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Oxygen: The End of the Beginning

Oxygen was discovered in 1772 by Carl Wilhelm Scheele. Oxygen was also discovered in 1774 by Joseph Preistley. Oxygen was also also discovered in 1774 by Antoine Laurent Lavoisier.

Success, as they say, has many fathers.

Before we sort out who deserves credit for what, I want to discuss just what it is that makes oxygen so important to the history of chemistry.

Had there been a periodic table in 1775, it would have looked like this:

Things have progressed nicely since the last time I posted one of these tables. We have several transition metals in a row, from manganese to zinc. Platinum and zinc are finally known to the world at large. Antimony, once lost along with the Sumerian culture, has been rediscovered. We even have a completed column in group 15, from nitrogen to bismuth. But I think the most telling detail is that more than a third of these elements have been discovered in a span of about 105 years.

That may not seem like a very rapid pace, but consider that it took the entire history of the human species up to about 1550 AD to discover the first fourteen elements. We're talking about tens of thousands of years here. Then, from 1669 to the 1774, eight new elements were discovered. Half of these eight were discovered in the 1770's alone. So the rate of discovery has increased exponentially.

This isn't a coincidence. The Scientific Revolution had begun in the 16th Century, and by the 17th Century is had begun to influence the course of alchemy. Hennig Brand discovered phosphorus in 1669, and although he wanted to keep it under wraps, there were simply too many scientists interested in what he had done and how he had done it. The rules changed, and after phosphorus, discovering a new element was something to be published in scholarly journals and books. Like the man said, the only difference between screwing around and science is writing it down. After 1669, people started writing this stuff down, and a great deal of progress was made. By 1780, there were enough elements on the board that chemists could actually begin to consider how they interrelated with one another. And that's why oxygen was so critical.

A lot of the elements I've already covered were appreciated in isolation. Copper is useful in its own right. You can fashion tools with it. Sulfur and phosphorus have interesting properties. Nitrogen and platinum were fascinating because of their reluctance to react with anything. Cobalt and nickel were important in themselves because they defied the notion of a set number of metals.

But in the 1770's, we've reached a point where chemists needed to understand the rules that governed the chemistries of these different elements. Hydrogen was generated when acid was added to certain metals. Chlorine was generated when hydrochloric acid was added to manganese dioxide. Manganese could be smelted from its ore, but with greater difficulty than other metals. Each new element raised questions about the chemical reactions that had produced it. And there would soon come a point where those chemical reactions would have to be understood before any new elements would be discovered.

Oxygen represents that crossroads. It wasn't enough to simply isolate some oxygen gas and write a paper about its physical characteristics or how it sustained a flame. Oxygen was a key to the whole game. Consider the following:

Oxygen is the most abundant element in the earth's crust, at nearly 47% by weight.

Oxygen makes up about 21% of the atmosphere. It's presence in the air is literally the only reason we breathe it.

Oxygen comprises 89% of the total mass of water.

Oxygen is what allows fire to burn.

Oxygen is the third most abundant element in the universe, at 1% by weight. (Okay, it's a distant third, but still...)

Oxygen is the most abundant element in the living things, at 65% by weight.

So you take all of the questions the Ancient Greek philosophers asked about the natural world, and how much of the answers depended on oxygen to some extent? The four classical elements--earth, air, fire, and water--all involved oxygen. The heavens contained substantial amounts of oxygen. All life is made of mostly oxygen. The ores we smelted for metal, that process relied upon the relative affinity for oxygen between metal and carbon.

I don't mean to suggest that the ancients would have understood everything if they had only known what oxygen was. My point is that any explanation for nature that doesn't include oxygen is woefully incomplete. In the 18th Century, chemistry was rapidly being built into a modern scientific discipline, but without an understanding of oxygen, it wouldn't get very far.

So on the one hand, you could say the discovery of oxygen was inevitable. It was all over the place, and sooner or later any path of chemical research was bound to run across it. On the other hand, it was also vital for the progress of science. It needed to be recognized and understood, and it needed to happen soon, or chemistry would slow to a crawl.

1) THE NEAR MISSES

Now you may wonder why it took so long for humanity to notice oxygen. Today, we all know the air is made up of about one-fifth oxygen and four-fifths nitrogen, and we take that knowledge for granted. But the air is all around us, and it's easy to think of it as a single homogeneous thing, if you even bother to think of it at all. Besides, there weren't a whole lot of other phenomena to compare it with.

Still, there were some people throughout the centuries that seemed to suspect the truth, but they never quite managed to put it all together. I don't want to spend too much time on them, but I think it's worth noting that there was a dim awareness of oxygen even before it was discovered.

