of atoms, and some molecules contained more than one atom of the same element. Hydrogen, we now know, contains two atoms and is the simplest possible molecule - since individual atoms do not exist under normal conditions.
Oxygen in its natural state also contains two atoms, but when these elements combine to form water the resulting compound molecule is made up of two atoms of hydrogen, and only one atom of oxygen.
From early times certain mystical symbols had been allocated to the known chemical substances. This practice was changed in the 19th century when scientists started using letters to represent the elements. These new symbols had a very special meaning.
Each represented one atom of a particular element, and it was now possible to represent chemical reactions by equations.
This system remains in use today. A hydrogen molecule, containing two atoms, is represented by H2. When more than one molecule is involved, this is indicated by a number in front of the symbol; thus 2H2 means two molecules. To represent the way in which water is formed by the combination of two molecules of hydrogen with one molecule of oxygen to form two molecules of water, the equation is written: 2H2 + 02 = 2H20 .