Heavy Metal
In 1643, Evangelista Torricelli, an Italian scientist, began to study air. Air must be made of something, he reasoned, and if so that something must press down upon the earth. Torricelli lived centuries before the discovery of atoms and molecules. However, he had probably studied the writings of the Atomists, a school of philosophers in ancient Greece, who believed that everything was made up of small particles called atoms. As a scientist, he was an observant man. He studied the changes in clouds and wind and weather. Air, he thought, must be a real substance, and if so it must exert a pressure on the earth's surface in the same way that stones and humans do. He devised a series of experiments to satisfy his curiosity, to prove that air had mass.

Torricelli commissioned the glassworks in his town to create a long, thin tube that was open at one end and closed at the other. He filled the tube with mercury, a metal element that exists in a semi-liquid state and is much heavier than water. Torricelli placed his thumb over the open end of the tube, turned the tube over, and lowered it into a bowl that was also full of mercury. If his hypothesis was correct, the air's pressure would press down on the mercury in the bowl and balance the weight of some of the mercury in the tube. If he was wrong, the mercury would flow out of the tube and into the bowl.

Much to Torricelli’s delight a great deal of mercury remained in the tube. He proceeded to conduct a series of many experiments, carefully measuring the height of the mercury column in the tube at various altitudes and under various weather conditions. At sea level, on cool, dry days, the column of mercury in the tube stood at a height of 760 millimeters (mm). At higher altitudes, the column of mercury in the tube fell below 760 mm. On warm, damp days at sea level the column of mercury rose above 760 mm. Torricelli reasoned that not only does air press down upon the earth but that altitude and humidity also influence the height of the mercury column in the tube.

Torricelli’s Tube Lives On
We still use instruments similar to Torricelli's "tube." They are called barometers. The rise and fall of a barometer’s needle or its column of mercury help us predict weather changes. Barometers without mercury are called aneroid barometers. They measure the pressure of the atmospheric gases as they press upon a sensitive vacuum-filled metallic drum.

Modern scientists have established a standard that permits them to compare and discuss air pressure. This standard is called "standard temperature and pressure," or STP. The established standard temperature is 59° Fahrenheit and the standard air pressure is 760 mm Hg. "Hg" is the chemical symbol for mercury. Air pressure of 760 mm Hg is called "one atmosphere." For his curiosity and ingenuity, the scientific unit, the Torr, was named in Torricelli’s honor; one millimeter of mercury equals one Torr. (1 mmHg = 1 Torr.)

Counting Molecules in Space Station Alpha
On earth the air we breathe is a blanket of trillions and trillions of gas molecules floating around us and almost 350 miles deep. The major atmospheric gases are nitrogen (N₂), oxygen (O₂), carbon dioxide (CO₂), water vapor (H₂O), argon (Ar), neon (Ne), helium (He), methane (CH₄), krypton (Kr), nitrogen oxide (N₂O), and hydrogen (H₂). Nitrogen represents more than 78% of the total mixture. Oxygen molecules make up more than 21% of the total mix of gases. The human body has both adapted to and depends upon the earth’s atmospheric pressure and unique mix of gases. If the percentage of oxygen falls sharply or drops due to a drop in total atmospheric pressure, health problems can arise.

For the astronauts to breathe and work and remain healthy, the space station’s life support system must maintain an earth-like atmosphere. The computer-monitored barometric instruments in the life support system can detect a sudden or even a gradual drop in air pressure. Such conditions would mean that not only are their fewer total gas molecules in the space station’s atmosphere, but there would also be fewer oxygen molecules to breath. In the space station, the astronauts must monitor and control, if necessary, the atmospheric pressure and the appropriate percentage of molecules of each kind of gas.

Astronauts Pollute Their Own Air
Not only do the astronauts use up the oxygen in the air; they also breathe out carbon dioxide, a poison, and water vapor. Water vapor is considered one of the atmospheric gases and a source of danger in the space station. Water vapor enters the station’s air both from the astronauts’ normal breathing and from the evaporation of water during space station experiments, cooking, and the drying of human perspiration.

Evaporation occurs as the individual H₂O molecules in water are heated. The water molecules become energized, their motion increases, and the bonds between them and the other water molecules break. With the introduction of heat, the individual energized water molecules slip from their liquid state and disperse among the gases in the air.

Condensation occurs when warm, energized water vapor molecules come into contact with cool objects. When cooled the water vapor molecules lose their energy and cling together forming droplets. Examples of condensation are the rain from clouds and the water droplets that form on pipes and the bathroom mirror while someone is taking a shower or bath.

Condensation can damage sensitive electrical circuits in computers and promote the growth of bacteria and molds within the space station. Too little water vapor in the atmosphere, on the other hand, creates an environment in which electrical sparks may occur. Special equipment has been designed to monitor and maintain the proper amount of water vapor in the space station’s atmosphere.