2nd Century BC: Philo of Byzantium observed that air is consumed during combustion. He suspended a glass vessel over a burning candle surrounded by water. As the candle burned and used up the oxygen, the water level inside the vessel began to rise to equalize the pressure. But he interpreted this loss as a transformation of air into fire, which then passed through the glass.

8th Century AD: A Chinese philosopher named Mao-Khoa speculated that the atmosphere is composed of two aspects. The Yin part (oxygen) was an impure portion which sustained combustion, while the pure Yang portion did not. I can’t find much information on Mao-Khoa, so you may want to take this with a grain of salt, but it sounds plausible that someone might try to apply Taoist philosophy to rationalize the dual nature of air.

1478-1519: Leonardo da Vinci observed that air is consumed during combustion and respiration. As he put it: "Where flame cannot live, no animal that draws breath can live."

1630: Jean Rey noticed that tin increases in weight when calcined (oxidized), and he speculated that the additional mass was absorbed from the air. He compared the infusion of tin by air to the moistening of sand by water.

ca. 1654: Following the invention of the vacuum pump, Robert Boyle conducted various experiments which proved that air is necessary for combustion. He also suggests that there is a “vital quintessence” in the air to explain air’s limited capacity to sustain flame and respiration.

1665: Robert Hooke proposes a theory of combustion in Micrographia. By his theory, air contained a a substance that could also be found in saltpeter (potassium nitrate), and a larger quantity of an inert substance.

Hooke seems to be on the right track, but this was only a theoretical model of the air. Until someone managed to isolate and characterize oxygen and/or nitrogen, this was all just an educated guess constructed to fit experimental results.

I say this because there would soon be a rival theory that explained the same facts with a completely opposite model....

1667: Johann Joachim Becher publishes Physica Subterranea, in which he proposes a new system of elements to replace the Aristotelian system. These elements included terra pinguis, which conferred oily, sulfurous and flammable properties to matter. In Becher’s model, flammable substances contained a great deal more terra pinguis than non-flammable substances, and as they burned, they released this terra pinguis, which was then absorbed into the air.

The terra pinguis concept was completely unreal, but it explained fire just as well as Hooke’s notion of air as a mixture of reactive and unreactive components.

1668: John Mayow publishes "De respiratione", an account of his nitroaerus experiments. His research demonstrated that combustion only depends upon a portion, which he names spiritus nitroaerus. Mayow also observed that antimony increases in weight when heated. He suggested that nitroaerus is present in saltpeter, specifically in the acid part (the nitrate ion of potassium nitrate). He went on to propose that all acids contain this nitroaerus, and all animals absorb nitroaerus when they breathe.

1678: Ole Borch prepares oxygen by heating saltpeter, but has no means to collect it or study it.

You may notice that there seems to be a lot of promise in the decomposition of potassium nitrate. Several chemists managed to produce oxygen by heating it, and John Mayow even gave it a name. This would seem to blow Becher’s terrra pinguis idea out of the water, except that no one was really investigating the oxygen as a discrete material. Gas chemistry was still getting started in the 17th Century, and a lot of these guys probably just thought oxygen was a vapor expelled by saltpeter. They may have thought it represented a principle that was also present in the air, but they didn’t necessarily think of it as a separate gas that was one of the ingredients of the air.

One of the problems of alchemy and early chemistry was that no one could seem to distinguish a substances composition from its properties. To them, a slice of cheese cake contained cheese, eggs, sugar and “deliciousness”. Qualities and ingredients were one and the same. That was what Becher was getting at with his terra pinguis idea. To him, it was silly to call fire an element, when the true element was the intangible quality that made fire possible. It didn’t really matter if terra pinguis was a physical substance or not. The point was that matter could absorb and expel through chemical reactions like fire.

In 1731, the German chemist Georg Ernst Stahl modified Becher's terra pinguis concept and renamed it phlogiston. So we often say the phlogiston theory was originated by Becher in 1667, although Stahl was the one who coined the term and popularized it several decades later.

I’ve been writing about the phlogiston theory for a while now, because the men who discovered hydrogen, nitrogen, chlorine, and manganese all subscribed to the theory, and so they explained their discoveries in that context. Hydrogen was thought to be a phlogiston-rich substance released from iron and other metals. Nitrogen was thought to be ordinary air saturated with as much phlogiston as it could possibly hold. Chlorine was thought to be hydrochloric acid stripped of is phlogiston content. And manganese, like all other metals, was thought to be a calx infused with enough phlogiston to give it metallic properties.

