Although air is invisible, its definite existence has been confirmed in various ways through our daily lives and scientific exploration. Despite being unseen, how can we recognize that air exists? This paper examines the reasons based on concrete data and scientific discoveries.First, while air is invisible to the eye, it demonstrates its existence through its effects. For example, the blowing of wind is direct evidence of air movement, namely air currents. Phenomena such as windmills turning and tree leaves swaying clearly show that a substance called air exerts force on objects.Furthermore, air has mass, which creates pressure. Although we may rarely be conscious of it in daily life, every object on Earth is constantly being pressed by the weight of the atmosphere. This pressure is called atmospheric pressure, and at sea level it is indicated by a value of approximately 1013.25 hectopascals (hPa). This means that a force of approximately 101,325 newtons (N) per square meter is being applied, specifically equivalent to a weight of over 10 tons. Despite such tremendous force, we don't feel this weight because it is balanced by the force from inside our bodies pushing outward.Torricelli, Galileo, and the Discovery of Atmospheric PressureThe existence of atmospheric pressure was first demonstrated in the 17th century by Italian scientist Evangelista Torricelli through experiments using mercury. Torricelli was born in Faenza, Italy, in 1608 and served as secretary to the mathematician Benedetto Castelli in Rome. His research caught the eye of Galileo Galilei, and in 1641, he received an invitation from Galileo to join him as a late-period disciple in collaborative research at Galileo's house in Arcetri.At this time, Galileo was already blind and in the difficult situation of house arrest following his trial by the Inquisition. However, his intellectual curiosity remained undiminished, and he continued his research through dictation. Torricelli was entrusted with solving one of the themes Galileo had been vigorously working on until the end: the long-standing mystery of why pumps could not draw water from wells deeper than about 10 meters. Scientists of the time attempted to explain this phenomenon with the idea that "nature abhors a vacuum" (horror vacui), but Galileo questioned this limitation with water.Even after Galileo's death, Torricelli continued working on this problem. It is said that just before Galileo died, they discussed using mercury instead of water. Mercury is a liquid much heavier than water, and Torricelli reasoned that if atmospheric pressure supported the column of water, the column of mercury should be much shorter.Torricelli then conducted a groundbreaking experiment: he filled a glass tube with mercury, inverted it, and stood it in a container of mercury. He discovered that the mercury stopped at a specific height, approximately 76 centimeters (760 millimeters). The pressure indicated by this mercury column was equal to the atmospheric pressure of the time, proving that air has weight. This experiment also demonstrated that a "vacuum" was created at the top of the glass tube, providing decisive evidence supporting the existence of air. The discovery of this "Torricellian vacuum" directly challenged the prevailing idea that "nature abhors a vacuum" and caused a great shock to the scientific community. Today, electric barometers developed from the principle of mercury barometers are widely used for high-precision atmospheric pressure measurements.Composition and Density of AirAnother important aspect supporting the existence of air is its composition. Dry air is a mixed gas mainly composed of nitrogen and oxygen. By volume, nitrogen (N₂) accounts for approximately 78.1 percent, and oxygen (O₂) for approximately 20.9 percent. The remaining approximately 1 percent includes argon (Ar) at about 0.93 percent and carbon dioxide (CO₂) at about 0.03 percent, along with trace amounts of various other gases. This composition has been precisely measured through analytical methods such as gas chromatography, revealing these ratios.Additionally, air has density. Under standard conditions (0°C, 1 atmosphere), the density of dry air is approximately 1.293 kilograms per cubic meter (kg/m³). This means that air occupying a space of one cubic meter has a mass of approximately 1.293 kilograms. This density allows, for example, airplanes to gain lift and fly, and balloons to float. In everyday examples, when you pump air into a bicycle tire, it becomes hard because the high-density air increases the internal pressure.Furthermore, air is also a medium for transmitting sound. Because sound travels as vibrations in air, we can have conversations and listen to music. The fact that sound does not travel in outer space also shows that the existence of air is essential for sound propagation.These scientific evidence and everyday phenomena confirm the existence of invisible air. The atmosphere surrounding us is the foundation supporting life activities on Earth, and understanding its physical and chemical properties provides essential knowledge for elucidating environmental problems and meteorological phenomena.References"Protect the Air" by Yasufumi Kawamura "The Mysteries of Air" edited by Yoshitaka Arafune, supervised by Shiro Yamazaki "The Study of 'Air'" by Shichihei Yamamoto "Properties of Air" supervised by Kazuo Taya, written by Tatsuji Fujitani "The Science of Air" by Hikaru Shoji