Joseph Priestly and the Discovery of Oxygen
Many chemical discoveries are made because some vey smart people became extraordinarily curious about the world around them.
And there is one thing in the world that is around us all the time, every minute of the day and night. - a substanue has surrounded every human being on the planet, for almost every minute of his or her life, sine the human rare began.
What is that substance? Air, of course. Yet it wasn't until a few hundred years ago that one man -- a priest, by profession, who had never taken a single chemistry course in his life -- became curious enough to discover the chemical nature of air and its importance to living things.
To become a chemist today requires alot of school. Most chemists go to college for 7 years or more, earning a bachelor's degree, master's degree, and a Ph.D. Then, with degree in hand, the young chemist goes to work am a research scientist for a company, or perhaps as a professor at a university.
But when Priestly was born over 250 years ago, amateur scientists routinely made many important chemical discoveries. For these men and women, chemistry was a hobby, not their profession.
If you were interested inchemistry, you didn't get a job 0s a chemist mra college degree in ubernistry. You didn't even study it in high school. You bought the apparatus you need -glassware and chemicals, mainly - conducted experiments, and reposed your results in scientific journals or by writing and publishing books.
Such was the case with Joseph Priestly. He was born in England in 1733, one of five children, the son of a clothes maker who earned a modest wage. His family were strict Calvinists, which meant they were very religious and attended church regularly.
Young Joseph liked school and war, a good student. He learned to speak and read Hebrew, Arabic, French, Italian, German, and several other languages.
He also studies physics, but chemistry was not widely taught in schools at -the time. Joseph was self-taught in chemistry, mainly through reading and, later on, conducting his own experiments.
Joseph, like his farnily, was religious and God-fearing. But he did not agree with the strict teachings of the Calvinist church, and instead became a Unitarian.
The Unitarians had a much more liberal church. They did not believe that Jesus Christ was the son of God. Their radical beliefs made Unitarians unpopular with the mainstream church.
There is a common misconception that scientists are not religious and don't believe in God. The truth is, some scientists are religious; and some aren't. Some believe in God; and some are atheists (meaning they do not believe in God).
Joseph decided to devote his life to the two pursuits that were most important to him, and they were science and religion. Since there were not many professional chemists in the 18th century, but there were professional pastors, Priestly becarne a minister in the Unitarian church.
Priestly was not a rich man, and he had a wife. and three children to support. He had a speech impediment - he stuttered - and did not do well as a pastor at first. Later he gained confidence as a minister and teacher. People overlooked his speech impediment because he could present interesting ideas in a clear, entertaining fashion.
Being a zninister was how Priestly earned his living. Science experiments were his hobby. He was a prolife author who wrote many books on both science aud religion. But even though he was a hobbyist, Priestly is today recognized as one of the most accomplished chemists of all time.
One of his first jobs was pastor of a Unitarian Church in Leeds. As it happens, the minister's house be was allowed to live in was located next door to a brewery. No doubt the smell of beer being brewed was strong and present all the time. Mary people enjoy this smell, which is caused by yeast fermenting grain, and is reminiscent of bread being baked.
Priestly knew that the brewing process generated large volumes of carbon dioxide. He arranged with the brewery to use their waste gases for his experiments.
Priestly put trays of water in the brewing vats. The water absorbed some of the carbon dioxide, making it bubbly. But it had a bad taste - probably from the yeast.
So Priestly made his own carbon dioxide in his laboratory, by mixing chalk with sulfuric acid. A glass tube ran from the beaker of chalk and acid into another beaker of water. As the carbon dioxide was produced, it ran through the tube and bubbled into the water.
This bubbly water was clean-tasting and pleasant to drink. In fact, it was slightly tart quite refreshing. Does it sound familiar to you? Priestly had invented carbonated water, also known as seltzer or club soda. The Royal Society awarded the Copley medal to Priestly in 1773 for this invention, which also got him elected to the French academy of Sciences in 1772.
But Priestly wanted to do more than create a refreshing drink for a hot day, He wanted to study carbon dioxide and other gases - and their properties - more closely.
For instance, he observed that carbon dioxide was heavier than air. If you pump carbon dioxide into a closed container, it sinks to the bottom. He also noticed that carbon dioxide put out flarnes. And of course, it. was soluble in water.
Like carbon dioxide, a large number of other gases are also soluble in water. This created a problem for Priestly. How could he study the gases without having them dissolve?
He solved this problem by building a piece of laboratory equipment he called the pneumatic trough. This cleverly designed apparatus consisted of a layer of mercury in a sealed glass vessel.
Mercury is a metal that exists as a dense liquid at room temperature. Because mercury is so much denser than water, gases do not dissolve in mercury as readily as they do in water. Also, because mercury is so dense, solids float on its surface.
Into his pneurnatic trough Priestly placed different materials. The solids floated on top of the layer of mercury.
