From the mid-seventeenth century to the mid-nineteenth century, scientists were using more "modern" methods of discovery, testing theories with their experiments.
One of the great controversies was the misery of combustion. Two chemists: Johann Joachim Becher and Georg Ernst Stahl proposed the phlogiston theory. This theory said that an "essence" (such as hardness or yellow color) should escape during the combustion process.
No one could prove the phlogiston theory. The first chemist to prove that oxygen is essential for combustion was Joseph Priestly. Oxygen and hydrogen were discovered during this period.
It was the French chemist Antoine Lavoisier who formulated the currently accepted theory of combustion.
This era marked a period when scientists used the "modern method" of testing theories with experiments.
This originated a new era, known as Modern Chemistry, which many refer to as atomic chemistry.