That invisible hand was coevolutionary life. At any moment, the atmosphere should fall, but for millions of years it doesn't. Since high oxygen levels are needed for most microbial life, and since microbial fossils are billions of years old, this odd state of discordant harmony has been quite persistent and stable. The Earth's atmosphere seeks a steady oxygen level much as a thermostat hones in on a steady temperature. Lower oxygen would be anemic, while greater oxygen would be too flammable.
Dead planets find equilibrium by geological circuits.
Gases, such as carbon dioxide, dissolve in liquids and can precipitate out as solids. Only so much gas will dissolve before it reaches a natural saturation. Solids can release gases back into the atmosphere when heated and pressed by volcanic activity. Thermodynamic entropy draws all chemical reactions down to their minimal energy level. The furnace metaphor breaks down.