According to Stephen Hawking, around 450 B.C. the Greek thinker Empedocles made the first step towards the discovery of air. Empedocles observed people using an odd ladle-like device. The device was round like a ball, but had holes in the bottom and a straw-like neck at the top. If the device was dunked in water, the ball portion would fill with water. More importantly, he noted that if the opening on the neck was clamped shut, then dunked in water, no water entered through the holes in the bottom of the device.
This was a fairly stunning observation. Empedocles guessed that there must be an invisible mass trapped in the device that prevented water from entering. This invisible mass was later to be identified as air.
To better understand this concept, you can try duplicating Empedocles experiment yourself. Take an empty gallon milk jug with the cap firmly in place. Cut a small hole near the top of the jug, make sure it is no bigger than the tip of your finger. Now cut or poke holes in the bottom of the jug. Then, while firmly holding your thumb over the hole near the top of the jug, dunk the jug in a sink or bowl of water. What happens?
Let me know how your experiment fares. Do you agree with Empedocles?