Water has a certain affinity for most gases. This affinity is more pronounced for some gases than for others; for instance, at atmospheric pressure and at ordinary temperatures, pure water will absorb 4 per cent, of its own volume of air, 4 per cent, of its volume of sulphureted hydrogen, or 100 per cent, of its volume of carbonic acid gas. By increasing the pressure on the water its capacity for absorption is increased in direct proportion. That is, if the pressure be increased to two atmospheres, the temperature remaining unchanged, pure water will absorb 8 per cent, of its own volume of air, 8 per cent, of its volume of sulphureted hydrogen or 200 per cent, of its volume of carbonic acid gas.

Heating water lessens its capacity for absorption in direct proportion to the amount of heat applied. The relative volume of gas absorbed is in all cases directly as the pressure and inversely as the temperature. Thus, if the pressure be increased it will absorb more gas, and if it be heated it will absorb correspondingly less gas. Water is saturated when it has in solution all the gas it can hold. If water is saturated with gas and the pressure is then increased or the temperature lowered, the capacity of the water to hold gas will be increased and it will absorb still more. If water is saturated with gas and the pressure is reduced or its temperature raised, the capacity of the water to hold gas will be reduced and some will be liberated.

It is due to the fact that increasing the pressure of water increases its capacity to absorb gases that necessitates frequent recharging of air chambers in pipe systems. Water usually enters a supply system from a pump or reservoir at atmospheric pressure, saturated with air. As the water becomes compressed, however, its capacity to absorb air is increased, hence, when passing an air chamber the water absorbs air from the chamber, which in turn gradually fills with water.

The fact that decrease of pressure liberates air from saturated waters determines the best place in a system to locate air chambers. When a faucet is opened the pressure of water at that point is considerably reduced; furthermore, in passing through the system of piping within the building the water has become slightly warmed; hence, if an air chamber is located immediately above the faucet, gases liberated from the water will rise into the air chamber and keep it charged.