Why did John Dalton argue that air is a mixture rather than a chemical compound?

John Dalton argued that the atmosphere is a physical mixture of gases—specifically approximately 80 percent nitrogen and 20 percent oxygen—rather than a chemical compound, based on his studies of meteorology and gas behavior.1

His conclusion was rooted in his development of the law of partial pressures, which posits that each constituent gas in a mixture exerts the same pressure it would if it were the only gas present in that volume. Dalton conceptualized that in a mixture of gases, atoms of different elements react indifferently toward one another, allowing each gas to behave independently. This physical independence supported his view that the atmosphere was not a chemically bonded compound but a mixture of distinct elemental gases.21