There were a number of problems with the phlogiston model, the most glaring being that metals were supposed to give up phlogiston when they were calcined, but they gained weight in the process. One feeble workaround for this was that phlogiston possessed “negative weight”. You might wonder why the scientific community would accept such a far-fetched defense, but phlogiston had a big upside. It answered a lot of questions about fire, combustion, and other chemical changes that depended upon the air. And it was so widely accepted that it brought the scientific community a lot closer together than they might have been otherwise. Thanks to phlogiston, they were all on the same page, even if it was the wrong one.

Nonetheless, a theory is only as valuable as it is useful. By the late 18th Century, it seems clear that scientists were more interested in propping up phlogiston theory than in seeking a more viable alternative. It may have served its purpose in the 1730′s, but by the 1770′s the scientists were serving the theory, when it should have been the other way around.

There were some skeptics out there. In 1756, the Russian chemist Mikhail Lomonosov heated metals in an airtight vessel and found that the total weight of the vessel and its contents remained constant. He denied the existence of phlogiston, since his metal samples could not have given up phlogiston since their weight was unchanged. However, he failed to publish his conclusions, and he failed to recognize that only a portion of the air would combine with metal.

So that was where things stood until oxygen was truly discovered. And to start us off on that topic...

2) CARL WILHELM SCHEELE

Aside from his role in the discovery of oxygen, Scheele is probably best known for his discovery of chlorine, and his research on manganese. But before that, he was working in a pharmacy in Upsala, Sweden. It was there that he prepared oxygen in 1771 and 1772. He did this by heating mercuric oxide, just as Priestly and Lavoisier would do in 1774.

But Scheele also obtained oxygen by heating other minerals, including silver carbonate, mercuric carbonate, potassium nitrate, and magnesium nitrate.

The common theme here is that these substances all contain oxygen, and they all decompose at relatively low temperatures. In the case of mercuric oxide, it’s just a matter of the mercury-oxygen bond being fairly weak.

2HgO → 2Hg + O2

A similar situation held true for silver carbonate and mercuric carbonate. First they decomposed into mercuric and silver oxide, and then they decomposed into oxygen and the free metal.

2Ag2CO3 → 2Ag2O + 2CO2 → 4Ag + O2 + CO2

2HgCO3 → 2HgO + 2CO2 → 2Hg + O2 + CO2

As for the nitrates, Scheele was just repeating what a lot of chemists had been doing for several decades. The nitrate ion is a good oxidizer, which means it can donate one of the oxygen atoms in its structure, thus becoming a nitrite ion.

2KNO3 → 2KNO2 + O2

I haven’t found an English translation of Scheele’s oxygen work, but I suspect the nitrate decomposition was what got him started. Gas chemistry was starting to become more important in the 1770′s, and he may have wanted to revisit the gas evolved from nitrates with a more modern viewpoint. This would have also led him to look for the same gas in other minerals that readily decompose on heating.

He also obtained oxygen by distilling a mixture of manganese dioxide and arsenic acid. I’m not really sure how that worked or what led him to think of it, but I’m suitably impressed.

I’ll go into greater detail with Priestley’s work because it was published in English, but I think it’s fair to say that Scheele’s work was at least as thorough and detailed as Priestley. More to the point, Scheele completed his research two years before Priestly even got started. So why doesn't Scheele get full credit?

For whatever reason, Scheele was slow in publishing his work. He compiled his research into a book entitled Chemical Observations and Experiments on Air and Fire. Maybe if he had know there were rival scientists out there, he might have been quicker about it, but as it was he didn’t send the book off to the publisher until 1775. . Maybe the book covered more material than just the oxygen work, or maybe it just took a really long time for him to write, or maybe he was too busy working on other projects (like chlorine and manganese) to find time for it. The point is that Scheele was dropping his manuscript in the mailbox right around the time Priestley's research was hitting the shelves.

To make matters worse, Scheele’s publisher took another two years to get the book printed. In 1776, Scheele wrote to his friend Torbern Bergman:

"I have thought for some time back, and I am now more than ever convinced, that the greater number of my laborious experiments on fire will be repeated, possibly in a somewhat different manner, by others, and that their work will be published sooner than my own, which is concerned also with air. It will then be said that my experiments are taken, it may be in a slightly altered form, from their writings. I have Swederus to thank for all this."

Swederus was Scheele's publisher. I really don't understand what went wrong. "Fire and Air" eventually saw the light of day, but not until 1777, long after Priestly and Lavoisier had staked their claims.

The other thing I find interesting was that Scheele was merely worried about another scientist repeating his experiments and getting the credit. Except it had already happened. Priestley's work had been available in 1775. There was no use blaming the publisher. Even if they could have done the job instantaneously, it still would have been too late.

I don't want to be too hard on the guy, but it feels like Scheele just didn't have a firm grasp of the importance of his discovery. He waited years to publish, his choice of publisher wasn't so hot, and he appears to have been completely unaware of what other scientists were up to. I don't know if this was a character deficiency or a one-time lapse in judgement. Ah well, he'll always have chlorine.

