The Last Stoic

High Falutin’ Convolutin’ Phlogiston

Posted in Uncategorized by munty13 on September 1, 2009

–Yosemite Sam picture courtesy of Warner Brothers

Before the caloric theory of combustion emerged in the 18th Century, there was the phlogiston theory. The theory was popularised by Johann Joachim Becher when he published the treatise “Physical Education” in 1667. Becher proposed that flammable objects contained a substance which he called phlogiston. An object without phlogiston does not burn. Metals and fire were considered to be rich in phlogiston, while the earth was considered to be poor. “Phlogisticated” substances are those that contain phlogiston and are “dephlogisticated” when burned.

I have unearthed a letter written by Walter Fitzroy regarding some of his thoughts about the phlogiston theory. It appears to be dated from around the time lines were being drawn by those caught between the prevailing caloric and phlogiston theories:

Phlogiston is present in all things that are combustible, and we are thankful that everything that contains phlogiston is not constantly in flames, and that it requires a release mechanism. I theorise that heat and fire are indeed release mechanisms for phlogiston and that it encourages phlogiston in the material to be released.

Similarly, heat is required to release the phlogiston from inflammable air to the calx during calcination. One must assume that the intense heat required for these processes is because phlogiston, by nature, is a very stubborn substance that is deeply imbedded in the substance it is stored in be it a metal or inflammable air.

As for why extra heat appears to be needed for returning calx to a metal then the reverse, a possible explanation is that to liberate phlogiston from air maybe more difficult than liberating it from the metal. It is very likely that the metal contains a higher concentration of phlogiston than the air that surrounds it.

Therefore, the release of phlogiston from metal is easier because there is a higher concentration, whilst in air the lesser concentration of phlogiston means that it requires more heat.

A blatant contradiction to the phlogiston theory though was the fact that a burning candle, supposedly rich in phlogiston, is extinguished by a glass shade. Critics pointed out that if the candle was burning phlogiston then the flame should not go out. Antiphlogistonists argued that this simple experiment proves the candle flame is using the air inside the glass for combustion – the glass has excluded the flame from the air outside. However, the phlogistonists countered that it is not so much the air that has been excluded, but that there is nowhere for the used phlogiston to go.

There being no escape for the phlogiston is the part of the theory which interests me. I have wondered aloud if the nitrogen gas which we find inside the glass after the combustion, was truly there before the combustion. The idea that the air is 79% nitrogen is pretty much based on these experiments of combustion. Is it possible that something happens to the burning phlogiston which remains inside the glass and thus transforms it into nitrogen? If a candle, or any other fuel for that matter, possesses phlogiston – then at least we can whittle the culprit down as far as something to do with hydrocarbons.

When a hydrocarbon burns in oxygen, the reaction will only yield carbon dioxide and water. When a hydrocarbon or any fuel burns in air, the combustion products will also include nitrogen.

The phlogiston theory was eventually dismissed though. In 1774 the French chemist Lavoisier stepped forward with oxygen – and the process of oxidisation – to explain combustion. Lavoisier emerged in a time where it was becoming more popular, and indeed more necessary, for experimenters to quantify the weight of every substance used throughout an experiment. Efforts were being made to reduce substances to their atomic components. Instruments used to measure experiments became incredibly precise, and notoriously expensive.

Instruments belonged only to the institutions which could afford them. Lavoisier was fortunate enough to act as a privatised tax collector (though an unfortunate career choice perhaps in light of the French Revolution and an appointment with Madamoiselle Guillotine) which provided plenty of money to satisfy the high costs of these instruments. Lavoisier was able to keep pushing the boundaries of these specialised measurements even further. Lavoisier possessed a balance so accurate it could measure 1 part in 400,000.

A substance was weighed, and then burned, and painstaking efforts were made to measure the weight of the ashes and gases produced. The results of these measurements were then compared. Phlogiston theory dictated that a substance should weigh less with the supposed loss of phlogiston. Examples though, such as sulphur and phosphorus, started to emerge in which substances weighed more AFTER they had burned.

The French physician Jean Rey, and the English philosopher Robert Boyle, among others, were also aware that metals, when they changed into earthy powders on heating, gained weight. 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. In an age where everything had to be quantified, a substance that weighed less than nothing simply would not do. The phlogiston theory was dropped like a hot coal.

It was nonsense, observed Lavoisier, “that one augments the weight of a body by taking part of its substance away from it.” This led Lavoisier to the conclusion that during combustion something had been taken from the air and added to the substance; namely oxygen.

Oxygen gas had previously been discovered by Joseph Priestley (1733-1804) on 1 August 1774 in the laboratory at Bowood House. There is perhaps a little irony that the greatest advocate of the phlogiston theory was also the one who discovered the element which would ultimately lead to the theory’s demise. (Though, to be fair, the Swedish chemist Carl Wilhelm Scheele may have discovered oxygen before Priestley but did not publish his results in time). Priestly at first gave the gas the catchy title of “dephlogisticated air”.

