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© Copyright 1996, Jim Loy
In ancient times, there were sometimes considered to be four elements: earth, air, fire, and water. In the heavens was the fifth element (the quintessence), the ether. This theory lacked the predictive power to make alchemy a science. Eventually, early chemists decided that combustion was the most important chemical reaction, that understanding combustion would actually revolutionize chemistry. Early on, they had the impressive insight that the corrosion of metals was a form of combustion. Another impressive insight was that respiration, in animals, is also a form of combustion. A common test of the goodness of any given air (gas) was to measure the time that a mouse would survive, breathing the air.
One early theory was that sulphur was the ingredient that caused combustion. After all, sulphur itself burned completely. It was not unreasonable to assume that any combustible substance contained sulphur. When wood burned, then sulphur was given off, into the air. But, the sulphur given off by wood was certainly different from the sulphur given off by solid sulphur. At least the smell was different.
Becher suggested a hypothetical substance, which he called "inflammable earth" which every flammable substance contains. Stahl called this mysterious, unknown substance "phlogiston" (pronounced flow-JISS-tunn). During combustion, phlogiston is given off into the air:
wood ---> calx (ash) + phlogiston (to the air)
iron ---> calx (rust) + phlogiston (to the air)
You and I know that this is not true, that instead various substances combine with oxygen during combustion. But, please suspend your righteous contempt for this phlogiston theory, and try to ignore your inside information, and look at the theory as a somewhat skeptical 18th century chemist would. I will continue to use some 18th century terminology, to keep your mind from leaping ahead to 20th century chemistry.
Wood ash is much lighter than the original wood. Iron rust seems much lighter than the original iron. It makes sense that much of the weight is given off into the air as phlogiston. The calx (plural "calces") is now the pure elemental substance, while each metal ends up being a compound of a basic calx and phlogiston.
Charcoal is almost completely consumed when burned (as is sulphur):
charcoal ---> phlogiston (to the air)
There were usually small residues, correctly recognized to be impurities. As greater efforts were employed to purify the charcoal (or sulphur), the less residue was left after combustion. And so, charcoal is almost pure phlogiston. In the case of sulphur, its calx is considered to be an air:
sulphur ---> calx (gaseous) + phlogiston (to the air)
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
Now it was discovered that 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 (the main defender of phlogiston theory) 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.
When wood burned in this new air, and you used enough wood so that the air that was left would no longer support combustion, then the air that was left was almost entirely fixed air. So, fixed air was in fact phlogisticated air, and what had previously been called "phlogisticated air" was, in fact, some other kind of air, an impurity of common air. 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
Mercury was so handy in forming dephlogisticated air, that most chemists were in danger of suffering from mercury poisoning. Cavendish was a truly bizarre person, and I suspect that this was due to mercury poisoning. The mercury experiment was delightfully reversible:
mercury + dephlogisticated air <--> precipitate per se
This precipitate per se would be a combination of a mercury calx and phlogiston. In this case, you don't need air to take up the phlogiston, as the mercury calx takes it up, instead. There is another, this one black, oxide of mercury. Priestley claimed that this was the true calx of mercury.
Several metals can be produced from their calces by heating the calx with charcoal:
calx + charcoal ---> metal + fixed air
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 ---> calx (water)
Again, we have given off no phlogisticated air. Is the resultant water not really the calx of inflammable air, but rather a combination of the true calx and phlogisticated air? But, water seems so basic a substance. And inflammable air, instead of charcoal, is now considered to be pure phlogiston. So:
water=phlogiston + dephlogisticated air
phlogisticated air=phlogiston + dephlogisticated air
And, phlogisticated air was considered fixed air. So we now revise our theory. Charcoal, of course, contains phlogiston (because it burns), and when burned, it gives off a gaseous calx combined with water. Water now takes the place of phlogisticated air. The various calces, while simpler substances than the metals to which they are related, may all be a combination of a simpler "base of pure earth" and water. This is how mercury can be changed into precipitate per se, without giving off any air:
mercury + dephlogisticated air ---> precipitate per se (base of pure earth + water)
Some metals may give off water, others do not:
metal + dephlogisticated air ---> calx (pure earth + water)
metal + dephlogisticated air ---> calx (pure earth) + water (gaseous or liquid)
Inflammable air is a better candidate for phlogiston than previous candidates. It burns more readily than either sulphur or charcoal. Before, we had some substances which contained some phlogiston (phosphorus, for example) which burned more readily than what we thought was pure phlogiston (charcoal). Now, our pure phlogiston (inflammable air) burns very readily.