3) JOSEPH PRIESTLEY

Joesph Priestley was an English clergyman who pursued science as a hobby. In 1767 he accepted a pastorate at Mill Hill Chapel in Leeds, and the parsonage just so happened to be next door to the Jakes and Nell Brewery. This was extremely handy for Priestley because it meant he had easy access to lots and lots of........... carbon dioxide, generated by the fermentation process used to make beer.

As you may recall, Jan Baptiste van Helmont discovered carbon dioxide by studying the gas generated by breweries, and here we see Priestley carrying on that research. He soon found that if he saturated water with carbon dioxide, it gave it a pleasant taste. So yeah, he's the guy who lived right next door to a brewery, and all he did with it was invent club soda.

What Priestley soon realized was that some gases can be collected over water, like hydrogen and nitrogen, while others, like carbon dioxide, are soluble in water, so they have to be collected over other liquids like mercury. This meant that there must be different types of "airs".

Now, this is old news to us, because we've already covered Joseph Black's carbon dioxide research in 1755, and Cavendish's hydrogen research in 1766, and Rutherford's nitrogen isolation in 1772. But in the midst of all this, Priestley had independently reached the same conclusion.

In 1773, Priestley left his ministry at Leeds to accept a patronage under the Earl of Shelburne. He moved into the Earl's estate of Bowood House, near Calne. There, he began devoting more of his time to science, although he still managed to help found Unitarianism.

He eventually published his research as a six-volume text entitled Experiments and Observations on Different Kinds of Air. Volume 2 is the one we're interested in, and it came out in 1775. Among the gases Priestley discovered were:

"nitrous air" (nitrogen monoxide, NO),

"dephlogisticated nitrous air" (nitrogen dioxide, NO2).

"marine acid air" (hydrogen chloride, HCl, the stuff you add to water to get hydrochloric acid)

"alkaline air" (ammonia, NH3)

He also isolated carbon monoxide, but apparently didn't recognize it.

But the main event was oxygen, which he obtained by heating "mercurius calcinatus" (mercuric oxide) on August 1, 1774.

Priestley collected the gas that evolved over water, and he soon found that fuel would burn more brightly in this gas than it did in air. In keeping with the phlogiston theory, he named this gas "dephlogisticated air". In other words, he saw this gas as air that was utterly free of phlogiston. By this logic, things burned more readily in this gas because it was so starved for phlogiston that it was eager to receive more and become more like atmospheric air.

Now what gave Priestley the bright idea to try this in the first place? To answer that, we ought to start with his account of the discovery, published in Experiments and Observations on Different Kinds of Air. Specifically, the Second volume, Section III, page 29-62.

[Note: There appear to be multiple editions of "E&ODKA", which were abridged or reorganized into fewer volumes. If you're looking to read this for yourself, I'm referencing the 1775 edition of volume 2, which I found here: I'm not sure why the instance of google isn't in English, but the book itself is, so you should be all set.]

Like a number of scientists before him, Priestley had heated nitrates to obtain oxygen. At the time, this hadn't meant much to him, except that all of the gases he had liberated from minerals ("factitious airs") had been "highly noxious", but the gas he got from nitre was not. It was conspicuous, but he didn't investigate any further. Also, he had heated these minerals in a gun barrel, which became corroded from all the different chemicals it was exposed to. So there was a possibility that the gun barrel's corrosion had affected the results of his nitre experiment, and yet he had no way to repeat the experiment since his gun barrel was ruined.

What Priestley needed was a burning lens, which could focus sunlight onto a glass vessel of mercury. This way, he could suspend a mineral sample inside the vessel and aim focused sunlight onto it. Any liberated gases would displace the mercury, allowing Priestley to observe and measure them. Eventually, he obtained just such a lens, and he soon went to town heating minerals to see what gases he could find.

In short, I don't think Priestley was really looking for anything in particular from mercuric oxide. He just knew that some minerals released different gases on heating, and he wanted to explore this general phenomenon as thoroughly as possible.

Once Priestley started playing with his new gas, he found that a candle burned more brightly in it than under atmospheric air. The only other gas he had found that behaved even close to this way was "nitrous air" (nitrogen monoxide, which I'm going to call "NO" from here on out.) Priestley knew this new gas couldn't be NO, because he had only been able to obtain NO from minerals that had been made by adding nitrous acid to metals. Nitrous acid hadn't been used to prepare his HgO, so that was that.

