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The Discovery of Carbon Dioxide and Oxygen

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Vincent Valentine, MD
University of Alabama Birmingham
Birmingham, AL, USA

In the Editor's Corner of the last two issues, we moved 300+ years into the 18th Century as Natural Philosophy evolved during the Renaissance. The Great Thinkers Copernicus, Kepler, Galilei and Newton challenged Aristotelian and Ptolemaic thoughts as well as the Catholic Church compelling us to gaze beyond the outer limits and into the heavens. From Newton to Werner with truly an international effort, though a bit Eurocentric, with many others not mentioned; the foundations of science and chemistry were laid as earlier scientists look away from the sky to describe the laws of nature and study the earth and its crust. This spawned the evolution of science, particularly chemistry and its separation from alchemy. In this issue, we will be further enlightened by the great discoveries and the importance of such discoveries for patients of the ISHLT from leading figures from Europe, especially France and England as we peer inward and down to the elements. The focus here is carbon dioxide and oxygen.

From the August Issue, Georg Stahl of Germany and his "fiery substance" was mentioned. He called this "imponderable substance," phlogiston believed to be released during combustion, respiration and calcination. When charcoal burned, it lost its phlogiston - a mysterious substance, weightless and imperceptible - escaped into the air, leaving behind "dephlogisticated" ashes. Before moving on, let's look back to Stephen Hales who shaped the evolution of French Chemistry. This English cleric and natural philosopher was curious about the fumes produced when various substances were heated in a flask. Initially, the only gas known in the 1700s was atmospheric gas. When Hales and others encountered foul-smelling fumes, the atmospheric air was thought to be polluted. Hales observed the release of fumes from burning plants. His theory was that air had somehow fixed in a plant and that burning set it free, thus air produced from burning plants was "fixed air." This theory was expanded to solid substances. What became important to Hales was the amount of fixed air bound up in various substances. He brought up this first step of experimental study on specific airs, a subject that later became known as pneumatic chemistry. Speaking of pneumatic, Hales was the first to measure blood pressure and invent a ventilator with a large bellows to improve air quality. But again, the important point was weighing the "fixed air" released from a reaction.

Now think about it. It seems quite troubling today that at times we cannot/do not weigh patients, their urine, their feces and be as precise as we should, especially at critical times when hospitalized in critical care units. But back to the matter at hand in the early 1700s, investigators were uninterested in the air or gas produced in a chemical reaction, but rather the rough amounts released from burning for example. Now meet Joe Black.

Joseph Black held chairs in anatomy, botany, chemistry and medicine in the Universities at Glasgow and Edinburgh, Scotland. Black was studying the importance of magnesia alba (magnesium carbonate) as an antacid. He quoted, "it mildly loosens the bowels" especially after overeating the wrong things. His thesis, De humore acido a cibis orto et magnesia alba (of the acid humor produced by food and of magnesia alba) was printed in June 1754. Black observed that the exact same weight loss occurred in two different experiments with magnesia alba. In one, he added an acid to this substance, air was given off and a residue remained, residue1 (R1). In the other, he heated it. An air was released and a different residue remained, residue2 (R2). Again, magnesia alba lost the same amount of weight in both instances. After Black observed this and concluded that the air released in both cases fixed in the magnesia alba was the same. Therefore, he examined the properties of this fixed air. He confirmed that this air was the same, but different from ordinary air. For example, you put a lit candle in magnesia alba air, it was immediately extinguished. One of his students showed that an animal forced to breathe this air would die. Black concluded that this was a different gas, not ordinary air, with its own properties. Black appropriated Hales' generic, "fixed air" for this specific new gas, today known as carbon dioxide. From this point on, chemists used "fixed air" to identify Black's new gas. This revolutionized the idea that there may be other gases to be discovered. Also, Black thinking outside the box made the following novel rationalization:

  1. Magnesium alba + acid = R1 + "fixed air"
  2. Magnesium alba + heat = R2 + "fixed air"

Black applied algebra to these chemical equations by subtracting equation two from equation one yielding: acid - heat = R1 - R2. Because he believed heat did not weigh anything, he ignored it. This leaves us with: acid = R1 - R2. By adding R2 to both sides of the equation, Black surmised that acid + R2 would result in R1. He proceeded with this experiment and found by adding acid to R2 did indeed resulted in R1. All of this was accomplished by his astute attention to the weights. By the late 18th century, weighing things was very important.

