Phlogiston Theory

• four elements: earth, air, fire, and water.

• One early theory was that sulphur was the ingredient that caused combustion. (Principles of Earth)

• All matter contained a fatty substance that gives it definition.

• Becher and Stahl called it a mysterious, unknown substance "phlogiston"

• During combustion, phlogiston is given off into the air:

• wood ---> calx (ash) + phlogiston (to the air)

• iron ---> calx (rust) + phlogiston (to the air)

• Wood ash is much lighter than the original wood.

• Iron rust seems much lighter than the original iron.

• Charcoal is almost completely consumed when burned:

• charcoal ---> phlogiston (to the air)

• charcoal is almost pure phlogiston.

• When wood burns in a sealed container, it will not burn to completion if there is not

enough air.

• Thus it was deduced that any given amount of common air (as it was called) could hold only so much phlogiston.

• Common air that could hold no more phlogiston was called "phlogisticated air."

• So:

• wood + common air ---> calx + phlogisticated air

• Some of this phlogisticated air would dissolve in water. Some of it would not.

• So there were now two kinds of air in this phlogisticated air.

• Neither of these two airs would support combustion.

• The air which would dissolve in water was called "fixed air."

• The other air was theorized to be the actual phlogisticated air.

• Joseph Priestley discovered another form of air.

• When mercury was heated in air, it formed a red substance, which became known as

"the precipitate per se."

• This precipitate per se, when heated with no air at all, changed back into mercury and this new air.

• This new air supported combustion much better than common air. Wood burned brighter, and more easily. Iron shavings actually burned rather than rusted.

• Common air was mostly composed of this impurity, which became known as "foul air" or "mephitic air."

• And Priestley called his new air "dephlogisticated air":

• common air=foul air + dephlogisticated air

• phlogisticated air=dephlogisticated air + phlogiston

• wood + dephlogisticated air ---> calx (ash) + phlogisticated air

• The mercury experiment was reversible:

• mercury + dephlogisticated air <--> precipitate per se

• This precipitate per se would be a combination of a mercury calx and phlogiston.

• Several metals can be produced from their calces by heating the calx with charcoal:

• calx + charcoal ---> metal + fixed air

This is:

• calx + phlogiston ---> metal + phlogisticated air

• Water was separated, by electrolysis, into two kinds of air, a new gas called "inflammable air" and dephlogisticated air:

• water ---> inflammable air + dephlogisticated air

• Inflammable air burns very easily, forming water:

• inflammable air + dephlogisticated air ---> (water)

• phlogisticated air=phlogiston + dephlogisticated air

• To summarize • water <---> inflammable air (phlogiston) +

dephlogisticated air• metal (pure base + phlogiston) +

dephlogisticated air ---> calx (pure base + water)

• charcoal (pure base + phlogiston) + dephlogisticated air ---> calx (fixed air + water)

• calx (pure base + water) + charcoal ---> metal (pure base + phlogiston) + fixed air

• metal (pure base + phlogiston) + water <---> calx + inflammable air (phlogiston)

• And we can now translate some of the archaic chemical words:

• an air=a gas (Common air was, of course, known to be a mixture of gasses from early on)

• fixed air=carbon dioxide (Charcoal is almost pure carbon)

• inflammable air=hydrogen• dephlogisticated air=oxygen• foul air (mephitic air)=nitrogen• calx=oxide (iron rust and finery cinder are two

different oxides of iron) • precipitate per se=mercuric oxide