19.
Some centuries ago, it was believed that anything that burns must contain material that people called phlogiston, that is to say, burning was explained as the release of phlogiston from the combustible material to the air.
Air was considered essential, as it had to provide a home for the released phlogiston. In addition, there would be a limit to the phlogiston transfer, since a given volume of air could absorb only so much phlogiston. When the air had become saturated, no additional amounts of phlogiston could leave the combustible substance, and the burning would stop. Burning would also stop when the combustible substance was emptied of all its phlogiston.
Despite the fact that the phlogiston theory was self-consistent, it was awkward because it needs imaginative, even mysterious, properties to be attributed to phlogiston. Besides, phlogiston was intangible, as no one had ever isolated it and experimentally determined its properties. Sometimes it seemed to show a negative weight as the residue left after burning weighed more than the material before burning. This was true, for example, when magnesium burned.
However, phlogiston sometimes seemed to show a positive weight, when, for example, woods burned, the ash weighed less than the starting material. And since so little residue was left when alcohol, kerosene, or high-grade coal burned, these obviously different materials were believed to be pure or nearly pure phlogiston.
Antoine Lavoisier, based on careful experimentation, was led to propose a different theory of burning, one that required a constituent of air-later shown to be oxygen- for combustion. Since the weight of the oxygen is always added, the weight of the products of combustion, including the evolved gases, would always be greater than that of the starting material.
Lavoisierâs interpretation was more reasonable and straightforward than that of the phlogiston theorists. The phlogiston theory, thus, became suspect, eventually fell into scientific disrepute, and was replaced by new ideas.
The text mainly discusses…
.