Henry's law, formulated by chemist William Henry in 1803, states, "at a constant temperature, the amount of a given gas that dissolves in a liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid." The solubility of a gas in a liquid depends directly on the partial pressure of the gas over the liquid.
Carbonated soft drinks provide an example of Henry's law. Before a can of soda is opened, the gas above the liquid inside the can is almost pure carbon dioxide at a pressure slightly higher than atmospheric pressure. Because the contents of the can are under pressure, a proportional amount of carbon dioxide is also dissolved in the liquid. When the can is opened and the pressure is released, some of the carbon dioxide gas escapes. A proportional amount of carbon dioxide also escapes from the liquid, arising as bubbles.