Or was it? Priestley briefly suspected that his mercuric oxide was a bad batch. But he obtained a second sample from a more reliable source, and the results were the same. In October 1774 the Earl of Shelburne invited him along on a trip to Continental Europe, and in Paris he discussed his experiment with the French chemist Antoine Lavoisier, and he purchased an even finer sample of mercuric oxide from Lavoisier's supplier.

In November, Priestley returned to his lab and got the same result from the Parisian HgO as all the others. Furthermore, Priestley was able to obtain the same gas from a completely different mineral: red lead, also known as minium, or lead oxide, Pb3O4. Like mercuric oxide, minium decomposes at the relatively low temperature of 500°C.

There was still a link between this new gas and NO. Both would sustain a candle flame, but Priestley soon found an important distinction. Oxygen was virtually insoluble in water, but NO would react with water (and the small amount of oxygen dissolved within it) to produce nitrous acid.

4NO + O2 + 2H2O ---> 4HNO2

When Priestley collected his oxygen sample over water, he found that it still sustained a candle flame, where NO could not possibly do this.

But even though the oxygen could sustain combustion, Priestley never considered the possibility that it was breathable, as atmospheric air was. He was convinced that it had to be another toxic gas, like NO, even if it wasn't NO itself. Priestley later confessed that it was prejudice that had blinded him to the truth. Phlogiston theory didn't account for a gas like oxygen, so Priestley wasn't looking for such a gas, even when he had it right in front of him. He turned his attention to other experiments until March 1, 1775, when he finally decided to compare oxygen to atmospheric air.

Priestley had found some time ago that NO would react with atmospheric air to produce nitrogen dioxide, a red-brown gas. The nitrogen dioxide could then be dissolved into water, leaving about 80% of the atmospheric air sample behind. Today, we understand that atmospheric air contains roughly 20% oxygen, and NO reacts exclusively with this portion.

2 NO + O2 ---> 2 NO2

Priestley knew that if he combined NO and air in a 1:2 ratio, he could reduce the air's volume down to 80%, and render it unbreathable. If oxygen was breathable, then it ought to produce a similar result. In this, Priestley was only partially correct. The reaction did take place, and brown nitrogen dioxide was formed in the vessel, and the final volume of the oxygen sample was diminished. But to Priestley's surprise, the remaining portion of the sample still sustained a candle flame.

This can be explained through stoichiometry. As we see in the reaction above, it takes two molecules of NO to react with one molecule oxygen. Let's say Priestley was working with 100 volumes of sample for these experiments. It doesn't matter how much one "volume" is. We're only worrying about proportions.

So Priestley had once combined 100 volumes of air with 50 volumes of NO. Every two volumes of NO could react with one volume of oxygen. We know air is about 20% oxygen, so 100 volumes of air contains 20 volumes of oxygen. So it would take 40 volumes of NO to "kill" it all. And since Priestley used 50 volumes of NO, there would still be 10 volumes left over. Chemists call this an "excess", because he used more than what he absolutely needed to get the job done. Like when they beat up that printer at the end of "Office Space." As long as it breaks, no one cares exactly how many times you have to hit a printer with a bat to break it. Unless you're some sort of bat efficiency expert. Anyway, the 80 volumes of air left over would be left alone, because NO doesn't react with any of it.

When Priestley tested his oxygen sample, he was combining 100 volumes of pure oxygen with 50 volumes of NO. Fifty volumes of NO is enough to "kill" 25 volumes of oxygen. The reaction stops, leaving 75 volumes of sample behind, but for a different reason. In the air test, the reaction stopped because there was no more oxygen for NO to react with. This time, the reaction stopped because the NO was used up. Priestley thought he had used an excess of NO, but he wasn't even close. The sample had been diminished, but it was still pure oxygen, and it could still sustain a flame just as well as it had done before.

Now oxygen had Priestley's full attention. On March 8, 1775 he put a mouse in a vessel of oxygen to settle the matter once and for all. Had the vessel been full of air, it would have been enough to keep a mouse alive for fifteen minutes. But the vessel contained pure oxygen, and so the mouse lasted thirty minutes. And it didn't even die at the end. It just passed out, and when Priestley took it over to the fire, it woke up again. Priestley recorded his astonishment in his book:

"By this I was confirmed in my conclusion, that the air extracted from mercurius calcinatus, &c. was at least as good as common air ; but I did not certainly conclude that it was any better; because, though one mouse would live only a quarter of an hour in a given quantity of air, I knew it was not impossible but that another mouse might have lived in it half an hour; so little accuracy is there in this method of ascertaining the goodness of air:"

Now that he understood what he was dealing with, Priestley began testing the oxygen to see how it stacked up to atmospheric air. Since a mouse had passed out in a sample, he assumed that it had been rendered unbreathable, so he treated it with NO to see what was left. To his surprise, the NO still reacted with the sample, and reduced its volume down to about 78% or so. The numbers don't quite line up with the previous experiment because the mouse had exhaled a bunch of carbon dioxide into the sample gas. Still, the sample was nearly pure oxygen, and Priestley still wasn't using enough NO to quench it.

