A property of gases, which states:

The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of the gas that is in contact with the liquid and the solubility coefficient of the gas in the particular liquid.

In other words, the amount of gas that can be dissolved into a liquid depends on the temperature, the pressure, and how well the particular liquid absorbs gases.

This is important in scuba diving because the air breathed is under pressure equal to the pressure of the surrounding water; hence, the deeper you go, the higher the air pressure, and the greater the amount of air that dissolves in the blood. A simple example of Henry's Law can be seen when a soft drink bottle is opened. When closed, the gas (carbon dioxide) in the bottle is under pressure and remains dissolved. When opened, the pressure is removed and the dissolved gas leaves the liquid rapidly in the form of bubbles. When a diver ascends too rapidly, this is what happens. The bubbles in the blood cause an extremely painful disorder known as decompression sickness.

The deeper a diver descends, the higher the pressure of the air he breathes. By Dalton's Law, the partial pressure of each of the gases mixed in the air also increases. Thus, the nitrogen being breathed is under higher pressure, and by Henry's Law more of it is absorbed into the blood than would be on the surface. Nitrogen acts as an anaesthetic in high concentrations, so when divers go deeper, they become subject to nitrogen narcosis.