Incidentally, one of the waste products of the production of iron is a hard black substance called "finery cinder." Finery cinder can be produced by passing steam over hot iron. This also produces inflammable air:
water (steam) + iron ---> finery cinder + inflammable air (phlogiston)
iron=calx (rust) + phlogiston
So, finery cinder is:
finery cinder=calx (rust) + water
To summarize the latest version of phlogiston theory, we have a few interesting equations:
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)
metal (pure base + phlogiston) + acid ---> salt (pure base + acid) + inflammable air
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)
foul air (mephitic air)=nitrogen
calx=oxide (iron rust and finery cinder are two different oxides of iron)
precipitate per se=mercuric oxide
Phlogiston theory evolved throughout the 18th century, because many experiments were being performed which needed to be explained. Most of these experiments were being performed by Antoine Lavoisier (the father of modern chemistry) and his followers (the Antiphlogistians). They would come up with an objection to phlogiston theory, and the Phlogistians (usually Priestley) would modify the theory to fit the new experiment. As the years went on, more and more Phlogistians became Antiphlogistians, until only Priestley was left.
As you can see above, phlogiston theory made some sense. But, the experiments which, more and more, convinced chemists that phlogiston was incorrect, were quantitative experiments. The Antiphlogistians measured the weight of every substance involved in the experiment, even the gasses. When iron rusts away completely, the rust actually weighs more than the original iron. When charcoal burns, the resultant carbon dioxide (fixed air) weighs more than the original charcoal. So, in every case, phlogiston would have to have a negative weight. This disturbing attribute convinced most of the last Phlogistians to abandon their theory.
The Phlogistians were not a bunch of cranks. They were serious scientists who helped resolve some of the basic questions of chemistry, by being the devil's advocate against the Antiphlogistians, and by discovering important experiments. And, for almost the entire 18th century, the phlogiston theory was more satisfying than the alternative, in some ways. The Antiphlogistians deduced that combustion was when a substance combined with oxygen (Priestley's dephlogisticated air). But, they could not tell why substances combined with oxygen, or even why all substances didn't just burn up. Phlogiston theory explained why, because some substances were rich in phlogiston, and burned. It was only later that more satisfactory explanations were discovered.
It may seem hard to believe that phlogiston theory, which is incorrect, was so persistent. How could it survive all of the attacks, and come back for more? I think the answer is that phlogiston theory is actually very close to the truth. If we consider a chemical's tendency to take up oxygen, and call its lack of oxygen "phlogiston," we can describe absolutely any chemical reaction involving oxygen. Instead of putting oxygen on one side of any chemical equation, we can put this anti-oxygen on the other side. It will always balance. One atom of phlogiston would always have an atomic weight of -16, and the weights will always balance, too. So, we can always construct a self-consistent phlogiston theory, even today.
See Considerations on the Doctrine of Phlogiston and the Decomposition of Water, by Joseph Priestley, LL.D., F.R.S., &c., &c..
How would you go about disproving the theory of phlogiston? We could merely burn carbon in oxygen, and accurately measure all of the gasses and solids, before and after. We would find that carbon+oxygen became carbon dioxide. We can determine that the final gas CONTAINS carbon dioxide. If we start with the right amounts of carbon and oxygen, we may be able to show that the final gas is nothing but carbon dioxide. I'm not sure how we quantitatively test for carbon dioxide. At least the weights will be right, which should be good enough.
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