Throughout recorded history, people have realized that air is critical to life. Hundreds of years before the birth of Christ, Greek philosophers suggested that air was one of four basic elements, along with fire, water, and earth. For nearly 2000 years most people, even scientists, believed in this idea, or some variation of it. Then in the 1600s and 1700s, the modern science of chemistry was born, and early chemists began to learn that air is made up of different gases. But the process of discovering these gases and understanding the ways that they react with each other took several decades, and it was not until the late 1800s that all of the gases present in air had been isolated and identified.
The first clue that certain gases could be distinguished from ordinary air was discovered around 1630 by Jan van Helmont, who introduced the term gas. During the rest of the century, scientists learned more about air, and the barometer was invented for measuring air pressure. During the 1700s, the individual gases contained in air were being investigated, but chemists were still confused about how to define them. Gases were called " airs" because scientists thought that they were just contaminated forms of air.
Chemists also argued about the role of different gases in the processes of combustion, or burning, and respiration, or breathing. Today we know that these processes use oxygen and produce carbon dioxide. But at that time, scientists had only a vague notion of these basic chemical reactions. At first they tested their ideas by such simple experiments as burning candles inside a glass jar with a limited supply of air, or enclosing birds and small animals in a jar to see how long they were able to breathe. German chemist George Ernst Stahl proposed that a substance called phlogiston was what gave materials their ability to burn. Supposedly, phlogiston was released from burning materials and absorbed by the air. Despite some controversy, the phlogiston theory was widely accepted for many years and resulted in the discovery of several gases.
Then in the 1770s, Antoine-Laurent Lavoisier developed the modern theory of combustion, which replaced the phlogiston theory. Lavoisier proved that air is composed mainly of two gases--one that supports combustion and one that does not. Lavoisier showed that during combustion a portion of the air (about one-fifth) combines with the substance being burned and adds weight to the resulting compound. The rest of the air, which is inactive, is left behind. Then Lavoisier reversed the process, showing that when the compound is decomposed, the original active portion of the air is released. When this portion is mixed with the inactive air, the mixture is the same as ordinary air. Lavoisier named the active gas "oxygen " and called the inactive portion of air azote..
Although Lavoisier clarified the process of combustion, other chemists before him had already discovered most of the gases contained in air and had begun studying their properties. In 1772 Carl Wilhelm Scheele discovered oxygen, which he called " fire air." He also showed that air contains another gas, "foul air," which does not support combustion. But publication of Scheele 's report was delayed, and Joseph Priestley is usually credited with oxygen's discovery. The gas that Lavoisier called azote is known today as nitrogen. It was first identified in 1772 by Daniel Rutherford
In the late 1700s, Henry Cavendish determined that air contains about 21 percent oxygen. More than a hundred years later, William Ramsay discovered that a small percentage of air--about 1 percent--is composed of inert gases, mainly argon. The remaining 78 percent of air is nitrogen, although carbon dioxide is also present in tiny amounts (about 0.03 percent). In a series of some 400 experiments, Cavendish also found that the composition of air is virtually the same, regardless of the air's geographic origin. His results were confirmed during the late 1800s, when more precise scientific studies showed that air's composition is the same all over Earth.
The air's gases are responsible for making the sky appear blue. Gas molecules scatter the Sun's light, and the blue color in the light's spectrum is scattered more thoroughly than other colors. Normally air also contains some moisture, or water vapor, which we measure as humidity, as well as small particles of dust suspended in the air's gases. Moisture and light combine to create such spectacular visual effects as rainbows and sunsets. Unfortunately, today's air also contains many pollutants such as ozone, sulfur dioxide, and nitrous oxide. Besides being dangerous to humans, these air pollutants can also injure plants and animals, damage buildings, and even affect the weather.
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