My biggest problem with the phlogiston theory is not so much that it is an invisible substance with a negative mass, but that I struggle trying to keep up with all the convoluted names given to the different elements, and compounds involved. For example we have “fixed air” for carbon dioxide, and “inflammable air” for hydrogen, and “dephlogisticated air” for oxygen. What on earth does “dephlogisticated” mean? Well, that would be the opposite of “phlogisticated”, and this is where I think it gets really interesting.

Please let me draw your attention to a paper written by Priestley in 1796, “Considerations on the Doctrine of Phlogiston, and the Decomposition of Water”. In it, Priestley can be caught saying that water is something “which I deem to be essential to the constitution of every kind of air”. He also confidently states that “I have, in my experiments on terra ponderosa aerata [barium carbonate] demonstrated that water constitutes about half the weight of fixed air [carbon dioxide].” In addition Priestley goes on to reveal more about what phlogiston actually is:

In all other cases of the calcination of metals in air, which I have called the phlogistication of the air, it is not only evident that they gain something, which adds to their weight, but that they likewise part with something.

The most simple of these processes is the exposing iron to the heat of a burning lens in confined air, in consequence of which the air is diminished, and the iron becomes a calx. But that there is something emitted from the iron in this process is evident from the strong smell which arises from it. If the process be continued, inflammable air will be produced, if there be any moisture at hand to form the basis of it.

From this it is at least probable, that, as the process went on in an uniform manner, the same substance, viz. the basis of inflammable air, was continually issuing from it; and this is the substance, or principle, to which we give the name of phlogiston.

Priestley is saying that phlogisticated air is generated during combustion. He is referring to phlogisticated air as a product of combustion. The air becomes phlogisticated by being saturated with phlogiston. You may or may not be surprised to learn that today we call “phlogisticated air” – nitrogen. This leads us on to the big question – which has hung around stubbornly from the very first moment you started reading – what the hell is phlogiston exactly?

From the above, please note that Priestley does not say that phlogiston is inflammable air (hydrogen) but rather it is the “basis of inflammable air”. Priestley continues:

When dephlogisticated [oxygen] and inflammable air [hydrogen], in the proportion of a little more than one measure of the former to two of the latter, both so pure as to contain no sensible quantity of phlogisticated air [nitrogen], are inclosed in a glass or copper vessel, and decomposed by taking an electric spark in it, a highly phlogisticated nitrous acid is instantly produced; and the purer the airs are, the stronger is the acid found to be.

If phlogisticated air be purposely introduced into this mixture of dephlogisticated and inflammable air, it is not affected by the process, though, when there is a considerable deficiency of inflammable air, the dephlogisticated air, for want of it, will unite with the phlogisticated air, and, as in Mr. Cavendish’s experiment, form the same acid. But since both kinds of air, viz. the inflammable and the phlogisticated, contribute to form the same acid, they must contain the same principle, viz. phlogiston.

Phlogistonists believed that the less calx which remained after the combustion of a metal, the more phlogiston it contained. Charcoal is almost completely consumed when burned, and so charcoal is considered to be almost pure phlogiston. Charcoal is almost pure carbon. When burning charcoal hardly any smoke or ashes are formed, only carbon dioxide and water vapour. If you were looking for phlogiston, I think it might pay to look very carefully at a lump of coal. In Section III “Other Objections to the Antiphlogistic Theory” Priestley finally reveals that which he believes phlogiston to be:

Though the new theory discards phlogiston, and in this respect is more simple than the old, it admits another new principle, to which its advocates give the name of carbone, which they define to be the same thing with charcoal, free from earth, salts, and all other extraneous substances; and whereas we say that fixed air consists of inflammable air and dephlogisticated air or oxygen, they say that it consists of this carbone dissolved in dephlogisticated air. Mr. Lavoisier says that “wherever fixed air has been obtained, there is charcoal.” They therefore call it the carbonic acid.

But in many of my experiments large quantities of fixed air have been procured where neither charcoal, nor any thing containing charcoal, was concerned, or none in quantity sufficient to account for it. When the purest malleable iron is heated in dephlogisticated air, or in vitriolic acid air, a considerable quantity of fixed air is formed.

Lastly, fixed air is procured in great abundance in animal respiration. It is true that fixed air is procured by exposing lime-water to atmospherical air, but it is never procured by this means in air confined in any vessel. There must, for this purpose, be an open communication with the atmosphere. but fixed air will be procured in great abundance by breathing air contained in the smallest receiver, and especially if the air be dephlogisticated. It must therefore be formed by phlogiston, or something emitted from the lungs, uniting with the dephlogistcated air which it meets there.

It may be said that since we feed in a great measure upon vegetables (and even animal food is originally formed from them) and this principle of carbone is found in all vegetables, this may be the substance that is exhaled from the lungs. But since, in this process, it forms the same substance that inflammable air from iron does with dephlogisticated air, or oxygen, it must be the same thing with it; and then this carbone will only be another name for phlogiston.

There we have it then. Priestley believed phlogiston was carbon. Therefore, it appears he understood phlogisticated air as being saturated with carbon, or to put it another way, that nitrogen was a compound which was made up with carbon.