Now we have the English clergyman, Joseph Priestly, a very successful gas chemist interested in the chemistry of combustion. Priestly was a believer of the phlogiston theory of combustion and applied it to explain his experiments. By the way, he was intrigued by Benjamin Franklin's experiments with electricity when Franklin visited England. Priestly wanted to understand how charcoal burns to ash, how metal changes to rust and how humans survive by breathing air. From the phlogiston theory, rusting of metals resulted from the loss of phlogiston from the metal. Then adding phlogiston back to the calx by reducing the calx or heating it the metal would return. The source of phlogiston was the problem. Phlogiston might come from charcoal burned to reduce the calx back to metal. With the invention of the large burning lens or magnifying glass, Priestly used such a lens to heat mercury calx and reduce it back to mercury metal. He was astonished when he was able to reduce the mercury calx back to a metal by superheating the calx with the sunrays from a lens without phlogiston. Then Priestly noticed that a gas was produced which was no surprise to him, but it was the nature of this gas. The most obvious property of this gas was that it readily supported combustion. Place a candle under a jar with it, the candle burned brightly. Also, this gas sustained a mouse for a long time under a jar. Eventually, the candle and mouse would die. Priestly, called this gas dephlogisticated air. Air with no phlogiston in it, therefore readily absorbs phlogiston.

Now we have, Antoine Lavoisier, a French lawyer, tax collector and chemist who invited Priestly to visit his private laboratory in Paris in 1774. Lavoisier had heard about Priestly's puzzling results. Lavoisier today, is considered the Father of Chemistry. Lavoisier had previously reduced mercury calx to mercury without charcoal. Lavoisier, after spending time with Priestly and his puzzling results, repeated the experiment with particular attention to the gas that strongly supported combustion. Lavoisier had incorporated Black's focus on measuring weights. He had been pondering over the question why metals gained weight when they rusted. With that, he began to question the phlogiston theory to explain the process of calcination and rusting. He noted the inconsistency of this theory simply based on the idea of metals gaining weight as they rusted. Lavoisier began to suggest that metals were fixing air as they rusted. But how can they do so and lose phlogiston at the same time. Stahl's Phlogiston theory did not support Lavoisier's new line of reasoning. Lavoisier had also observed that when one burns phosphorus a vapor was released. If one collects this vapor and condenses it, the result is an acid. The weight of the acid is always greater than the weight of the consumed phosphorus. Lavoisier rationalized that phosphorus was fixing air into itself. It was possible that the weight was coming from the water vapor in the air. He repeated this experiment. This time he poured out the acid, then filled beaker with water to the same level where the acid had been. He weighed the water. This weighed less than the acid. He subtracted the weight of the water from the weight of the condensed acid. Now he had the weight of the condensed acid without the condensed water vapor. This weight was still more than the original phosphorus. Rationale - phosphorus was indeed fixing air when it burned. When Priestly, explained his findings to Lavoisier, Lavoisier was prepared for the notion that combustion involved the fixing of air into the substances that burned. After Priestly and Lavoisier discussed these points, Priestly repeated his experiments by reducing the mercury calx back to mercury with a burning lens and a charcoal fire. This time he paid closer attention to the gases produced. The resulting gases from these two different experiments were different. The gas produced from reducing the mercury calx back to mercury with the burning lens supported combustion. But, the gas produced from reducing the mercury calx with the charcoal fire did not support combustion. The candle in a jar with this gas when out immediately. The gas from the lens experiment was insoluble in water. The gas from the charcoal experiment easily dissolved in water. Lavoisier had known that the gas from the charcoal experiment was Black's "fixed air." He concluded that the air from using the lens was "pure" common air, because like common air, it supported combustion, but better. Lavoisier presented this to the French Academy in the Spring 1775 - when metals like mercury rust to form a calx, their gain in weight is due to the addition to the metal of the purest part of the air we breathe. At the same time in England, Priestly had figured out a way to make clear that the gas produced with a lens was a new gas, not common air. When Lavoisier reported his findings to the Academy, Priestly felt insulted. This gas was not named, and he believed it to be a new gas. After all it was Priestly who first observed that the gas from the lens experiment was not pure common air, but a new gas - yet it was Lavoisier who named the new gas oxygen. He named it in 1777 because Lavoisier thought of it as the cause of acidity - oxygen - acid maker. In retrospect, this was an error, this name should have been given to hydrogen, the real acid maker.

The real advance where Lavoisier truly deserves credit is with the idea that matter can neither be created nor destroyed, but can be transformed from one kind into another which is known today as the law of conservation of matter. For bringing this law into the chemistry, he has been revered as the Father of Modern Chemistry. He dispensed with the imponderable substance of phlogiston by rationalizing that combustion was the fixing of oxygen and not the release of phlogiston. Still, a persistent problem remained at this time, which was the belief that heat was a weightless substance. Lavoisier believed that heat was a weightless element that combined with air.

This is an example of change in scientific rational thought that takes time with many trials and tribulations repeated from a variety scientists and reasonable thinkers. Major changes often do not occur suddenly from a single person's insight where everything becomes clear. In the quest for the truth, it can be messy and never easy. Breakthroughs occur after many investigators contribute to a complex sequence of events over time that eventually result in a consensus which for that matter is no different than today. Thus we have oxygen and carbon dioxide, its applicability today in life, to the ISHLT and for the world is obvious. ■

Disclosure Statement: The author has no conflicts of interest to disclose.

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