Combustion

Probably the earliest reasonably scientific attempt to explain combustion was that of Johannes (or Jan) Baptista van Helmont, a Flemish physician and alchemist who lived from 1580 to 1644. Van Helmont observed the relationship among a burning material, smoke and flame and said that combustion involved the escape of a "wild spirit" (spiritus silvestre) from the burning material. This explanation was later incorporated into a theory of combustion—the phlogiston theory—that dominated alchemical thinking for the better part of two centuries.

According to the phlogiston theory, combustible materials contain a substance—phlogiston—that is emitted by the material as it burns. A non-combustible material, such as ashes, will not burn, according to this theory, because all phlogiston contained in the original material (such as wood) had been driven out. The phlogiston theory was developed primarily by the German alchemist Johann Becher and his student Georg Ernst Stahl at the end of the seventeenth century.

Although scoffed at today, the phlogiston theory satisfactorily explained most combustion phenomena known at the time of Becher and Stahl. One serious problem was a quantitative issue. Many objects weigh more after being burned than before. How this could happen when phlogiston escaped from the burning material? One possible explanation was that phlogiston had negative weight, an idea that many early chemists thought absurd, while others were willing to consider. In any case, precise measurements had not yet become an important feature of chemical studies, so loss of weight was not an insurmountable barrier to the phlogiston concept.