Mercury is especially suitable for use in manometers (and barometers) since it has a low vapor pressure at ambient temperature, does not wet glass, and has a high density. Manometers have also employed other liquids such as linseed oil or water. The choice of liquid affects the reading on the meter. Boyle's law states that the ratio of the partial pressures of two gases varies in proportion to their absolute temperatures. At constant pressure, the volume of one gas relative to another changes if the temperature changes. For example, if the volume of an air bubble in water increases, this shows that the air is becoming less dense and thus its pressure is dropping. On the other hand, if the volume of the air bubble decreases, this shows that the air is becoming more dense and thus its pressure is rising. In order to measure the absolute pressure of a single gas, you need to know its partial pressure. This can be done using an instrument called a pneumatic gauge. These are very accurate devices that employ an amount of mercury in a sealed container. As the pressure of the gas applied to one side of the container increases, the mercury rises to compensate. When the pressure reaches 100 kPa (10 mmHg), no more mercury will rise. The pressure is recorded by some form of pointer attached to a scale.