A eudiometer is a laboratory device that measures the change in volume of a gas mixture following a physical or chemical change. Description Depending on the reaction being measured, the device can take a variety of forms. In general, it is similar to a graduated cylinder, and is most commonly found in two sizes: 50 mL and 100 mL. It is closed at the top end with the bottom end immersed in water or mercury. The liquid traps a sample of gas in the cylinder, and the graduation allows the volume of the gas to be measured. For some reactions, two platinum wires (chosen for their non-reactivity) are placed in the sealed end so an electric spark can be created between them. The electric spark can initiate a reaction in the gas mixture and the graduation on the cylinder can be read to determine the change in volume resulting from the reaction. The use of the device is quite similar to the original barometer, except that the gas inside displaces some of the liquid that is used. History In 1772, Joseph Priestley began experimenting with different "airs" using his own redesigned pneumatic trough in which mercury instead of water would trap gases that were usually soluble in water. From these experiments Priestley is credited with discovering many new gases such as oxygen, hydrogen chloride, and ammonia. He also discovered a way to find the purity or "goodness" of air using "nitrous air test". The eudiometer functions on the greater solubility of NO2 in water over NO, and the oxidation reaction of NO into NO2 by air oxygen: 2 NO + O2 → 2 NO2. A quantity of air is combined with NO over water, and the more soluble compound NO2 dissolves, leaving the remaining air somewhat contracted in volume. The richer the air was in oxygen, the greater was the contraction. Marsilio Landriani was studying pneumatic chemistry with Pietro Moscati when they attempted to quantify Priestley's nitric acid test for air quality. Landriani used a pneumatic trough in the form of a tall, graduated cylinder over water. As it measured the salubrity of air, he called it a eudiometer An associate of Moscati's, Felice Fontana also designed a eudiometer on the same principles and quantified the salubrity of the air. The eudiometer with the nitrous air test was the way Jan Ingenhousz verified that the bubbles given off under water