He then used a lens, which he called a burning lens, to focus the rays of the sun on the materials. Perhaps you have used a magnifying to focus sunlight on a piece of paper or a dry brown leaf long enough to make it smolder and burn.
The intensified sunlight heated the solids, causing them to give off gases. The gases generated by the heat naturally were trapped within the space above the liquid mercury in the sealed vessel.
Using the pneumatic trough and burning lens, Priestly collected and studied numerous gases. These include nitrous oxide, ammonia, sulfur dioxide, hydrogen chloride, hydrogen sulfide, silicofluoride, and carbon monoxide. (Carbon monoxide is a poison gas found in automobile exhaust fumes, which is why you should never leave a car running in a closed garage.)
Paying for equipment and chemicals was expensive, and Priestly did not earn a lot of money as a minister. But then he had a lucky break.
William Petty, who was Earl of Shelburne and lived in a lavish estate at Calme in Wiltshire, invited Priestly to become his personal librarian at two and a half times the salary he earned as a minister. The work was light, giving Priestly plenty of time to continue his experiments.
(Such an arrangement is not as unusual as it may sound. In 18th century England; it was common for rich private citizens to subsidize science, just as artists and musicians were subsidized in Germany other nations. Thruoug this system of patronage, many great works of science and art were created that might not otherwise have been made - the compositions of Mozart and Hayden, for example.)
Unfortunately, Priestly's writings discussed religion as well as science, and his unconventional religious views - Priestly said there was no spirit or soul apart from matter, and that God was a single entity rather than father, son, and holy ghost - were unpopular. One of his books was officially burned in England in 1785.
The Earl of Shelburne had political ambitions, and he was afraid that his close association with Priestly would hurt his chances of getting elected to office. They ended their relationship on amiable tenns.
Priestly moved to Birmingham England with his wife and children, but continued to have troubles because of his outspoken and unconventional religious beliefs. The family sailed to the United States where Priestly befriended and was much admired by Thomas Jefferson. He lived the last ten years of his life writing and experimenting in peace on a farm in Pennsylvania, where he died in 1804 at the age of 71.
Light a match and hold it to a piece of paper; the paper burns. Now hold the match to a piece of metal. The metal does not burn, Why?
In Priestly's day, chemists believe that flammable materials -- matter that could catch fire and burn -- contained an invisible substance called phlogiston.
A material that lacked phlogiston, such as a. rock or piece of metal, would not burn. A material that contained phogiston, such as wood or paper, burned readily.
The belief was that combustion drew the phlogiston out of the material as it burned. Once all the phlogiston was drawn out of the material during combustion, it could not burn any longer. That was why ashes in a fireplace would not burn further; all the phlogiston they had contained was used up.
Ironically, while Pnistely's most important chemical discovery helped proved the phlogiston theory to be false, Priestly himself continued to believe in phlogiston for the rest of his life -- long after the rest of the scientific community did not.
Remember the pneumatic trough? Priestly's most important discovery came when he placed in it a brick-red compound called calc. Calx is formed by heating mercury. And Priestly used plenty of mercury in his experiments.
The chemical name for ca1x is mercuric oxide. It consists of an atom of mercury attached to an atom of oxygen.
The bond between oxygen and other atoms is often easily broken. When Priestly focused sunlight on calx with his burning lens, it broke that bond.
The mercury seperated from the oxygen into shining silvery globules. The oxygen gas collected above the mercury in the rest of the vessel.
Priestly performed a number of experiments to determine the properties of this gas. Burning embers thrust into the container would burst into flame and bum vigorously. The gas supported combustion much more aggressively than ordinary air. Why?
Priestly reasoned that somehow the gas he had discovered drew up phlogiston from the flammable material. This gas or "air" would not have the capacity to absorb phlogiston unless its own phlogiston content was low or nonexistent -- in the same way a sponge that is dry absorbs water readily, while a sponge that is soaked cannot sop up more water.
Therefore he called the gas "dephlogisticated air." French chemist Antoine-Laurent Lavosier who duplicated Priestly's experiments and tried to take credit for them, named the gas oxygen, from Greek words meaning "acid producer."
(Lavosier mistakenly believed that all acids contain oxygen. We now know they do not. Hydrochloric acid, for instance, contains one molecule of hydrogen attached to one molecule of chlorine.)
Further, Priestly found that mice placed into a container of oxygen became friskier. He breathed the pure oxygen and found it made him light-headed. He imagined that breathing oxygen to get this light, lively feeling might become a small thrill in which people might indulge. We know that today unwary people inhale certain vapors to get such a "high."
Recall that Priestly also experimented with carbon dioxide and had used it to make soda water. He know placed his nice inside a container filled with carbon dioxide instead of oxygen. Instead of becoming livelier, they suffocated. Apparently oxygen could support animal life, while carbon dioxide could not.