Priestley slept on this, then decided to try it again. The NO still reacted with the gas. Later, he attempted to measure just how much NO was needed to completely quench the oxygen sample, and came to the conclusion that it would always take five times more NO than was needed for atmospheric air. Thus, oxygen was five times "better" than "common air".

Priestley went on to examine other minerals to see if they might release this same "dephlogisticated air" upon heating, but he was now satisfied that it was fit for respiration. So much so, that he tried breathing it himself. From Section V, p.102:

"My reader will not wonder that, after having ascertained the superior goodness of dephlogisticated air by mice living in it, and the other tests above mentioned, I should have the curiosity to taste it myself. I have gratified that curiosity by breathing it, drawing it through a glass syphon, and by this means I reduced a large jar full of it to the standard of common air. The feeling of it to my lungs was not sensibly different from that of common air, but I fancied that my breast felt peculiarly light and easy for some time afterwards. Who can tell but that, in time, this pure air may become a fashionable article in luxury? Hitherto only two mice and myself have had the privilege of breathing it."

Priestley's words were somewhat prophetic, except that breathing pure oxygen became more of a medical treatment than a recreational pastime. Well, I guess Michael Jackson's oxygen chamber was a luxury, but he was trying to prolong his life, not have fun. On the other hand, I understand that Homer Simpson sleeps nude in an oxygen tent, which gives him sexual powers.

[Above: Homer Simpson sleeps nude in an oxygen tent, which he believes gives him sexual powers.]

4) ANTOINE LAVOISIER

Of the three men credited with discovering oxygen, Lavoisier's claim is by far the weakest. Still, he gets a boost because of all three chemists, he was the only one who truly understood what oxygen actually was.

While Priestly was still preaching in Leeds, and Scheele was plugging away at his manganese research, Antoine Lavoisier was turning his attention to the chemistry of combustion. His experiments indicated that the phlogiston theory didn't hold up under scrutiny.

Now we might well wonder why it took this long for someone to speak up about this, but as Priestly had observed, prejudices and preconceived notions can allow a good scientist to overlook the truth. And while laboratory procedure had improved dramatically over the past century, chemists of the 18th Century still had a ways to go as far as making accurate measurements. Priestly had been careful about measuring the volumes of his gas samples, but he wasn't necessarily weighing everything. If he had, he might have weighed the mercuric oxide he heated to produce his "dephlogisticated air", and compared it to the metallic mercury that was left behind.

To be fair, it's a lot easier to weigh things in a laboratory in 2015 than it was in 1774. An electronic balance is a piece of cake. You put your sample on and wait for the readout to settle down. A beam balance is a pain in the butt. Priestly may not have had the means to weigh gases to any degree of accuracy, and he may not have seen the point. Volume certainly seemed more important when it came to gases, and the world was just beginning to learn that there were more than one kind of gas. It wasn't obvious that they should all have different densities. Besides, Priestly was an ascetic who pursued chemistry as a hobby. There were surely limits on how much time and money he was willing to invest in an investigation.

On the other hand, scrutiny was Lavoisier's specialty, and he could afford to apply it. Lavoisier is often known as the "father of modern chemistry", but he doesn't have a lot of important chemical discoveries or innovations to back that up. His great strength was in the rigorousness of his experiments, and in his insights as a theoretician. Lavoisier strove to make accurate measurements, so when he repeated the experiments of others, he could see things that the others may have missed. He could then take those refined observations and get to the bottom of what was going on.

On November 1, 1772, Lavoisier wrote a sealed letter to stake his claim on his discovery: That sulfur and phosphorus gain weight on combustion.

"About eight days ago I discovered that sulfur in burning, instead of losing weight, on the contrary acquires weight; that is to say, from a pound of sulfur it is possible to obtain more than a pound of sulfuric acid, allowance being made for the moisture of the air. The same is the case also with phosphorus. This augmentation of weight arises from a prodigious quantity of air that is fixed during the combustion,and which combines with the vapours.

"This discovery, which I have confirmed by experiments that I consider to be decisive, led me to think that the result which is observed in the combustion of sulfur and phosphorus might probably take place in regard to all substances which acquire weight by combustion and calcination, and I was persuaded that the augmentation of weight in metallic calces is due to the same cause.