The antiphlogistians always suppose azote, or phlogisticated air, to be a simple substance, though I think abundant evidence has been given (and more will be found in my last memoir, printed in the Transactions of the Philosophical Society at Philadelphia), that it is composed of phlogiston and dephlogisticated air.

You can find Priestley’s entire treatise here thanks to:

So Priestly thought that nitrogen was a compound made-up of carbon and oxygen. Not forgetting aswell that he also thought water was “essential to the constitution of every kind of air”. Obviously, carbon monoxide and carbon dioxide are made-up of carbon and oxygen too. Carbon monoxide will support combustion, but carbon dioxide will not, and neither does nitrogen. The carbon in carbon dioxide has been fully vapourized, whereas in carbon monoxide it has not.

I am at once grateful that the names used in the phlogiston theory are so over-descriptive. These names are not simply designed to be high falutin’ for the sake of it, they give us a real chance of deciphering what it was that these early experimenters understood about the air around us.

Some important chemists of the time agreed with Priestley’s idea that nitrogen was a compound body formed from oxygen. Berzelius and Sir H Davy both held the view that nitrogen was a compound; Berzelius went as far as proposing the name “Nitricon”. Berzelius later dropped the hypothesis following Davy’s series of experiments which failed to decompose the compound.

I found a very interesting article from the archives of the Transactions and Proceedings of the Royal Society of New Zealand 1868-1961. The article is headed “Discovery of Argon” and dated 1896:

…Recent investigations made by Lord Rayleigh and Professor Ramsay, which have not only resulted in conclusively establishing the compound nature of atmospheric nitrogen, but also in showing that the substance with which it is associated is a gas previously unknown, to which they have given the name of argon. But the question, What is argon? still remains to be solved. So far as present researches into its chemical character have been carried, it is found to possess properties of so peculiar a description as to raise questions of paramount importance for chemistry.

Argon… is supposed to be a tri-atomic form of nitrogen, as ozone is a bi-atomic form of oxygen; and many circumstances already known—for example, its concurrent appearance in nature with nitrogen, the difficulty of separating them, their common inertness—exaggerated in argon—their common lines in the spectra, their double spectra, and the outer resemblance of their benzine compounds as shown in Berthelot’s experiments —are said to lend strength to this hypothesis.

The discovery of argon paved the way for the discovery of the noble gases in the atmosphere. The six noble gases that occur naturally are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn). These gases are well known for their very low chemical reactivity. Neon lights are famous throughout the world for illuminating places you can buy a beer, but each one of the noble gases can also be used in the lamps. Each gas generates a different colour of the rainbow.

Oh. One more thing before I go… (I’m waving a cigar and hunched between the collars of my raincoat in my best impression of Columbo) ….but there were a couple of things about Priestley’s work which still bug me. Priestley was not one for rash explanations of experiments. In spite of what appears to be blatant proof to the contrary, Priestley remained unconvinced that water was simply constituted of hydrogen and oxygen.

Another pretended proof that water is composed of dephlogisticated and inflammable air, is that when the latter is burned slowly in the former, they both disappear, and a quantity of water is produced, equal to their weight. I do not, however, find that it was in more than a single experiment that water so produced is said to have been entirely free from acidity…

If there be a redundancy of inflammable air in this process, no acid will be produced, as in the great experiment of the French chemists, but in the place of it there will be a quantity of phlogisticated air. A considerable quantity of water is always produced in these decompositions of air. But this circumstance only proves that the greatest part of the weight of all kinds of air is water. I have, in my experiments on terra ponderosa aerata demonstrated that water constitutes about half the weight of fixed air.

Priestley is confident however that “the greatest part of the weight of all kinds of air is water”,and that water makes up half the weight of carbon dioxide. Maybe Priestley was on to something. Here’s me thinking that the experiments which show the decomposition of water prove exactly what it is made of. I thought it was unquestionable. But Priestley’s reservations show that it just depends on how you look at it.

Priestley is also famous for inventing the world’s first fizzy drink when, in 1772, he carbonated water. One volume of carbon dioxide will dissolve in one volume of water at standard room temperature and pressure to form carbonic acid. The carbon dioxide simply slips into the bath and appears to displace nothing. That’s gotta mean something, right?

On April 15, 1770, Joseph Priestley recorded his discovery of Indian gum’s ability to rub out or erase lead pencil marks. He wrote, “I have seen a substance excellently adapted to the purpose of wiping from paper the mark of black lead pencil.” These were the first erasers which Priestley called a “rubber”.

Indian rubber is an old friend of mine. I have wondered in the past why a solvent that dissolves Indian rubber was named ether, when the name was already taken by alchemists for the substance of the very Universe itself. And here’s Priestley using the rubber to wipe-out the mistakes he wrote with his pencil. The lead in pencil is of course made from graphite – a form of carbon!

Many thanks:
Elements of chemistry By Edward Turner, Franklin Bache
From Atomos to Atom By Andrew G. van Melsen


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