Would plants also die in a carbon dioxide atmosphere? Priestly placed a plant inside a vessel filled with carbon dioxide. But the plant did not die. It lived and flourished. Even more surprising, when Priestly put mice in this vessel after the plant had been there a. few months, they too thrived!
What was happening? Apparently the plant replenished the air with the oxygen the nice needed to live. Priestly had discovered nothing less than the cycle that sustains all life on Earth.
In this cycle, animals breathe in oxygen and expel carbon dioxide. Plants, through a process we call photosynthesis, breathe in this carbon dioxide and exhale oxygen. Priestly was the first scientist to record this observation in his writings.
"The injury which is continually done to the atmosphere by the respiration of such a large number of animals is in part repaired by the vegetable creation," Priestly wrote. This balance keeps the atmosphere breathable. If there were no plants, the atmosphere would eventually turn to pure carbon dioxide, ond you and I would have no breathable air left. We would die.
Priestly's discoveries, most of which revolved around oxygen and other gases, not only contributed to our understanding of chemistry but also had many practical applications in medicine, food, and manufacturing.
Oxygen is of course his most important discovery. There are over a hundred elements on the Earth, and oxygen is the most common. Half of the atoms on Earth are oxygen. Air is a mixture of gases, one-fifth oxygen and most of the rest nitrogen.
Oxygen has many uses, and the most important of these may be health care. Once Priestly demonstrated that air contained oxygen, and that oxygen was necessary for life, other scientists found ways to make and purify oxygen for medical uses.
Perhaps you have an older relative who is "on oxygen. "Often as people get older, they have difficulty breathing. They can't get enough of the oxygen their body needs from breathing the air. The doctor may recommend that they breathe pure oxygen several tinnes during the day to help them get all the oxygen they need. The medical oxygen is stored in metal cylinders and kept by their bedside.
Oxygen is also used in the manufacturing of cheiricals and metals. To make many metals, ore is heated at high temperatures using large burners. Oxygen is injected into the burner fuel to make it burn hotter.
How do we make oxygen for medical, industrial, and other applications?
There are several different methods. The most common is cryogenic separation. Cryogenic means "at low temperatures."
As Priestly and others discovered, air is a mixture of several gases, but it is mostly nitrogen and oxygen. At extremely cold (but slightly different) temperatures, nitrogen and oxygen become liquids.
Air is chilled to subzero temperatures. As the temperature drops, oxygen becomes liquid and is collected. The nitrogen is still a gas. The air is then made colder until the nitrogen liquefies. It too is collected. Other gases, including argon and carbon dioxide, are also produced through this cryogenic separation.
Aside from oxygen and carbon dioxide, Priestly made many other discoveries with practical applications. One gas he discovered, nitrous oxide, made him light-headed when he inhaled it, even more so than oxygen did. Years later, nitrous oxide became the first surgical anesthetic. Known today as "laughing, gas," it is used by many dentists to help their patients tolerate the pain of drilling and other dental procwures.
Carbon dioxide is widely used in the food industry for freezing meat, poultry, and other food products. Cola, root beer, and all other sodas are made simply by adding sugar and flavorings to Priestly's basic carbonated water. So in a sense Priestly was the father of the modern soft drink industry.
On a trip to London, Priestly met Benjamin Franklin, with whom he became good friends. Priestly's exposure to Franklin, who was well known for his electrical experiments, got Priestly interested in electricity as well.
Priestly decided to write a book on the history of electrical research, for which he did many experiments of his own. In one experiment he discovered that carbon was a good conductor of electricity. Today carbon is used in almost every small battery manufactured. If you buy a battery at the drug store or supermarket, that battery has a carbon rod in its center.
Priestly was also one of the first clietnists to see that electricity could play an important role in chemistry. Years later, an entire scientific discipline - electrochemistry - was developed to study how electricity could be used to cause chemical reactions.
While vvriting his books, Priestly did many of his own drawings in pencil. He found that a certain tree sap, which explorer Charles La Condamine brought back with him from an expedition to South America, was good at rubbing out lemd pencil marks. Priestly was the first to recommend the sap for this application and the name he gave it - rubber, for its ability to rub out the marks - is still used today.
One final note: Thene is a saying among scientists at universities, "Publish or perish." They mean to say that publishing the results of your work is essential to building your reputation as an accomplished practitioner in your.field. Well, Joseph Priestly was smart enough to publish his discovery of dephlogisticated air when he first conducted his experiments in England in l774. And it's a good thing he did so promptly.
Why? As it thappens, a German-born chemist, Carl William Scheele, had discovered oxygen a couple of years earlier, in 1772. But he made the mistake of not publishing his results until 1777. And in science, the discovery of a theory or invention is typically credited to the one who publishes his findings first. So history gives Priestly, not Scheele, the credit as the discoverer of oxygen today.