"Experience has completely confirmed my conjectures. In reducing litharge in closed vessels by means of Hale's apparatus, I observed that at the moment when the calx became converted into metal, a considerable quantity of air was disengaged, and that the volume of this air was a thousand time greater than that of the litharge. This discovery appearing to me one of the most interesting of those that have been made since Stahl, I considered it necessary to establish my claim to having made it by placing this deposition in the hands of the Secretary of the Academie to remain private until the time when I publish my experiments.”

LAVOISIER. Paris November 1, 1772

Now, the mere fact that substances gain weight when heated in closed vessels was nothing new. Robert Boyle had once written a paper suggesting that the weight gain was caused by the "principle" of fire penetrating the glass of the vessel. This sounds pretty far-fetched, but you have to remember that Boyle lived in the dying days of alchemy. Remember what I said about cheesecake? Sugar and eggs couldn’t pass through glass, but for all anyone knew “deliciousness” probably could.

Lavoisier wasn't buying it, and he devised an experiment in which he calcinated tin and lead in sealed vessels of different sizes. In the smaller vessels, the calcination process stopped after a certain amount of time, because there was less air present to sustain the reaction. In the larger vessels, which contained more air, the calcination proceeded further. In each reaction, the total weight of the vessel and its contents was unchanged. If the reaction had been drawing matter from beyond the vessel, rather than from the air inside, then the size of the vessel should have made no difference.

Furthermore, when the vessels were unsealed after the experiment, air could be heard rushing in, and the weight of the vessel was increased accordingly. The larger vessels sucked in more air, and the smaller vessels sucked in less air.

Lavoisier then realized that only a portion of the air was being consumed during the calcination of metal. He wasn't sure of the specifics, but he was willing to conclude that the air must be a mixture of gases, or possibly a chemical compound. He announced this conclusion to the French Academy of Sciences in November 1774.

And this is where the waters get muddy, because Lavoisier had met with Joseph Priestly in October of 1774. Remember? Priestly discussed his mercuric oxide experiments with Lavoisier, and he even acquired a sample of mercuric oxide from Lavoisier's chemical supplier. Later in November, Lavoisier began experimenting with mercuric oxide himself. Coincidence? Maybe...

Oh, and guess who else was in contact with Lavoisier around this time? That's right, Carl Wilhelm Scheele, the other discoverer of oxygen. On September 30, 1774, he sent Lavoisier a letter about his oxygen research.

Scheele's business was simple. He knew Lavoisier had access to better equipment, and he was hoping Lavoisier might use it to heat silver carbonate to produce oxygen gas.

Lavoisier never acknowledged Scheele’s letter, but we know he received it on October 15, 1774, because it was found among his effects after his death. And then a month later, he was pursuing a similar line of research. Coincidence? Maybe...

The problem here is that Priestly and Scheele deserve co-credit for discovering oxygen because they did so independently of each other, around the same time. Lavoisier would merit co-discoverer credit under these same conditions, except he was in contact with both Priestly and Scheele right before he published his own research. If he was merely following their lead, then he doesn't deserve any credit.

But Lavoisier failed to acknowledge Priestly or Scheele when he published his work. Was this an intentional snub? Was he trying to claim full credit and hoping no one would call him on it? Or did he truly fail to notice the connection between their experiments and his own? Or did he just forget his visit from Priestly altogether? He may have received Scheele's letter, but did he ever sit down and read it?

[”And that’s the story of how I discovered oxygen all by myself, with no outside help whatsoever! Er, you believe me, right, Marie? Right?”]

The general consensus is that Lavoisier was pulling some shenanigans. Even if he was truly innocent, he still comes across as a bit of a jerk in all this. The thing is, even if Lavoisier was cribbing from Priestley and Scheele, it only took him a few weeks to connect the dots. Meanwhile Priestley went home to England and was so mystified by oxygen that he set it aside for four months to work on other things. And even after he found that it was five times more breathable than air, he still couldn't quite put it together.

This is what I meant before when I said oxygen had to be discovered for chemistry was to progress much further. Lavoisier had spent two years questioning the phlogiston theory before he met Priestley. That alone gave him a better understanding of the decomposition of mercuric oxide. Priestly and Scheele, however, were die-hard phlogistonists. The great irony is that the two of them discovered oxygen, yet they couldn't accept it for what it was. Lavoisier wasn't shackled to the phlogiston tradition, and so he could make greater progress in a shorter amount of time.

Regardless of where Lavoisier got the idea to play with mercuric oxide, the point was that it represented the reverse of the calcination experiments he had done. He had proven that a portion of air was absorbed by metals during calcination. So if he took a calx and heated it to decomposition, he could then release that same portion of the air and study it.

The results of Lavoisier's work presented in what is now called the "Easter Memoir", because he read it to the French Academy on Easter Sunday, April 16, 1775. It's proper title was "Memoir on the Nature of the Principle Which Combines with Metals During Calcination."

