We are surrounded by thousands of different materials. By the action of chemical processes some of them could be broken down into substances completely different from the initial ones. But there is also some which couldn't be broken down by any chemical means. A 17th century scientist Robert Boyle tried to discover which of the known substances could be broken down and which not. The first ones were called compounds and the second ones were called by him simple bodies or as we say today - elements. Soon it emerged that all chemical compounds consist of some limited number of different elements. Water, for example, which is a compound could be broken down into two elements: oxygen and hydrogen. It must be accentuated here that the chemical characteristic of the compound is different from the characteristics of the elements it is bound of. Look at the chart and compare the chemical characteristics of water, oxygen, and hydrogen.

Roert Boyle was a subscriber for the atomic theory. He did the research on the effect of air resistance, its pressure and on changes of its volume while changing its pressure. He discovered the law describing dependence between these two quantities (Boyle's law). You surely remember it from school (it should be somewhere at the beginning of your notes from lessons about thermodynamics). This law was experimentally discovered by Boyle but formulated later by Marriotte. The experiments with pressure were guiding later scientists, who were researching air structure. The experiments proved that air is composed of separate, moving atoms, which stay, in quite a distance from each other. Thanks to such structure, air can change its volume considerably. Pressure, which Boyle researched, is caused by the movement of particles, which collide with other things influencing them with some force. When air's volume is smaller, there are more collisions in each square centimetre of surface (what Bernoullie's researches showed later). This interdependence Boyle discovered in an experimental way.

Chemistry as a science, at the beginning of the 18th century, was an assembly of different, chaotic rules. The same was with chemical nomenclature. The great French chemist Antoine Laurent Lavoisier developed chemical nomenclature based on the names of simple substances while giving names to complicated substances composed of those simple ones. But before he was able to systematise chemical nomenclature he had to make changes in chemistry. He proved that all elements could occur in three states of aggregation: gaseous, liquid and solid. He showed that while burning, substances combine with oxygen. He also proved that water could not change into other substances as many scientists thought. After a longer research he managed to prove that water consisted of oxygen and hydrogen. By decomposing water he discovered that the weight ratio between oxygen and hydrogen was always 8:1. It was the direction indicating that world consisted of atoms. Lavoisier believed in it, but he didn't develop his study of atom. The one who did it was John Dalton.