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Article of the Month: The History of Air

What is Air Made of? How do we know?

Air is not one thing but many things. It is a mixture of separate molecules, each with their own properties.

We know that oxygen is present in air. If not, breathing as we know it would be impossible. We can infer that other things are present in air as well. A flame burns brightly in the presence of oxygen. As oxygen is added to the flame, the flame burns even brighter. Imagine if oxygen was the most common molecule in the air. What would it be like to light a match in such an environment?

The following history shows how philosophers and scientists viewed matter-the stuff our world is made of-as it relates to air. It also shows how scientific processes were used to find out what air is made of.

Consider the following questions as you read:

Philosophers were the first to try and understand matter. Early Greek philosophers reasoned that all things on Earth came from four basic building blocks: air, earth, water and fire. Aristotle (384-322 B.C.) popularized this idea and added to it. In Aristotle's view, air was not a mixture. It was a fundamental building block and could not be reduced to a simpler form. This explanation lasted for roughly two thousand years.

The early philosophers proposed explanations but did not always test their ideas by doing experiments. Alchemists, the first chemists, did many experiments. The alchemists mainly concerned themselves with creating useful chemicals-medicines, metals, and more. Alchemy was often a secret activity. In many of the alchemists' written histories, important details of their experiments were left out. Because of the secretive nature of the experiments, one could not always believe their results. For example, many alchemists claimed to be able to turn ordinary substances into gold or silver, but not one could do so publicly, nor would they furnish "recipes".

The most influential alchemist lived and worked near what is now Baghdad, Iraq. Jabir al Hayyan (Persia, circa 721-815) created hundreds of new chemicals. Unlike others who hid their findings, he published many detailed recipes. Jabir's contribution to philosophy was also influential. He proposed that all metals on Earth were composed of mercury (a liquid metal) and sulfur (a yellow powder). Later, Paracelsus (Switzerland, 1493-1541) added salt as the third component of all matter, including nonmetals. Both agreed that water and earth played a role in determining the nature of matter, in combination with the other primary ingredients.

Jabir's and Paracelcus's ideas, along with the ideas of Aristotle, appealed to many European philosophers. They were simple and, to the minds of those at the time, logical.

Until the 17th century, philosophers had explained matter but they did not always test their explanations with experiments. Galileo Galilei (Italy, 1564-1642) inspired 17th century philosophers to conduct experiments that could be repeated by others. The steps of his experiments were clearly written. He analyzed his results with mathematics. He supported his results with evidence.

This was the beginning of a scientific revolution.

Galileo's attention to these details inspired others to put Aristotle's ideas to the test. Robert Boyle (Ireland/England, 1621-1691) helped disprove the idea that a vacuum-the absence of air-could not exist. Boyle created vacuums and observed that, without air, a flame would not burn.

Boyle helped make popular the idea that ideas must be tested and the methods of testing must be clear. In 1661, he wrote:

".the surest way is to Learn by particular Experiments."
-The Sceptical Chymist

This type of thinking was revolutionary. It sparked an age of rapid scientific discovery.

Another great experimenter was the Flemish Jan Baptist van Helmont (Belgium, 1579-1644). Van Helmont introduced the word "gas" to common usage. One gas in particular caught his attention-the one created by burning and fermentation (fermentation is the process that causes bread to rise). We know this gas today by the name carbon dioxide. Van Helmont's observations showed that not all gases were the same. Van Helmont's gas gave water a sour taste. As was discovered later, it was actually quite pleasing to drink. Today we add carbon dioxide to soda. This makes the bubbles that give carbonated beverages their distinctive flavor.

Many years passed before anyone proposed that carbon dioxide might be a separate part of air. Yet today we can see this might be a logical conclusion. If burning and fermentation produce carbon dioxide, it must go somewhere. In fact, carbon dioxide levels in the atmosphere have risen steadily in recent years. Human societies burn more and more fuel sources to generate electricity and power motor vehicles. All of this adds to the levels of carbon dioxide in the air.

Oxygen was discovered long before it had a name. John Mayow (England, 1640-1679) showed that a mouse inside a glass container consumed "vital particles" from the original air. As the container had been inverted over a pan of water, the water level rose inside the glass. (The mouse was not directly in the water-it hung from the top of the glass in a basket-like contraption.) He supposed that the amount of the vital particles that were consumed by the mouse would equal the amount of space (volume) that the rising water displaced. Many years later, Joseph Priestley (England, 1733-1804) used mice in a different way to make a related point. He created the gas we know today as oxygen and added it to regular air in a closed container. He knew that a mouse would live no more than fifteen minutes with regular air in a closed container. Yet, after the mouse had spent 30 minutes in the air and oxygen mixture, he retrieved it, unharmed!

Perhaps the cleverest of the chemists in this age of discovery was Antoine Laurent Lavoisier (France, 1743-1794). He did experiments that were very similar to Priestley's; however, he considered the consequences in more detail. This life-giving gas must be a separate component of air, he reasoned. He called it oxygen. Lavoisier saw from his experiments and the works of others that oxygen made up about one-fifth (20%) of regular, dry air. The remainder of air he called "azote" (literally meaning "no life", because animals could not live in it. It did not allow a flame to burn.). This gas would later be called nitrogen. Lavoisier concluded that about four-fifths (80%) of air was made of nitrogen. His conclusion was not completely accurate-air contains other molecules as well, including carbon dioxide. Yet he was the first to suggest that air contained at least two separate-and very different-chemicals.

Lavoisier named another gas, studied carefully by his contemporary Henry Cavendish (England, 1731-1810). This gas was explosive and, when ignited, produced water. Lavoisier named it hydrogen (literally, "water-producing").

Cavendish also discovered a significant component of atmospheric air-a gas that reacted with nothing and did not appear necessary for living organisms. He did not realize the extent of his discovery and did not name this gas, the third most common in the atmosphere. He believed it to be experimental error (incorrectly, as it turns out). Almost one hundred years later, it would be called Argon.

The current estimation of gases in air is as follows:

Gas % in air by volume
Nitrogen (N2) 78%
Oxygen (O2) 21%
Argon (Ar) Almost 1%
Carbon Dioxide (CO2) < 1%
Hydrogen (H2) < 1%
Other gases < 1%