As Lavoisier explained, the unique thing about mercury's calx was that it could be decomposed without using charcoal to reduce it. Still just to eliminate uncertainty, he went ahead and reduced it along with some charcoal, just to prove that it was indeed a calx. With that out of the way, he isolated the oxygen, and determined the same properties that Priestly had found at about the same time.

But the important question was this: Where was the phlogiston? Lavoisier had shown that mercuric calx could be broken down into metallic mercury, a gas, and nothing else. By the phlogiston theory, metals were thought to be dephlogisticated calxes, so no phlogiston there. The gas couldn't contain phlogiston because it supported combustion even better than atmospheric air. That's why Priestly had named it "dephlogisticated air".

So if you can combine dephlogisticated air and dephlogisticated calx and produce a calx, then phlogiston apparently plays no role in any of it. One month later, Lavoisier's memoir was published in the Journal de Physique's May 1775 issue. Not long after that, Priestly read about it, and he realized that Lavoisier had goofed.

Like Priestly, Lavoisier had tested his oxygen sample by mixing it with NO gas in a 2:1 ratio. Again, this ratio would be sufficient to render atmospheric air unsuitable for respiration. Lavoisier's oversight was that he only performed this test to see if a reaction took place, and to measure how much gas was left over. He did not check the remaining oxygen to see if it was still capable of supporting a flame. Consequently, Lavoisier didn't realize just how different from atmospheric air oxygen really was.

But Priestly knew, because he had made the same mistake once. This led to a rather awkward exchange where Priestly wrote about Lavoisier's work to point out his error, but dancing around the topic of their meeting in October 1774, and Lavoisier's failure to mention Priestly in his Easter Memoir. Lavoisier wrote a follow-up paper in 1776, and though he made sure to acknowledge Priestley's contributions this time, he still maintained that he alone had been the one with the correct conclusion.

Nonetheless, the Easter Memoir went through some revisions before the French Academy finally published it in 1778. By then, Lavoisier had taken Priestley's advice to heart, and he soon realized that the gas he had obtained from mercuric oxide was similar to common air, or better than common air. As he wrote:

"This seems to prove that the principle which unites with metals when they are calcined and causes them to increase in weight is nothing else than the purest part of the air which surrounds us, which we breathe, and which during calcination passes from a condition of expansibility to that of solidity; if it is obtained in the form of fixed air from metallic reductions in which charcoal is employed, this is due to the combination of the charcoal with the pure part of the air, and it is very probable that all metallic calces would give, like that of mercury, only this eminently respirable air, if one could reduce them all without addition, as precipitated mercury is reduced... "

This only cemented Lavoisier's argument against the phlogiston theory. The same portion of the air that went into metals during calcination was also being released during reduction. The reaction was just too simple for phlogiston to be involved. Whatever role charcoal played in the reaction, it must have been an intermediary between the air and the metal. And since the reaction could be easily explained without phlogiston, what good was the theory?

Lavoisier named the gas "vital air" for a time, but in 1777 he gave it a new name: oxygène. This was in reference to Lavoisier's belief that oxygen was a component of all acids. The Greek word ὀξύς (oxys) means "sharp", and was meant to refer to the sharp or sour taste of acids, while -γενής (-genēs) means "producer" or "begetter". The acid connection had some legitimacy, since nitric, sulfuric, perchloric, phosphoric, and acetic acids all had oxygen in them. But hydrochloric acid doesn't contain oxygen, thus disproving Lavoisier's theory. Unfortunately, the truth about hydrochloric acid didn't come to light for a few dozen years, and by then the name "oxygen" had become too entrenched to change.

On the other hand, oxygen is such an important element that the meaning behind its name is virtually irrelevant. "Oxygen", "oxide", "oxidation", and "oxy" all refer to the element itself now, and not the sharpness or acidity it was named for.

Lavoisier’s credit for co-discovering oxygen would be dubious if we were talking about any other element. But oxygen was more than just a cool new gas. It overturned the last vestige of alchemical thought, and paved the way for the Chemical Revolution. For the first time ever, human beings understood what fire was. No more vague analogies or an abstract models. Oxygen was real, and fire was what really happened combined with something very quickly. Priestly didn’t figure that out. Scheele didn’t figure that out. Lavoisier did.

Sure, he was kind of a butthole about the whole thing, but the important thing is that I don’t have to type the word “dephlogisticated” anymore when I write about this stuff. Lavoisier can claim to have discovered the moon for all I care.

[”All the other scientists were like ‘Duh, duh what am big glow thing in sky?’ But then I climbed in my rocketship and proved it was the moon--! Honey? Honey? Are you sighing because you’re so impressed with my exploits?”]

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