Phlogiston Theory

The failure to understand combustion was an insurmountable obstacle to real progress in chemistry. Any theory of chemical change must be able to explain combustion and phlogiston was the first real attempt to do so.

The fact that wood turns to ashes and metals become soft powders when heated and can be changed back into metals in the presence of charcoal is hard to reconcile without imagining the addition or subtraction of some substance.

Phlogiston was that substance.

The theory was simple, and although having serious contradictions, was better than no theory at all. Besides, despite the quantitative work of Galileo and Newton, the importance of quantitative measurements had not yet been impressed upon the chemists.

The phlogiston theory was really quite simple.

Metals and combustible substances contained an imponderable substance known as phlogiston which was released into the air along with caloric. Air had a limited capacity to absorb phlogiston.

Since phlogiston was an imponderable substance, itŐs properties were incapable of being detected by senses and could be contradictory.

Sometimes it had weight, sometimes it had negative weight, and sometimes it had no weight at all.

Phlogiston theory explained many facts about combustion

1. combustibles lose weight when burning because they lose phlogiston
2. a flame goes out in an enclosed space because air becomes saturated with phlogiston
3. charcoal leaves little residue upon burning because it is nearly pure phlogiston
4. a mouse dies in an airtight space because the air becomes saturated with phlogiston
5. some metal calxes turn to metals when heated with charcoal because phlogiston from charcoal is restored to the calx

A serious problem was that the calx formed when a metal such as magnesium burns weighs more than the metal from which it formed, just as a rusty nail weighs more than the nail.

The supporters of the phlogiston theory answered this by postulating that metallic phlogiston has negative weight while other combustibles contain phlogiston with positive weight.

Adding a special postulate such as this signaled a theory in trouble and led to the ultimate demise of the theory.